POSIVA OY

POSIVA 98-13

Solubilities of uranium forTILA-99

Kaija Ollila

VTT Chemical Technology

Lasse Ahonen

Geological Survey of Finland

November 1998

Mikonkatu 15 A, FIN-001 00 HELSINKI, FINLAND

Phone (09) 2280 30 (nat.). (+358-9-) 2280 30 (int.)

Fax (09) 2280 3719 (nat.). (+358-9-) 2280 3719 (int.)

~,,.... KEMIANTEKNIIKKA V I I Teollisuusfysiikka

1 (1) TUTKIMUSSELOSTUS NRO KET2058/98

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Tilaus

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JAKELU

VTI KEMIANTEKNIIKKA Teollisuusfysiikka

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9809/97/MVS

Erikoistutkija Kaija Ollila, (09) 456 6341

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Raportissa "Solubilities of uranium for TILA-99"

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Arto Muurinen

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r,

, Puh.vaihde (09) 4561 , . Faksi (09) 456 6390 Http:iJwww. vtt.fi/ ··

Posiva-raportti - Posiva Report

PosivaOy Mikonkatu 15 A, FIN-001 00 HELSINKI, FINLAND Puh. (09) 2280 30- lnt. Tel. +358 9 2280 30

Tekija(t) - Author(s)

Kai ja Ollila, VTI Chemical Technology Lasse Ahonen, Geological Survey of Finland

Nimeke- Title

SOLUBILITIES OF URANIUM FOR TILA-99

Tiivistelma - Abstract

Toimeksiantaja(t)- Commissioned by

Posiva Oy

Raportin tunnus - Report code

POSIV A 98-13

Julkaisuaika- Date

November 1998

This report presents the evaluation of the uranium solubilities in the reference waters of TILA-99. The behaviour of uranium has been discussed separately in the near-field and far-field conditions. The bentonite/ ground water interactions have been considered in the compositions of the fresh and saline near-field reference waters. The far-field groundwaters' compositions include fresh, brackish, saline and very saline, almost brine-type compositions. The pH and redox conditions, as the main parameters affecting the solubilities, are considered.

A literature study was made in order to obtain information on the recent dissolution and leaching experiments of uo2 and spent fuel. The latest literature includes studies on uo2 solubility under anoxic conditions, in which the methods for simulating the reducing conditions of deep groundwater have been improved. Studies on natural uraninite and its alteration products give a valuable insight into the long-term behaviour of spent fuel. Also the solubility equilibria for some relevant poorly known uranium minerals have been determined.

The solubilities of the selected solubility-limiting phases were calculated using the geochemical code, EQ3/6. The NEA database for uranium was the basis for the modelling. The recently extended and updated SR '97 database was used for comparison. The solubility products for uranophane were taken from the latest literature.

The recommended values for solubilities were given after a comparison between the calculated solubilities, experimental information and measured concentrations in natural groundwaters. The experiments include several U02 dissolution studies in synthetic groundwaters with compositions close to the reference groundwaters.

Avainsanat- Keywords

uranium, U02, solubility, simulated near-field and far-field groundwaters, oxidizing, reducing conditions

ISBN ISSN ISBN 951-652-051-0 ISSN 1239-3096

Sivumaara- Number of pages Kieli- Language 57 English

Posiva-raportti - Posiva Report

PosivaOy Mikonkatu 15 A, FIN-001 00 HELSINKI, FINLAND Puh. {09) 2280 30 - lnt. Tel. +358 9 2280 30

Tekija(t) - Author(s)

Kai ja Ollila, VTI Kemiantekniikka Lasse Ahonen, Geologian tutkimuskeskus

Nimeke - Title

TILA-99 URAANIN LIUKOISUUSARVOT

Tiivistelma -Abstract

Toimeksiantaja(t)- Commissioned by

Posiva Oy

Raportin tunnus - Report code

POSIV A 98-13

Julkaisuaika - Date

Marraskuu 1998

Tassa raportissa esitetiliin uraanin liukoisuusarvot TILA-99 turvallisuusanalyyseissa kaytettavissa referenssivesissa. Uraanin liukenemista on arvioitu erikseen Hihialueolosuhteissa ja kaukoalueella. Lahialueen referenssivesien koostumuksissa on otettu huomioon bentoniittilpohjavesi vuoro­vaikutuksia. Mukana ovat makea ja suolainen lahialuepohjavesi. Kaukoalueen referenssivedet ovat makea, murto-, suolainen ja erittain suolainen, lahes 'brine' -pohjavesi. Vesien pH- ja redox­olosuhteiden vaikutuksia liukoisuuteen arvioidaan.

Aluksi tehtiin kirjallisuustutkimus, jossa kaytiin lapi uusimpia U02:n ja kaytetyn polttoaineen liukenemis- ja eluutiokokeita. Niissa on tutkittu mm. U02:n liukoisuutta pelkistavissa olosuhteissa. Kokeissa on kehitetty menetelmia syvan pohjaveden redox-olosuhteiden simuloimiseksi. Luonnon­analogiatutkimuksissa on selvitetty uraniniitin muuntumista hapettavissa olosuhteissa, jonka perus­teella voidaan arvioida kaytetyn polttoaineen kayttaytymista pitkalla tahtaimella. Eraiden huonosti tunnettujen relevanttien uraanimineraalien liukoisuutta on tutkittu.

Valittujen liukoisuutta rajoittavien faasien liukoisuudet laskettiin geokemiallisen mallin, EQ3/6, avulla. Termodynaamisena peruslahtotiedostona oli NEA:n uraanitiedosto. Vertailun aikaan­saamiseksi kaytettiin myos uudempaa SR '97 lahtotiedostoa. Uranofaanin liukoisuustulo otettiin uusimmasta kirjallisuudesta.

Mallinnuksen avulla saatuja liukoisuuksia verrattiin kokeellisen tutkimuksen tuloksiin ja luonnon pohjavesissa mitattuihin pitoisuuksiin. Kokeellinen tutkimus sisa1taa U02:n liukenemiskokeita synteettisissa pohjavesissa, joiden koostumus on lahella referenssivesia. Vertailujen pohjalta esitettiin liukoisuusarvot.

Avainsanat- Keywords

uraani, U02, liukoisuus, simuloidut Hihialue- ja kaukoaluepohjavedet, hapettavat ja pelkisUivat olosuhteet

ISBN ISSN ISBN 951-652-051-0 ISSN 1239-3096

Sivumaara- Number of pages Kieli - Language 57 Englanti

7

TABLE OF CONTENTS

Abstract

Tiivistelma

1 INTRODUCTION .................................................................................................. 9

2 REFERENCE GROUNDWATERS ..................................................................... 1 0

3 SOLUBILITIES OF URANIUM IN THE NEAR-FIELD ......................................... 13

3.1 Reducing conditions .................................................................................. 13

3.1 .1 Dissolution mechanisms of spent fuel .............................................. 13

3.1.2 Solubility-limiting solid phases .......................................................... 14

3.1.3 Solubility studies of U(IV) in different parametric conditions ............. 15

3.1.4 Calculated solubilities in the near-field groundwaters ....................... 20

3.1.5 Experimental dissolution studies with U02 and spent fuel

in synthetic groundwaters ................................................................ 23

3.1 .6 Summary and recommended solubility values for uranium

in the near-field groundwaters under reducing conditions ................ 26

3.2 Oxidizing conditions .................................................................................. 28

3.2.1 Dissolution mechanisms of spent fuel .............................................. 28

3.2.2 Solubility-limiting solid phases .......................................................... 29

3.2.3 Calculated solubilities in the near-field groundwaters ....................... 31

3.2.4 Experimental dissolution studies with U02 and spent fuel

in synthetic groundwaters ................................................................ 33

3.2.5 Summary and recommended solubility values for uranium

in the near-field groundwaters under oxidizing conditions ................ 35

4 SOLUBILITIES OF URANIUM IN THE FAR-FIELD ............................................ 37

4.1 Reducing conditions .................................................................................. 37

4.2 Oxidizing conditions .................................................................................. 38

4.3 Measured uranium contents in natural groundwaters ................................ 39

4.4 Recommended solubility values for uranium in the far-field groundwaters 43

5 SUMMARY ......................................................................................................... 45

8

REFERENCES ............................................................................................................ 47

APPENDIX 1 ............................................................................................................... 54

APPENDIX 2 ............................................................................................................... 55

APPENDIX 3 ............................................................................................................... 56

APPENDIX 4 ............................................................................................................... 57

9

1 INTRODUCTION

The purpose of this study is to evaluate the solubilities of uranium in the reference groundwaters of TILA-99. The solubilities of all the other elements of importance are discussed in Nuorinen et al. 1998/. Uranium is dealt with separately, because there is extensive literature on the U02- groundwater system during the years after the TV0-92 safety assessment Nieno et al. 1992/. Several experimental solubility studies for U02 in synthetic groundwaters with compositions close to the reference waters have been per­formed /Ollila 1995, 1996, 1997, 1998/. In those experiments, the methods for simulat­ing the reducing conditions of deep groundwater have been improved. Studies dealing with uraninite as a natural analogue for the long-term behaviour of spent fuel uo2 have been published.

The near-field ground waters include fresh and saline compositions, which were planned based on experimental and modelling studies on the bentonite/ ground water interaction. The far-field ground waters include fresh, brackish, saline and very saline, almost brine­type compositions. They are based on groundwater studies within the site investigations for spent fuel. The pH and redox conditions, as the main parameters affecting the solu­bilities, are considered.

The solubilities are calculated using the geochemical code, EQ3/6. The NEA thermody­namic database /Grenthe et al. 1992/ serves as the basis for the calculations. Some new data concerning the relevant uranium minerals and aqueous species has been published after the NEA publication. This data is used in some calculations.

Finally, the calculated solubilities will be compared with the uranium concentrations measured in the dissolution and leaching studies of unirradiated uo2 materials and spent fuel, as well as the concentrations measured in natural groundwater conditions. These include an overview of Finnish groundwaters.

The solubilities are presented as recommended values. The aim is to give well-justified solubility values with less conservatism, as they are compared with the TV0-92 Nieno et al. 1992/ and TILA-96 Nieno and Nordman 1996/.

10

2 REFERENCE GROUNDWATERS

The Finnish reference conditions were chosen based on the site investigation data from the four candidate sites studied for the final disposal of spent nuclear fuel: Olkiluoto in Eurajoki, Hastholmen in Loviisa, Romuvaara in Kuhmo, and Kivetty in Aanekoski. Ta­ble 2-1 shows the compositions of the granitic reference groundwaters.

Table 2-1. Composition of the granitic reference groundwaters (RE refers to reducing condi­tions and OX to oxidizing conditions) /Vuorinen and Snellman 19981.

ALLARD FRESH-RE FRESH-OX BRACKISH-RE Basic logpC02=-4.0 logpC02=-3.5 HH-KR3

Depth m 336 Eh measured m V -216 Eh calculated m V <-300 -200 Eh calculated m V -280 Ionic strength M :::.0.006 :::.0.006 :::.0.2 pH 8.3 8.8 ** 8.4 ** 7.7 Alkalinity meq/L 2 1.07 1.49 1.61 DIC mg/L 19.4 Si02 mg/L 8 0.8 1.4 8.3 TOC mg/L 1.3 Ca mg/L 18 5,1 10 680 Na mg/L 52 52 52 2 600 Mg mg/L 4.3 0.7 2.8 340 K mg/L 3.9 3.9 3.9 26 Fe( tot) mg/L 2.3 Fe(II) mg/L max. 2.7 ** 2.3 AI mg/L 0.67 0.67 0.030 Mn mg/L 0.68 0.68 2.3 Ba mg/L 0.55 0.55 Sr mg/L 0.196 0.196 6.986 Cs mg/L 0.034 0.034 0.004 B mg/L 0.2 NJ4 mg/L 0.15 0.15 1.4 Cl mg/L 53 52 47 5 290 F mg/L 5.2 5.2 1.6 Br mg/L 16 I mg/L 0.41 0.41 0.05 N03 mg/L 0.02* N02 mg/L 0.02* P04 mg/L 1.1 * 1.1* S(-II)tot mg/L max. 3 ** 0.11 so4 mg/L 9.6 9.6 ** 9.6 710 H-3 TU 0.8 U(tot) Jlg/L 7.52 U-234/U-238 5.22 Rn Bq/L 2 200 C-14 pM 15 N2(g) m1/L 40.0 * C02(g) milL 1.14 * H2(g) ml/L 0.003 * CH4(g) ml/L 0.025 * TDS mg/L :::.150 :::.140 :::.140 :::.9 800

*) uncertain values due to analytical problems. **) these parameters are affected by the redox buffers acting in the system.

***) due to lack of sound knowledge

SALINE-RE BRINE-RE OL-KRI OL-KR4 613.5 861 -270 -3 *

:::.0.5 :::< 1.3 8.3 7.8** 0.3 0.2

1 3.3 5.3 2 4000 15 700 4 800 9 750 56 110 21 22 0.92 2.45 0.77 2 0.056 0.003 0.61 2,2 <0.5 35 160 <0.02 0.12 0.92 0.9 0.24 0.03 14 800 43 000 1.2 1.6 105 348 0.85 0.6 0.02 * <0.02 * 0.17 * 0.04 * 0.03 0.05 0.84 <1 2.8 0.8 0.02 0.054

1.27 44* 43.6 3 38 *** .... 480

0.05 268

26,2 990 :::.24 000 :::.70 000

11

The 'brine' reference groundwater is nearly brine groundwater from Olkiluoto KR 4 (TDS ~ 70 g/1). Blank spaces in Table 2-1 indicate missing values (not analyzed or in­cluded in the composition).

For estimating the near-field reference conditions, experimentally measured laboratory data from the bentonite/groundwater interaction was used /Muurinen et al. 1998/. Table 2-2 shows the near-field reference waters and the range of pH values to consider. The chosen pH range is large and is based on the variations encountered in natural groundwa­ter and the effects seen in the bentonite groundwater interaction experiments. The ben­tonite-groundwater system is discussed in more by Vuorinen and Snellman /1998/.

Table 2-2. Composition for the near-field reference waters /Vuorinen and Snellman 1998/.

FRESH- FRESH- SALINE-NEAR- SALINE-NEAR-OX NEAR-RE ox NEAR-RE

Eh chosen Np(V) UOz/U409 Np(V) UOz/U409 pH 7.0-10.0 7.0-10.0 7.0-9.0 7.0-9.0 Alkalinity meq/L 7.0 7.0 1.4 1.4 Ionic strength M z0.3 z0.3 z}.1 ""'1.1

TDS mg/1 14 500 14 500 34 500 34 500 SiOz mg/L 23 23 10 10 Na - " - 4 400 4 400 12 000 12 000

K - " - 50 50 90 90

Ca - " - 190 190 1 600 1 600

Mg - " - 58 58 340 340

AI - " - 0.8 0.8 0.2 0.2

Mn - " - 0.7 0.7 2.6 2.6

Ba - " - 0.8 0.8 2.7 2.7

Sr - " - 0.3 0.3 67 67

Cs - " - 0.04 0.04 0.2 0.2

B - " - 0.3 0.3 1.7 1.7

NH4 - " - 0.2 0.2

Cl - " - 420 420 17 000 17 000

F - " - 6.2 6.2 3.5 3.5

Br - " - 3.0 3.0 140 140

I - " - 0.5 0.5 2.3 2.3

N03 - " - 0.3? 0.3? 0.7? 0.7?

NOz - " - 0.1? 0.1? 0.1? 0.1?

P04 - " - 1.3? 1.3? 0.2? 0.2?

S( -II)tot - " - 4.0 4.0

so4 - " - 9 400 9 400 3 200 3 200

?) questionable values due to analytical problems in analysis of the ground water samples

12

Due to the choice of each reference water having a range of pH, oversaturation of calcite occurred at higher pH values. In order to avoid having an oversaturation of calcite in the reference waters they were equilibrated in respect of calcite. This, on the other hand, resulted in somewhat lower values of alkalinity in the high-pH area than has been en­countered in the sampled high-pH groundwaters.

The near-field reference waters, especially the fresh one, indicated a high saturation of calcite. As presently there is not enough knowledge of the bentonite porewater system, the high saturation was accepted, but alongside also calcite-limited Ca and Ctot concentra­tions were used in modelling the solubilities. The detailed data is given in Nuorinen and Snellman 1998/.

For the reducing near-field groundwaters, the redox range -200 ... -400 mV was consid­ered. The effect of canister corrosion products was simulated by calculating the solubili­ties at the magnetite/hematite redox potential, Table 2-3. For the reducing far-field groundwaters, the redox constraints used were based on the redox processes interpreted to occur at the investigation sites. The Eh values were obtained by modelling (EQ3/6) for the chosen pH ranges, Table 2-3.

The redox control under oxidizing conditions was the the oxygen-fixed gas fugacity. The fugacity was chosen so that the predominant oxidation state for neptunium was the Np(V). The modelling exercise resulted in choosing the 0 2 fugacity of 1 o-10 for all oxi­dizing reference conditions, see Table 2-3.

Table 2-3. Calculated (EQ3/6) Eh values (25 °C) for the near- and far-field groundwaters at the pH range considered and the controlling processes indicated.

Reference water redox control pH Eh [V]

fresh near-field oxid. log fOz = -10 7.0 - 10.0 0.667 - 0.490

......... ;;··························;~~·········· ···~~-g~~·1i1~ih·~~~ii·1~················ ··7~a···:···~o~a·············· ··~o"."2s4···~···:·o:4·r3···············

...................................................................................................................................................................................................... saline near-field oxid. log fOz = -10 7.0 - 9.0 0.667 - 0.549

·········;;·························;~:··· .. ····· ···~~·g~~·ii'i~ih"~~~iii~················ ···7~a···:···9:c;················ ... ~o·:254···~···~o:3·73···············

fresh far-field oxid. log fOz = -10 7.0 - 10.0 0.667 - 0.490

·········;;·························;~.··········· ···~·~i.j;h"id~~~~~p·h~1~/py~·ii·~··· ···6:9···-:_····io~~·············· ···~a·.'2o·5····_:···:·o:4·i·s···············

...................................................................................................................................................................................................... brackishfar-field red. goethite/Fe 7.2 - 9.0 -0.205 - -0.343

...................................................................................................................................................................................................... saline far-field red. sulphide/sulphate/pyrite 7.0 - 9.2 -0.195 - -0.340

13

3 SOLUBILITIES OF U IN THE NEAR-FIELD

3.1 Reducing conditions

3.1.1 Dissolution mechanisms of spent fuel

Under reducing conditions, i.e. in the redox regime where U(IV) is the predominant oxidation state, the U02 (or U409) matrix of spent fuel is considered to be thermody­namically stable. This means that the fuel itself is the solubility-limiting phase. The dis­solution reaction,

U02 + (n-2) H20 ~ U(OH)n <4-n)+ + (n-4) H+

is chemical in nature /Johnson et al. 1988, Shoesmith and Sunder 19911 and it attains equilibrium in the absence of oxidants. U4+ is hydrolyzed or complexed in solution. The reaction, U02(fuel) ~ U(IV)aq is very slow. Bruno et al. /1988/ have measured a disso­lution rate of- 4 · 10-5 g m-2 d-I, which is independent of pH (7 ~ pH ~ 11). The con­centration of uranium in solution is limited by the solubility of the dissolving solid, be­ing very low (~10-9 mol/1) in the absence of oxidants. For these conditions, a solubility­based dissolution model has been developed to determine the rate of spent fuel dissolu­tion /Shoesmith and Sunder 1991/. In this model, the mobilization of most fission prod­ucts and actinides is controlled by the solubility of the U02 solid. The release, however, of sparingly soluble radionuclides depends on their solubilities.

The uo2 in the fuel is crystalline. Therefore, the solubility of crystalline uo2 is consid­ered to limit solubility under strongly reducing conditions in the absence of oxidants. The solubility of U02 clearly increases even under mildly reducing (oxidizing) condi­tions due to surface oxidation. When oxidation is confined to the region below uo2.33 (U 307 ), the fluorite structure is maintained. Under these redox conditions the oxidative dissolution rates are anticipated to be extremely low, and dissolution can be considered still chemical /Shoesmith and Sunder 1991 I. Another uranium oxide considered as a solubility-limiting phase in the solubility-limited dissolution model is the mixed valence oxide, U409 (U02.2s), which can be formed by the oxidation ofU02 without significant disruption of the spent fuel matrix /Lemire and Garisto 1989/. This phase is formed by diffusion of oxygen into the U02 matrix /Garisto and Garisto 1985/. U409 has been identified as an oxidation product in grain boundaries of spent fuel (175 - 195°C) /Einziger et al. 1992, Thomas et al. 1993/. No U307 was observed to form. The presence of impurity cations and fission products would seem to inhibit the oxidation of U02 in spent fuel to oxides higher than U409.

14

3.1.2 Solubility-limiting solid phases

Only U02 or U40 9 needs to be considered as the solubility-limiting phases under re­ducing conditions. Coffinite, USi04, may become stable relative to U02 in groundwa­ters with a high silica content /Langmuir 1978/. Nearly insoluble U02 (or USi04) limits the solubility in low-Eh environments. At intermediate Ehs, however, surface oxidation and dissolution is clearly enhanced, particularly when carbonate is present. The uranium concentration in solution is still low. The complex formation with carbonate affects the relative stabilities of the oxidation states of uranium. This makes the U(VI) stable in more reducing conditions than in the absence of carbonate and increases solubility. The relative amounts of the U(VI)-carbonate complexes and the U(IV)-hydroxide complexes depend on carbonate content, pH and Eh /SKI Project-90 19911. Bruno et al. /1998/ have calculated the solubility curve for U02(fuel) depending on the bicarbonate content in water, Figure 3-1. The calculations were performed by using the bentonite pore-water composition, at pH= 9.21 and Eh= -200 mV. According to the graph, the solubility markedly increases at a free carbonate content of 10-3 mol dm-3 due to the stabilization ofthe U02(C03)34- complex.

: 1.00E-04 (.) t/)

C) ..2 1.00E-05 -..... 0 ..... 5' 1.00E-06 .....

1.00E-07 --t====+===~-+---~

1.00E-05 1.00E-04 1.00E-03 1.00E-02

[HC03-]free (log sea le)

Figure 3-1. Solubility of uranium as a junction of the .free bicarbonate concentration /Bruno et al. 19981.

The solubility of U02 depends on the crystallinity and the size of the particles of the solid. In the NEA uranium database /Grenthe et al. 1992/, two oxides ofU(IV): U02(c) (uraninite) and U02(am) have been included. U02(c) corresponds to a well-crystallized solid. The data for U02(am) reflects the properties of an amorphous solid that is ob­tained by precipitation in alkaline aqueous solutions. In the SKB uranium database, an additional uranium dioxide, U02(fuel), has been included /Bruno et al. 1986, Puigdo­menech and Bruno 1988/. It is considered as an intermediate solid between U02(c) and U02(am), which corresponds to the average particle size ofU02 in spent fuel (1-5 Jllll).

15

U02(fuel) was assumed to limit the solubilities of uranium under reducing conditions in the TV0-92 safety assessment Nieno et al. 1992/.

U02 (or U409) is the solubility-limiting phase for uranium, if there is no other solid phase with a lower solubility. Grambow /1998/ has presented that the supposed solubil­ity difference between spent fuel U02 (1 o-7 mol/1) /SR 95/ and pure U02 (1 o-9 mol/1) could provide a thermodynamic driving force of phase transformation. U02(pure) would precipitate as a secondary phase controlling the uranium concentration in solution. Moreover, in silica-containing groundwaters, coffinite (USi04), can be more stable than U02 depending on the dissolved silica levels. Langmuir /1978/ concluded that natural occurrences of coffinite suggest it may be stable relative to U02( c) when dissolved silica levels in groundwater exceed 1 o-3 M. A somewhat lower limit, 2.6 · 104 M, has been presented by Goldhaber et al. /1987/. The reaction between coffinite and U02(c) may be written:

USi04( c) + 2 H20 = U02( c) + fuSi04

Based on the NEA database, the log K for the reaction would be -3.17. Accordingly, the silica concentration, 6.8 · 104 M, would make coffinite more stable than U02(c). Typi­cal dissolved silica concentrations in Finnish groundwaters vary from 1 · 104 to 3 · 104 M Nuorinen and Snellman 1998/. The alteration of uraninite to coffinite (coffinization) has been investigated in the natural analogue studies /Janeczek and Ew­ing 1992/. Uraninite may be significantly altered under hydrothermal (100-300 °C) re­ducing conditions through dissolution, loss of Pb and other elements, and by coffiniza­tion. These processes are well documented at the natural analogue sites at Oklo and Ci­gar Lake. The conditions favourable for coffinization of U02 are high silica concentra­tion and low Eh.

The key thermodynamic data required to predict the aqueous behaviour ofU(IV) are the hydrolysis constants. For uranium, well documented thermodynamic data are available /Grenthe et al. 1992/. They selected the hydrolysis constant, log ~14 =- 4.5, for the reac­tion:

U4+ + 4 H20 {:::) U(OH)4(aq) + 4 H+ .

This selection has been discussed in Kristallin-1 by Bemer /1995/, because the solubility determinations reported in recent literature give varying values. He suggested using the Rai et al. /1990/ data, which predicts lower stability for U(OH)4(aq). Bruno et al. /1998/ selected the value, log K= -5.3, for use in SR'97. The recent solubility determinations found in the literature are discussed in the following section. The formation constants derived from those studies for U(OH)4(aq) are presented for comparison. The NEA thermodynamic data for uranium evaluated by /Grenthe et al. 1992/ is mainly adopted in this study for solubility calculations.

3.1.3 Solubility studies of U(IV) in different parametric conditions

The solubility of U(IV) has been studied by several authors. The extent of the dissolu­tion is very dependent on the morphology of the solid phase. The solubility of amor­phous uranium dioxide /Bruno et al. 1987, Rai et al. 1990/, hydrous U(IV) oxide /Rai et

16

al. 1995/, crystalline U02(s) Najima et al. 1995/, unirradiated U02(fuel) /Torrero et al. 1991, Ollila 1995, 1996/ has been determined in the latest literature. The solubility de­terminations have been made in different aqueous media as a function of pH and ionic strength:

- 0.008, 0.1, 0.5, 1, 5 M NaCl04, pH 2- 12

- 1, 5 M NaCl, pH 2-12

- HC03-- CO/-- OH-- H20 system

- (1 - 9.8) · 1 o-3 M NaHC03

- deionized water, pH 2- 12

The dissolution experiments performed in synthetic groundwaters of uo2 and spent fuel will be discussed in Section 3 .1.5.

Solubility determinations made at 25 oc are considered in this report. However, the re­ported data varies over orders of magnitude. In practice, there are difficulties in main­taining the reducing conditions in the experiments, problems with sorption and with re­moval of fine particles from solution /Grenthe et al. 1992/. In earlier studies, the use of analytical methods of insufficient sensitivity has probably caused problems. U(IV) is very easily oxidized to U (VI) in the presence of trace oxygen content, and oxidation in­creases with increasing pH. This was clearly observed, when testing a method for the determination of U oxidation states in anaerobic (N2) aqueous solutions /Ollila 1996/. The oxidation state of uranium was determined for anaerobic (N2) aqueous solutions (including two synthetic groundwaters) after contact with unirradiated uo2 pellets for 500 days in the glove box. According to the analyses, the uranium was mainly(> 90 %) at the hexavalent state. A conclusion was made, that a typical level of trace oxygen of < 1 ppm in the gas of the glove box filled with inert gas (N2) is enough to cause slightly oxidizing conditions for uranium in the absence of reducing agents.

Bruno et. al. /1987 I measured the solubility of amorphous U02 in a pH range of 2 to 10.5 in 0.5 M NaCl04. U02(s) was precipitated by reducing an acidic uranium(VI) per­chlorate solution with H2(g), using Pd as the catalyst, followed by a pH adjustment with NaOH. The solubility of this material was 10-4.4 M. It was independent of pH in the pH range 5.5 to 10. The U(IV) speciation in this pH range in the absence of other com­plexation agents (i.e. carbonate) can be explained by assuming the formation of the hy­drolysis complex U(OH)4. Based on these solubility measurements, the hydrolysis con­stant, log ~14 = -5.4 ± 0.2, was derived.

The solubility measurements of Rai et al. /1990/ for amorphous U02 gave results, that were considerably lower. They conducted experiments with amorphous U02 in deoxy­genated deionized water in the pH range 2- 12. The solubility was approached from both the oversaturation and undersaturation directions. Precautions were taken to mini­mize oxidation during the experiments. The experiments were performed in a glove box with a N2 atmosphere (a few ppm of oxygen). Iron powder and Eu2

+ were used to elimi­nate 0 2 and to maintain reducing conditions (low Eh) in the aqueous phase. The meas­ured uranium solubilities at pH values > 4 were around 1 o-8 M, which was also the de­tection limit of their analytical method. A fair number of samples, especially between pH 10-12 showed higher uranium concentrations. Arthur and Apted /1996/ presented the

17

measurements of Bruno et al. /1987/ and Rai et al. /1990/ with the earlier results of Gayer and Leider /1957/ and Ryan and Rai /1983/ in the same graph, see Figure Al-l (App.1 ). The insufficient control of the redox state of uranium, the influence of reducing agents (metallic Fe) on the crystallinity ofU02(s), sorption ofU(IV) species onto possi­ble oxide alteration products ofFe0 have been presented as possible reasons for the large difference between the experiments. The estimated upper-limit value for the equilibrium constant for the formation of U(OH)4 was several orders of magnitude lower (log K < -12.0) than earlier presented.

Figure A1-2 (App.1) shows the solubility ofU02(s) measured by Yajima et al. /1995/, compared with the results ofBruno et al. /1987/ and Rai et al. /1990/. The solubility of U02(s) was examined in dilute NaCl solutions in the pH range from 2 to 12. The ionic strength of each sample was adjusted to 0.1 M by adding NaC104. Both oversaturation and undersaturation experiments were carried out. The U(IV) stock solution was pre­pared by electrolytic reduction from uranium nitrate solution. The oxidation state of uranium in the experiments was maintained as tetravalent by adding sodium hydrosul­phite (Na2S204) as the reducing agent to the solutions in a prepurified Ar atmosphere. In the undersaturation experiments, the U02(s) sample was identified as crystalline, while in the oversaturation experiments the crystallization of the precipitations ofU02(s) pro­gressed gradually. At pH above 3, the uranium solubilities were essentially independent of pH (Figure Al-2, App. 1). The results obtained by oversaturation and undersaturation experiments showed good agreement. The progress of crystallization had little effect on the solubility. The data of this study is 4 to 6 orders of magnitude lower than the results for the solubility of U02(am) reported by Bruno et al. /1987/, and 0.5 to 2 orders of magnitude lower than the results of Rai et al. /1990/ for U02(am). Based on these solu­bility measurements, the hydrolysis constant, log ~ 14 = -9.0 ± 0.5, was calculated.

Torrero et al. /19911 studied the effect of high salinity on the solubility of crystalline unirradiated U02(s). They used three different ionic media: 0.008 M NaC104, 1 M and 5 M NaCl. The XPS observations of the reacted solids did not show the presence of solid surface phases other than uranium dioxide, with an upper oxidation state ofU02.1. The solid phase had a maximum particle size of 50 J..lm. Reducing conditions were ensured by flushing H2(g) through the test solutions in the presence of a Pd catalyst. The solu­bilities measured in different media in these undersaturation experiments as a function of pH are presented in App. 2. The hydrolysis constant, log ~14 = -5.7 ± 0.1, derived from the solubility measurements in NaC104 (Figure A2-l, App. 2), is comparable to the one obtained in the work ofBruno et al. /1987/ for a solid of an amorphous morphology. The dependence on pH of the measured U02(s) solubility in chloride solutions (Figure A2-2, App. 2) is practically the same at the two different NaCl concentrations. Only in the pH range between 6 and 7 is there a slight difference. For the pH values higher than 6, the measured uranium concentration reached the same value obtained in the perchlo­rate medium. The log ~14 = -5.5 ± 0.2 was calculated from the measurements at both ionic strengths.

The effect of carbonate concentration on the solubility of amorphous U02, has been studied by Rai et al. /1995/. The solubility was determined in HC03, C03, and mixed OH-C03 solutions extending to high carbonate concentration. The procedure consisted of precipitating the U02 · xH20(am), suspending it in appropriate carbonate solutions,

18

and equilibrating the suspensions for different periods. The experiments were conducted in the presence of Fe powder to maintain low redox potential in solution in an atmos­pheric control chamber with an Ar atmosphere. The solubility ofU02(am) increased in both high carbonate and bicarbonate solutions, see Figure A3-1 (App. 3), which shows the results of the precipitation tests in Na2C03 solutions (0.01 M NaOH). At carbonate concentrations below 1 o-2 M, the solubility did not increase with the increase in carbon­ate. The measured aqueous U concentrations decreased with the increase in the equili­bration period from 3 days up to 43 days. The reason was discussed by the authors to be the presence ofU(VI) which was subsequently reduced to U(IV) with time. The possible effects of metallic Fe, as presented earlier (p. 17), cannot be excluded. The reason for the increase in solubility for U02(am) in high carbonate solutions was explained by the presence of the U(IV) pentacarbonate complex, U(C03)s6-.

The effect of bicarbonate concentration on the solubility of unirradiated crystalline U02 pellets has been studied under anaerobic (N2) conditions by the author /Ollila 1995/. The experiments were performed in (1 - 9.8) · 10-3 M NaHC03 solutions. This range covers the range of the carbonate contents of the reference waters of TILA-99, the highest car­bonate content of 7 · 10-3 M being in the near-field fresh groundwater. No reducing agents were added to the aqueous phase. The oxygen content in the N2 atmosphere of the glove box was low,< 1 ppm. The solubility did not increase with the increase in bi­carbonate in this range of the content, see Figure A4-1 (App. 4). There was a slight in­crease in solubility compared with the solubility in deionized water, Figure A4-2 (App. 4). This is possibly due to the formation of U(VI)-carbonate complexes (cf. Fig­ure 3.1). The oxidation state of uranium in the aqueous phase was determined at the end of these experiments /Ollila et al. 1996/. The results of the analyses showed that the dominant oxidation state of uranium was the hexavalent state. The measured solubilities were compared with the calculated solubilities obtained with EQ3/6 using NEA's and SKB's databases for uranium (App. 4). The concentrations at steady state were lower than the theoretical solubilities of U02(fuel), but clearly higher than the solubilities of well-crystallized U02(c). They were at the level of the theoretical solubility of the mixed valence oxide, U409(c) (U02.2s).

Table 3-1 summarizes the solubility determinations discussed in this section.

19

Table 3-1. Solubility determinations of U(JV) in anoxic conditions.

Solid phase Aqueous Solubility Hydrolysis Reference media constant

log ~u

U02(am) 0.5MNaC104 10-4.4 M -5.4 ± 0.2 Bruno et al. pH 5.5- 10 1987

uo2 · xH20 (am) deionized water ::= 10-8 M ::= -12 Rai et al. 1990 (Fe powder or EuCh) pH 5- 10

crystalline U02(s) 0.1 MNaCl ::= 10-9 M -9 ± 0.5 Yajima et al. (0.0 1 M Na2S204) 1995 pH 4- 12

crystalline unirradi- 0.008 M NaC104 5.0 (± 1.3) · 10-8 M -5.7 ± 0.1 Torrero et al. ated U02(s), particle pH 5- 10 1995 size::;; 50 Jlm

" 1 MNaCl 7.9 (± 1.6) · 10-8 M -5.5 ±0.2 " pH 6- 10

" 5 MNaCl 5.0 (± 2.0) · 10-8 M -5.5 ±0.2 " pH 6- 10

uo2 · xH20 (am) 0.001-0.01 M Na2C03 ::= 10-8 M - Rai et al. 1995 (0.01 M NaOH) Fe powder

crystalline unirradi- deionized water 8.2 (± 0.7) ·10-9 M* - Ollila 1995 ated U02(s) pellets pH7.0

" 0.001 M NaHC03 1.7 (± 0.1) ·10-8 M* - .. pH9.1

" 0.002 M NaHC03 2.2 (± 0.2) ·10-8 M* - " pH9.1

0.0045 M NaHC03 1.8 (±0.1) ·10-8 M* - " pH9.1

" 0.0098 M NaHC03 2.2 (± 0.2) ·10-8 M* - " pH9.1

*) the bracketed value is the standard deviation of the measured U concentrations at steady state (App. 4)

20

3.1.4 Calculated solubilities in the near-field groundwaters

The EQ3/6 code (Version 7.2b) developed by the Lawrence Livermore National Labo­ratory lW olery 1992al was used to calculate uranium solubilities in the reference groundwaters. The EQ3/6 code is a theoretical model based on the premise of chemical equilibrium conditions. The DataO.com.R2 thermodynamic database by Lawrence Livermore National Laboratory lW olery 1992al, the accompanying database in the EQ3/6 package, was mainly adopted in this study. This composite data file includes the compilation of thermodynamic data for uranium minerals, aqueous species and gases evaluated by the Nuclear Energy Agency /Grenthe et al. 1992/. A recently extended and updated database by Bruno et al. /1998/, SR '97, was used for comparison. This data­base includes the SKBU thermodynamic data for uranium /Bruno and Puigdomenech 1989/. The difference between the DataO.com.R2 and SR '97 databases is the selection of the formation constant for U(OH)4(aq), as discussed earlier. The SR '97 (log K= -5.3) results in lower solubilities than the DataO.com.R2 (log K = -4.5). The recent measure­ments of the U(IV) solubility support the value used in the SR '97 database, see Table 3-1. An even lower stability for U(OH)4(aq) has been predicted /Rai et al. 1990, Yajima et al. 1995/. Otherwise these databases are in agreement, as the data affecting the solu­bilities under reducing conditions is considered. The SR '97 database includes U02(fuel), representing the solid with the average particle size of U02 in spent fuel. This phase has not been included in the NEA database.

The correction of the ionic strength was performed with the B-dot equation /Helgeson et al. 1969/. The activity coefficients in this option are treated as functions of the ionic strength but not of the specific aqueous solution composition. This is realistic in dilute solutions where the ionic strength is less than or equal to 1.0 molal /Wolery 1992b/. The granitic groundwater compositions considered in this study are sufficiently dilute, the highest ionic strengths ranging from 0.3 ... 0.6 M, for this correction to be applicable.

As discussed earlier, the U02 (or U409) matrix of spent fuel is considered to be thermo­dynamically stable under reducing conditions, the solubility of U02 being very depend­ent on the crystallinity and the size of the particles of the solid. Consequently, U02 (c), U02(fuel) and U409 were used as the solubility-limiting phases in the calculations. U40 9 is also considered because this oxide can be formed by the oxidation of U02 without significant disruption of the matrix. U02(am) is formed by precipitation from solution.

The most important parameter affecting the calculated solubilities is the Eh and the car­bonate content of the aqueous phase. Under reducing conditions, in the absence of car­bonate, the solubility is independent of Eh. The pH has no influence in the pH range of 6-10. This was shown by several experimental solubility studies (Table 3-1). The pres­ence of carbonate makes the situation more complicated. One of the reference ground­waters, the fresh near-field groundwater has a high carbonate content (7·10-3 M). It is somewhat decreased, when equilibrated with calcite (3 ·1 o-3

) Nuorinen and Snellman 1998/. Bruno et al. /1998/ calculated (EQ3/6) that the solubility of uranium (U02(fuel)) markedly increases at free carbonate contents higher than 1 o-3 M (Figure 3-1 ). The rea­son was the stabilization of the U(VI) carbonate complex, U02(C03) 3 4- in the negative redox regime. Figure 3-2 gives the calculated solubilities (SR '97 database) as a func­tion of Eh in both near-field groundwaters. The U02(fuel) was assumed to be the solu­bility-limiting phase.

21

1 o-3 -r-------------J--' U02(fuel} 1

10-4

I I ' I

I

10-6 J / I I

I 4 1 o-7 ·~-~-==~·.----A.--

-400 -300 -200 -100

Eh [mV]

- fresh near-field (pH 8.6) saline near-field (pH 8.1)

Figure 3-2. Solubilities of uranium as a function of Eh in the near-field groundwaters.

In fresh near-field groundwater with a high carbonate content (7 · 10-3 M), the calculated solubility starts to increase early in the negative redox regime due to U02(C03)3 4-, while in saline near-field ground water with a moderate carbonate content ( 1.4 · 1 o-3 M), the solubility remains low. Unfortunately, only limited experimental data was available for the solubility of U(IV) compounds in bicarbonate solutions under reducing conditions. Rai et al. /1995/ concluded that the solubility of U(IV) hydrous oxides increases in both high bicarbonate {> 1 o-1 M) and carbonate (> 1 o-2 M) solutions due to the presence of U(IV) pentacarbonate complexes, U(C03)56-· In the solubility experiments of unirradi­ated U02 pellets in (1 - 9.8) · 10-3 M NaHC03 solutions under anaerobic (N2) condi­tions, there was a slight increase (2.4 fold) in solubility compared to the solubility in deionized water {Table 3-1 ). No increase in solubility was observed with the increase in bicarbonate in the studied range (App. 4) /Ollila 1995/. Puigdomenech et al. /1990/ sug­gested the formation of a mixed U(IV)-hydroxide carbonate complex, U(OH)3C03-, in reducing conditions.

The solubilities in the near-field groundwaters were calculated at the magnetite/hematite redox potential (-254 ... -413 m V, pH 7.0 ... 9.0) in order to simulate redox conditions when groundwater gets into contact with canister corrosion products. In the EQ3/6 simulation, the equilibrium with hematite is assigned to 02(g) and the equilibrium with magnetite is the constraint assigned to Fe2

+. Additionally, the solubilities were calcu­lated at Eh= -200 m V. Tables 3-2 and 3-3 summarize the results of the modelled solu­bilities. The values in brackets give the solubilities for in-respect-of-calcite equilibrated groundwaters. This has a lowering effect at higher Eh (-200 m V) in fresh near-field groundwater, where the U(VI) carbonate complexation increases the solubility.

22

Table 3-2. Modelled solubilities for uranium in fresh near-field groundwater under reducing conditions (values in brackets: calcite-equilibrated groundwaters).

Limiting solid Solubility, M Solubility, M pH 8. 6/Eh -200 m V pH 8. 6/Eh -350 m V*

··n~1-;;o~·~~~~·R2···········1··sii··;97··························· ··n~i~o:~~;;,:m··········1··sii··;97··································

uo2 (c) 1.5 . 10-9 8.5 . 10-10 3.9 . 10-10 7.6. 10-11

(4.3 . 10-10) (1.1 . 10-10

)

U409 (c) 2.3 . 10-8 1.3 . 10-8 1.1 . 10-7 2.1 . 10-8

(8.1 . 10-9) (1.9 . 10-9

)

uo2 (fuel) - 1.4. 10-6 1.3 . 10-7

(1.8 . 10-7)

~---------r-----~-----------------~------------------------Aqueous U02(C03)3 (73%) U02(C03)34- (91%) U(OH)4 (aq) U(OH)4 (aq) speciation U(OH)4 (27 %) U(OH)4 (9 %)

Table 3-3. Modelled solubilities for uranium in saline near-field groundwater under reducing conditions.

Limiting solid Solubility, M Solubility, M pH 8.1/Eh -200 m V pH 8.1/Eh -350 m V*

··n~l;;ii"~~;;,~ia··········T·sR"··;9i·························· ··n~i~ii·~~;;,:m·········rsii··;97··································

3.8 . 10-10 7.4. 10-11 3.8 . 10-10 7.4. 10-11

2.0 . 10-9 1.1 . 10-7 2.1 10-8

uo2 (fuel) 1.3 . 10-7 1.2 . 10-7

r---------r-----------r-----------~-------------------------Aqueous U(OH)4 (aq) U(OH)4 (aq) U(OH)4 (aq) U(OH)4 (aq)

speciation

*) magnetitelhematite redox potential

23

1Q-6 ..,.-----------------,

........ 10-7 6 ~ :.0 :::J 0 CIJ

:::> 1 o-s ~.. 11

'~ ,. ~

I I "

, I

I

1 Q-9 +----~----r------,---------1

7 8 9

pH

10 11

· fresh near-field saline near-field

Figure 3-3. Solubilities of uranium as a function of pH in the near-field groundwaters (Eh= -200 m V).

As can be seen in Tables 3-2 and 3-3, the solubilities given by the SR '97 database are lower, when the U(IV) hydrolysis reaction determines the solubility. In fresh near-field groundwater, especially at high pH, the calculated solubility (DataO.com.R2) at Eh= -200 m V is increased due to the formation ofU02(C03)34-, Figure 3-3. At the mag­netite/hematite redox potential, the U(VI) carbonate complexation has no effect on the solubility. In saline near-field groundwater, the solubility decreases with the increase in pH according to the modelling calculations.

The aqueous speciation under reducing conditions is dominated by the U(OH)4(aq) in both fresh and saline near-field groundwaters. Only in the case of higher Eh with fresh composition, is the dominant complex the U02(C03)34-·

3.1.5 Experimental dissolution studies with U02 and spent fuel in synthetic groundwaters

The dissolution and leaching experiments in synthetic groundwaters with varying com­position were performed using unirradiated uo2 materials, like sintered polycrystalline U02 fuel in pellet or crushed form, U02 powder, simulated fuel (Simfuel), and high­active spent fuel. These experiments, especially with spent fuel, have often been planned for studying dissolution mechanisms, as well as dissolution rates, e. g. flow-through tests ITait and Luht 1997 I. However, the steady-state concentrations measured in the static dissolution experiments with sufficiently long contact periods, can be used for comparison with the calculated solubilities. Bruno et al. I 1997 I interpreted these steady­state concentrations in terms of apparent solubility limits. Similar difficulties, as was discussed in connection with U(IV) solubility studies (3.1.3), in maintaining reducing conditions are related to these experiments. In the studies with spent fuel the effects of

24

alpha- and gamma-radiolysis cannot be excluded. The majority of the studies with spent fuel were performed under oxidizing conditions. Tables 3-4 and 3-5 present the steady­state concentrations measured for U in the anoxic experiments with unirradiated U02 and spent fuel, respectively. The concentrations measured for other actinides and fission products have been included. These solubilities are discussed by Vuorinen et al. /1998/.

Table 3-4. Measured steady-state solution concentrations (M) in the dissolution experiments of unirradiated uo2 (s) in synthetic groundwaters under anoxic conditions.

Solid phase Aqueous media Test method Steady-state Reference solution concen-

tration (M)

crystalline, Allard groundwater static (batch) 1.9 (± 0.2) · 10-8 M* Ollila 1995 unirradiated I= 0.005 M 550 days U02(s) pellets [HC03-] 0.002 M anaerobic (N 2)

pH9.0 27-29 oc

" Bentonite groundwater " 2.7 (± 0.5) · 10-8 M* " I= 0.014 M [HC03- ] 0.009 M pH8.9

" modified Allard ground- static (batch) 1.7 (± 0.6) · 10-9 M* Ollila, 1997, 1998 water, I= 0.003 M 60 days calcite equilibrium, anaerobic (N z) [HC03-] 0.001 M reducing species: pH 9.0 ... 9.4 (S2-) Ehmeas.: -300 ... -250 m V 27-29 oc

" saline groundwater static (batch) 1.8 (± 0.4) · 10-9 M* " I= 0.5 M 270 days no [HC03-] anaerobic (N z) pH 8.8 ... 9.6 reducing species: Ehmeas.: -300 ... -200 m V (S2-), 27-29 oc

natural UOz, Allard groundwater static (batch) 1-2 · 10-7 M Quifiones et al. Simfuel pH 8.5 ... 9 290 days 1998

anaerobic (Nz) H2 bubbling (Pt) 25 oc

Unirradiated saline groundwater static ""'10-8 M Stroes-Gascoyne VOz (SCSSS), I= 1.37 M, 450 days et al. 1993 Candu fuel pel- [HC03-] 1.6 mM Ar-3%H2 - flushed lets pH7 titanium vessels,

100 oc

Simfuel " " ::::: 10-8 M "

Pu-doped fuel " " == 10-8 M " (0.5 wt% of fissile Pu)

Cigar Lake Ore: " " == 10-8 M " uraninite voz.zs - uo2.33

*) the bracketed value is the standard deviation of the measured U concentrations at steady state

25

Table 3-5. Measured steady-state solution concentrations (M) in the dissolution experiments of spent fuel in synthetic groundwaters under anoxic conditions.

Solid phase Aqueous media Test method Steady-state Reference solution concentration

(M)

FueVclad segments, Bicarbonate ground- static (sequential), U: (2.2- 2.9) · 10-7 M F orsyth 1997 PWR reference rod, water (Allard), contact time: burnup: I= 0.005 M 1555- 2722 days, Np: (2.8- 6.9) · 10-11 M 43 MWd/kgU [HC03-] 1.9-2.4 mM anoxic using ground- Pu: (1.1-1.6) ·10-10 M (specimens: 7.10, pH 7.8-7.9 water after prolonged (contact period 3) 7.11, 7.12) contact with crushed ---

rock, Tc: 6 · 10-9 M (average) 20-25 °C Forsyth and

Werme 1992

FueVclad segments, Bicarbonate ground- static (sequential), U: 5.8 · 10-8- 4.0 · 10-7M Forsyth 1997

BWRfuel, water (Allard), contact time: burnup: I= 0.005 M 370 days, Np: (4.6- 9.6) · 10-12 M Forsyth and 41-48 MWd/kgU [HC03-] 0.7-0.8 mM anoxic by means of Pu: 4.1 · 10-11

-- Eklund 1995 (specimens: 11.6, pH 9.3-9.4 flowing H2/ Ar, 1.1 · 10-10 M 11.9, 11.15) 20-25 °C Cm: 8.4 · 1 o-14

-- Werme and 6.7 · 10-13 M Spahiu 1998 (contact period 6)

Candu fuel, synthetic groundwater, static, U: 10-8 - 10-7 M Tait et al. segments I= 0.27 M contact time: Tc: 10-9

- 10-8 M 1991 pH 7.5-7.8 490 days, Sr: 5 · 10-8 M [HC03 -] 0.9 mM Ar/3%H2/0.02%C02 - Cs: 5 · 10-7 M

flushing, carbon steel coupons, titanium autoclaves, 95 °C

Candu fuel, saline groundwater static, U: 10-8 - 10-7 M Stroes-fragments (SCSSS), I= 1.37 contact time: Gascoyne

[HC03 -] 1.6 mM 450 days et al. 1993 pH7 Ar-3%H2 - flushing,

titanium autoclaves, 100 °C

Generally, the steady-state concentrations are reached for U and also for the other acti­nides rapidly in the experiments under anoxic conditions. In the dissolution experiments with U02 pellets in Allard and saline groundwater at low Eh (-300 ... -200 m V), the con­centration ofU stabilized in a few days /Ollila 1997, 1998/. The actinide concentrations in the experiments with spent fuel have been found to be independent of contact time. The results in Allard groundwater suggest that a precipitation of an actinide-rich phase has occurred /Werme and Spahiu 1998, Forsyth and Werme 1992/. The small amounts of precipitates made the identification difficult.

Measured uranium concentrations range between 1 o-9 • • • 4 · 1 o-7 M in all experiments

with different solid phases in different aqueous media, see Tables 3-1, 3-4 and 3-5. Only the solubility of amorphous U02 /Bruno et al. 1987 I is higher. The lowest solubilities (10-9 M) were measured in the anoxic dissolution experiments with unirradiated U02 (s)

26

in the presence of reducing agents N ajima et al. 1995, Ollila 1997, 1998/. The compo­sition of the aqueous phase, e.g. salinity, seems to have a minor effect in the studied conditions. The redox conditions obviously play a dominant role. Typically, under an­aerobic conditions, the measured steady-state concentrations of U vary 1 o-8

• • • 1 o-7 M. In contrast to the results of modelling, the solubility experiments of unirradiated U02 pel­lets under anaerobic conditions did not show any clear increase in solubility in bentonite groundwater due to the high carbonate content (0.009 M) /Ollila 1995/. This is in agreement with the solubility experiments in NaHC03 solutions with, varying bicarbon­ate content under similar conditions, see Table 3-1. The solubility stays at a low level in spite of the increasing HC03- content. A small amount of carbon dioxide was lost from the test vessels with a resulting increase in pH (9.1) and a decrease in the total carbonate content (6-12 %) /Ollila 1995/. In bentonite groundwater, a precipitation of calcite and eo-precipitation of uranium cannot be excluded. As mentioned earlier, there is limited experimental data available for the solubility of U(N) compounds in bicarbonate solu­tions under reducing conditions.

3.1.6 Summary and recommended solubility values for uranium in the near­field groundwaters under reducing conditions

In deep natural groundwaters and in groundwater in contact with the repository's engi­neered barriers, the pH and redox conditions are such that U02 is thermodynamically stable. Under these anoxic reducing conditions U(N) is dominant and the fuel matrix itself, U02 (U409), of spent fuel is the solubility-limiting phase. U(N) primarily forms hydroxide complex in the aqueous phase. The U 0 2 in the fuel is crystalline. Therefore the solubility of crystalline U02 is considered to limit the solubility in reducing condi­tions. The solubility is very low, ~ 1 o-9 M, in the absence of oxidants. The solubility of U02 clearly increases even under mildly reducing conditions due to surface oxidation. Another uranium oxide, often considered as a solubility-limiting phase in the solubility­limited dissolution model, is the mixed valence U(N)(VI)-oxide, U40 9 (U02.25), which can be formed by the oxidation ofU02 without significant disruption of the fuel matrix. The complex formation with carbonate affects the relative stabilities of the oxidation states by stabilizing U(VI) in more reducing conditions than in the absence of carbonate. The relative amounts of the U(VI) carbonate complexes and the U(N) hydroxide com­plexes depend on the carbonate content, pH and Eh of the groundwater.

Consequently, only U02 and U409 needs to be considered as the solubility-limiting phases in the near-field groundwaters under reducing conditions. The solubility of U02 depends on the crystallinity and the size of the particles of the solid. U02(fuel) reflects the solubility of an intermediate solid between well-crystallized and amorphous solids, which corresponds to the average particle size ofU~ in spent fuel.

The important parameter affecting the modelled solubilities is the selection of Eh. In fresh near-field ground water with a high carbonate content, the calculated solubility starts to increase early in the negative redox regime due to the stabilization of U02(C03) 3 4-, especially at higher pH. The solubility in the saline composition remains low. The simulation of the effects of canister corrosion products, magnetite and hema­tite, resulted in strongly reducing conditions for uranium (Eh= -350 m V). In this case

27

the U(IV) hydrolysis reaction determines the solubility. Unfortunately, only limited ex­perimental data is available for the solubility of U(IV) compounds in bicarbonate solu­tions. The solubility studies with unirradiated uo2 pellets under anaerobic conditions, which obviously represent mildly reducing conditions for uranium, did not show any increase in solubility with the increase in bicarbonate in the range covering the contents of the reference ground waters.

For uranium, there is well-documented data available by Grenthe et al. /1992/, which served as the basis for the calculations. A recently extended and updated database by Bruno et al. /1997/, SR '97, was used for comparison. The solubilities given by SR '97 are somewhat lower, when the U(IV) hydrolysis reaction determines the solubility, due to the different selection of the U(IV) hydrolysis constant. The several recent measure­ments of the U(IV) solubility in different parametric conditions support the lower solu­bilities.

The measured uranium solubilities range between 10-9 ... 4 · 10-7 M in the experiments with different crystalline solid phases, including unirradiated uo2 materials, natural U02 and spent fuel U02, in different aqueous media. The redox conditions obviously play a dominant role. In the leaching studies with spent fuel, the radiolysis effects can­not be excluded. The composition of the aqueous phase, even if it was NaC104 or syn­thetic groundwater (or higher salinity), had a minor effect.

The recommended solubilities for U are given in Table 3-6. The solubility-limiting solid phase is thought to be U02(fuel) or U409(c). The modelled solubilities are in good agreement with the experimental solubility studies.

Table 3-6. Recommended U solubilities in the near-field groundwaters under reducing con­ditions.

Limiting solid Fresh near-field Saline near-field pH8.6 pH8.1

············································•········································· ············································-············································ Solubility, M Solubility, M Solubility, M Solubility, M Eh= -200mV Eh= -350mV Eh= -200 m V Eh= -350mV

(magn./hem.) (magn./hem.)

U409 (c) 10-8 ••• 10-7 M 10-8

••• 10-7 M 10-8 ••• 10-7 M 10-8

••• 10-7 M U02(C03)3 4- U(OH)4(aq) U(OH)4(aq) U(OH)4(aq) U(OH)4(aq)

uo2 (fuel) 10-7 •• .10-6 M 1 · 10-7 M 1 · 10-7 M 1 · 10-7 M

U02(C03)34- U(OH)4(aq) U(OH)4(aq) U(OH)4(aq) U(OH)4(aq)

Recommended values 1 · 10-7 M 1·10-7 M

28

3.2 Oxidizing conditions

3.2.1 Dissolution mechanisms of spent fuel

Even though non-oxidizing conditions are expected to prevail in the near-field, the pos­sibility that alpha-radiolysis will maintain oxidizing conditions at the fuel surface needs to be considered. The conditions inside the iron canisters will be complex due to pro­gressing iron corrosion and alpha-radiolysis of the intruding water. Oxidative dissolu­tion occurs only if the 0/U ratio at the surface of uo2+x is higher than 2.33 (U307, the stoichiometric limit of the fluorite structure). The surface oxidation state is influenced by the geochemical environment and by radiolysis.

Under oxidizing conditions the solubility of uranium is affected by the dissolution mechanisms of the fuel matrix. uo2 is not thermodynamically stable. The oxidation of uo2 leads to the formation of progressively higher oxidation states of uranium and an oxidized surface layer with a composition of U 02+x , where 0 ~ x ~ 1. This phase is sparingly soluble under relatively reducing conditions. Once the oxidation state reaches U02.33 , uranium solubility may increase by several orders of magnitude. The U02/U02+x matrix is unlikely to achieve a solubility limit, although the alteration prod­ucts do. Hence the system will evolve to form thermodynamically stable secondary phases. The formation of the alteration products depends on the composition of groundwater. The solubility of uranium increases until saturation with respect to the al­teration product is reached. The uranyl phases in equilibrium with groundwater will ul­timately control the concentration of uranium in solution. In the near-field conditions the mechanisms of dissolution are highly dependent on the extent of oxidation caused by alpha-radiolysis.

The study of uraninite and its alteration products provided valuable insight into the long-term behaviour of spent fuel in the natural environment /Finch and Ewing 1990, 1992/ /Janeczek and Ewing 1993/ /Cramer and Smellie 1994/. The most common natu­rally occurring uranium mineral, uraninite (U02+x), chemically and structurally resem­bles spent fuel U02. Uraninite is stable over a wide range of pH in reducing to moder­ately oxidizing environments. Although the mechanisms of corrosion of spent U02 fuel may differ somewhat from that of unirradiated uo2 ' the long-term oxidative alteration pathway for uo2' uraninite and spent nuclear fuel is considered to follow similar pat­terns /Bruno et al. 1995/. The following scheme has been proposed for the oxidation and alteration of the spent nuclear fuel matrix in a near-field groundwater/bentonite me­dium:

1) Initial surface oxidation of the U02 matrix:

uo2 + (x/2) 02 <=> uo2+x

2) Full oxidation to uranyl oxide hydrates, including sometimes Ca2+ or K+:

U02+x + ((1-x)/2) 02 + 2H20 <=> U03 · 2H20 (schoepite)

6U02+x + 02 + Ca2+ + (16-6x) H20 <=> Ca(U02)604(0H)6 · 8H20 + (10-12x) W

(becquerelite)

29

3) Final alteration to either silicates and/or phosphates depends on the Si02/P04 ratio in the contacting waters. In the case of a bentonite/ ground water system the formation of silicates is favoured:

Ca(U02)60 4(0H)6·8H20 + 6Si02 + 2Ca2+ + ?H20 <=> 3Ca[(U02)(Si030H)h·5H20 + 4W

(uranophane)

The corrosion of spent fuel and U02 under oxidizing conditions has been widely stud­ied. Nevertheless, there was only a few studies available in literature, with sufficiently long contact times to analyze the solid phases through the alteration pathway. Wronkiewicz et al. /1992, 1997 I studied the alteration behaviour of unirradiated U02 pellets in a silica-bicarbonate simulated groundwater (pH 8.4, [HC03-] 0.002 M, TDS 255 mg/1) under atmospheric conditions (90 °C) during a contact time of 10 years. The end result was the development of a paragenetic sequence of alteration phases, in which the formation of schoepite was a transient event and the alteration then proceeded to the formation ofbecquerelite, soddyite, uranophane and other Ca-D-silicates. A conclusion was drawn that the groundwater composition plays a critical role in determining which secondary minerals will form during spent fuel alteration under disposal conditions.

3.2.2 Solubility-limiting solid phases

In the long term, the thermodynamically most stable phases are uranyl silicates and uranyl phosphates. Their formation is very dependent on the concentrations of silica and phosphates in the groundwater. They also may incorporate trace elements which are relevant in the repository environment /Grambow et al. 1990/. According to the min­eralogical observations, the most abundant secondary uranium phase in granitic envi­ronments in Scandinavia is uranophane /Bruno et al. 1995/. In Finland, in HyrkkoHi Cu-U mineralization, which is studied as a natural analogue for copper canisters, urano­phane has been analyzed probably as a secondary uranium phase under oxidizing condi­tions /Marcos and Ahonen 1998/. Schoepite is thermodynamically stable in water with low activities of dissolved Ca and silica.

In the short term, the kinetically favoured solid phase is schoepite. The initial decompo­sition of uraninite produces uranyl oxide hydrates, not higher anhydrous oxides (like U307, U30s) /Finch and Ewing 1992/. Uraninite seems to be an analogue of irradiated U02 of spent fuel in terms of oxidation mechanisms. The presence of impurity cations (e.g. Pb, Ca) in uraninite and fission products in spent fuel seem to stabilize the cubic structure and inhibit oxidation to oxides higher than U409. There is no convincing evi­dence for the occurrence of natural U307 /Janeczek et al. 1996/. The structural limit of oxidation for spent U02 fuel has been shown to be U409 by several oxidation tests and reaction product analyses of spent fuel /Einziger et al. 1992, Thomas et al. 1993/.

Unfortunately, there is a limited knowledge of the solubilities of uranyl silicates, which are abundant and important for controlling the concentration of uranium in natural wa­ters. Only a few relevant uranium minerals have been included in the NEA database. No uranyl silicates were selected. Uranophane and soddyite are included in the composite data file (DataO.com.R2). Uranyl silicates are less soluble than uranyl oxide hydrates and the replacement of uranyl oxide hydrates by uranyl silicates lowers the concentra-

30

tion of uranium in solution. The solubilities of uranyl phosphates are generally below the solubilities ofuranyl silicates /Finch and Ewing 1992/.

In the recent literature, the solubility equilibria ofbecquerelite, soddyite and uranophane have been studied /Casas et al. 1992, 1994, Finch et al. 1995/. The relative stability fields calculated by Bruno et al. /1995/ for schoepite, becquerelite, soddyite and urano­phane based on the new thermodynamic data by Casas et al. I 1992/, suggest that schoepite, although kinetically favoured, is thermodynamically unstable in natural groundwaters. The most probable thermodynamic reaction path for the evolution of spent fuel, Figure 3-4, in granitic/bentonite groundwater was suggested. The initial ki­netically favoured oxidation of uo2 to uo2+x and schoepite is followed by evolution towards uranophane.

_:I I

Log (Ca2+) -10 f

-15

• ·--- ... becquerelite

soddyite

-zo~~~~--~~~--~~ -zo -ts -to -5 0

Log (H4Si04)

Figure 3-4. Thermodynamic reaction pathway for the alteration of schoepite in grani­tic/bentonite groundwater (pH 8) /Bruno et al. 19951.

Schoepite has been observed to form as a secondary solid phase in some dissolution and leaching studies of unirradiated uo2 pellets and spent fuel under oxic conditions. Schoepite or a closely related compound (dehydrated schoepite, U03·0.8H20) was iden­tified in long-term leaching experiments of spent fuel in deionized water /Forsyth et al. 1990, Stroes-Gascoyne et al. 1986/. In a parallel experiment in synthetic groundwater (Allard), no such deposit was found /Forsyth et al. 1990/. Schoepite was also identified, probably as a secondary phase, in the dissolution experiments with powdered uo2 in deionized water, in 0.002 M NaHC03 solution and in synthetic groundwaters (Allard groundwater, bentonite groundwater) after a contact time of 280 days. After a contact time of 1300 days, the concentrations of U in synthetic groundwaters decreased. In Al­lard groundwater, a uranyl silicate, possibly sodium boltwoodite, was identified /Ollila and Leino-Forsman 1993/. Wilson and Shaw /1987/ and /Wilson 1990ab/ studied the dissolution of spent fuel in J -13 water at 25 and 85 °C. The composition of this silica­bicarbonate water (pH 8.4) resembles the composition of Allard groundwater, the silica content being higher (1.2 mM). At room temperature, secondary amorphous precipitates

31

formed. The compositions pointed to uranyl silicates (possibly hai­weeite: Ca(U02)2(Sh05)3 · 5 H20. At 85 °C, uranophane was identified. Bates et al. /1990/ studied the reaction of unirradiated U02 in both powdered and pellet form with dripping silica-bicarbonate groundwater (EJ-13, pH 8.4, [HC03-]:2mM, Si: 1.6 mM) at 90 °C. These experiments indicated that the uo2 matrix transformed into a series of secondary phases. First, the formation of schoepite was observed on the surface ofU02 . Thereafter, the uranium release decreased and a second set of secondary phases was ob­served. The latter phases included boltwoodite (K(H30)U02(Si04)·nH20), uranophane ( Ca(U02)2(Si030H)2· SH20), sklodowskite (Mg(U02)2(Si030H)2· SH20), compreignacite (K2U60 19·11H20) and schoepite (U03·2H20). Finn et al. /1993, 1994/ studied the dis­solution of spent fuel under similar conditions. Soddyite and schoepite were found in the leachates.

3.2.3 Calculated solubilities in the near-field groundwaters

Under oxidizing conditions, schoepite and uranophane were considered as solubility­limiting solid phases in the modelling calculations. The composite database (DataO.com.R2) was used. The redox constraint used was the oxygen-fixed gas fugacity (log 0 2 fug. = -1 0) Nuorinen and Snellman 1998/.

The thermodynamic data for uranophane has not been included in the NEA database. The composite database includes the log K= 17.29 for uranophane /Langmuir 1978/. This value results in solubilities which are at the level of the solubilities of schoepite or higher. Generally, uranyl silicates are presented to be less soluble than schoepite /Finch and Ewing 1992/. The recent determinations of the solubility product for uranophane found in literature show some discrepancy, Table 3-7, but they are in agreement with Finch and Ewing /1992/ in predicting lower solubility for uranophane.

Table 3-7. The solubility products for uranophane.

log Kso = 17.29 (Data0.com.R2) /Langmuir 1978/

log Kso = 9.42 /Nguyen et al. 1992/

log Kso = 7.8 /Casas et al. 1994/

log Kso = 11.7 /Casas et al. 1997 I

The solubilities calculated as a function of pH (7 ... 1 0), assuming schoepite and urano­phane to be the solubility-limiting phases, are given in Figure 3-5. The solubilities of both solid phases are higher in the fresh near-field groundwater due to the high bicar­bonate content. The pH has a small effect in the fresh composition, while in the saline composition the solubilities increase at higher and lower pH values. Table 3-8 summa­rizes the solubilities and aqueous speciation for uranium in the near-field oxidizing

32

groundwaters. The selection of the log K for uranophane has a clear effect on the solu­bility.

10-2

10-3

~ 10-4

>. ~ :.0 10-5 ::J 0 en ::::> 10-6

10-7

10-8 7

.._ schoepite-fresh ----------e---. 0... , ---0 _

0 __ 0 __ ~~9J.8ane-fresh

' _...._.

"\ \

~ *' _.... _.... -;choepite-saline

\ uranophane-saline "' ___ -£

8

.... -9

pH

10 11

Figure 3-5. Solubilities of uranium as a function of pH in fresh and saline near-field ground­waters under oxidizing conditions (uranophane, log K = 11. 7).

Table 3-8. Modelled solubilities for uranium in the near-field groundwaters under oxidizing conditions.

Limiting solid

Schoepite

Uranophane log K= 11.7 log K= 9.42 log K= 7.80

Fresh near-field pH 8. 6/log 02 fug. = -10

Saline near-field pH 8.1/log 02fug.= -10

···s~~~b-ilir;··M··r···············-~~~;:~·-·············· ····s~z;;btittY:""ii··-1·-···············-~~~;:~·-·············· 7.89 ° 10-4 U02(C03)3 4- (91 °/o) 1.28 . 10"5 (U02hC03(0H)3 -(52 %)

(U02)2C03(0H)3-(6 %) U02(0H)2 (aq) (26%)

U02(C03)/- (3%) U02(C03)34- (13 %)

3.90 . 10-4 6.14·10"6

4.55 . 10"5 U02(C03)3 4- 4.61 . 10"7 U02(C03)3 4-

7.40 . 10"6 7.16 . 10"8

33

3.2.4 Experimental dissolution studies with U02 and spent fuel in synthetic groundwaters

A general result of the dissolution experiments with unirradiated U02 pellets and spent fuel performed in the presence of atmospheric oxygen is that the solution concentrations of uranium and other actinides (Pu, Np, Am, Cm) attain a steady state after a certain time period /Forsyth and Werme 1992, Werme and Spahiu 1998, Grambow 1989, Wil­son 1990bc/, suggesting that actinide releases are solubility-controlled. Table 3-9 pres­ents the steady-state concentrations measured in those experiments. The concentrations measured for other actinides have been included for comparison. These solubilities are evaluated in another report by Vuorinen et al. /1998/.

In Allard groundwater, the results of the experiments with unirradiated U02 pellets /Ollila 1995/ and with spent fuel /Forsyth and Werme 1992, Forsyth 1997/ in the pres­ence of air are in good agreement. The steady-state is typically attained at the concentra­tion of (1-3) · 10-5 M. This is difficult to correlate with the solubility calculated assum­ing chemical equilibrium. It is one order of magnitude lower than that given by the solu­bility of schoepite /Ollila 1995/. Secondary alteration products were not found in these experiments. The reason may be the small amount of possibly forming alteration prod­ucts, which is difficult to analyze. Schoepite was identified, probably as a secondary phase in Allard groundwater, only in the agitated dissolution experiments with U02 powder using a high SN ratio. The surface ofU02 powder was oxidized in air prior to the initiation of these experiments, which may have had an effect on the dissolution mechanisms. The uranium concentration dropped at a longer contact time. A uranyl sili­cate (possibly sodium boltwoodite, (Na,K)(H30)U02(Si04)·H20) was found when ana­lyzing the solid phase /Ollila and Leino-Forsman 1993/. The solution concentration in the experiments with uo2 pellets and spent fuel appears to be controlled with respect to some kind of oxidized phase. It has previously been observed in evaluating solubility­limiting (steady-state) factors, that assuming a redox potential controlled by the U 301/U 30s equilibrium, a good agreement could be obtained between calculated and measured data /Grambow 1989, Forsyth and Werme 1992, Ollila 1995/.

The dissolution experiments in bentonite groundwater with U02 pellets /Ollila 1995/ suggested a different solubility control. This synthetic groundwater simulates the effects of bentonite in fresh granitic groundwater /Snellman 1988/. It has a high bicarbonate content of 9 mM. Nevertheless, the steady-state value for uranium was relatively low, < 10-6 M, being at the level of the solubilities ofuranyl silicates. No secondary phase of U was identified. The coprecipitation of U with calcite cannot be excluded. Calcite pre­cipitates were identified in the aqueous phase at the end of the experiments.

The steady-state measured in the leaching tests with spent fuel in J-13 groundwater is at the same level as in Allard groundwater /Wilson 1990abc/. J-13 is a silica- bicarbonate groundwater, resembling Allard groundwater in composition, see Table 3-9. In the ex­periments performed in stainless steel vessels, lower U release was measured, possibly caused by the presence of iron. Uranophane was identified at 85 °C as a secondary phase. In the leaching experiments in salt brine /Gray 1987 I with iron coupons, a de­crease in the solution concentration of U was observed.

34

Table 3-9. Measured steady-state solution concentrations (M) in the dissolution experiments of uo2 and spent fuel in synthetic groundwaters under oxidizing conditions.

Solid phase Aqueous media Test method Steady-state Reference solution concentra-

tion (M)

crystalline, Allard groundwater air, 25 °C, 1.24 (± 0.14) · 10-5 M* Ollila 1995 unirradiated I= 0.005 M static (batch), U02(s) pellets [HC03-] 2mM 2000 days,

pH 8.4 SN= 1.8 m-1 **

" " air, 25 °C, 1.61 (± 0.18) · 10-5 M* " static (batch), 1500 days, SN= 18m-1

" 0.002 M NaHC03 air, 25 °C, 4.62 (± 0.50) · 10-5 M* " pH8.0 static (batch),

2000 days, SN= 1.8 m-1

" Bentonite ground- " 8.34 (± 0.90) · 10-7 M " water I= 0.014 M [HC03-] 9 mM pH8.9

PWR,BWR, Bicarbonate ground- air, 20 - 25 °C, U: 7.00 · 10-6 M- Forsyth 1995 fuel/clad seg- water static (sequential), 2.96 · 10-5 M (table 6-6, ments, (Allard), I= 0.005 M 337- 1366 days

Np: 3.15 · 10-10-

p. 42) 27.0-48.8 pH 8.3-8.5 MWdlkgU 3.77 · 10-9 M Forsyth and (Series 3, 7, 11) Pu: 5.01 . 10-10

- Werme 1992 1.16 · 10-8 M Cm: 3.79 · 10-13

- F orsyth 1991 1.76 · 10-11 M

PWR bare fuel, J-13 (silica- bicar- air, 25 °C, U: 5.3 · 10-5 M Wilson 1990a 27MWdlkgU bonate groundwater), semi-static,

Np: 1.1 · 10-8 M oxidized, [HC03-] 2 mM 1200 days, 0/M= 2.21- Si: 1.2 mM unsealed fused, Pu: 8.2 · 10-9 M 2.33 pH 7.7- 8.5 silica vessels Am: (3.3-4.1) · 10-10 M

PWR bare fuel, " air, 25 °C, U: (4- 8) · 10-6 M Wilson 1990b 27,30 semi-static, Np: 2.4 · 10-9 M MWd/kgU 200 days, Pu: 8.8 . 10-10

-

(series 2) unsealed fused 4.4 · 10-9 M silica vessels Am: 1.5 · 10-10 M

Cm: 2.6 · 10-12 M

PWR bare fuel, " air, 25 °C, U: 1.3 · 10-6 M Wilson 1990c 27,30 semi-static, Np: 1.3 · 10-9 M MWdlkgU 200 days, Pu: 4.0 · 10-9 M (series 3) sealed stainless steel Am: 1.6 · 10-10 M

vessels Cm: 5.0 · 10-12 M

" " air, 85 °C, U: 6.3 · 10-7 M " semi-static, Np: 7.9 · 10-10 M 200 days, Pu: 4.0 · 10-11 M sealed stainless steel Am: 5.0 · 10-13 M vessels Cm: 5.0 · 10-15 M

35

Table 3-9 continuing ...

Solid phase Aqueous media Test method Steady-state Reference solution concentration

(M)

PWR fuel frag- EJ -13 (silica - bicarbon- air, 90 °C, Pu: 1.4 · 10-10- Finn et al.

ments, ate groundwater), unsaturated test 5.0 · 10-9 M 1993, 1994 30,43 MWdlkgU [HC03-] 2mM method, Am: 1.6. 10-9

-

Si: 1.6 mM 120 days, 5.4 · 10-8 M pH 8.4 sealed stainless Cm: 1.6 · 1 o-11

-

steel vessels 8.2 · 10-9 M

PWR fuel frag- Salt brine air, 25 - 30 °C, U: 10-4 M Gray 1987 ments, static, Pu: 10-8

- 10-7 M 28 MWdlkgU 180 days,

iron absent

" " air, 25 - 30 °C, U: 10-6 " static, Pu: 10-10

- 10-9 M 180 days, iron coupons

*) the bracketed value is the standard deviation of the measured U concentrations at steady state

**) SN: a ratio of pellet surface area (geometric) to water volume

3.2.5 Summary and recommended solubility values for uranium in the near­field groundwaters under oxidizing conditions

Under oxidizing conditions, the solubility of uranium is affected by the dissolution mechanisms of the spent fuel matrix. uo2 is not thermodynamically stable, but oxidizes and alters to stable secondary products which control the concentration of uranium in the aqueous phase. The long-term oxidation pathway of spent fuel is considered to fol­low similar patterns with natural uraninite. The study of uraninite and its alteration products provided valuable insight into the long-term behaviour of spent fuel.

In the short term, the kinetically favoured solid phase is schoepite, while in the long term, the thermodynamically most stable phases are uranyl silicates and uranyl phos­phates depending on the ground water composition. In a bentonite/ ground water system the formation of silicates is favoured. There is limited knowledge of the solubilities of uranyl silicates, although, e.g. uranophane, is according to mineralogical observations the most abundant secondary uranium phase in granitic environments in Scandinavia. No uranyl silicates were included in the NEA uranium database. Generally, uranyl sili­cates are less soluble than uranyl oxide hydrates, like schoepite, and the replacement of uranyl oxide hydrates by uranyl silicates lowers the uranium concentration in ground­water. The development of a sequence of alteration phases has also been observed under laboratory conditions in a few dissolution studies of uo2 and spent fuel in synthetic groundwater with sufficiently long contact times. The groundwater composition seems to play a critical role in determining which secondary minerals form.

36

Consequently, schoepite and uranophane were considered as solubility-limiting phases in the near-field ground waters in the modelling calculations. The solubility product for uranophane was taken from the recent determinations found in the literature. The pH (7 ... 1 0) had a small effect in the fresh composition, while in the saline composition the solubilities increase at higher and lower values. The recommended solubilities are given in Table 3-10.

The measured uranium concentrations at steady state in the dissolution experiments with unirradiated uo2 pellets and spent fuel in synthetic groundwaters range between 10-6

••• 5 · 10-5 M. They are typically lower than the solubilities of schoepite. The solubil­ity-limiting solid phases have been rarely identified.

The recommended values are average values from the experimental information and the modelling results of schoepite and uranophane solubilities.

Table 3-10. Recommended U solubilities in the near-field groundwaters under oxidizing con­ditions.

Limiting solid Fresh near-field Saline near-field pH 8. 6/log 02jug. = -10 pH 8.1/log 02jug. = -10

....................................................................................... ······················································································ Solubility, M Solubility, M

Schoepite 8 . 104 1 . 10-5

U02(C03)3 4- (U02)2C03(0II)3-(U02)2C03(0II)3- U02(0II)2 (aq)

U02(C03)3 4-

Uranophane 10-5 .. .104 M 10-7 .. .10-5 M

U02(C03)3 4- U02(C03)3 4-

Recommended 1 · 104 M 5 · 10-5 M values

37

4 SOLUBILITIES OF URANIUM IN THE FAR-FIELD

4.1 Reducing conditions

In the far-field, i.e. in the chemically and physically undisturbed bedrock outside the re­pository, the behaviour of uranium, assumed to be released from the repository, would be essentially the same as the behaviour of uranium in natural boorock.

Uranium occurs in a variety of minerals. The most abundant uranium mineral is uran­inite, uo2+x (0.01 < X < 0.25), in reducing conditions. Natural uraninite crystallizes in the cubic system (fluorite structure type). The nominal composition is close to uo2+x ' although the content of impurities (e.g. Pb, Th, Ca, Si and rare earth elements) may lead to rather complex compositions. Crystallographic evidence suggests that uraninites are most probably a mixture of two phases, uo2.00- 2.07 and uo2.23-2.25 /Janeczek et al. 1996/. Pitchblende is a term used to describe a microcrystalline variety of uraninite. Uraninite from Palmottu, which is a natural analogue site in south-west Finland, repre­sents the U02-Th02 solid solution /Blomqvist and Kaija 1998/. Uraninite in Palmottu has been partially altered to a fine-grained uranium silicate mineral in the late-stage metamorphic (hydrothermal) conditions (about 1.8 Ga ago). At low temperatures, cof­finite (USi04) may become stable in silica-containing groundwaters, if dissolved silica levels are high enough. Relatively high silica concentrations (> 5 · 104 M) were calcu­lated to make coffinite more stable than U02 (see Section 3.1.2). The highest silica content in the reference far-field ground waters is 1.4 · 104 M.

In the NEA database, uraninite, U409(c) (U02.2s) and U02(am) are included. Uraninite represents a well-crystallized solid with a very low solubility. The mixed valence oxide, U409, is not formed by precipitation from the solution, but by diffusion of oxygen into the U02 matrix. U40 9(c) is considered in this connection to simulate a slightly oxidized U02 under mildly reducing conditions. The only precipitable oxides in the NEA data­base are U02(c) and U02(am). The solubilities ofU02(am) calculated with the present data are high compared with the contents of uranium found in deep natural groundwa­ters.

Uraninite and U409(c) were used as solubility-limiting solid phases in the modelling calculations in the far-field conditions, Table 4-1. The solubilities of coffinite (USi04) were calculated for comparison, although the silica concentration in the reference groundwaters is low. The solubilities are determined by the U(IV) hydrolysis reaction, the dominant species being U(OH)4(aq). The solubilities given by the SR '97 database are lower due to the different U(IV) hydrolysis constant. The solubilities are independ­ent of pH and Eh in the studied ranges (7 ... 1 0). The composition of ground water has a minor effect.

In the dissolution experiments with unirradiated crystalline uo2 pellets in synthetic groundwaters (Allard groundwater, bentonite groundwater) under anaerobic conditions (N2), the measured uranium concentrations at steady state were at the level of U409 solubilities, being clearly higher than the solubilities of well-crystallized U02(c) and below the solubilities of U02(fuel). The concentrations ranged (1.7 ... 2.7) · 10·8 M (see Table 3-4). ·

38

Table 4-1. Modelled solubilities for uranium in the far-field groundwaters using the DataO.com.R2 and SR '97 databases. The latter values are shown in brackets.

Solubility Fresh Brackish Saline Brine limiting pH6.9 ... 10.1 pH7.2 ... 9.0 pH7.0 ... 9.2 pH7.8

solid phase Eh -205 ... -418 m V Eh -205 ... -343 m V Eh -195 ... -340 m V Eh-300mV

uo2 (c) 3.9. 10-10 3.9 . 10-10 3.8 . 10-10 3.7 . 10-10

(7.7 . 10-11) (7.6 . 10-11

) (7.6 . 10-11)

U409 (c) 6.5 . 10-8 5.5 . 10-8 4.7 . 10-8 1.0 . 10-7

(1.1 . 10-8) (1.2 . 10-8) (8.6 . 10-9)

USi04 2.5 . 10-8 1.6 . 10-9 7.2. 10-9 2.8 . 10-9

(2.4. 10-9) (3.3 . 10-10) (3 .3 . 1 o-10

)

The composition of the aqueous phase did not have an influence on solubility. In the experiments under reducing conditions, in the presence of reducing agents (S2), the measured uranium concentrations in synthetic groundwaters (Allard and saline ground­water) were one order of magnitude lower (Table 3-4).

4.2 Oxidizing conditions

As before in the oxidizing near-field, schoepite and uranophane were considered as the solubility-limiting phases in the oxidizing far-field. Schoepite is the kinetically favoured solid phase in the short term, but it is thermodynamically unstable in natural waters /Bruno et al. 1995/. In the long term, the thermodynamically most stable phases are uranyl silicates, e.g. uranophane and soddyite. The uranyl silicates are abundant and im­portant for controlling the concentration of uranium in natural groundwaters /Finch and Ewing 1992/. Uranophane (acicular form) has been identified, probably as a secondary uranium mineral, under oxidizing conditions in HyrkkoHi Cu-U mineralization /Marcos and Ahonen 1998/. Schoepite and a uranyl silicate (possibly N a-boltwoodite) have been observed to form as secondary phases in the dissolution experiments with powdered U02 pellets in Allard groundwater under air-saturated conditions /Ollila, Leino-Forsman 1993/.

Figure 4-1 shows the calculated solubilities of schoepite and uranophane as a function of pH (Data0.com.R2) in Allard groundwater, which is the reference groundwater for the oxidizing far-field. The solubility product of uranophane is based on the recent de­termination by Casas et al. /1997/. The solubility of schoepite increases slowly with the increase in pH at higher pH values. On the contrary, the solubility of uranophane in­cre~ses rapidly at lower pH values.

39

1Q-2 -r----------------,

1Q-3

\ Schoepite __ .... •__:_\ _____________ _

1Q-5 \. "' Uranophane

•. ._ .• - .• --..ta.

1 Q-6 -'-----.---...,.....---.------r----l

7 8 9 10 11

pH

Figure 4-1. Calculated solubilities of uranium as a function of pH in Allard groundwater (uranophane, log K= 11. 7) under oxidizing conditions (log 0 2.fugacity= -10).

As discussed earlier, the measured steady-state concentrations in the experiments with unirradiated U02 pellets /Ollila 1995/ in air-saturated Allard groundwater were (1-2) · 10-5 M, being one order of magnitude lower than the solubility of schoepite. The steady-state concentration is close to the solubility ofU30 7, (3 · 10-5 M). No secondary phases, possibly controlling solubility, were found in the aqueous phase.

In natural conditions, in HyrkkoHi, the measured contents of U in oxidizing ground water samples were 1 o-6

. • . 1 o-5 M, which is in good agreement with the calculated solubility ofuranophane (log K= 11.7).

4.3 Measured uranium contents in natural groundwaters

Figure 4-2 shows an overall picture of the total uranium concentrations in Finnish groundwaters plotted as a function of sampling depth measured along the drillhole. The data comprises:

a) data from Palmottu natural analogue study site /Blomqvist and Kaija 1998/

b) data from two drillholes from HyrkkoHi native copper occurrence /Marcos and Ahonen 1998/

c) data from the site investigations of Posiva Nuorinen and Snellman 1998/ /Ruotsalainen and Snellman 1996/

d) data from drilled wells in Helsinki area I Asikainen and Kahlos 1979/.

a .ri bil = ~ -~ -= ..= ~ J.

= =

40

U, p.M 0.00001 0.0001 0.001 0.01 0.1 1 10 100

0

~

100 0

•• • 200 0 0

0 • 300 o e 0 ,

0 0 ~ •• 400 eh:>

0 8 • . o

500 0

600 0

-700

• • 0

800

900 ~----------------------------------~----------------~

• P:Overburden

0 P:Southwestern Flow

e Romuvaara

Syyry

• P:Stagnant Flow

P:N orthwestern Flow

e Veitsivaara

0 Olkiluoto

P:Eas tern Flow

~Hyrkkoli

e Kivetty

"Helsinki"

Figure 4-2. Uranium concentrations as a function of approximate sampling depth in different groundwaters in Finland (P= Palmottu).

41

The Palmottu site in south-west Finland comprises more than 30 drillholes penetrating a small uranium occurrence. The drillholes are 100-300 meters in depth, except for one of 550 meters. Based on the available hydrogeological information /Blomqvist and Kaija 1998/, four different hydrogeo1ogica1 units were identified, as shown in Figure 4-2 (P: Stagnant Flow, P: Eastern Flow, P: Southwestern Flow, P: Northwestern Flow). The indicated groundwater samples were mainly taken from permanently isolated (0.5 - 1.5 years before sampling) drillhole sections.

High uranium concentrations of up to 1 o-5 M have been analyzed in samples from groundwaters in the Palmottu eastern and south-western flow system. The bedrock asso­ciated with these flow systems consists of mica gneiss and a granite pegmatite, which has a relatively high hydraulic conductivity, especially in its contact with mica gneiss. The rocks are typically hematite-rich. The most common fracture mineral is calcite (often U -bearing). Kaolinite and pyrite are also frequently found. Other fracture miner­als encountered include ferric hydroxide, siderite, and uranophane. The measured Eh values vary from slightly negative to clearly positive, suggesting oxidizing conditions for uranium. All high uranium concentrations have been analyzed in samples taken from drillholes in the uppermost 100 meters of the bedrock, while the surface waters (overburden) are very uranium-poor.

Slightly elevated salinities (TDS about 1 g/1) have been encountered in the groundwaters of deeper boreholes of the Palmottu study site. Compared with the fresh waters of the upper bedrock, these waters are interpreted to be stagnant. They are typically anoxic and reducing, also sulphide-bearing waters have been encountered. Uranium concentrations in the stagnant waters are low,< 5 · 10-8 M, even though the water samples are from the uranium-rich environment. The main rock types are migmatitic mica gneiss, granitic pegmatites and amphibolites. The main uranium minerals are uraninite and coffinite.

The HyrkkoHi site /Marcos and Ahonen 1998/, originally found and studied within a uranium ore prospecting campaign, has been later studied as an analogue for copper canisters because of the existence of native copper in the crystalline bedrock. The site is situated in Nummi-Pusula, south-western Finland. As in Palmottu, the bedrock belongs to the Svecofennian schist belt. The main rock types in HyrkkoHi are quartz-feldspar gneiss and amphibolite with uranium-bearing granite pegmatite veins. The groundwater conditions are extremely oxidizing. The Eh values, which were measured both from sur­face and 'in situ', were +400 ... +500 mV. The pH values are around 7. The measured uranium concentrations were 1 o-6

• . . 1 o-5 M. Uranophane has been identified as one of the U (VI) minerals.

Within the site characterization program of Posiva Oy, groundwaters from deep drill­holes have been sampled during the preliminary site investigation phase from the sites Syyry, Olkiluoto, Romuvaara, Kivetty and Veitsivaara. The hydrogeochemical data from the preliminary site investigations is presented by /Lampen and Snellman 1993/. The hydrogeochemistry of the ground waters from the detailed site investigations are reported by /Ruotsalainen and Snellman 1996/.

In the Syyry and Olkiluoto sites, fresh, brackish and saline groundwaters have been ob­tained. The brackish groundwater in Syyry contains dissolved hydrogen and hydrocar­bons, the conditions are strongly reducing and the measured uranium concentrations are low, 1 o-9 M. In Olkiluoto, the saline ground water contains dissolved sulphide and meth-

42

ane, and the redox conditions are reducing. The uranium concentrations in deep saline waters are about 1 o-9 M ... 1 o-8 M, while the upper fresh and brackish waters may con­tain about 1 o-7 M uranium.

In fresh groundwater conditions in Romuvaara, V eitsivaara and Kivetty the concentra­tions range from 1 o-9 to 3 · 1 o-7 M. Reducing conditions are generally indicated by the Eh measurements, as well as by the presence of dissolved sulphide, methane and hydro­gen. The highest uranium concentrations, up to 3 · 1 o-7 M, have been measured in Kivetty. Some possible explanations for the elevated uranium concentration might be the rock type, with higher uranium and thorium contents than in Romuvaara, and the ground water with fairly low concentrations of redox buffering species /Ruotsalainen and Snellman 1996/.

The highest uranium concentrations measured in drilled wells in the Helsinki area have been plotted in Figure 4-2 for comparison. Asikainen and Kahlos /1979/ have reported anomalous high (3 · 1 o-5 M) concentrations in more than ten drilled wells.

In Cigar Lake groundwaters, uranium concentrations in the reduced zone vary in the range 10-9 ... 10-7 M /Bruno and Casas 1994/ in /Cramer and Smellie 1994/. The 1.3-Ga-old uranium deposit at Cigar Lake is located in northern Saskatchewan, Canada. The uranium mineralization is found inside the clay-rich matrix of hydrothermally al­tered sandstone at a depth of 450 meters below surface, at the contact between the sand­stone and the older basement rocks. The uranium mineralization comprises primarily of uraninite (U02) and pitchblende (amorphous U02) with subordinate coffinite (USi04). In order to determine the solubility-limiting phase for uranium, Bruno and Casas /1994/ calculated uranium solubilities for all the Cigar Lake groundwaters. Equilibrium was assumed with U02, U307, U409, coffinite and another USi04 polymorph. From com­parisons between measured and predicted solubilities it was not possible to determine the phase.

QJ

a.

5

0

-s

5

U-40g(s)

7 pH

8 9

Figure 4-3. Pourbaix diagram showing measured redox potentials (pe) vs. pH for the Cigar Lake groundwaters /Bruno and Casas 19941.

43

Figure 4-3 gives the predominance of the uranium oxide phases as a function of pH and pe. The measured pe and pH data from all the Cigar Lake reference groundwaters plot in the narrow U30 7 field. It has been concluded that the waters are in equilibrium with a slightly oxidized uraninite, represented by the stoichiometry of the U307 phase /Bruno et al. 1997/. However, at pH values 7 ... 8 U409(s) would seem to become dominant. The mineralogical characterization of the samples of the solid phase suggests stoichiometries that are between U409 and U307 phases. The efforts to identify U307 in samples failed /Janeczek and Ewing 1992, 1993/. Laboratory measurements of the solubility of uran­inite from Cigar Lake, using a granitic groundwater under reducing conditions, gave a value of ( 4 ± 1) · 1 o-8 M /Bruno and Casas 1994/.

At Cigar Lake, there is no mineralogical data regarding the presence of secondary U(VI) phases even in the clay halo zone where redox potentials as high as 200 m V have been measured. The measured uranium concentrations are in the range 10-9 ... 10-7 M, which corresponds to the expected uranium concentrations under anoxic conditions. Bruno et al. /1987/ have suggested a control by the association ofU(VI) to Fe(III) oxydydroxides, which are present in the clay.

The measured uranium concentrations in deep saline groundwaters of the Carnmenellis granite (U.K.) vary over 8 · 10-11

••• 1.8 · 10-7 M /Edmunds et al. 1987/ in /Bruno et al. 1987/. In Stripa granite, uranium concentrations are 1n the range of 4 · 10-8 M ... 3.7 · 10-7 M /Bruno et al. 1987/.

4.4 Recommended solubility values for uranium in the far-field groundwaters

In the far-field, the solubility of uranium can be considered to be similar to the solubility of uranium in the groundwater in natural bedrock. The most abundant uranium mineral under reducing conditions is uraninite. The composition of uraninite is rather complex. It is most probably a varying mixture of two phases, U02.oo-2.01 and U02.23-2.2s, and of impurity cations. It has different morphologies as well. Both properties have an influ­ence on the concentration in the aqueous phase. This is in agreement with the reported measured concentrations in natural ground waters, which vary over many orders of mag­nitude, 8 · 10-11

••• 3.7 · 10-7 M. The highest values probably represent slightly oxidizing conditions.

The pure phases, U02(c) and U409(c) are included in the NEA thermodynamic database. The well-crystallized uraninite, U02(c), is the solubility-limiting phase under strongly reducing conditions. U409 (c) (U02.2s) is considered to be the solubility-limiting phase under moderately reducing conditions. The recommended value is based on the solubil­ity ofU409(c). The solubilities are independent of pH and Eh in the studied range. The composition of groundwater has a minor effect. The measured solubilities for U in the dissolution experiments with unirradiated crystalline uo2 pellets in synthetic ground­waters under anaerobic conditions (N2) were in good agreement with the calculated U409 solubilities.

Under oxidizing conditions, schoepite is the kinetically most favoured solid phase in the short term. In the long term, the thermodynamically most stable phases are uranyl sili-

44

cates, e.g. uranophane, which are abundant and important for controlling the uranium concentration in natural groundwaters. The measured steady-state concentrations in the experiments with unirradiated crystalline U02 pellets in air-saturated Allard groundwa­ter are ( 1-2) · 1 o-5 M. The recommended value is an average value from the experimen­tal and modelling results, and measured uranium concentrations in natural groundwaters (e.g. HyrkkoHi).

Table 4-2. Recommended U solubilities in the far-field groundwaters.

Limiting solid Reducing conditions Limiting solid Oxidizing conditions ALL COMPOSITIONS FRESH (Allard groundwater)

pH 7 ... 10/Eh -200 ... -400 m V pH 8 ... 9/log 02 fug. =-1 0 ······································································· ········································· ············································································

Solubility, M Solubility, M

U02(c) 8 . 10-11 ... 4. 10-10 Schoepite 1 · 104 M U(OH)4(aq) (U02)2<:03(0H)3-

U02(<:03h2-U02(<:03)3 4-

U409 (c) 10-8 ... 10-7 M Uranophane 1 · 10-5 M U(OH)4(aq) U02(<:03)22-

(U02h<:03(0H)3-U02(<:03)3 4-

Recommended 1·10-7 M Recommended s. 1o-5 M value value

45

5 SUMMARY

The purpose of this study was to evaluate the solubilities of uranium in the reference waters ofTILA-99. The near-field groundwaters included fresh and saline compositions and the far-field groundwaters included fresh, brackish, saline and 'almost brine' com­positions. The pH and carbonate concentration, and redox conditions were considered to be the main factors affecting the solubilities. First, a literature study was made in order to gain information on the recent dissolution and leaching experiments with unirradiated U02 materials and spent fuel, as well as natural analogue studies. The latest literature includes, e.g., studies on U02 solubility under reducing conditions, and studies on uraninite and its alteration products under oxidizing conditions which provide a valuable insight into the long-term behaviour of spent fuel. Alteration products of U02, possible solubility-controlling phases for uranium, have been identified only in a few dissolution experiments with uo2 and spent fuel with sufficiently long contact times.

Secondly, the solubilities were calculated using the geochemical code EQ3/6. For ura­nium, there is a well-documented NEA thermodynamic database /Grenthe et al. 1992/. The recently extended and updated database, SR '97 by Bruno et al. /1997/, was used for comparison. A different selection of the U(N) hydrolysis constant in SR '97 lowers the solubilities under reducing conditions. This selection is supported by the recent U(N) solubility studies. Otherwise the databases are in good agreement.

In the reducing near-field, the spent fuel matrix itself, U02 (U409), was considered to be the solubility-limiting phase. The U(N) dominates and the solubility is low. However, the calculated solubility starts to increase early in the negative redox regime in fresh near-field groundwater with a high carbonate content due to the stabilization of U(VI) carbonate complex, especially at higher pH. Only limited experimental data is available for the solubility of U(N) compounds in carbonate solutions under reducing conditions. The solubility studies with unirradiated U02 pellets under anaerobic condi­tions did not show any increase in solubility with the increase in bicarbonate in the range covering the contents of the reference ground waters. The simulation of the effects of canister corrosion products, magnetite and hematite, resulted in strongly reducing conditions. Under these conditions the U(N) hydrolysis reaction determines the solu­bility.

In the oxidizing near-field, U02 is not stable, but alters to stable secondary products de­pending on the groundwater composition. The oxidative alteration pathway of spent fuel was considered to follow similar patterns to natural uraninite. The thermodynamically most stable phases in the long term in the groundwaterlbentonite medium are probably uranyl silicates, e.g. uranophane. In the short term, the kinetically favoured solid phase is schoepite. There is limited knowledge of the solubilities of uranyl silicates. None was selected into the NEA database. The solubility products found for uranophane from the literature show some scatter. The recommended values for solubility were taken as aver­age values from the experimental information and the modelling results of schoepite and uranophane solubilities.

In the far-field, the solubility of uranium can be considered to be similar to the solubility of uranium in the groundwater in natural granitic bedrock. The most abundant uranium

46

mineral under reducing conditions is uraninite. Coffinite may become stable in ground­waters with a high silica content. The measured concentrations of uranium in natural groundwaters vary 10-10

••• 10·7 M, reflecting the varying composition of uraninite. The calculated solubilities did not show a major variation within the water compositions. This is in good agreement with the uo2 dissolution experiments under anaerobic and reducing conditions. The recommended value was based on the solubility of U409(c). Under oxidizing conditions, schoepite and uranophane were considered to be solubility­limiting phases.

The solubility values recommended in this work should be regarded as conservative.

47

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Ollila K 119951, Solubility of unirradiated U02 fuel in aqueous solutions- comparison between experimental and calculated (EQ3/6) data. Helsinki, Finland: Nuclear Waste Commission of Finnish Power Companies. Report YJT-95-14. 27 p. ISSN-0359-548X.

01/ila K, 0/in M and Lipponen M 11996/, Solubility and oxidation state of uranium under anoxic conditions (N2 atmosphere). Radiochimica Acta 74, pp. 9-13.

Ollila K /1996/, Determination ofU oxidation state in anoxic (N2) aqueous solutions­method development and testing. Helsinki, Finland: Posiva Oy. Posiva Report 96-01. 33 p. ISBN 951-652-000-6.

Ollila K 11997/, Dissolution of unirradiated U02 fuel in synthetic saline groundwater, Experimental methods and preliminary results. Helsinki, Finland: Posiva Oy. Posiva Report 97-09. 28 p. ISBN 951-652-034-0.

Ollila K 11998/, Dissolution of unirradiated U02 fuel in synthetic groundwater- Prog­ress report '97. Helsinki, Finland: Posiva Oy. Posiva Report 98-06. 17 p. ISBN 951-652-044-8.

51

Puigdomenech 1 and Bruno J. I 19881, Modelling uranium solubilities in aqueous solu­tions: Validation of a thermodynamic data base for the EQ3/6 geochemical codes. Stockholm, Sweden: Swedish Nuclear Fuel Waste Management Co. SKB Technical Re­port 88-21. 62 p. ISSN 0284-3757.

Puigdomenech 1, Casas 1 and Bruno J. 119901, Kinetics of U02(s) dissolution under reducing conditions: Numerical modelling. Stockholm, Sweden: Swedish Nuclear Fuel Waste Management Co. SKB Technical Report 90-25.22 p. ISSN 0284-3757.

Quiiiones J., Garcia-Serrano, Serrano J.A., Diaz-Arocas P. and Almazan J.L.R. 119981, Simfuel and U02 solubility and leaching behaviour under anoxic conditions. In: Scien­tific Basis for Nuclear Waste Management XXI (eds. I. G. McKinley and C. McCombie), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 506. pp. 247-252. ISBN 1-55899-411-4.

RaiD., Felmy A.R. and Ryan J.L. 119901, Uranium(IV} hydrolysis constants and solu­bility product ofU02 · xH20(am). Inorg. Chem. 29, pp. 260-264.

Rai D., Felmy A.R., Moore D.A. and Mason MJ. 119951, The solubility of Th(IV) and U(IV) hydrous oxides in concentrated NaHC03 and Na2C03 solutions. In: Scientific Ba­sis for Nuclear Waste Management XVIII, Part 2 (eds. T. Murakami and R.C. Ewing), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 353. pp. 1143-1150. ISBN 1-55899-253-7.

Ruotsalainen P. and Snellman M 119961, Hydrogeochemical baseline characterization at Romuvaara, Kivetty and Olkiluoto, Finland. Helsinki, Finland: Posiva Oy. Posiva Working Report P ATU -96-91 e. 109 p.

Ryan J.L. and Rai D. I 19831, The solubility of uranium(IV) hydrous oxide in sodium hydroxide solutions under reducing conditions. Polyhedron 2, No. 9, pp. 947-952.

Shoesmith D. W. and SunderS. 119911, An electrochemistry-based model for the disso­lution ofU02. Stockholm, Sweden: Swedish Nuclear Fuel Waste Management Co. SKB Technical Report 91-63. 97 p. ISSN 0284-3757.

SKI Project-90 119911, Stockholm, Sweden: Swedish Nuclear Power Inspectorate. SKI Technical Report 91:23. Vol. 1. 239 p.

Snellman M 119881, Chemical conditions in a repository- experimental and modelling studies. In: Third Finnish- German Seminar on Nuclear Waste Management (ed. L. Lamberg), Espoo, Finland: VTT Symposium 87. pp. 146-168. ISBN 951-38-3175-2.

SR 95 /19951, Template for safety reports with descriptive example. Stockholm, Swe­den: Swedish Nuclear Fuel Waste Management Co. SKB Technical Report 96-05. 263 p. ISSN 0284-3757.

Stroes-Gascoyne S., Johnson L.H, Beeley P.A. and Sellinger D.M 11986/, Dissolution of used Candu fuel at various temperatures and redox conditions. In: Scientific Basis for Nuclear Waste Management IX (ed. L.O. Werme}, Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 50. pp. 317-326. ISBN 0-931837-15-4.

Stroes-Gascoyne S., Tail J.C., Porth R.J., McConnell J.L. and Due/os A.M 11993/, Comparison of the dissolution behaviour of various U02 samples in saline solution at 100 °C under reducing conditions. In: Scientific Basis for Nuclear Waste Management

52

XVII ( eds. A. Barkatt, and R.A. V an Konynenburg), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 333. pp. 425-430. ISBN 1-55899-232-4.

Tail J.C., Stroes-Gascoyne S., Hocking W.H, Duclos A.M, Porlh R.J. and Wilkin D.L. 11991/, Dissolution behaviour of used Candu fuel under disposal conditions. In: Scien­tific Basis for Nuclear Waste Management XN (eds. T. Abrajano, Jr. and L.H. John­son), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 212. pp. 189-196. ISBN 1-55899-104-2.

Tail J.C. and Luhl J.L. 119971, Dissolution rates of uranium from unirradiated U02 and uranium and radionuclides from used CANDU fuel using the single-pass flow-through apparatus. Toronto, Ontario, Canada: Ontario Hydro. Report No: 06819-REP-01200-0006 ROO. 40 p.

Torrero ME., Casas 1, Aquilar M, de Pablo J., Gimenez J. and Bruno J. /19911, The solubility of unirradiated uo2 in both perchlorate and chloride test solutions. Influence of the ionic medium. In: Scientific Basis for Nuclear Waste Management XN ( eds. T. Abrajano, Jr. and L.H. Johnson), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 212. pp. 229-234. ISBN 1-55899-104-2.

Thomas L.E., Einziger R.E. and Buchanan HC. /1993/, Effect of fission products on air-oxidation ofLWR spent fuel. J. Nucl. Mater. 201, pp. 310-319.

Vieno T., Haulojiirvi A., Koskinen L. and Nordman H /1992/, Safety analysis of spent fuel disposal. Helsinki, Finland: Nuclear Waste Commission of Finnish Power Compa­nies. Report YJT -92-33E. 254 p. ISSN 0359-548X.

Vieno T. and Nordman H /1996/, Interim report on safety assessment of spent fuel dis­posal TILA-96. Helsinki, Finland: Posiva Oy. Posiva Report 96-17. 176 p. ISBN 951-652-016-2.

Vuorinen U. Kulmala S., Hakanen M, Ahonen L. and Carlsson T. /1998/, Solubility database for TILA-99. Helsinki, Finland: Posiva Oy. Posiva Report 98-14 (to be pub­lished), ISBN 951-652-034-0.

Vuorinen U. and Snellman M 119981, Finnish reference waters for solubility, sorption and diffusion studies. Helsinki, Finland: Posiva Oy. Posiva Working Report 98- (to be published).

Werme L. 0. and Spahiu K I 19981, Direct disposal of spent nuclear fuel: comparison between experimental and modelled actinide solubilities in natural waters. Journal of Alloys and Compounds, Vol. 271-273, pp. 194-200. ISSN 0925-8388.

Wilson C.N. and Shaw HF. 119871, Experimental study of the dissolution of spent fuel at 85 °C in natural ground water. In: Scientific Basis for Nuclear Waste Management X (eds. J.K. Bates and W.B. Seefeldt), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 84. pp. 123-130. ISBN 0-931837-49-9.

Wilson C. N. I 1990a/, Results from long-term dissolution tests using oxidized spent fuel. In: Scientific Basis for Nuclear Waste Management XN (eds. T. Abrajano, Jr. and L.H. Johnson), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 212. pp. 197-204. 197-204ISBN 1-55899-104-2.

53

Wilson C.N. /1990bl, Results from NNWSI Series 2 bare fuel dissolution tests. Rich­land, Washington, USA: Pacific Northwest Laboratory. Report PNL-7169 (UC-802). 74p.

Wilson C. N. I 1990c/, Results from NNWSI Series 3 spent fuel dissolution tests. Rich­land, Washington, USA: Pacific Northwest Laboratory. Report PNL-7170 (UC-802). 101 p.

Wolery T.J. /1992/, EQ3/6, A software package for geochemical modeling of aqueous systems (version 7.0). Lawrence Livermore National Laboratory, USA: UCRL-MA-11 0662, PT 1-4.

Wolery T.J., /1992bl, EQ3NR, A computer program for geochemical aqueous specia­tion-solubility calculations: Theoretical manual, user's guide, and related documentation (version 7.0). Lawrence Livermore National Laboratory, USA. 246 p.

Wronkiewicz D.J., Bates J.K, Gerding T.J., Veleckis E. and Tani B.S. 119921, Uranium release and secondary phase formation during unsaturated testing of uo2 at 90 °C. J. Nucl. Mater. 190, pp. 107-127. ISSN 0022-3115.

Wronkiewicz D.J., Buck E.C. and Bates J.K 119971, Grain boundary corrosion and al­teration phase formation during the oxidative dissolution of U02 pellets. In: Scientific Basis for Nuclear Waste Management XX (eds. W.J. Gray, and I.R. Triay), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 465. pp. 519-525. ISBN 1-55899-369-X.

Yajima T., Kawamura Y. and Ueta S. /19951, Uranium(IV) solubility and hydrolysis constants under reduced conditions. In: Scientific Basis for Nuclear Waste Management XVIII, Part 2 (eds. T. Murakami and R:C. Ewing), Pittsburgh, Pennsylvania, USA: Mat. Res. Soc. Symp. Proc. Vol. 353. pp. 1137-1142. ISBN 1-55899-253-7.

-3

-4

-5

-6

-7

-8

-9

6

~ Bruno et al. (1987) ~ Gayer and Leider (1957)

f»:.l Rai et al. (1990) t::::.::::J Ryan and Rai (1983)

7 8 9

54 App. 1

10 11 12 13 14

pH

Figure Al-l. Comparison of experimental U02(am) solubilities under reducing conditions. The solid line presents the calculated solubility using the EQ3/6 thermodynamic data file, DataO.com.R22. A best-fit line to the experimental data of Rai et al. 11990/ is shown by the dashed line /Arthur and Apted 1996/.

~ ~--~~-------------------------------. 'I% 0

-3 1-

--9

-10 -

0

oo 0

0

e Thlsstudy

0 Ral(1990)

0 Bruno (1987)

0

0 ooo

0 ~ 0 rfl,g,_ o• o o ~ -·· ,.. . .. ' .~ ..... ,. . . ., ..

••• -11 L---~·----~·-----L-·--~·----~·-----~·--~

0 2 4 6 8 10 12 14

pH

Figure Al-2. Comparison of solubility data under reducing conditions obtained by Yajima et al. /1995/ for U02(s) [filled symbols] with the data of Rai et al. 11990/ and Bruno et al. 11987/ for U02(am).

log S

-3

-s

-6

-7

-B

D

55

0 (ClO~-)=O.DDB •ol d•-3 ---- Theoretical •odel

0

B ID pH

App. 2

12

FigureA2-1. Solubility ofunirradiated U02(s) in 0.008 M NaC104 under reducing conditions /Torrero et al. 19911.

log 5

. -3 I~ (Cl-)=1 •ol d•-31 .. 6~'8.~ .... (Cl-):5 •ol d•-3

-<4

• -s 66

-6 .. ~

-7 v.6 .... ;''" .,e. -8

0 2 I) B 10 12 pH

Figure A2-2. Solubility of unirradiated U02(s) in 1 and 5 M NaCl under reducing conditions /Torrero et al. 19911.

56 App. 3

-2

o 3 day -3 1- A 9 day

x 43 day

-4 1--~ -51- X 0

5 0

:::> -6 ~ ~ Ol 0 0 0 QO 66

.2 0 0 0 X 6 0 0 0 6 -7- l;. .0.

l;. 6 X

l;. X

-8 1- l;. X X • ~

6 ~ X

-9 I I X _! I

-4 -3 -2 -1 0

log Na2C03 (molal)

Figure A3-1. Aqueous uranium concentrations in filtrates from U02 · xH20(am) suspensions at different equilibration periods under reducing conditions in Na2C03 solutions containing 0.01 M NaOH /Rai et al. 19951.

..---. s 0 .s

:::>

57

10-7

10-8

10-9

10·10

U02 (fuel) (SKBU)

U02_25 (NEAU)

• 1 mM NaHC03

10_11 .___ __ ___,_ ___ ___,_ ___ ........... __ ____. A 2 mM NaHC03

0 200 400 600 800 () 4.5 mM NaHC03

Q 9.8 mM NaHC03 Time [days]

App.4

Figure A4-l. Solubility of unirradiated U02 fuel in NaHC03 solutions (pH= 8.8-9.1) under anaerobic conditions (N2 atmosphere). The reference lines give the calculated (EQ3/6, NEA and SKB databasesfor U) solubilities of uranium oxides /Ollila 19951.

..---. 0:::::::::

0 .s :::>

10-7

10-8

10-9

10·10

1 o-11 L--__ ___.. ___ __._ ___ _._ __ ___,

0 100 200

Time [days]

300 400

U02(fuel) (SKBU)

U40 9(c) (SKBU)

U02.25 (NEAU)

U02(c) (NEAU)

U02(c) (SKBU)

Figure A4-2. Solubility of unirradiated U02 fuel in de ionized water (pH= 7. 0) under anaerobic conditions (N2 atmosphere). The reference lines give the calculated (EQ3/6, NEA and SKB databases for U) solubilities of uranium oxides /Ollila 19951.

LIST OF REPORTS 1 (3)

POSIVA REPORTS 1998, situation 11/98

POSIV A 98-01

POSIV A 98-02

POSIV A 98-03

POSIV A 98-04

POSIV A 98-05

Bentonite swelling pressure in strong NaCl solutions - Correlation of model calculations to experimentally determined data Ola Karnland Clay Technology, Lund, Sweden January 1998 ISBN 951-652-039-1

A working groups conclusions on site specific flow and transport modelling J ohan Andersson Golder Associates AB, Sweden Henry Ahokas Fintact Oy Lasse Koskinen, Antti Poteri VTTEnergy Auli Niemi Royal Institute of Technology, Hydraulic Engineering, Sweden (permanent affiliation: VTT Communities and Infrastructure, Finland) Aimo Hautojiirvi Posiva Oy March 1998 ISBN 951-652-040-5

EB-welding of the copper canister for the nuclear waste disposal­Final report of the development programme 1994-1997 HarriAalto Outokumpu Poricopper Oy October 1998 ISBN 951-652-041-3

An isotopic and fluid inclusion study of fracture calcite from borehole OL-KR1 at the Olkiluoto site, Finland Alexander Blyth, Shaun Frape University of Waterloo, Waterloo, Ontario, Canada Runar Blomqvist, Pasi Nissinen Geological Survey of Finland Robert McNutt McMaster University, Hamilton, Ontario, Canada April1998 ISBN 951-652-042-1

Sorption of iodine on rocks from Posiva investigation sites Seija Kulmala, Martti Hakanen Laboratory of Radiochemistry Department of Chemistry University of Helsinki Antero Lindberg Geological Survey of Finland May 1998 ISBN 951-652-043-X

POSIV A 98-06

POSIV A 98-07

POSIV A 98-08

POSIV A 98-09

POSIV A 98-10

POSIV A 98-11

LIST OF REPORTS

Dissolution of unirradiated U02 fuel in synthetic groundwater­Progress report '97 Kaija Ollila VTT Chemical Technology June 1998 ISBN 951-652-044-8

2 (3)

Geochemical modelling of groundwater evolution and residence time at the Kivetty site Petteri Pitkiinen, Ari Luukkonen VTT Communities and Infrastructure Paula Ruotsalainen Fintact Oy Hilkka Leino-Forsman, Ulla Vuorinen VTT Chemical Technology August 1998 (to be published) ISBN 951-652-045-6

Modelling gas migration in compacted bentonite -A report produced for the GAMBIT Club P.J. Nash, B.T. Swift, M. Goodfield, W.R. Rodwell AEA Technology plc, Dorchester, United Kingdom August 1998 ISBN 951-652-046-4

Geomicrobial investigations of groundwaters from Olkiluoto, Hastholmen, Kivetty and Romuvaara, Finland Shelley A. Haveman, Karsten Pedersen Goteborg University, Sweden Paula Ruotsalainen Fintact Oy August 1998 ISBN 951-652-047-2

Geochemical modeling of ground water evolution and residence time at the Olkiluoto site Petteri Pitkiinen, Ari Luukkonen VTT Communities and Infrastructure September 1998 (to be published) ISBN 951-652-048-0

Sorption of cesium on Olkiluoto mica gneiss, granodiorite and granite Tuula Huitti, Martti Hakanen Laboratory of Radiochemistry, Department of Chemistry, University of Helsinki Antero Lindberg Geological Survey of Finland September 1998 ISBN 951-652-049-9

POSIV A 98-12

POSIV A 98-13

POSIV l\ 98-· I 4

LIST OF REPORTS

Sorption of plutonium on rocks in ground waters from Posiva investigation sites Seija Kulmala, Martti Hakanen Laboratory of Radiochemistry, Department of Chemistry, University of Helsinki Antero Lindberg Geological Survey of Finland September 1998 (to be published) ISBN 951-652-050-2

Solubilities of uranium for TJLA .. 99 Kaija Olli/a VTT Chemical Technology Lasse Ahonen Geological Survey of Finland November 1998 ISBN 951-652-051-0

Solubility database for TILA-99 Ulla \luorinen VTT Chemical Technology Seija Kulmala, Martti Hakanen University of Helsinki, Laboratory of Radiochemistry Lasse Ahonen Geological Survey of Finland Torbjorn Carlsson VIT Chemical Technology November 1998 ISBN 951-652-052-9

3 (3)