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Journal of Hazardous Materials 235 236 (2012) 17 28

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rn al h om epage: www.elsev ier .com/ loc ate / jhazmat

eview

review on immobilization of phosphate containing high level nuclear wastesithin glass matrix Present status and future challenges

ranesh Senguptaaterials Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

i g h l i g h t s

Technical review.High level nuclear waste immobilization within phosphate glasses.Integration of data from laboratory scale experiments, plant scale observations and natural rock information.

r t i c l e i n f o

rticle history:eceived 10 April 2012eceived in revised form 12 July 2012ccepted 18 July 2012

a b s t r a c t

Immobilization of phosphate containing high level nuclear wastes within commonly used silicate glassesis difficult due to restricted solubility of P2O5 within such melts and its tendency to promote crystalliza-tion. The situation becomes more adverse when sulfate, chromate, etc. are also present within the waste.To solve this problem waste developers have carried out significant laboratory scale research works in

vailable online 27 July 2012

eywords:igh level nuclear waste

mmobilizationhosphate glasseview

various phosphate based glass systems and successfully identified few formulations which apparentlylook very promising as they are chemically durable, thermally stable and can be processed at moderatetemperatures. However, in the absence of required plant scale manufacturing experiences it is not pos-sible to replace existing silicate based vitrification processes by the phosphate based ones. A review onphosphate glass based wasteforms is presented here.

2012 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182. Generation of phosphate rich HLW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.1. Bismuth process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2. PUREX process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3. P2O5-HLWs storage as neutralized solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.4. P2O5-HLWs storage as calcined powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3. Phosphate in silicate melts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.1. Lessons learnt from natural melts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2. Difficulties associated with phase separations within HLW loaded silicate melts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4. Networking within phosphate melt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225. Immobilization of phosphate containing HLW within glass matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6. Future challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E-mail address: praneshsengupta@gmail.com

304-3894/$ see front matter 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2012.07.039

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

dx.doi.org/10.1016/j.jhazmat.2012.07.039http://www.sciencedirect.com/science/journal/03043894http://www.elsevier.com/locate/jhazmatmailto:praneshsengupta@gmail.comdx.doi.org/10.1016/j.jhazmat.2012.07.039

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8 P. Sengupta / Journal of Hazard

. Introduction

Eco-friendly and proliferation resistant management of spentuels is one of the most sensitive issues in the world today. Optionsvailable at the back-end of nuclear fuel cycles are either

(i) direct disposal of spent fuel within suitable deep geologicalrepositories (better known as open fuel cycle), or

ii) reprocessing it to extract valuables, followed by immobilizingthe process generated nuclear high level liquid wastes (HLWs)within suitable inert matrice(s) for its interim storage anddisposal inside deep geological repositories (better known asclosed fuel cycle). Reaserch is also being pursued to segregateand separately irradiate long lived minor actinides and fissionproducts within critical and subcritical reactors to convert theminto shorter lived radionuclides (partitioning and transmuta-tion; advanced fuel cycle; Fig. 1).

Recently, closed fuel cycle option has gained more importances it is environment friendly and economical [1]. In a simpli-ed way, reprocessing begins with dismantling of water cooledpent fuel bundles into pins and separating irradiated pellets fromircaloy (ZrSn alloys)/Al/stainless steel (FeCrNi alloys) clads

protective metal jacket surrounding the fuel pellets) throughhopping (mostly by mechanical shearing) and dissolving themall slices within concentrated acids (Fig. 2). The resultant solu-ions so obtained are highly hazardous as they contain various

Fig. 1. Schematic diagram showing basic differences betw

aterials 235 236 (2012) 17 28

radioisotopes (radioactivity more than few Ci/litre and possesnearly 99% of the total radioactivity witnessed in a given nuclearfuel cycle) of fissile and fertile materials, minor actinides, fissionelements, activation products, etc. extracted from the spent fuels(Table 1; [2]). Needless to say, the actual compositions (chem-istry and radiochemistry) of HLWs depend on reprocessing routeadopted as well as on spent fuel compositions [37] and their irra-diation history (type of reactors, pellet-cladding interactions [8],neutron flux, burn up, cooling period, etc. [9]). Fig. 3 shows the fourdifferent possible modes of occurrences of radionuclides within aspent fuel. For example they can occur (i) within the irradiatedmatrix fuel, (ii) along the grain boundaries, pores, cracks, dislo-cations, etc., (iii) diffused inside clad and (iv) within the gap inbetween fuel and clad. With passage of time, relative contributionsof each of the radionuclides on the overall radioactivity of the spentfuel changes due to differences in their respective half-lives (T1/2;Table 2). Among all the radioisotopes present, those having T1/2 ofthe order of years to decades and capable of getting incorporatedwithin tissues or organs (e.g. 90Sr) are biologically most dangerousones. On the other hand, radionuclides having high radiotoxicity,geochemical mobility, and long half-lives (e.g. 99Tc, 129I, 79Se, 135Cs,239Pu, 237Np, 235U, 36Cl, and 14C) are elements of deep concern fromenvironmental pollution point of view [10,11]. Therefore, for pro-

tection of biosphere, HLWs need to be concentrated and contained.

In general, HLWs are concentrated by evaporation and neutral-ized by addition of alkali (NaOH), and stored in large undergroundsteel tanks. Although such storage is acceptable for few years, but

een open and closed (advanced) nuclear fuel cycles.

P. Sengupta / Journal of Hazardous Materials 235 236 (2012) 17 28 19

Cooled

spent fuel

Shearing Chopping and

dissolution Reprocessing Product

conversion

U storage/

disposal

U oxide

Compacti on/ Alloy formation

U nitrate solutio n

Vitri fic ati on

Np, Am, Cm, re maining fission products

Pu nitrate solutio n

Product

conversion

Pu oxideMOX fuel

fabricatio n

Repository

s in th

opeaFaRrccmaddfimedtmc

TR

Fig. 2. Schematic diagram showing major step

n long time scale the wastes need to be immobilized within appro-riate inert host matrices (wasteform), stored and disposed offxtremely carefully within suitable deep geological repositories sos to isolate them from biosphere for as many as 104106 years.or conditioning of the HLWs, various amorphous (borosilicatend aluminosilicate glasses), crystalline (synthetic rock SYN-OC, titanate ceramics, phosphate ceramics (monazite, apatite andelated phases), calcines, alloys, etc.) and crypto-crystalline (glasseramics based on sphene, zirconolite, monazite, zircon, etc.) matri-es [1224] have been proposed but the final selection of wasteformaterials depends on several scientific and technological merits

nd demerits associated with HLW compositions [25,26], producturability factors, processing constraints [2733], service conditionemands [34,35], etc. Thus identification of a suitable wasteformor any given HLW is a difficult task, and too much of expectationsn terms of its long term performance within geological repository

ake the selection procedure even tougher. It is argued that thenvironment within deep geological repository (constructed at a

epth of 5001000 m from the surface) is expected to be harsh dueo simultaneous interplay between (i) thermal field, (ii) thermo-

echanical field, (iii) biological field, (iv) hydrological field, (v)hemical field and (vi) radiation field [36].

able 1adionuclides commonly found within spent nuclear fuel.

Major actinides 234U, 235U, 236U, 237U, 238U232Th236Pu, 237Pu, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu

Minor actinides 237Np, 239Np241Am, 242Am, 243Am242Cm, 243Cm, 244Cm, 245Cm, 246Cm

Fission materials 79Se, 85Kr, 87Rb, 89Sr, 90Sr, 93Zr, 95Zr, 95Nb 99Tc, 107Pd, 115In149Sm, 151Sm, 133Xe, 140Ba, 134Te, 93Mo, 106Ru, 106Rh, 107

Activation products 3H, 10Be, 14C, 24Na, 36Cl, 39Ar, 55Fe, 59Ni, 60Co, 63Ni, 93Mo,

e back-end part of closed nuclear fuel cycles.

Of the various matrix-options available, sodium borosilicateglasses are mostly favored as they offer wide compositional flexibil-ities, high order of product durabilities and are easy to manufacturethrough remote and robotic operations. Extensive scientific andtechnological database already exist for the system and the glassesare well accepted by general public. However in reality, borosili-cate glasses cannot be considered as a universal wasteform matrixas in many cases HLWs contain elements having poor solubilitywithin it [37,38]. A good example of this is phosphate rich HLWs(P2O5-HLWs) which have only 23 wt% (P2O5) solubility withinborosilicate melts [39]. The only possible way to condition suchHLWs within conventional borosilicate glass metrices is to dilutethem to a level so that the concentration(s) of the troubleshootingelement(s) become lower than their respective solubility limits. Butsuch an approach finally increases the volume of the vitrified wasteproducts and makes the waste management procedure more costly.An alternative approach to this is to find out a novel amorphousmatrix having higher solubility for P2O5-HLWs. Toward this a num-

ber of efforts have been made across the world over the last fiftyyears, but so far limited progresses have been made to produce suchmatrices in actual plant scale on regular basis. The probable reasonbehind this is lack of complete comprehensions of the problem, its

, 126Sn, 129I, 131I 135Cs, 137Cs, 141Ce, 142Ce, 144Ce, 144Pr 144Nd, 147Sm, 147Pm, 148Sm,Pd, 140La, 154Eu

93mNb, 94Nb, 99Tc, 108mAg, 113mCd, 121mSn, 205Pb, 210Po

20 P. Sengupta / Journal of Hazardous M

Ff

aubwbhdittfimStc

2

t(d

TT

ig. 3. Schematic diagram (not to scale) showing elemental distribution within auel-clad assembly after irradiation.

ssociated difficulties and bottle neck situations. Hence for betternderstanding of the subject and to take right steps towards feasi-le solutions it is absolutely necessary to put together the lessonse have learnt from past experiments. So far no such attempt has

een made and the present article tries to fill up this lacuna. Itas compiled all relevant past experimental results, from 1950s tillate, in a single source so as to enable the researchers, the waste

mmobilizers and the policy makers to judge the current state-of-he-art. The review starts by summarizing the various reprocessingechniques leading to P2O5-HLW generations, followed by the dif-culties encountered while immobilizing the same in borosilicateatrices (also discusses observations made from natural rocks).

ubsequently a detailed note on the various wasteform matricesried in the laboratory scale is described and finally certain futurehallenges associated with the problem are mentioned.

. Generation of phosphate rich HLW

Spent fuel reprocessing was first done in 1940s, and since

hen various aqueous (wet; hydrometallurgical) and non-aqueousdry; pyrometallurgical) routes have been established or beingeveloped, keeping in mind the future requirements [40]. In past,

able 21/2 (half-lives) of selected radionuclides present within spent nuclear fuel.

Isotope T1/2 Isotope T1/2 Isotope T1/2

Relatively short T1/289Sr 54 days 95Zr 65 days 95Nb 39 days103Ru 40 days 103Rh 57 min 106Rh 30 s131I 8 days 133Xe 8 days 134Te 42 min140Ba 13 days 140La 40 h 141Ce 32 days

Intermediate scale (year to decades) T1/2 (in years)3H 12.3 85Kr 10 90Sr 29106Ru 1 137Cs 30 144Ce 1.3147Pm 2.3 238Pu 85.3 241Am 440224Cm 17.4 55Fe 2.73 60Co 5.27

Longer (century and above) T1/2 (in years)99Tc 2 106 129I 1.7 107 239Pu 24,000240Pu 6500 243Am 7300 14C 573036Cl 301,000 39Ar 269 59Ni 76,00063Ni 100 93Mo 4000 94Nb 20,300

aterials 235 236 (2012) 17 28

P2O5-HLWs were generated due to two different reprocessingroutes (a) bismuth process [41] and (b) PUREX process [40]. Sub-sequently for safe temporary storage, these wastes were eitherstored as neutralized liquid or as calcined powder. Short descrip-tions on each of these items are given below. It may be addedhere that besides the above mentioned two reprocessing routes,currently various other innovative extraction processes involvingoctyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide;trialkyl phosphine oxides, etc. are being tried to reprocess spentfuels more efficiently. These may also yield P2O5-HLWs in future.

2.1. Bismuth process

In this process, decladded spent fuels were dissolved within con-centrated HNO3 solutions containing Binitrate and H3PO4 so asto precipitate Pu4+ together with Biphosphate. The precipitateswere then centrifuged and dissolved in HNO3, to which KMnO4was added to convert Pu4+ to PuO22+ and kept it in solution whileallowing Biphosphate to precipitate out. In subsequent stages,Pu was precipitated using ferrous sulfate (FeSO4), ferrous sulfa-mate [Fe(SO3NH2)2], sulfamic acid (HSO3NH2), etc. while ensuringformation of uranyl sulfate [UO2(SO4)22] such that U did notcoprecipitate with Pu. The precipitates were separated from fissionproducts and sulfate containing solution through centrifugation,and were subjected to decontamination in order to reduce overall-activity; treated with Nabismuthate, H3PO4, HNO3, HF, lan-thanum salts, NaOH, oxalic acid, H2SO4, (NH4)2SO4, etc. to finallyobtain Pu in the form of solid PuLa oxide. From this mixed oxide,metallic Pu was derived via Punitrate formation. P2O5-HLWs gen-erated at different stages were transferred to underground storagetanks [42].

2.2. PUREX process

Although Bi-process was efficient enough to extract Pu in pureform but it could not extract useful U, and also the volume of wastegenerated was huge due to repetition of precipitation cycles. Thisled to adaptation of continuous PUREX (Plutonium Uranium RedoxExtraction) solvent extraction process in plant scale where tributylphosphate (TBP; saturated in hydrocarbon (e.g. kerosene, etc.)) wasemployed to coextract Pu and U from HLWs generated after dis-solution of oxide-, carbide-, nitride-based spent fuels [40]. In thisprocess, chopped fuel pins were dissolved in concentrated HNO3solution from which U (UO22+) and Pu (Pu4+) were coextractedand subsequently separated by reducing Pu4+ to Pu3+ (either byferrous sulfamate or by hydroxylamine) while keeping UO22+ insolvent phase. P2O5-HLWs obtained at the end of the process weretransferred to underground tanks.

2.3. P2O5-HLWs storage as neutralized solutions

Storage of P2O5-HLWs within steel tanks for decades led tovarious physicochemical changes including (i) thermal and radi-olytic breakdown of organic/inorganic compounds, (ii) generationof hydrogen and other gases, (iii) release of large amounts ofheat due to interaction between ferrocyanides and nitrates, etc.To avoid unwanted corrosion of storage tanks, the acidic liq-uids are now being neutralized with NaOH. However, ageing ofthese HLWs together with addition of fresh waste batches, andoccasional evaporation to adjust HLW concentrations and vol-ume, lead to segregation of the mass in the form of supernateliquids, saltcakes and insoluble sludges ([42], Fig. 4). During the

course of evaporation, soluble Na salts present within supernateliquids (e.g. nitrate, nitrite, aluminate, hydroxide, etc. [43]) getcrystallized out in the form of salt cake, which is dominantlyNaNO3 mixed with Na7F(PO4)219H2O, Na3FSO4, Na2C2O4, NaF,

P. Sengupta / Journal of Hazardous M

Sludge ejkaite, clarkeite, nitratine, goethite, magh emite, amorphous

solids (Na, Al, P, O and C), spinels, quartz, Na- Al silicate s

Salt cakes Na -nitrate + chloride, fluoride, phosphate, sulphate

Supernate liquid Na salts (nitrate, nitrite, alu minate, hydroxide)

High level waste storage tan k

Fv

NtltrcegAecsnta

2

cctavAasapscmBaes(aN

3

3

f

ig. 4. Schematic diagram (not to scale) showing possible modes of compositionalariations within high-level waste storage tanks.

a3AlF6, etc. [44]. Radioactivity present within supernate solu-ion and salt cake is essentially due to the presence of Cs, Tc andimited amount of Sr and transuranic elements. The sludge, onhe other hand is much more hazardous as it retains most of theadionuclides present within HLW (excepting Cs) and essentiallyonstituted of Cejkaite (Na4(UO2)(CO3)3). Apart from this, pres-nce of clarkeite (Na[(UO2)O(OH)](H2O)01), nitratine (NaNO3),oethite [-FeO(OH)], magnetite (-Fe2O3), amorphous solids (Na,l, P, O and C), (Fe, Cr, and Ni) oxides, SiO2, Na Al silicates,tc. have also been reported [45]. Thus, supernate liquid and saltakes together constitute low activity waste (LAW) and whereasludge constitute high level waste (HLW). The radioactive compo-ents present within LAW are recovered through isotopic dilution,ransuranic precipitation, ultrafiltration and ion-exchange processnd are added to HLW fraction [46].

.4. P2O5-HLWs storage as calcined powder

Apart from storing as liquid, P2O5-HLWs are also stored as cal-ined powder. Reprocessing of Al/Zircaloy/stainless steel/graphiteladded spent fuels following PUREX process results in genera-ions of (a) F-rich and (b) Al-rich wastes [47]. To these (i) Al-nd Ca-nitrates are added for fluoride complexations as well asolatility suppression, and (ii) H3BO3 is put to prevent insoluble -l2O3 and -Al2O3 formation. For volume reduction, P2O5-HLWsre sprayed into hot (400600 C) fluidized bed whereupon anionsuch as nitrates, carbonates, etc. decompose leading to formationnd growth of multi-oxide calcined particles, which are then pipedneumatically into underground storage tanks. The final compo-ition of calcined wastes therefore depends on initial P2O5-HLWompositions and subsequent chemicals added during powder for-ations. For example, Al-calcines are usually rich in Al2O3 and

2O3, while Zr-calcines are enriched in CaF2, ZrO2, Al2O3, CaOnd B2O3 [48]. Similar calcination route has also being consid-red for treating NaNO3 rich waste streams (commonly referred asodium bearing waste, SBW) which mostly arise from site clean-updecontamination and decommission) activities [47]. Composition-lly SBW is a HNO3 solution with relatively high concentrations ofO3, Na+, Al3+, K+, P2O5, and SO42 [49].

. Phosphate in silicate melts

.1. Lessons learnt from natural melts

As mentioned earlier, borosilicate glasses are generally pre-erred for HLW immobilization. However, natural analogue studies

aterials 235 236 (2012) 17 28 21

backed up by experimental observations on equivalent syn-thetic melts show that P2O5 even if present in small quantities(12 wt%) can (i) promote liquid immiscibility (basic and acidicmelts [50,51]), (ii) control trace element partitioning within melts[52,53], (iii) reduce melt viscosity [54,55], (iv) shift liquidusboundaries towards silica deficient domains within variably poly-merized silicate melts [56] and (vi) depress solidus [57]. Ramaninvestigations in SiO2P2O5 system show the presence of iso-lated non-polymerizedP2O5 (basic) rich domains connected topolymerizedSiO2 (acidic) melts through formation of P O Si link-ages [58]. In case of aluminosilicate system, the situation changesas juxtaposition of Al3+ and P5+ promote quartz (SiO2)berlinite(AlPO4) substitution:

Al3+ + P5+ = 2Si4+ (1)within melts. Bulk compositional analyses of silica rocks showphosphate solubility within multicomponent aluminosilicatemelts depends on A/CNK (Al2O3/(CaO + Na2O+ K2O); [59]) or ASI(Al/(Li + Na + K + Rb + 0.5Ca); [60]) molar ratios and are usuallyhigher in peraluminous (A/CNK > 1) melts in comparison to met-aluminous (A/CNK = 1) ones. Wolf and London [61] estimatedthat meta-aluminous haplogranite melts (ASI = 1.0) can dissolve0.1 wt% P2O5 at 750 C whereas in similar condition peralumi-nous melts (ASI = 1.35) can incorporate 0.7 wt% P2O5. Thus as far aswasteform designing is concerned peraluminous melts have morepromise than others. The reason behind such variations in phos-phate solubility is linked to its respective melt structures. Withinperalkaline melts (A/CNK < 1) phosphate addition increases itspolymerization through associating alkali metals with phosphatetetrahedra and thereby decreasing the number of NBO [6264].On the other hand, phosphate in peraluminous melts stabilizesberlinite component and thereby enhances apatite solubility [59].This scenario however becomes complicated when fluxing ele-ments (e.g. F and B), alkali (Li, Rb, and Cs) and alkaline earths(Be, Sr, and Ba) are present within the system. It has been con-firmed from both petrological as well as experimental studiesthat these elements play contrasting roles in terms of promot-ing crystallization. For example, formation of F Al clusters withinsilicate melts decreases berlinite (AlPO4) activity [65] whereasreverse happens with development of BAl complexes (due tocompetition for tetrahedral positions). Similarly, high Li contenttogether with virgilite (LiAlSi5O12) substitution are known to sta-bilize berlinite solid solutions through enhancing the AlPO4SiO2mutual solubility. In addition to these, crystallization of apatite,berlinite, amblygonitemontebrasite, lithiophilitetriphylite, spo-dumene, petalite, etc. instead of potential silicate minerals withinacidic melts containing phosphate is not uncommon.

3.2. Difficulties associated with phase separations within HLWloaded silicate melts

As observed from natural analogues and equivalent syntheticmelt experiments, phosphate is found to promote phase separa-tions (liquid immiscibility as well as crystallization) within wasteglass melts even if present in minor amounts [66]. Such phase sep-arations are not coveted from HLW immobilization point of view asit deteriorates product durability factors severely and also hampersplant scale manufacturing processes significantly. The situationbecomes even more stringent when additional poorly soluble (insilicate melt) anions like sulfates, chromate, halides, etc. are alsopresent within the HLW [31,67]. In case of P2O5-HLWs, presence ofsulfate anion within the waste is very common as reducing agents

such as ferrous sulfate, and ferrous sulfamates are used for Pu cationreduction. It has been noted that due to poor solubility of sulfateanion within silicate melts (

22 P. Sengupta / Journal of Hazardous Materials 235 236 (2012) 17 28

ooHf(asdap((sp[atHmttdKomoe

4

(ldobPOitabn

Fig. 5. Formation of yellow phase within borosilicate glass matrix.

ut (formed essentially due to release of oxygen upon reductionf multivalent cations like Fe, Ni, Mn, etc.) during vitrification ofLW [38,6872]. The yellow phase is essentially a cluster of sul-

ates, chromates and aluminates of which water soluble thenarditeNa2SO4) is the most prominent one and acts as a sink for haz-rdous radionuclides ( emitting, heat generating, radiotoxic)uch as 137Cs and 90Sr. Fig. 5 (hand specimen scale) shows theevelopment of yellow phase within borosilicate melt upon excessmount of sulfate containing waste loading. Apart from this, yellowhase formations also severely impede vitrification process as theyi) do not allow the release of gaseous reaction products from melt,ii) corrode melter liners, (iii) diverts electrical field and can evenhort electrodes, (iv) go on accumulating and occupying significantortion of vitrification furnace unless physically separated out, etc.3032,73]. Adaptation of high temperature vitrification process islso not a solution for such type of problems as the decompositionemperatures for salt layers are usually very high (above 1300 C).ence the best solutions for such cases is to go for an alternativeatrix where it is possible to maintain the liquidus temperature of

he melt at least 100 C below than the plant operating tempera-ures (1100 C) and the melt should not undergo phase separationuring production and even under prolonged idling conditions.eeping all these factors in mind a group of wasteform designerspted for P2O5 glasses (instead of silicate glasses) as an alternativeatrix since previous structural studies revealed that the system

ffer wider range of network structures suitable for radionuclidentrapment.

. Networking within phosphate melt

Long back Zachariasen [74] and van Wazer [75] identifiedPO4)3 tetrahedra as the basic unit of P2O5 glasses which uponinking through covalent P O P bonds and others were found toefine various types of network structures (Table 3 and Fig. 6). Eachf the tetrahedra, results through formation of sp3 hybrid orbitalsy the outer electron (3s23p3). Usually, the fifth outer electron of

gets promoted to 3d orbital where together with 2p electron of forms strong -bond. Within each (PO4)3 tetrahedra, the P ion

s linked to three bridging oxygens (OB) through P OB bonds and

o one terminal oxygen (OT) by doubly bonded P OT bonds. Forny given glass, the P OT bonds are always shorter than P OBonds and their respective lengths depend on P2O5 contents andature of the modifying cations [76]. Cross linking of three out of

Fig. 6. Phosphate structural units reported from glasses (Qi: i represents number ofbridging oxygen per tetrahedron).

the four vertices of (PO4)3 tetrahedra leads to the formation ofQ3 ultraphosphate structural group, which upon additions of mod-ifying oxides (e.g. alkali oxides (R2O), alkaline earth oxides (RO),etc.), get depolymerized through conversion of bridging oxygens(P O P) to non-bridging oxygens (P O R) [77]. Thus in case ofbinary xR2O (or RO)(1 x)P2O5 glasses (where x is in mole frac-tions), with increasing molar R2O/P2O5 ratios (from 0 to 1 to 2 tofinally 3) or [O]/[P] ratios (from 2.5 to 3 to finally >3.5), the struc-tural groups (represented by Qi where i represents the numberof bridging oxygens per tetrahedron) passes from cross linked Q3

(ultraphosphate; O/P = 2.5) to Q2 (metaphosphate; O/P = 3.0) to Q1

(pyrophosphate; O/P = 3.5) to Q0 (orthophosphate; O/P = 4.0), keep-ing P always in four fold coordination [78,79]. Recently it has been

noted that depending on the chemical nature, the cations presentwithin a given phosphate glass can act either as network modifier(e.g. Na+ and Ca2+) or network former (e.g. Al3+, Bi3+ and Nb5+; [80]and their coordination numbers may change with compositional

P. Sengupta / Journal of Hazardous Materials 235 236 (2012) 17 28 23

Table 3Compositional dependence of phosphate structure binary in binary xR2O (or RO)(1 x)P2O5 (x in mol%) system.

Structure Ultraphosphate Mesophosphate Polyphosphate Pyrophosphate Orthophosphate

Compositionalrange

0 < x < 0.5 x = 0.50 x > 0.50 x = 0.67 x = 0.75

Dominatingstructural groups

Q2 and Q3 Q2 form chainsand rings

Q2 chains terminatedby Q1 tetrahedra

Phosphate dimmers, a pair oflinked Q1 tetrahedra

Isolated Q0

Proportion of f(Q2) = (x/(1 x))3

f(Q1) = ((2x 1)/(1 x))2

f(Q0) = ((3x 2)/(1 x)) and1

mpvfilHfb(0ha

5g

gwfrwbhmesctdi

TC

structural groups f(Q ) = ((1 2x)/(1 x))

and f(Q ) = ((2 3x)/(1 x))

Molar O/P ratio 2.5 3 >3

odifications [81]. Thus, in principle a variety of glasses can berepared within phosphate based systems, some of which do havearious advantageous properties suitable for plant scale HLW vitri-cation such as (i) low melting temperatures; (ii) fast melting, (iii)

ow viscosity, and (iv) ability to dissolve wide range of elements.owever from conventional melt processing point of view, glass

ormations is typically limited from vitreous P2O5 to compositionsetween the metaphosphate and pyrophosphate compositionsi.e. in case of binary xR2O (or RO) (1 x)P2O5 systems within

< x < 0.550.60). Keeping these aspects in mind various attemptsave been made to condition P2O5-HLWs within phosphate glassesnd a brief description on this is given below.

. Immobilization of phosphate containing HLW withinlass matrices

Recognizing the various advantageous features of P2O5-basedlasses, attempts were made (in 1960s) to condition Na rich HLWsithin Naphosphate glasses. These wasteforms however suffered

rom poor chemical durability, rapid devitrification and high cor-osiveness in molten stage which made them less attractive withinasteform designer community [8285]. The poor chemical dura-

ility of the waste glasses was attributed to dominance of easilyydrolysable P O P bonds, which upon hydration get depoly-erized through formation of terminal OH groups [86]. Bunker

t al. [87] studied the interactions between different hydrogenpecies, (H2O, OH and H+ or H3O+) and phosphatic glasses and con-

luded that metaphosphate glasses dissolve congruently through awo stage process involving (a) surface hydration (due to wateriffusion) and (b) subsequent release of metaphosphate chains

n solution (due to surface reactions). Subsequent experimental

able 4ompositions (wt%) of some well studied simulated waste loaded phosphate glasses.

Oxide Hanford mixed waste [91] Hanford LAW [130]

SPP22 Pb SPP23 Pb IP30LAW IP3

Na2O 21.17 17.36 Al2O3 18.39 21.08 1.3 11P2O5 35.55 42.34 52.2 52B2O3 4.49 - 0.0 0BaO 0.01 0.01 CaO 5.25 5.61 0.0 0CeO2 0.21 0.20 Cr2O3 0.04 0.07 0.1 0CuO 4.17 4.33 Fe2O3 .51 0.52 20.0 10K2O 0.94 0.78 0.0 0MnO 0.43 0.45 0.0 0PbO 0.67 0.53 SO3 1.62 a 2.9 2SiO2 3.79 4.32 0.2 0ZrO2 2.73 2.35 Cl 0.0 0.0 0.2 0F 0.0 0.0 0.5 0

a Spiked with SO3: 217 wt%.

f(Q ) = ((3 4x)/(1 x))

3.5 >3.5

studies showed that addition of Al2O3 improves chemical durabilityof phosphate glasses [88] and this re-established the credibil-ity of the system. In fact, former Soviet Union came up with anew Na Al phosphate glass formulation for immobilizing its sul-fate containing HLWs [89]. 27Al-(MQ) MAS NMR spectroscopicanalysis of aluminophosphate glasses showed that replacementof P O P bonds by Al O P linkages is principally responsiblefor the improved chemical durability of the matrices [90]. Theidea of employing phosphate glasses for waste immobilizationwas once again tried by United States, and this time it yieldedgood results in terms of conditioning Na and sulfate rich mixedwaste from Solar Evaporation Basin of Hanford ([91]; Table 4).Parallel to this, Kushinikov et al. [92] and Matyunin and Jar-dine [93] tried to immobilize Pu rich waste, both in the formof PuO2 powder and Pu nitrate solution, within Na Al phos-phate glass (Na2O: 24 wt%, Al2O3: 22 wt% and P2O5: 54 wt%)and respectively could load 0.240.31 wt% (for PuO2 powderwaste) and 0.70.8 wt% (for Pu nitrate waste) of waste homoge-neously. Lately, Donald et al. [94] considered Na Al phosphateglasses (41.0Na2O20.5Al2O338.5P2O5 (mol%)) for encapsulatingCa phosphate ceramic (containing chlorapatite (Ca5(PO4)3Cl) andspodiosite (Ca2(PO4)Cl)) loaded with chloride enriched HLWs [95].They studied the devitrification tendency of the encapsulating glass(AlPO4, Na3PO4, NaAlP2O7 and Na7(AlP2O7)4PO4) and hardly foundany detrimental effect of the same on the overall properties of thefinal monolithic wasteform. Further, to improve thermal stabil-ity of the material and suppress devitrification, several trials were

taken with FePO4 (as a source of Fe2O3), ZnO and B2O3, of whichthe later (>2 mol% B2O3) yielded the best result [94,96]. The reasonbehind such improvement is obviously better linkages of (PO4)3

tetrahedra with trigonal (B3) and tetrahedral (B4) borate units [97].

INEEL-SBW [131]

0LAW-A IP30LAW-C IP40WG IP40WG-CCIM

.3 1.3 10.9 11.6.2 52.2 51.7 50.0.0 0.0 0.1 0.0

.0 0.0 0.9 0.9

.1 3.1 0.1

.0 17.0 10.3 10.6.0 0.0 3.0 3.2.0 0.0 0.3 0.0

.9 2.9 1.4 1.8.2 0.2 0.0 0.0

.2 0.2 0.3 .5 0.5 0.3 0.0

2 ous M

Btp

BpwiiErasp(ttpch[(rwtdbaoNa

twfcl(iNthmaflidot

TS

4 P. Sengupta / Journal of Hazard

ut once again, the encapsulation suffered from hydrolyzation ofhe network as borate units are also found to be hygroscopic ashosphate units are [98].

Parallel to the studies on Na Al phosphate glasses, Sales andoatner [99] came up with a new formulation from Pb Fe phos-hate glass system to immobilize chloride rich HLWs. Pb and Feere purposefully added to the phosphate glass system to decrease

ts melting temperature and viscosity, and to increase its chem-cal durability and resistance towards devitrification [100,101].xperimental studies indicated that addition of PbO should beestricted within 3566 wt% as beyond this range phosphate glassesre either prone to devitrification or are highly viscous in moltentage. Hence Sales and Boatner [99] selected an intermediate com-osition of 45 wt% PbO55 wt% P2O5 and added 9 wt% Fe2O340PbO10Fe2O350P2O5 (mol%)) to further improve its resistanceowards corrosion and devitrification, and to increase its softeningemperature. Thus they could ultimately formulate a pyrophos-hate glass (Fe2Pb(P2O7)2 and Fe3(P2O7)2; O/P molar ratio 3.7)apable of incorporating 1015 wt% of simulated waste and stillaving a leach resistance better than borosilicate waste glasses102104]. Mssbauer [105] and electron paramagnetic resonanceEPR; [106,107]) studies indicated that Fe2+ cations are mostlyestricted within interstitial positions with octahedral coordinationhereas Fe3+ cations can occupy both substitutional and intersti-

ial positions with octahedral coordination. The improved chemicalurability is due to replacement of hygroscopic P O P linkagesy P O Fe bonds [108]. With increasing waste limit, the over-ll pyrophosphate structure (P2O7)4 was gradually replaced byrthophosphate (PO4)3 and metaphosphate (PO3) species, andaFeP2O7 and SiP2O7 crystallized within the matrix adverselyffecting its chemical durability.

Although PbFe phosphate glass exhibited high potentialityowards waste immobilization but still it suffered from (i) lowaste loading capacity (

ous M

tPssI3wwmpaeg(((a

ow(tgaa[BgrobtaI[oirtAwNapcbsh

ohrmFmp1ftwao

c

P. Sengupta / Journal of Hazard

hat although additions of both (R2O and RO) in multicomponent2O5Fe2O3FeORyO2 (R = Li, Na, K, Mg, Ca, Ba, or Pb and y = 1 or 2)ystems induces depolymerization, R2O hardly modifies networktructures, thermal properties and redox state whereas RO does.t has been noted that up to 26 mol% of CsCl, 31 mol% SrF2, and4 mol% CsCl + SrF2 could be loaded within Fe-phosphate glassesithout much sacrifice on product durability aspects [124,125],hereas for higher loading (e.g. Cs2O of 55 mol%, [126]) theatrix becomes moisture sensitive and the glass-transition tem-

erature reduces drastically. In case of chemical durability, R2Odditions reduce it following the series K > Na > Li whereas pres-nce of RO enhances it. Bulk crystallizations of Fe-phosphatelasses are also observed to take place at much lower temperatures600 C) in presence of R2O (3040 mol%) than when RO is present700 C). For example, in case of Cs2O doped Fe-phosphate glasses36 mol% Cs2O26 mol% Fe2O3P2O5) CsFe(P2O7), Cs7Fe7(PO4)8O2nd Cs3PO4are found to crystallize out at 583 C [127].

Phosphate glasses have long been known for better solubilityf sulfate [128] and sulfate containing HLW [68,129]. Howeverhen tried for sulfate and phosphate containing mixed HLW

17 wt% sulfate; Hanford mixed waste) it is observed that reten-ion of sulfate within otherwise homogeneous Na2OAl2O3P2O5lasses (loaded with 38 wt% of simulated HLW, 1050 C)re rather poor [91]. To understand the reason, Binghamnd Hand [114] carried out in-depth experimental studies on(1 x)(0.6P2O50.4Fe2O3)] + xRySO4 (where R = Li, Na, K, Mg, Ca,a, Pb; x = 00.5; y = 2 and 1) and P2O5Al2O3Na2OFe2O3SO3lasses and noted that sulfate solubility (log[SO3]) is linearlyelated to normalized cation field strength index (z/a2), the-retical optical basicity th, and [O]/[P] molar ratio. It haseen postulated that under general plant scale operating condi-ions (1150 C), Fe-phosphate glasses for which (z/a2) < 1.2;

th > 0.5; [O]/[P] > 3.8; or P2O5 content

2 ous M

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omrstucbboatyotdattdawtptfocmaat

A

do

R

6 P. Sengupta / Journal of Hazard

s known to be much more aggressive in case of phosphatic meltss compared to silicate ones. As a solution towards this, one mayuggest to modify phosphate melt compositions by adding Al2O3,2O3, etc. [138,139] or by putting chemical diffusion barriers on thelloy components [140,141], but the actual progress in this direc-ion can only be done after getting feed back from plant operators.ue to lack of such data or experiences it is also not possible toecide various important processing parameters such as type ofurnace need to be used, nature of melter feed (slurry or calcined

ass), requirement of external agitation (using stirrer, bubbler,tc.) for homogenization of the melt, etc. Although judging fromxperimentally determined viscosity, electrical conductivity datane can suggest to adapt induction furnace based technologiesut it will not be prudent to decide on this without having doneequired plant scale testing. Such an approach will also help instablishing flexibility within process flow sheets as variations inLWs and/or melter feed chemistry and various processing param-ters can impact melting rate and steady state melting conditionsignificantly.

All said and done, another big challenge in final adaptationf phosphate melt based vitrification technique in plant scale isental blockage. Over the decades huge investments of time and

esources have been made to establish borosilicate glasses as auitable waste form matrix. Thus waste immobilizers are reluc-ant to adopt any new vitrification technique in plant scale levelntil and unless all routes of modifying borosilicate glass matri-es are tried. On one hand the possibility of modifying existingorosilicate glass compositions with better compositional flexi-ility, durability and processing characteristics cannot be ruledut but on the other hand it is also equally possible to identifyn alternative matrix having wider processing windows, greaterolerance towards waste stream compositional fluctuations andet having acceptable product durability properties. In such casesf course one has to take decision after giving proper weightageo the overall costs of developing a new wasteform along withuly required acquisition of repository performance data, licensingmendments, production change-over, revision of waste accep-ance requirements, identification of new consistency/durabilityest specifications, rescheduling storage, disposal plans, etc. But theata and information required to do such appraisals is still lackingnd until and unless these are made available the preferred choiceill be to stick to already established vitrification technique. Thus

o establish the credentials of phosphate glass based vitrificationrocess in plant scale it is important to assure all required informa-ion and experiences on a faster track for the already identified bulkormulations and the newer ones [142,143]. Additionally, in casef waste streams containing significant amount of low-solubilityomponents, due attentions should be given to glass-compositeaterials also [144,145]. Finally, it is hoped that with such an

pproach, practicing professionals and interested students will beble to appreciate the intricacies of the subject and come forwardo solve the problem in future.

cknowledgements

Author gratefully acknowledges Alexander von Humboldt Foun-ation, Germany. Prof. Andrew J. Daugulis is thanked for carryingut editorial responsibility very effectively.

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A review on immobilization of phosphate containing high level nuclear wastes within glass matrix Present status and futu...1 Introduction2 Generation of phosphate rich HLW2.1 Bismuth process2.2 PUREX process2.3 P2O5-HLWs storage as neutralized solutions2.4 P2O5-HLWs storage as calcined powder3 Phosphate in silicate melts3.1 Lessons learnt from natural melts3.2 Difficulties associated with phase separations within HLW loaded silicate melts4 Networking within phosphate melt5 Immobilization of phosphate containing HLW within glass matrices6 Future challengesAcknowledgementsReferences