increased fuel burn up in a candu thorium reactor using weapon grade plutonium

Nuclear Engineering and Design 236 (2006) 1778–1788

Increased fuel burn up in a CANDU thorium reactor usingweapon grade plutonium

Sumer Sahin a,∗, Kadir Yıldız b, Hacı Mehmet Sahin a, Necmettin Sahin b, Adem Acır a

a Gazi Universitesi, Teknik Egitim Fakultesi, Besevler, Ankara, Turkeyb Nigde Universitesi, Aksaray Muhendislik Fakultesi, Aksaray 68000, Turkey

Received 27 October 2005; received in revised form 23 January 2006; accepted 23 January 2006

Abstract

Weapon grade plutonium is used as a booster fissile fuel material in the form of mixed ThO2/PuO2 fuel in a Canada Deuterium Uranium(CANDU) fuel bundle in order to assure the initial criticality at startup.

Two different fuel compositions have been used: (1) 97% thoria (ThO2) + 3%PuO2 and (2) 92% ThO2 + 5% UO2 + 3% PuO2. The latter isused to denaturize the new 233U fuel with 238U. The temporal variation of the criticality k∞ and the burn-up values of the reactor have beenctsb

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alculated by full power operation for a period of 20 years. The criticality starts by k∞ = ∼1.48 for both fuel compositions. A sharp decrease ofhe criticality has been observed in the first year as a consequence of rapid plutonium burnout. The criticality becomes quasi constant after theecond year and remains above k∞ > 1.06 for ∼20 years. After the second year, the CANDU reactor begins to operate practically as a thoriumurner.

Very high burn up could be achieved with the same fuel material (up to 500,000 MW·D/T), provided that the fuel rod claddings would be replacederiodically (after every 50,000 or 100,000 MW·D/T). The reactor criticality will be sufficient until a great fraction of the thorium fuel is burnt up.his would reduce fuel fabrication costs and nuclear waste mass for final disposal per unit energy drastically.2006 Elsevier B.V. All rights reserved.

. Introduction

.1. Civilian use of weapon grade plutonium

During the cold war large quantities of weapon grade plu-onium have been accumulated in the nuclear warheads. It iseported that the US maintains forces that keep ∼7600 nucleararheads deployed at an estimated cost of US$ 7.8 billion perear (Oelrich, 2005). The nuclear arsenal of Russia is estimatedo exceed >10,000 nuclear warheads.

It is of general interest to reduce plutonium inventories,ecause of the serious public and political concern in the worldbout misuse of this plutonium and about accidental release ofighly radiotoxic material into the environment. It, therefore,ecomes necessary to keep the plutonium under strong security.oday’s main goal probably should be to reduce the separatedmounts of plutonium as soon as possible.

∗ Corresponding author. Tel.: +90 312 212 43 04/490 63 09 (Home);ax: +90 312 212 43 04.

E-mail address: [email protected] (S. Sahin).

One alternative for the management of plutonium is to incin-erate it in reactors. On the other hand, if the plutonium wouldbe fueled in reactors in the form of uranium/plutonium mixedoxide (MOX), second generation plutonium is produced. Thiswould not help to reduce plutonium inventories.

1.2. Thorium/plutonium mixed fuel

A possible solution to this problem is to incinerate pluto-nium in combination with thorium. Because, one produces onlyuranium from thorium, which is a significantly benign materialcompared to plutonium with respect to safeguarding consider-ations as well as radio toxicity. For thorium by itself is not anuclear fuel, excess plutonium must serve as a booster fissilefuel material in a thermal reactor, fueled mainly with thorium.

The thorium cycle produces 233U, which from a non-proliferation point of view, is preferable to plutonium for tworeasons. Firstly, it is contaminated with 232U, which decays togive highly active daughter products. This has already a highlevel of deterrence and would make handling and diversion dif-ficult. Secondly, the 233U could easily be denaturized by adding

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S. Sahin et al. / Nuclear Engineering and Design 236 (2006) 1778–1788 1779

some 238U to the thorium, which would then eliminate the pos-sibility to make a critical assembly with the fuel. The quantityof 238U could be fine-tuned so as to be sufficient to denaturizethe 233U, but not so much as to produce a significant quantityof plutonium. The thorium option not only produces electricity,but also replaces the plutonium with denaturized 233U. Hence,thorium may play a larger role in the nuclear fuel cycle. Thesuperior nuclear properties of 233U over 235U and 239Pu in ther-mal reactors imply potential advantages to 233U in advancedterrestrial reactors and also in space craft reactors.

There have been several proposals to make commercial uti-lization of weapon grade plutonium in nuclear reactors. A coor-dinated research project has been conducted through Interna-tional Atomic Energy Agency (IAEA) to asses the potential ofcommercial utilization of weapon grade plutonium in thoriumbased nuclear reactors (IAEA, 2003). Ponomarev-Stepnoi andTsourikov (1997), Ponomarev-Stepnoi (1997) and Koudriatsevet al. (2003) report on options in fast breeder reactors, pres-surized water power reactors, and high temperature gas cooledreactors. Alekseev et al. (1997) consider molten salt cooled reac-tors for incineration of weapons-grade plutonium. In February1995, MINATOM of Russia and General Atomics (USA) havefirst signed the Agreement for the development and design of amodular helium reactor and a gas turbine (GT-MHR) to be con-structed in Russia (Kiryushin et al., 1997). Then, the design of aGas Turbine-Modular Helium Reactor (GT-MHR) of 600 MWthpteCW2csta2

Drothu(Lut2rngesona

Fig. 1. Cross-sectional view of the fuel channel: (I) original CANDU squarelattice cell and (II) equivalent diameter, used in calculations (dimensions are incentimeters, not in scale).

For enhanced inherent safeguarding purposes, an easy andelegant way will be to add 5% natural uranium to the mixedTh/Pu fuel.

2. Description of the problem

The investigations in the present study are based on aGENTILLY-II design (Woodhead and Ingolfsrud, 1975) modi-fied to a CANDU–thorium reactor, as described in Alkan (2003),IAEA (1979) and Altınok (1993). The reactor core has 380 fuelbundles. Fig. 1 shows a typical CANDU fuel channel wherethe fuel bundle is contained in the calandria tube. The calan-dria tubes, made of zircalloy-2, are arranged in square arrays inD2O moderator at low temperature (<71 ◦C) and at ∼100 kPapressure. The nuclear heat generated in the fuel rods is trans-ferred via the D2O coolant contained in the pressure tubes to thepower conversion system. Pressure tubes are made of Zr–Nb.D2O coolant enters the tubes at 266 ◦C and leaves at 310 ◦C ata pressure of ∼10 MPa (Lamarsh and Baratta, 2001). The pres-sure and calandria tubes are separated through a gap in order toassure an effective heat barrier between D2O moderator at lowtemperature and D2O coolant at high temperature. The gap isfilled with CO2 gas.

In the CANDU reactor, each fuel channel contains 37 fuelrods in the fuel bundle zone, as shown in Fig. 2. The cladding oftag

ower has been started in frames of “The Agreement betweenhe Government of the United States of America and the Gov-rnment of the Russian Federation on Scientific and Technicalooperation in the Management of Plutonium that has beenithdrawn from Nuclear Military Programs” signed on July

4, 1998 (Kodochigov et al., 2003). The expected electricalonversion efficiency of the power plant is ∼50%. Japanesecientists have investigated the potential of weapons-grade plu-onium utilization in light water reactors (Akie et al., 1994),nd conducted experiments with mixed fuel (Yamashita et al.,001).

On the other hand, the higher neutron economy of Canadaeuterium Uranium (CANDU) reactors compared to light water

eactors (LWRs) gives an inherent advantage in the exploitationf rich world thorium reserves which are estimated to be abouthree times more abundant than the natural uranium reserves. Itas been shown that with a uranium enrichment grade of >1%,tilization of thorium in CANDU reactors can become viableBoczar et al., 2002a,b; Critoph, 1976; Jagannathan et al., 2001;oewen et al., 2001). In previous studies, different aspects of thetilization of LWR spent fuel in CANDU reactors mixed withhoria (ThO2) have been analyzed (Sahin et al., 2004a,b; Alkan,003). In the present work, the neutronic analysis of a CANDUeactor with a mixed fuel made thorium and weapon grade pluto-ium is presented. The main purpose of the utilization of weaponrade plutonium instead of low enriched uranium is the incin-ration of the former and so reduction of the nuclear weapontockpile, as already addressed above. Of course, the fuel costf uranium will be much lower than that of weapon grade pluto-ium, unless the latter is subsidized or provided through politicalrrangements.

he rod is made of zircalloy-4. Table 1 shows the compositionnd atomic density of the reactor materials. During the investi-ations, two different fuel compositions have been chosen:

1780 S. Sahin et al. / Nuclear Engineering and Design 236 (2006) 1778–1788

Fig. 2. Placement of 37-fuel rods in the bundle (dimensions are in millimeters, not in scale).

1. The mixed fuel is composed of thoria (ThO2) as the mainfuel and few % of weapon grade PuO2 as booster materialfor reactor criticality.

2. The mixed fuel is composed of thoria (ThO2) as the mainfuel and few % of weapon grade PuO2 as booster materialfor reactor criticality. Furthermore, 5% natural UO2 has beenadded to denaturize the new 233U fuel with 238U. The latterwill assure enhanced safeguarding of the uranium compo-nent.

The isotopic composition of the weapon grade plutonium hasbeen adopted from IAEA (2003), and shown in Table 2. In themixed fuel, a small amount of weapon grade plutonium willserve to burn significant quantities of thorium fuel in CANDUreactors. This would allow wide exploitation of world thoriumreserves, based on the well-established conventional CANDUreactor technology and would avoid large development costsand efforts for a new technology line.

3. Numerical calculations

The neutron transport calculations have been performed withthe help of SCALE5 system (Petrie, 2004) in S8-P3 approx-imation by solving the Boltzmann transport equation in one-dimensional geometry with the transport code XSDRNPM(Greene and Petrie, 2004) using the 238-neutron groups datallet

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power refueling scheme to provide sufficient reactivity dur-ing reactor operation without reactor shutdown, a modestexcess reactivity will be sufficient for long-time operation.The on-power refueling results in a continuous distributionof fuel burn up in the core. A fuel bundle can be reshuf-fled in the core until discharge reactivity kend is attained. Thekend is typically 1.05–1.06 considering the leakage from thecore and parasitic absorption in the core. These considera-tions have been outlined in a recent work (Chapter 2.2, Sahinet al. (2004b)), in detail, and hence have not been repeatedhere.

In a series of successive calculations, the temporal variationof the atomic densities N of the fissionable fuel isotopes in aCANDU fuel bundle during reactor operation are evaluated fordiscrete time intervals �t = 10 days, under consideration of thenuclear reactions through neutron capture and radio-active trans-formation processes of the actinides, as described in Fig. 3 ofSahin et al. (2004b).

3.2. Lattice calculations for the fuel rod and fuel channel

Resonance self shielding calculations and cross-section pro-cessing are performed with the help BONAMI and NITAWL-III first for the CANDU fuel rod as a micro lattice, shownin Fig. 2B. These codes allow cell calculation with a max-iere

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ibrary, derived from ENDF/B-V (Jordan et al., 2004). Theibrary has 148 fast and 90 thermal (above 3 eV) groups, andnables very fine energy resolution for multi-group neutronransport calculations.

The resonance self-shielding weighted cross-sections haveeen processed with the help of the CSAS control moduleLanders et al., 2004) using BONAMI (Greene, 2004) for unre-olved resonances and NITAWL-III (Greene et al., 2004) foresolved resonances.

.1. Strategy for criticality and burn-up calculations

The initial fuel charge starts with a high keff value forCANDU reactor. As the CANDU reactor uses an on-

mum of three zones, defined as fuel, cladding and mod-rator regions, and applies the Bondarenko method for theesonance treatment with some corrections for heterogeneousffects.

At the next step, one-dimensional neutron transport calcu-ations are conducted with the help of the SN transport codeSDRNPM for the CANDU fuel channel in Fig. 1, as a rep-

esentative macro lattice for the reactor core. The quadraticuel channel is transformed to a volume and material equiv-lent cylindrical macro lattice for each fuel rod row in radialirection separately. Due to the irregularities of the CANDUundle in Fig. 2A, moderator volume per fuel rod reveals someinor variations in radial direction in the fuel zone, and the

rrangement of the 37 fuel rods does not comply exactly with aeometrically hexagonal structure. A general purpose code sys-


Table 1Composition and atomic density of the reactor materials

Material Elements Atomicdensities(1024 cm−3)

99% ThO2 + 1% PuO2232Th 9.24444E-03a

16O 1.87494E-02239Pu 9.80265E-05240Pu 6.25291E-06

98% ThO2 + 2% PuO2232Th 9.15106E-0316O 1.83073E-02239Pu 1.96053E-04240Pu 1.25058E-05





92% ThO2 + 3% PuO2 + 5% UO2232Th 8.59080E-0316O 1.88288E-02239Pu 2.94079E-04240Pu 1.87580E-05238U 5.03775E-04235U 3.62304E-06

Zircaloy-4 (Petrie et al., 2000) Zr 3.10250E-03Sn 4.57972E-05Fe 6.63273E-06Cr 3.15844E-06Hf 3.15844E-07

Zr-2.5Nb (Sahin et al., 2004b) Zr 4.15000E-02Nb 1.06400E-03

Zircaloy-2 (Sahin et al., 2004b) Cr 4.28200E-05Fe 5.53200E-05Zr 4.20600E-02Sn 6.42300E-04

D2O D 6.68700E-02O 3.34300E-02

a Read as 9,24444.10−03.

Table 2The composition of the fissionable isotopes in the fresh fuel (IAEA, 2003)

Isotopes aWeapon grade plutonium (%)

239Pu 94.0240Pu 6.0

a IAEA, 2003, p. 55, Table 3.3.6.

tem, such as SCALE5, may at first appear not directly applicablefor CANDU configurations due to irregularity of the CANDUbundles. However, SCALE5 allows a multitude of fast computerruns for a differentiated analysis of nuclear energy productionin one-dimensional geometry with a very fine energy resolutionin neutron 238-groups.

In order to consider the irregularity of the basic cell geometryin the CANDU fuel bundle, separate micro cell calculations areconducted for each fuel rod row from I to IV in the bundle underconsideration of variable moderator volume for each individualrod. During time calculations, the variations of the densities ofall fissionable nuclides are calculated for each fuel rod locationseparately. In this way, four different cross-section sets havebeen processed in the fuel zone for a higher precision in thecalculations.

Following the lattice calculations for the basic fuel rod cell,the micro cell weighted and resonance treated cross-sectionsare then used for the neutron transport calculations with the SNcode XSDRNPM for the fuel channel. The cluster geometryis divided into separate zones with similar neutronic charac-teristics, such as, fuel zone (four radial regions with minorvariations of moderator/fuel ratio), structures, CO2 gap andmoderator zones, as shown in Fig. 1. A reflecting bound-ary condition in the centre and a white boundary conditionat the outer periphery of the cylindrical moderator region areapplied. Multiplication factors (k ) for the fuel channel arecbl

ple

3

lttctcsfaTlpbta2pkm2

∞alculated for a core height of 594 cm, with the help of auckling factor for an approximate consideration of the axialeakage.

For reactor dimensions exceeding several neutron mean freeaths, one fuel channel can be considered as a representativeattice element for the entire reactor core to follow temporalvaluation of burn-up characteristics.

.3. Criticality and burn-up calculations

In large LWRs and CANDU reactors, k∞ value of a macroattice is very close to keff of the entire reactor. In previous work,he multiplication factor k∞ of the CANDU macro lattice andhe multiplication factor keff of the CANDU reactor includingore and reflector with finite dimensions and under considera-ion of the neutron leakage have been calculated for different fuelompositions. Comparisons of corresponding k∞ and keff havehown that the difference between k∞ and keff remains <1%or all fuel compositions, both at the beginning of life (BOL)nd at the end of life (EOL) of the reactor (Sahin et al., 2004a).his is also a good indication that the neutron leakage is very

ow due to the great dimensions of the CANDU reactor cou-led with an efficient neutron reflector. Hence, criticality andurn-up values of the fuel channel with a white boundary condi-ion at the periphery can represent the reactor core. Calculationsre conducted for a total reactor power level of 640 MWel and180 MWth (IAEA, 1979), using two different mixed fuel com-ositions, described in Chapter 2. Variation of the criticality∞ and burn-up values of the fuel channel with the same fuelaterial has been pursued for a full power operation period of

0 years.


Fig. 3. (a and b) Temporal variation of the lattice criticality k∞: (1) 99% ThO2 + 1% PuO2; (2) 98% ThO2 + 2% PuO2; (3) 97% ThO2 + 3% PuO2; (4) 96% ThO2 + 4%PuO2; (5) 95% ThO2 + 5% PuO2; (6) 94% ThO2 + 6% PuO2.

3.3.1. General behavior of criticality and burn-up forvariable fuel compositions

In a CANDU reactor, fuel exploitation can be extended untilreaching an infinite lattice reactivity down to around k∞ = 1.06.At the first step, the ThO2/PuO2 composition has been variedwith 1–6% PuO2. Fig. 3(a and b) shows the temporal behavior ofreactor criticality k∞ over the operation period. At start up, thecriticality will be supported mainly by the fissile 239Pu isotope.Hence, a higher PuO2 content in the mixed fuel leads to higherlattice criticality in the early years of reactor operation, where ithas the dominant contribution to the fission neutron production,which can be observed in Fig. 3(a). However, 239Pu isotope willbe burnt out gradually in the early years of reactor operation andits share on fissile neutron generation decreases. During thisperiod, the criticality drops rapidly, depending on the content ofthe 239Pu isotope in the initial mixed fuel. A modest contributionto fission neutron production will be supplied by 241Pu.

In long term, the criticality will then be made possible moreand more through the newly generated and accumulated 233Uisotope. The latter will directly be produced through a neu-tron capture in 232Th. Therefore, higher ThO2 and consequentlylower PuO2 content in the mixed fuel will lead to higher critical-ity values in later years of the reactor operation whence 239Puisotope will be burnt out due to the higher 233U production. Forthat reason, a crossover of the criticality values can be distin-guished in Fig. 3(b) in a fine resolution, where a mixed fuel withl

tent after few (up to 5) years. Actually, with >3% PuO2 in themixed fuel, a k∞ value >1.06 can be sustained over long termand reactor operation could be extended unprecedented long.

A PuO2 content of 1% would not be sufficient because thereactor would die out after few months. A PuO2 content of 2%could perhaps be still tolerated, but it has not been preferreddue to the small reactivity swing of k∞ < 1.06 after 1 year ofreactor operation. The reactor would have sufficient criticality(k∞ > 1.06) over 20 years with >3% PuO2.

3.3.2. Criticality and burn up for selected fuel compositionsStarting with a higher PuO2 content in the mixed fuel will be

disadvantageous for long term criticality (after >5 years). There-fore, the rest of the study will be conducted with 3% PuO2 inthe mixed fuel. Fig. 4(a and b) show the infinite lattice criticalityk∞ and the fuel burn-up values as a function of the plant opera-tion period by full power for the two different fuel compositionsof Section 2. It starts by k∞ = 1.489 for 97% ThO2 + 3% PuO2and k∞ = 1.479 for 92% ThO2 + 5% UO2 + 3% PuO2. One canobserve a steep decrease of the criticality in the first year. Thecriticality becomes quasi constant after the second year of plantoperation period and remains above k∞ > 1.06 over 20 yearswith soft, minor fluctuations.

By sufficient plutonium content in the mixed fuel, viz., >3%PuO2, the generation and accumulation of 233U can reach such ahigh level within few years, that the continuous nuclear fuel burnu
ower PuO2 content catches that one with a higher PuO2 con- p will be fully compensated by new 233U fissile fuel produc-


Fig. 4. (a) Temporal variation of the lattice criticality k∞ and the fuel burn-up grade and (b) temporal variation of the lattice criticality k∞ fuelled with (solid lines:97% ThO2 + 3% PuO2; broken lines: 92% ThO2 + 5% UO2 + 3% PuO2).

tion, accompanied with thorium depletion. This opens prospectsof attaining very high burn-up levels with same fuel material.After few years, fissile plutonium isotopes diminish almost com-pletely. In later years, the reactor criticality could be sustainedthrough continuous conversion of 232Th to 233U in sufficientquantities, which becomes possible due to the excellent neutroneconomy of a CANDU reactor, until a significant fraction ofthorium will be burnt up. This process can continue as long asmaterial damage criteria would allow the use of the same fuelrod.

The possibility of long term utilization of the same fuel isvalid for both mixed compositions of Section 2. The mixed fuelwithout natural UO2 reveals slightly higher long term criticalityvalue than that one with natural UO2 due to the higher ThO2 con-tent, and consequently higher 233U production. However, alsothe mixed fuel with natural UO2 has a sufficiently high criti-cality value to comply the requirements for a very long reactoroperation period, based on thorium fuel utilization.

The reactor will have large reactivity margins in first yearof operation, viz., at the range of ∼30–40%, which needs tobe compensated. Several possibilities can be considered for thispurpose:

• Utilization of a multitude of control rods. This may be the firstobvious remedy, however, it would be a very crude method,and would consume a great fraction of the neutron and may

• Utilization of burnable poison in the form of boric acid. Thiswould also consume a great fraction of the precious excessneutrons.

• A more elegant way may be given through insertion of a seriesof transmutation fuel (TF) rods with thoria only. Then, theprecious excess neutrons will convert the additional 232Th in233U which then can be fed into the fuel cycle in later stagesduring the renewal of the fuel rod cladding in the course ofmultiple fuel utilization. The insertion of the TF rods can beselected in such a way to support a certain degree of fissionpower flattening in the reactor core. Therefore, it would beworth to study this type fuel cycle scenarios in later studies.However, such a scenario is out of the scope of the presentwork.

3.3.3. Considerations for extended burn upThe study shows that the nuclear fuel exploitation with the

ThO2/PuO2 fuel will not be limited by the reduction of the criti-cality, but only through fuel rod destruction and material damageby high burn up. Conservative burn-up values in a heavy waterreactor (HWR), LWR and FBR are of the order of <10,000(∼7000), 30,000–40,000 and 100,000 MW·D/T, respectively.This corresponds to fuel consumption rates of 142, 33–25 and10 g/MW D for the respective reactor types. Previous studieshave reported that the peak discharge exposure must apparentlybe in excess of 200,000 MW·D/T in order to reach burn upst
lead to severe neutron flux distortions in the core. hat are high enough to fully exploit the economic potential


of a mixed-oxide (MOX) fuel system (Leggertt and Omberg,1987). Although such burn-up levels have not yet been achievedin conventional reactors, there is no reason to expect that suchperformance can not be attained (Waltar and Deitrich, 1988).These observations may be true for fast reactors or pebble bedmodular reactors, however, and probably not for the fuel ele-ments of current CANDU reactors and LWRs. But the trends inMOX fuel development indicate that UO2 fuel elements couldbe designed to withstand the necessary burn ups for potentialadvantages (Leggertt and Omberg, 1987; Waltar and Deitrich,1988).

Standard CANDU reactors fueled with natural uranium canreach burn up only below 10,000 (∼7000) MW·D/T due to therapid fuel depletion and related criticality loss (IAEA, 1979).On the other hand, a CANDU reactor, fueled with the inves-tigated mixed ThO2/UO2/PuO2 fuel, could attain a burn-uplevel of ∼42,000 and ∼107,000 MW·D/T in 2 and 5 years,comparable to that in a LWR and fast breeder reactor (FBR),respectively, as shown in Fig. 4a. Then, fuel consumption ratesshrink down to ∼24 and ∼9 g/MW D from 142 g/MW D byutilization of weapon grade plutonium instead of natural ura-nium. Although such high burn-up values are not applicable forthe present CANDU reactors, previous work by Leggertt andOmberg (1987) and Waltar and Deitrich (1988) report aboutthe prospects of fuel element development to withstand burn-uplevels up to >200,000 MW·D/T. Theoretically, sufficient reactorc∼

tf5gcfns

(

123

4

3

waSaD

Fig. 5. Density variations of the peripheral fuel row of the main fissionableisotopes in the bundle with 97% ThO2 + 3% PuO2.

sition will be lost in the heterogeneous bundle. Under the verysoft neutron spectrum in a CANDU reactor, most of the ther-mal neutrons are absorbed in the peripheral fuel rods and onlya small fraction of them can reach the central fuel rod. Hence,fuel burn up and breeding and all other nuclear transformationsproceed at the periphery faster than in the center.

Figs. 5 and 6 show the variation of the weight densities ofthe fissionable heavy metal isotopes in the peripheral and inthe central fuel rods for the mixed fuel compositions with 97%ThO2 + 3% PuO2, respectively. Figs. 7 and 8 show the variationof the weight densities of the fissionable heavy metal isotopesin the peripheral and in the central fuel rods for the mixed fuelcompositions with 92% ThO2 + 5% UO2 + 3% PuO2, respec-tively. Fissile primary fissile isotope 239Pu falls sharply in thefirst and second year of plant operation time, where the impor-tance of 233U for reactor criticality increases rapidly. Throughsuccessive neutron capture, some 241Pu (during 3–5 years) and235U (>2 years) will also be observed.

After the first year, the amount of the initial 239Pu fuel in theperipheral rods becomes almost insignificant so that the 233Uaccumulated becomes the main fissile element for the reactorcriticality.

Gradual depletion of the metallic density of the dominant232Th and 238U isotopes is depicted in Fig. 9 for both mixedfuel compositions over the investigated period. This depletionreduces the macroscopic absorption cross-section of the fuel roda

riticality could still be maintained for burn-up levels as high as500,000 MW·D/T with the same fuel material.On the other hand, the concerns for limited lifetime due

o fuel rod destruction can be circumvented by replacing theuel rod cladding at definite intervals (for example after every0,000–100,000 MW·D/T) accompanied with the removal ofaseous fission products through a simplified reprocessing pro-ess. Then, the same fuel material could be used until a greatraction of the thorium fuel is burnt up. This would reduce theuclear waste per unit energy output drastically with a fuel con-umption rate as low as 2 g/MW D.

The most obvious advantages can be summarized, as followsSahin et al., 1989, 1991):

. Higher nuclear fuel exploitation.

. Significant reduction of the fuel rod fabrication costs.

. Drastic reduction of nuclear fuel reprocessing per unit of totalenergy production.

. Finally and the most important one is that the extended burn-up grade for the same fuel mass implies drastic reduction ofthe nuclear waste material per unit energy output for finalwaste disposal.

.4. Fissile fuel density

During reactor operation, fissionable isotopes in the bundleill be subject of continuous transformation via neutron inter-

ction and radioactive decay, which are described in Fig. 3 of¸ahin et al. (2004b), in detail. At startup, the composition fuel inll rods is uniform and independent of its position in the bundle.uring the reactor operation, the uniformity of the fuel compo-
nd has a positive effect on k∞.


Fig. 6. Density variations of the central fuel row of the main fissionable isotopesin the bundle with 97% ThO2 + 3% PuO2.

Fig. 7. Density variations of the peripheral fuel row of the main fissionableisotopes in the bundle with 92% ThO2 + 5% UO2 + 3% PuO2.

Fig. 8. Density variations of the central fuel row of the main fissionable isotopesin the bundle with 92% ThO2 + 5% UO2 + 3% PuO2.

It is interesting to investigate the temporal behavior ofthe cumulative fissile material. Fig. 10 shows the variationof the accumulated densities of all pertinent fissile isotopes(233U + 235U + 239Pu + 241Pu) in the central and in the periph-eral fuel rods over power plant operation period of 10 years.One can observe first a rapid decrease of the cumulative fissiledensities in earlier years. However, the gradient of the fissilematerial decrease will be strongly mitigated after few years sothat fuel generation and depletion become nearly balanced. Thereactor begins to operate practically as a thorium burner after2 years. Concerning reactor criticality, minor depletion of thecumulative fissile isotopes is compensated by reduced absorp-tion in 232Th and 238U so that k∞ > 1.06 can be maintained over∼20 years. Theoretically, the reactor would have sufficient crit-icality until the entire thorium fuel is burnt up, provided thatthe fuel rods could be fabricated to withstand such high burn-uplevels (>200,000 MW·D/T), as reported in Leggertt and Omberg(1987).

3.5. Fission power profile in the bundle

Fission power production density (FPPD) is linearly propor-tional to fission rate. Fig. 11 shows the fission rate profile forthe 92% ThO2 + 5% UO2 + 3% PuO2 mixed fuel compositionin the fuel bundle, normalized to one neutron per 1 cm bundlehs

eight. FPPD for the 97% ThO2 + 3% PuO2 mixed fuel compo-ition without UO2 has a very similar pattern. At BOL with fresh


Fig. 9. Gradual depletion of the metallic density of the dominant 232Th and 238Uisotopes: (1) 97% ThO2 + 3% PuO2; (2) 92% ThO2 + 5% UO2 + 3% PuO2 (solidlines, central fuel row; broken lines, peripheral fuel row).

fuel, the FPPD is continuous due to the uniform fuel composi-tion in the bundle. One observes a significant power depressiontowards the center of the bundle, viz. down to <20% in the cen-tral rod, compared to periphery, caused by the drastic depressionof the thermal flux in the strongly absorbing fuel region withplutonium component. As plutonium burns up rapidly, at theperiphery faster than at the center, after a full power operationof ∼1 year, the power profile flattens gradually and the powerdensity in the central rod becomes ∼3/4 of that in the periphery.FPPD becomes discontinuous due to the non-uniform fuel com-position arisen from variable fuel burn up and variable nucleartransmutation processes in different fuel rows in radial direction.

Due to the strongly non-uniform fission power production inthe early stages of plant operation, the peripheral fuel rods (18 of37 rods in a bundle) will be heated more than the internal rods.Hence, temperature limits in the fuel rods will prevent to run thereactor at full plant power in the first year. As half of the fuel rodsare located at the periphery of the fuel bundle, the reactor will runat 50–60% of full power in the early phase. However, along witha rapid depletion of 239Pu, the discontinuity of the FPPD will bemitigated from 1 year towards 5 years of operation. For the restof the reactor life, 233U becomes the dominant fissile isotope inthe reactor. As the neutron absorption in uranium is less thanin plutonium, the non-uniformity of FPPD remains significantlyreduced. The exploitation of thorium and consequently of 233Ucontinues quasi-uniform after an equilibrium stage of 1–2 years.U

Fig. 10. Temporal variation of the accumulated densities of fissile isotopes(233U + 235U + 239Pu + 241Pu) in the fuel bundle: (1) 97% ThO2 + 3% PuO2; (2)92% ThO2 + 5% UO2 + 3% PuO2 (solid lines, central fuel row; broken lines,peripheral fuel row).

efficient fuel exploitation. This will also allow the reactor to runnearly at full power with respect to temperature limits.

3.6. Safeguard aspects of the mixed fuel

The main purpose for the utilization of this mixed fuel inreactors is to burn and reduce the existing weapon grade pluto-nium stockpiles. In the course of reactor operation, the fuel willproduce uranium and plutonium components in situ, both ofwhich are naturally managed during the entire operation periodof the reactor in a way that avoids all proliferation concerns. Thenon-proliferation problems will arise mainly:

• during the multiple utilization of the same fuel in the reactor;• and in the course of the fuel discharge from the reactor to the

storage.

Safeguarding of the fuel is described below for both elements indetail.

3.6.1. Plutonium componentThe most efficient way to denaturize weapon grade plutonium

is to increase the amount of even 240Pu and 242Pu isotopes. Then,the high spontaneous fission rate of them continuously causes
niform FPPD in the fuel bundle would be the ideal case for


Fig. 11. Fission rate in the fuel zone of a bundle with 92% ThO2 + 5% UO2 + 3%PuO2 (normalized to one neutron per 1 cm height).

abundant neutron generation which would then prevent a nucleardetonation (Manson et al., 1981). The spontaneous fission half-life T1/2 of the plutonium isotopes is given in Meyer et al. (1977)as T1/2 = 5.5 × 1015, 1.4 × 1011 and 7.1 × 1010 years for 239Pu,240Pu and 242Pu, respectively. Hence, the intensity of sponta-neous fission neutron generation in 240Pu and 242Pu is about40,000 and 77,000 times higher than in 239Pu so that the pres-ence of a moderate level (>10%) of 240Pu and/or 242Pu wouldalready be sufficient to denaturize the plutonium to a non-prolificlevel.

Due to the strong thermal flux depression in the center of theCANDU bundle, the fuel burn up and nuclear transformationprocesses occur in the central rod slower than in the peripheralfuel rods. However, even in the central rod, the accumulation of240Pu and the burn out of 239Pu progresses so fast that within fewmonths, the weapon grade plutonium will totally be degradedto reactor grade plutonium. The nuclear fuel is then denaturizedand remains then non-prolific.

One can observe in Figs. 5–8 that there are inherently suffi-ciently high 240Pu and 242Pu isotopes generated in the mixed fuelthereafter. After few years, the 239Pu isotope will almost dimin-ish totally and only a small fraction of fissile 241Pu will be presentin the mixed fuel so that the 240Pu and 242Pu will be dominatingisotopes in plutonium. The naturally occurring denaturization ofthe generated plutonium in the reactor is always kept below anyproliferation level over the rest of entire operation period.

3

U

sufficiently high amount of the non-fissile 238U isotope in orderto degrade the uranium component to a low to moderate enrichedfuel, which will then absolutely prevent to make a nuclear bombfrom uranium. A comparison of the 233U quantity in Figs. 7 and 8(from 0 up to <0.09 g/cm3) with the non-fissile 238U quantity inFig. 9 (from ∼0.4 down to >0.3 g/cm3) shows clearly that 238Uremains the dominating isotope in uranium (from 100% downto >80%), which remains at low to moderate enriched level andtherefore will not be suitable for military applications over theentire operation period. In addition to the original non-fissile238U, there will be a gradual build-up of a new non-fissile 234Uisotope which will also support the denaturized character of thefuel. Through a successive neutron capture, the latter will partlybe converted to a new fissile isotope 235U.

With natural uranium in the mixture, some new 239Pu willbe generated. A comparison of the 239Pu quantity in Figs. 5–8shows that the depletion time of 239Pu in the ThO2/PuO2/UO2mixed fuel will be extended through new supply. However, thereare sufficiently high 240Pu and 242Pu isotopes present in pluto-nium over the rest of the operation period (after few months)to keep plutonium reactor grade and therefore not suitable formilitary applications.

On the other hand, even in the case if no natural UO2 is con-tained in the fuel, there will be always some 232U production inthe uranium fuel as an inevitable byproduct during the produc-t 233 232

asIq2

spfiklw

tch

4

•

•
.6.2. Uranium component
From the viewpoint of criticality, a small quantity of naturalO2 (5%) in the mixed fuel would guarantee the presence of

ion cycle of U. Then U, even at minor quantities, becomeslready a sufficiently high deterrent factor in handling and diver-ion due to the hard �-ray emission, as mentioned in Section 1.n fact, alone the presence of 232U in the fuel can be consideredualified for safeguarding. It can be noted that denaturizing of33U through natural uranium will cause a series of concessions,uch as a decrease of the fuel cycle effectiveness, an increase ofarasitic neutron capture and fissile material consumption, andnally a certain degree of decrease in the multiplication factor∞, as one can see clearly in Fig. 4(a and b) where the brokenines denote lower k∞ and burn-up values for the mixed fuelith denaturized 233U.In summary, the fuel is denaturized at multiple levels, viz.,

hrough the hard �-ray emission of 232U, through non-fissileharacter of the 238U, 240Pu and 242Pu isotopes, and throughigh spontaneous fission rate in the 240Pu and 242Pu isotopes.

. Conclusions

The main output of the study can be summarized as follows:

A few % (∼3%) of weapon grade PuO2 mixed with tho-ria would make possible to run a CANDU reactor overunprecedented long operation periods without the need forfuel renewal. At the startup, the criticality is quite high withk∞ ∼ 1.48. In the first∼2 years, it will drop towards an asymp-tote well above k∞ > 1.06 by an operation at full plant power.A criticality value of k∞ ∼ 1.06 for the CANDU fuel lattice isconsidered to be sufficient for a continuous reactor operation.After ∼2 years, the build up of 233U will be high enough tosustain alone the reactor operation. New 233U will continu-


ously be produced to compensate the burnt up fuel due to theexcellent neutron economy of the CANDU reactor design.Then the reactor will begin practically as a thorium burner.

• Very high burn-up levels (>200,000 MW·D/T) could beattained for a given fuel mass, which would reduce drasti-cally the fuel fabrication costs as well as the nuclear wastemass per unit energy output, if fuel rods could be fabricatedto endure such high burn-up levels.

• Weapon grade plutonium will be rapidly degraded to reactorgrade plutonium within few months, and will remain non-prolific thereafter.

• Addition of few % (∼ 5%) of natural UO2 to the mixed fuelwill provide full inherent safeguarding also for the generateduranium over the entire plant operation time.

Acknowledgment

This work has been supported by the Research Fund of theGazi University, Project #07/2003-14.

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increased fuel burn up in a candu thorium reactor using weapon grade plutonium

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