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Nuclear Engineering and Design 300 (2016) 330–338

Contents lists available at ScienceDirect

Nuclear Engineering and Design

jou rn al hom epage : www.elsev ier .com/ locate /nucengdes

study on transmutation of LLFPs using various types of HTGRs

azuki Koraa,∗, Hiroyuki Nakayaa, Hideaki Matsuuraa, Minoru Gotob,higeaki Nakagawab, Satoshi Shimakawab

Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka-shi, Fukuoka-ken, JapanJapan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki, Ibaraki-ken, Japan

i g h l i g h t s

We propose utilization of a variety of HTGRs for LLFP transmutation and storage.The transmutation performance of four types of HTGRs was examined and compared.Some types of HTGRs show preferable characteristics for LLFP transmutation.

r t i c l e i n f o

rticle history:eceived 9 October 2015eceived in revised form 20 January 2016ccepted 30 January 2016

. Transmutation

a b s t r a c t

In order to investigate the potential of high temperature gas-cooled reactors (HTGRs) for transmutationof long-lived fission products (LLFPs), numerical simulation of four types of HTGRs were carried out. Inaddition to the gas-turbine high temperature reactor system “GTHTR300”, which is the subject of ourprevious research, a small modular HTGR plant “HTR50S” and two types of plutonium burner HTGRs“Clean Burn with MA” and “Clean Burn without MA” were considered. The simulation results show thatan early realization of LLFP transmutation using a compact HTGR may be possible since the HTR50S can

transmute fair amount of LLFPs for its thermal output. The Clean Burn with MA can transmute a limitedamount of LLFPs. However, an efficient LLFP transmutation using the Clean Burn without MA seems tobe convincing as it is able to achieve very high burn-ups and produce LLFP transmutation more thanGTHTR300. Based on these results, we propose utilization of variety of HTGRs for LLFP transmutation andstorage.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

The management of high-level radioactive waste (HLW) ismportant for continuation and development of nuclear energy.eological disposal is currently considered to be the most practicalethod for final disposal of HLW. Most of the fission products con-

ained in HLW decays within 1000 years. However, some nuclidesave half-lives longer than 200,000 years, and they are called long-

ived fission products (LLFPs). List of LLFP nuclides is shown inable 1 (Nishihara, 2010). Management of LLFP nuclides such as9Tc and 129I requires special attention because they have relatively

igher solubility in water and may leak into the environment. Inrder to minimize the risk, partitioning and transmutation of LLFPsre suggested (Kondo et al., 1994).

∗ Corresponding author. Tel.: +81 928023503.E-mail address: kora k@nucl.kyushu-u.ac.jp (K. Kora).

ttp://dx.doi.org/10.1016/j.nucengdes.2016.01.030029-5493/© 2016 Elsevier B.V. All rights reserved.

Japan Atomic Energy Research Institute (JAERI), which is one ofthe predecessors of Japan Atomic Energy Agency (JAEA), startedthe research and development of partitioning nuclear waste in1973 (Kubota and Nakamura, 1985). The necessity of separatingHLW into four groups was reported, and research and develop-ment focusing on the separation started in 1985 (Yamaguchi andKubota, 1987). In this process, HLW is separated into four groups;minor actinides (MAs), Sr-and-Cs, Tc-and-platinum-group-metalsand other-fission-products. After the separation, MAs and LLFPs aretransmuted into short lived or stable nuclides. As for the trans-mutation, fast breeder reactors (FBRs) (Naganuma et al., 2006)and accelerator driven systems (ADSs) (Tsujimoto et al., 2000) hadbeen studied. Effective transmutation of MAs using those systemswith fast neutrons was seemed to be viable. However, it was notthe case for LLFPs. This is because LLFP nuclides have large cap-

ture cross-section for thermal neutrons rather than fast neutrons(Fig. 1). Therefore, thermal reactors are preferable in the case oftransmuting LLFPs. Studies on LLFP transmutation using light waterreactors (LWRs) had been done in the past (Budi Setiawan and

K. Kora et al. / Nuclear Engineering an

Table 1List of LLFP nuclides.

Nuclide Half-life [y] Mass in spentfuel* [g/ItHM]

Cross-section**

[barn]

79Se 295,000 6.15 50.0493Zr 1530,000 958 2.24099Tc 211,000 1050 23.68107Pd 6500,000 312 9.192126Sn 230,000 30.6 0.009008129I 15,700,000 242 80.07135Cs 2300,000 522 8.304

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* Nuclide content in PWR spent fuel (45 GWd/tHM, 5 year cooling).** Thermal neutron capture cross-section (JENDL-4.0 Maxwellian averaged).

itamoto, 2001), but the LLFP transmutation performance of LWRsas found to be limited (Liu et al., 2013). Our research group has

een investigating the use of an alternative thermal reactor; theigh temperature gas-cooled reactor (HTGR).

A HTGR is a graphite-moderated helium-cooled reactor withmproved safety and thermal efficiency compared to conventionalWRs. It can also provide heat source for hydrogen produc-ion, seawater desalination and other industrial use (Yan et al.,014). Another important feature of the HTGR is the use ofristructural-isotropic fuel (TRISO) particles. They are highly resis-ant to irradiation, and very high burn-ups, up to 740 GWd/t, can bechieved with these particles (Rodriguez et al., 2003). The powerensity is low compared to LWR, so large volume is needed for

ts reactor core. The neutron flux is also lower, but the HTGR isonsidered to be suitable for LLFP transmutation because of thearge core size. The large core provides a sufficient space to load

large amount of transmutation targets. In addition, the neutronoderator and coolant are made of low neutron absorbing materi-

ls which enable efficient transmutation by reducing unnecessaryoss of neutrons. In our previous study, we conducted investiga-ion on the transmutation of LLFPs using the HTGR (Nakaya et al.,011). Considering the transmutation of 99Tc using a “Gas Turbineigh Temperature Reactor” (GTHTR300) whose conceptual designad been created by JAEA (Nakata et al., 2002), the calculationesults showed that a GTHTR300 unit can transmute 4–6 timeshe amount of 99Tc produce by itself. Additional study showedhe effectiveness of transmutation of other major LLFP nuclidesnd the possibility of utilizing GTHTR300 as LLFPs storage andransmuter (Kora et al., 2014). In terms of neutron economy, mostf the LLFPs contained in spent fuel (SF) in Japan can be stored

n one HTGR core, so that their radiotoxicity can be graduallyeduced.

Other types of HTGRs are under development by JAEA. “HTR50S”s a small-sized HTGR for steam supply and power generation,

Fig. 1. Cross-sections of major LLFP nuclides (JENDL-4.0 300K).(Single column).

d Design 300 (2016) 330–338 331

which is expected to be on the market in 2030s (Ohashi et al.,2010). “Clean Burn” is a concept of utilizing a HTGR for plutoniumincineration aimed at achieving burnup of 500 GWd/t (Fukaya et al.,2014). There are two options for Clean Burn; “Clean Burn with MA”in which MAs are incorporated with the plutonium, and “CleanBurn without MA” in which most of the MAs are removed. Thetransmutation performance is expected to vary for such types ofHTGRs.

The purpose of this study is to further investigate the poten-tial of the HTGRs for LLFP transmutation and propose a variety ofschemes for transmuting LLFPs. The transmutation performanceof the four types of HTGRs; “HTR50S”, “GTHTR300”, “Clean Burnwith MA” and “Clean Burn without MA” is examined by numericalanalysis. Following factors; neutron flux, transmutation rate andburn-up are calculated to understand the characteristics of eachreactor.

2. Calculation methods

2.1. Core design

The HTR50S is a prismatic block type HTGR. Its core consistsof 30 fuel columns, 13 control rod guide columns and 18 reflectorcolumns. Each column has 6 hexagonal graphite blocks piled up ina row. One fuel block of HTR50S is made up of 33 fuel pins and agraphite block. Each fuel pin contains 14 cylindrical fuel compactswith 10 mm inner diameter and 26 mm outer diameter. The fuelcompacts are covered with a graphite sleeve 4 mm thick. TRISO fuelparticles are incorporated into the fuel compacts with a graphitematrix. The TRISO particles consist of a spherical kernel containingUO2 surrounded by a four layer coating. The enrichment of UO2ranges from 4.3 to 9.4 wt.% depending on the location, but it isassumed to be fixed to 6.1 wt.% for the following calculation.

The GTHTR300 has an annular core with 90 fuel columns, 55inner and 36 outer reflector columns, 18 inner and 12 outer controlguide column. In a fuel column, 8 fuel blocks are stacked, and eachfuel block holds 57 fuel pins. One fuel pin is made of 12 cylindricalfuel compacts with 9 mm inner diameter and 24 mm outer diam-eter. Fuel compacts are covered with a graphite layer 1 mm thick.The enrichment of UO2 is set to 14 wt.%.

The Clean Burn is based on the GTHTR300 design, except thatit employs plutonium as fuel instead of uranium. Because the plu-tonium nuclides have larger neutron absorption cross section, thenumber of fuel columns is increased from 90 to 144 and the pluto-nium inventory in each fuel block is reduced in order to decreaseneutron capture and enhance neutron moderation. In addition,yttria-stabilized zirconia (YSZ) is mixed with plutonium to improveproliferation resistance. Two options are considered for Clean Burnfuel composition. For Clean Burn with MA, 58.6 wt.% is fissile pluto-nium and 14.3 wt.% is MAs. Previous studies indicate that the MAssignificantly deplete the criticality of the core, but there is a possi-bility of additional transmutation of MAs. For Clean Burn withoutMA, 67.8 wt.% is fissile plutonium and 0.8 wt.% is MAs. Due to theremoval of most of MAs, previous studies suggest that very highburn-up and efficient plutonium incineration can be achieved.

2.2. Simulation models

We created four simulation models based on HTR50S,GTHTR300, Clean Burn with MA and Clean Burn without MA. Thegeometry of the models is shown in Fig. 2. The model configuration

and fuel composition of each model is shown in Tables 2 and 3,respectively. For the sake of simplicity, only one layer of the fuelblocks is modeled for each type of reactors, and reflective bound-ary condition is set for top and bottom of the layer. The neutron

332 K. Kora et al. / Nuclear Engineering and Design 300 (2016) 330–338

Fig. 2. Geometry of simulation models. (2-column).

Table 2Model configuration.

HTR50S GTHTR300 CB with MA CB without MA

Thermal power(MWt)

50 600 600 600

Heavy metalinventory (ton)

1.1 7.7 1.2 1.0

Discharge burn-up(GWd/t)

33 69 370 520

Number of fuelcolumns

30 90 144 144

Number of blocksper column

6 8 8 8

Number of fuelpins per block

33 57 57 57

Height of blocks(cm)

58.0 100 105 105

Height of fuel pins(cm)

54.6 95.0 95.0 95.0

Inner radius of fuelcompact (cm)

0.50 0.45 0.45 0.45

Outer radius of fuelcompact (cm)

1.30 1.20 1.20 1.20

Packing fraction (%) 30.0 33.0 31.6 31.6Fuel kerneldiameter (�m)

600 550 300 300

Buffer thickness(�m)

60 140 150 150

IPC thickness (�m) 30 25 35 35SiC thickness (�m) 25 40 35 35OPC thickness(�m)

45 25 40 40

Table 3Fuel composition (wt%IHM).

HTR50S GTHTR300 CB with MA CB without MA

235U 6.1 14.0 0.0 0.0238U 93.9 86.0 0.0 0.0237Np 0.0 0.0 4.6 0.0238Pu 0.0 0.0 1.3 1.5239Pu 0.0 0.0 51.0 59.1240Pu 0.0 0.0 20.8 24.1241Pu 0.0 0.0 7.6 8.8242Pu 0.0 0.0 4.9 5.7241Am 0.0 0.0 8.2 0.8242mAm 0.0 0.0 0.0 0.0243Am 0.0 0.0 1.5 0.0Fissile nuclides 6.1 14.0 58.6 67.8MA nuclides 0.0 0.0 14.3 0.8

leakage for axial direction is ignored due to the reflective boundarycondition. From preliminary examinations, its effect on reactivityis estimated to be approximately 1% �k/kk′. As we intend to com-pare the transmutation performance of the models under the samecondition, the effect of the simplification is so small to be negligiblefor this purpose. Furthermore, burnable poisons and control rodsare not considered in the models because they have little impacton achievable burn-up (Fukaya et al., 2014) and LLFP transmutationrates (Liu et al., 2013).

K. Kora et al. / Nuclear Engineering and Design 300 (2016) 330–338 333

Fig. 3. Geometry of LLFP pins in f

2

L

Fig. 4. Geometry of LLFP pins in reflector blocks. (Single column).

.3. Loading LLFPs

In order to manage LLFPs independently of the fuel, we assumeLFPs to be in the form of pins. A LLFP pin consists of a graphite rod

uel blocks. (Single column).

surrounded by a LLFP layer and a graphite sleeve layer. The amountof LLFPs is adjusted by changing the inner diameter of the LLFPlayer. To minimize changes to the core design, two ways of loadingLLFP pins into the HTGR cores are considered. The first method isloading them into the fuel blocks. As shown in Fig. 3, three fuelpins from each fuel block are replaced with three LLFP pins. Thesecond method is loading LLFP pins in the reflector blocks. 18 or48 reflector blocks are loaded with LLFPs, and 33 or 57 LLFP pinsare inserted into each block as shown in Fig. 4. Remaining reflectorblocks are reserved for control rod guide column as in the originaldesign.

For the former, the neutron importance is high, that is, loadingLLFPs into the fuel blocks has large impact on reactivity. Neu-tron flux in such region is maintained to keep the fission reactionrate constant, so neutron absorption by LLFPs is compensatedand large transmutation rate is expected. For the latter, load-ing LLFPs has small impact on reactivity. The neutron flux is notretained and the transmutation rate may not be as large as theformer, but large amount of LLFPs can be loaded. Therefore, thismethod is expected to be suitable for storing large amount ofLLFPs.

2.4. Calculation codes

In this study, following calculation codes are used; MVP,MVP-BURN (Nagaya et al., 2005) and ORIGEN (Croff, 1980). TheMVP code is a continuous-energy Monte Carlo transport codefor neutron and photon transport calculations. The MVP-BURNis a code system which utilizes the MVP code and an auxiliarycode BURN, and is used for calculation of build-up and decayof nuclides under irradiation. The MVP code is used to evaluateneutron flux and effective cross-sections to be used in ORIGENcalculations. The ORIGEN is a different type of code systems for

calculating the buildup and decay of radioactive materials. Inthis study, it is used to assess the results of MVP-BURN. Cross-sections are taken from JENDL-4.0 (Shibata et al., 2011) for allcalculations.

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. Results and discussion

.1. Neutron flux

Because LLFP nuclides have relatively large neutron captureross-sections for thermal neutrons, high thermal neutron flux isxpected to produce effective LLFP transmutation. To grasp theransmutation performance of the HTGRs, we first calculated thenitial neutron flux in regions where LLFP pins are supposed to benserted, i.e. the regions colored in purple in Figs. 3 and 4. The neu-ron flux spectra in fuel blocks for each HTGR are compared in Fig. 5.t shows that, although HTR50S has the least thermal output, it hashe largest thermal neutron flux. This is probably due to the facthat the HTR50S has the least amount of fissile material in the core.he thermal output of the HTR50S is 1/12 of the GTHTR300, but thessile inventory is even smaller, approximately 1/16 in weight. Dueo the difference, the HTR50S requires high neutron flux to produceufficient fission reaction. On the other hand, the thermal neutronux of Clean Burn HTGRs is smaller than HTR50S or GTHTR300. Theeason is that neutron capture cross-section of plutonium fuel forhe Clean Burn HTGRs is larger than that of uranium fuel for HTR50Sr GTHTR300.

As shown in Fig. 6, the neutron flux spectra in reflector blocks forach HTGR are of the same order of magnitude when no LLFPs are

oaded. However, the neutron flux is likely to change significantly

hen LLFPs are loaded. The neutron importance in reflector regions low, so neutron flux is expected to drop due to neutron absorptiony LLFPs as mentioned in Section 2.3.

ig. 5. Neutron flux spectra in fuel blocks for each model (640 neutron energyroups). (Single column).

ig. 6. Neutron flux spectra in reflector blocks for each model (640 neutron energyroups). (Single column).

d Design 300 (2016) 330–338

3.2. Burn-up calculations for LLFP transmutation

Due to concerns over potential radiotoxicity and public expo-sure, transmutation of LLFP nuclides such as 129I, 135Cs and 99Tcis desirable. To simplify the calculation, we consider 99Tc, which isthe most abundant LLFP nuclide, to be the representative of all LLFPnuclides. The melting point of metallic technetium is sufficientlyhigher than operating temperature of HTGRs, so we assume theform of 99Tc to be pure metal in the calculations. Another researchis required to devise appropriate loading methods for 129I and 135Cssince they are chemically unstable and have low melting points.Burn-up calculations of 99Tc transmutation with the four modelswere performed using the Continuous-energy Monte Carlo trans-port code MVP-BURN. The nuclear data was taken from JENDL-4.0.

The following four items were evaluated; net transmutationrate, cycle length, net transmutation and burn-up. The net transmu-tation rate is the net reduction of 99Tc in one reactor core per unittime. The cycle length is the duration of time for one fuel cycle. Fuelshuffling is not considered in this study, so the reactor starts oper-ation with all fresh fuel and continues operation until the effectivemultiplication factor (keff) drops to 1.0. The net transmutation is thenet reduction of 99Tc in one reactor core, i.e. the mass of reductionof 99Tc in LLFP pins minus the mass of buildup of 99Tc in fuel parti-cles in one cycle. The burn-up is the achievable maximum burn-upwhen a certain amount of LLFPs is loaded into the core. The cyclelength and the burn-up depend on the initial mass of target LLFPs,in addition to the types and the mass of nuclear fuel.

The calculation results for transmutation of LLFPs in fuel blocksare shown in Table 4, Figs. 7 and 8. Results with cycle length shorterthan 90 days are omitted since such short cycle length is not favor-able for operation of nuclear power plants for economic reasons.The figures indicate that, when no LLFPs are loaded, the net trans-mutation is negative due to the buildup of LLFPs in fuel particles.As the mass of initial LLFPs in fuel block is increased, the neutroncapture by LLFP nuclides increases and so does the net transmu-tation rate. However, the rise in neutron capture causes the keffto decrease and the cycle length becomes shorter. It is especiallyobvious in the case for the HTR50S model as it cannot sustain crit-icality when 1.0 t of 99Tc is loaded into the fuel blocks. As for theClean Burn without MA models, the net transmutation increasesuntil a certain point, and decreases from the point onward due tothe excessive reduction of the cycle length. The net transmutationis the largest when 2.0 t of initial 99Tc is loaded into the fuel blocks.

The net transmutation rate is approximately the same for theGTHTR300, Clean Burn with MA and Clean Burn without MA eventhough calculation results of neutron flux in the previous chap-ter suggested the transmutation rate of the GTHTR300 model tobe higher than those of Clean Burn models. The cause of this isthe resonance capture. In the fuel block loading, there is a certainlevel of epithermal neutrons in the LLFP region. As 99Tc has a largecapture resonance peak at 5.6 eV, it contributes to the increase oftransmutation rates. The neutron flux between 1 eV and 10 eV ofClean Burn models is slightly higher than that of GTHTR300 model,and it creates additional transmutation for Clean Burn models. As aresult, the transmutation rates of GTHTR300, Clean Burn with MAand Clean Burn without MA are approximately equal.

The net transmutation and the net transmutation rates of LLFPsin reflector blocks are shown in Table 5, Figs. 9 and 10. In this loadingmethod, the maximum mass of 99Tc which can be loaded into thecore without considering the neutronic properties is approximately24 t for HTR50S and 257 t for GTHTR300 and Clean Burn. In orderto see the trend of transmutation, burn-up calculation is done for

initial target mass up to 15 t for HTR50S and 30 t for the others. Thenet transmutation and the net transmutation rate for the HTR50Smodel are small, but sufficient considering its thermal output. Itcan perform transmutation even with large amount of initial LLFPs.

K. Kora et al. / Nuclear Engineering and Design 300 (2016) 330–338 335

Table 4Calculation results of LLFP transmutation when LLFP is loaded into fuel blocks.

Model Initial target mass(metric ton)

Net transmutationrate (kg/year)

Transmutedfraction (%/year)

Cycle length(days)

Net transmutation(kg)

Burn-up(GWd/t)

HTR50S 0.0 −0.467 – 679 −0.869 33.30.1 1.58 1.58 348 1.51 17.5

GTHTR300 0.0 −5.48 – 893 −13.4 73.60.1 −1.93 −1.93 832 −4.40 68.61.0 13.6 1.36 551 20.5 45.42.0 24.9 1.24 358 24.4 29.5

CB with MA 0.0 −5.37 – 724 −10.7 3820.1 −1.52 −1.52 652 −2.71 3431.0 13.0 1.30 287 10.3 151

CB without MA 0.0 −5.32 – 832 −12.1 5080.1 −1.41 −1.41 796 −3.07 4861.0 14.1 1.41

2.0 23.9 1.19

4.0 36.8 0.919

Fig. 7. Net transmutation rate (solid lines) and cycle length (dotted line) when LLFPsare loaded into fuel blocks. (Single column).

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ig. 8. Net transmutation (solid lines) and burn-up (dotted line) when LLFPs areoaded into fuel blocks. (Single column).

s for the Clean Burn without MA model, the net transmutation isarge but the net transmutation rate is approximately 60% of thatf the GTHTR300 model. Although the neutron fluxes in reflectorlocks are almost the same for the GTHTR300 model and the Cleanurn models, the calculation results of net transmutation rate forhe Clean Burn models are lower. The reason is that the neutronmportance of reflector blocks in the Clean Burn models is low due

o the increased number of fuel columns. In other words, the num-ers of fuel blocks that are not close to reflector blocks increased, soewer neutrons are delivered to the reflector blocks to compensateor the absorption by LLFPs.

597 23.1 364431 28.2 263168 17.0 103

The changes for HTR50S, GTHTR300 and Clean Burn withoutMA in Figs. 9 and 10 trend to be stable with the increase of ini-tial target mass. This is because of self-shielding effect of LLFPs.The initial target mass is increased by thickening the LLFP layers.As transmutation is less likely to occur in the inner part of LLFPregion than the surface part, the transmutation of additional LLFPsis relatively smaller. In particular, the transmuted fraction declinesfrom 1.45%/year to 0.145%/year as initial target mass is increasedfrom 1.0 t to 30 t. The Clean Burn without MA model produces largenet LLFP transmutation per cycle because it has the highest fis-sile enrichment and it contributes to the longest fuel cycle. As forthe Clean Burn with MA model, the transmutation and the trans-mutation rate are lower. This is due to the extra transmutation ofMAs, which reduces the keff of the reactor and hinders the neu-tron capture by LLFPs. Since the keff of this model is low, loadinglarge amount of LLFPs causes significant decrease in cycle lengthand net transmutation. Simultaneous transmutation of MAs andLLFPs might be possible, but further studies are needed to assess itspracticability.

The method of loading LLFPs into fuel blocks is only effectivefor loading a small amount of initial target. However, it requireslittle changes to the original design of the reactor since there is noneed to bore holes in graphite blocks for LLFP pins. In contrast, themethod of loading LLFPs into reflector blocks is effective for widerange of initial target mass. In addition, the net transmutation perfuel cycle is generally larger as the cycle length is longer in thismethod. There are three possible causes for this; there is more fuelin the core because fuel pins are not replaced with LLFP pins; theLLFPs in reflector blocks does not have negative effect on the keff asmuch as the LLFPs in fuel blocks; and the LLFPs in reflector blockscan capture neutrons leaking from the core.

3.3. Verification using ORIGEN

In order to verify the calculation results of MVP-BURN in Sec-tion 3.2, transmutation rates were calculated for each model usingORIGEN2.2. The initial loading mass of 99Tc was assumed to be 0.1 tfor HTR50S and 1.0 t for others. Preparation of ORIGEN one-groupcross-section library was done in three steps. First, 640-groupcross-section library based on JENDL-4.0 was created using NJOYnuclear data processing system (MacFarlane and Muir, 1994). Sec-ond, 640-group neutron flux spectrum in the LLFP region in each

core was calculated using MVP. Third, one-group cross-sectionlibrary was created by multiplying the 640-group cross-sectionlibrary with the 640-group neutron flux spectrum. The neutron fluxis assumed to be constant during one year irradiation. The buildup

336 K. Kora et al. / Nuclear Engineering and Design 300 (2016) 330–338

Table 5Calculation results of LLFP transmutation when LLFP is loaded into reflector blocks.

Model Initial target mass(metric ton)

Net transmutationrate (kg/year)

Transmutedfraction (%/year)

Cycle length(days)

Net transmutation(kg)

Burn-up(GWd/t)

HTR50S 0.0 −0.467 – 735 −0.941 32.80.1 0.776 0.776 615 1.31 27.51.0 2.09 0.209 488 2.79 21.82.0 2.32 0.116 464 2.95 20.75.0 2.46 0.0492 444 2.99 19.8

10 2.56 0.0256 430 3.03 19.215 2.65 0.0177 417 3.03 18.6

GTHTR300 0.0 −5.49 – 882 −13.3 68.80.1 −2.12 −2.12 854 −4.96 66.61.0 14.5 1.45 627 24.9 48.92.0 22.5 1.12 514 31.6 40.15.0 31.7 0.633 381 33.0 29.7

10 36.9 0.369 300 30.4 23.415 40.0 0.267 273 29.9 21.320 41.5 0.207 242 27.5 18.930 43.4 0.145 215 25.6 16.8

CB with MA 0.0 −5.36 – 742 −10.9 3710.1 −2.27 −2.27 704 −4.38 3511.0 10.4 1.04 481 13.7 2402.0 15.1 0.754 337 13.9 1685.0 18.9 0.378 105 18.9 52.7

CB without MA 0.0 −5.30 – 886 −12.9 5120.1 −2.33 −2.323 861 −5.49 4981.0 10.3 1.03 778 21.9 4502.0 15.1 0.757 744 30.9 4305.0 17.9 0.405 689 38.2 398

10 22.6 0.226 659 40.7 38115 23.7 0.158 659 41.5 37020 24.6 0.123 633 42.7 36630 25.5 0.0850 624 43.6 361

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(Liu et al., 2013), ADS (Yang et al., 2004) and FBR (Naganuma et al.,

ig. 9. Net transmutation rate (solid lines) and cycle length (dotted line) when LLFPsre loaded into reflector blocks. (Single column).

f LLFPs in fuel region is ignored for this comparison. Fig. 11 showshe transmutation rates of LLFPs when loaded into fuel blocks andhen loaded into reflector blocks.

The differences between transmutation rates calculated byRIGEN and MVP-BURN are 0.1%/year at most. The cause of under-stimation of ORIGEN is that neutron flux is assumed to be constanturing the whole irradiation period. In MVP-BURN, neutron flux

ncreases as fuel burns, and the transmutation rates increases like-ise. On the other hand, the overestimation of ORIGEN is caused in

he preparation of ORIGEN library. As MVP-BURN calculates contin-ous energy neutron flux, dips appear at resonance cross-sectionaxima and peaks appear at resonance cross-section minima. The

roducts of neutron flux and cross-section i.e. reaction rate is sup-ressed in MVP-BURN due to such peaks and dips. However, whenhe flux is converted into 640-group neutron flux spectrum for

Fig. 10. Net transmutation (solid lines) and burn-up (dotted line) when LLFPs areloaded into reflector blocks. (Single column).

creation of one-group ORIGEN library, the neutron flux in eachenergy group is averaged and this leads to increase of ORIGENtransmutation rates.

In spite of such factors, the difference between those two codesis sufficiently small. Overall, it is confirmed that the transmutationcalculation by MVP-BURN is appropriate.

3.4. Comparison with existing options

The transmutation performance of the HTGR is compared withexisting transmutation studies; pressurized water reactor (PWR)

2006). Since the PWR is an existing type of reactors, it can savetime and cost of the transmutation scheme. The drawback is thatits core design is optimized for power generation. Therefore, only

K. Kora et al. / Nuclear Engineering an

Fig. 11. Transmutation rates of LLFP nuclides calculated by ORIGEN and MVP-BURN.(Single column).

Table 6Comparison of transmutation performance.

HTGR*1 PWR*2 ADS FBR*3

Initial targetmass (kg/GWt)

1667 50,000 134 377 369

Nettransmutationrate(kg/GWt/year)

24.1 72.3 9.64 26.6 1.02

Transmutedfraction(%/year)

1.45 0.145 7.21 7.06 0.276

*1 Calculation results of GTHTR300 in Section 3.2 normalized by thermal output.

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Liu, Kun, et al., 2013. Studies on LLFP transmutation in a pressurized water reactor.Nucl. Sci. Technol. 50 (6), 581–598, http://dx.doi.org/10.1080/00223131.2013.

*2 Two-batch refueling scheme. Assuming thermal efficiency to be 34%.*3 Sum of simultaneous transmutation of 99Tc and 129I.

ittle space is available for transmutation purpose. On the otherand, the ADS is potentially a useful transmutation system, but itsechnology is not well-established. In the case of using the FBR,ransmutation of LLFPs is severely limited due to the fact thatresence of LLFPs degrades the breeding ratio. LLFP mass trans-uted in the FBR is considered to be slightly greater than that

roduced.Transmutation of 99Tc in each option is summarized in

able 6. Mass of LLFPs is normalized by thermal output. Ithows that the HTGR excels in initial target loading capacitynd compares favorably in transmutation rate. However, in termsf transmuted fraction, the HTGR is inferior to the PWR orhe ADS.

Due to the limitation of initial target loading, the main objec-ive of the conventional transmutation concept is to achieve highransmutation rate with small quantity of initial target mass.igh neutron flux is required for this concept. Utilization of theTGR is an alternative approach to LLFP transmutation. Insteadf loading small amount of LLFPs into a high neutron flux envi-onment, store large amount of LLFPs in the HTGR core andarry out transmutation. In this way, it is possible to achieveigh transmutation rate. Its large loading capacity and inher-nt safety features make it favorable for transmutation of legacyLFPs.

It should be noted, however, that the maximum thermal outputf the HTGR is limited to be on the order of 600 MWt in order toafely remove heat in case of an accident. Therefore, multiple unitsay be required to increase the transmutation rate. Furthermore,

artitioning of LLFPs can take a long time, so preparation of largenitial target LLFPs may delay the startup of transmutation.

d Design 300 (2016) 330–338 337

3.5. Future prospects

As of March 2015, there is approximately 17,000 t of SF in Japan(METI, 2015). The SF is considered to contain 32 t of 129I, 135Cs and99Tc. Because one HTGR can hold tons of LLFPs in its reactor, it maybe possible to use the HTGR as temporary LLFP storage. It should beable to generate electricity and store a large amount of LLFPs whilereducing the mass through transmutation. Furthermore, storingLLFPs inside a reactor allows the use of their decay heat as additionalheat source.

In order to utilize the HTGR for such purpose, further studiesare needed to evaluate other factors. For example, efficient parti-tioning of LLFPs from SF and secure containment for target LLFPs inHTGR core need to be developed. Safety of modified core designsin terms of reactivity control and heat removal is also a concern.In addition, increase of MAs in spent HTGR fuel due to LLFP trans-mutation should be evaluated, and economic efficiency of the LLFPtransmutation scheme must be assured.

4. Conclusion

In order to investigate the potential of the HTGR for transmuta-tion of LLFPs, the transmutation capability of four types of HTGRs;HTR50S, GTHTR300, Clean Burn with MA and Clean Burn withoutMA was studied by using MVP-BURN and validated by ORIGEN. Fol-lowing factors; neutron flux, transmutation rate and burn-up werecalculated to see the characteristics of each reactor. The Clean Burnwithout MA model has the largest net transmutation of LLFPs perfuel cycle for most cases. The transmutation performance of theClean Burn with MA model is lower compared to Clean Burn with-out MA model due to extra MAs in the fuel which act as poison.The HTR50S produced small but positive net LLFP transmutation.From these results, it can be said that early realization of LLFP trans-mutation using a compact HTGR and efficient LLFP transmutationusing a high burn-up HTGR seem to be viable. Moreover, there is apossibility of utilization of such types of HTGRs as temporary LLFPstorage.

Acknowledgements

This work is supported by MEXT/JSPS KAKENHI Grant-in-Aidfor Scientific Research (B)15H04230. The authors wish to acknowl-edge Takashi Yasumoto, Shohei Kochi, Kohei Tanouchi, Kotaro Kuboand Yasuko Kawamoto from Kyushu University whose insight andexpertise greatly assisted the research.

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