Naoyuki TAKAGI a & Hiroshi SEKIMOTO a a Research Laboratory for Nuclear Reactors, Tokyo Institute of

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1 This article was downloaded by: [ ] On: 22 March 214, At: 3:52 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Nuclear Science and Technology Publication details, including instructions for authors and subscription information: Feasibility of Fast Fission System Confining Long-Lived Nuclides Naoyuki TAKAGI a & Hiroshi SEKIMOTO a a Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, 152 Published online: 15 Mar 212. To cite this article: Naoyuki TAKAGI & Hiroshi SEKIMOTO (1992) Feasibility of Fast Fission System Confining Long-Lived Nuclides, Journal of Nuclear Science and Technology, 29:3, , DOI: 1.18/ To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

2 Journal of NucLEAR SciENCE and TECHNOLOGY, 29(3], pp (March 1992). Feasibility of Fast Fission System Confining Long-Lived Nuclides Naoyuki TAKAGI and Hiroshi SEKIMOTO Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology* Received September 13, 1991 The feasibility of fast fission system confining long-lived nuclides without other supporting system as synergetics for fuel sustainment and waste incineration was studied from the aspects of nuclear material balance and neutron economy. The continuous utilization of fast fission system which confines all actinides in the reactor but discharges all FP will lead to huge accumulation of radioactive wastes such as 129 1, 135 Cs, 17 Pd, 93 Zr, 99 Tc, 12 "Sn and 79 Se in the far future. Then we studied the feasibility of the system that these long-lived seven FP are also confined in the reactor with actinides. In this scheme, all the long-lived nuclides to be disposed of were exposed with neutrons in the reactor and removed as different nuclides after nuclear transmutation. As the wastes stored in the repository was composed of only shorter-lived nuclides, total amount of radioactive wastes in the repository was suppressed to be less than a few tons per 3 GWt reactor. KEYWORDS: long-lived wastes, fission products, actinides, confinement, fuel cycle, fast reactors, incineration, transmutation, nuclear quasi-equilibrium, feasibility studies I. INTRODUCTION In our previous preliminary study, the feasibility of actinides-confining fission energy system was investigated for a nuclear quasiequilibrium state in the future l. We have assumed continuous fission energy utilization extending into far future, taking all fission energy resources contained even in the ocean into account. The results indicated that fast reactor (especially having hard spectrum) can confine all actinides with natural resource continuously supplied to maintain criticality. This can be continued until all U and/or Th resources will be exhausted. Assuming long-term contribution of fast reactors to our society, the long-lived FP, such as 129 {, 135 Cs, 99 Tc etc., contained slightly in the spent fuel will accumulate largely in the repository in the far future because of their extremely low decay rate compared to their production rate. Therefore the unnecessary long-lived radioactive FP may be better to be confined in the reactor similar to the actinides and to be transmuted to the shorter or stable isotopes by nuclear reactions. Many feasibility studies for incineration of longlived nuclides had been performed in 197s, and in recent years the incentives are reexamined to find the most acceptable solution for back-end nuclear wastes among various laboratoriesc 2 J. The sufficiently high decay rate of radioactive wastes stored in the repository makes it possible to attain the nuclear quasi-equilibrium state where there is no increase of the radioactive wastescsj. One objective of this study is to analyze the waste amount accumulated in the repository for the fuel cycle confining only actinides. It provides us the information to find desirable fuel cycle scheme for waste amount reduction. Our most concern, another objective, is on the feasibility evaluation of stand alone fast fission system confining long-lived nuclides, considering from * -okayama, Meguro-ku, Tokyo

3 Vol. 29, No. 3 (Mar. 1992) 277 long-term view point. We discuss the fuel cycle confining only actinides and the fuel cycle confining longlived nuclides (all actinides and long-jived FP) in Chaps. II and Ill, respectively. II. RADIOACTIVE WASTE ACCUMULATION IN ACTINIDE-CONFINING FUEL CYCLE At first, we analyzed the equilibrium nuclide composition of fuel in the reactor and of wastes stored in the repository for fuel cycle confining actinides (without recycling FP) to investigate nuclide which becomes dominant under the nuclear equilibrium state in the future. 1. Analytical Model The conditions in this analysis are as follows: (1) Liquid metal fast reactor was operated under fixed thermal rating with continuous fuel loading. (The fuel loaded to the reactor was either natural U or 232 Th.) (2) The spent fuel discharged from the reactor were reprocessed and separated into ( i) actinides (including Ra) to be re- ACTlNIDES FAST REACTOR I... t. ENERGY PRODUCTION cycled, (ii) radioactive FP and a-decay daughters Tl""'Fr to be stored in the repository and (iii) stable nuclides to be removed from the fuel cycle. (3) The actinides were confined in the reactor and thoroughly incinerated by fission reactions, so that it was expected 1% burn-up (about 1 MWd/g) of heavy metal. And the radioactive nuclides except for actinides were stored in the repository after isotope separation. (There were no actinides in the repository.) (4) The stored wastes in the repository were periodically separated into radioactive and stable nuclides. Then the active nuclides were restored to the repository and the stable nuclides were removed from the fuel cycle. The nuclides flow in this model is shown in Fig. 1. It was assumed that the isotope separation processes for spent fuel and wastes were practiced without loss and time lag. The natural uranium or thorium was supplied to the reactor, and only stable nuclides were removed from the fuel cycle. The closed fuel cycle was formed concerning with radioactive nuclides. REPOSITORY RADIOACTIVE FAB.: FUEl FABRICATION SEP.: REPROCESSING AND SEPARATION FP Fig. 1 Actinides confining fuel cycle In this paper, "stable nuclides" indicate nuclides whose half lives are more than the age of universe (1.5 x 1 1 yr) as well as strictly stable ones. Figure 2 shows half lives of FP, actinides and the a-decay daughters, treated in this analysis. A broad gap is observed over the half life of 1 7 ""' 1 1 yr for FP for the atomic number less than 7. The half life of 87 Rb, positioned on the right-side of this gap, is about 3 times of the universe age, and the relative hazard for the same number of atoms to 129 I, positioned on the left-side of the gap, is only about 4 X w- derived from MPC (Maximum Permissible Con- -83-

4 278 ]. Nucl. Sci. Techno!., 9?'{ uclides he a Yier than Ra are confined in the reactor ' [3!..... D. D..... t. :.... ;... C) ;:::: :::::: ::::: u ::::: < ~ Half life of nuclide (year) Fig. 2 Half lives of FPs, actinides and their a-decay daughters centration < 4 J ). The thermal rating of the reactor was set to be 3,3 MW which is approximately equivalent to 1.2 t/yr fuel loading to the reactor and the same rate of spent fuel removal. The removal rate for each nuclide was obtained as a product of it's number density and a discharge constant which is described below. The power density and neutron flux were set to be 32 W /em' and 2.X1 15 n/cm 2 s, respectively. The nuclear data were employed from ORIGEN2 data library (advanced-oxide recycle fuel core for liquid metal cooled fast reactor) which includes 127 actinides-daughters and 857 FP, and the average neutron numbers released per fission were taken from JENDL-3. The equilibrium composition Rreac. in the reactor for a given fuel loading rate s under continuous fuel management is given by the following material balance equation : -[ <ifj(e)t(a))e+d()., y)+ Rreac.Jnreac.=S, (1) where < >E denotes the integration over neutron energy, ifj(e) is the neutron energy flux, T(a) the matrix of cross sections, D(J., y) the matrix of decay constants and fission yields and Rreac. the discharge constant matrix for the reactor. The Rreac. is a diagonal matrix whose diagonal elements were set to be zero for trans-radium and set to be 2.2x1o-s s 1 for others, namely Tl""'Fr and FP. Here Rreac., ;; = means that nuclide i is confined in the reactor. In 2/ Rreac.. ;; implies half life for density decrease ascribed to discharge operation for nuclide i, and Rreac.,;;=2.2x1o-s s 1 meets the discharge half life of about 1 yr. On the other hand, the equilibrium composition in the repository Rrep. is given by - [D(J.)+ Rrep.JRrep. =Rreac.Rreac., ( 2) where Rrep. is the discharge constant matrix for the repository. The right-side is supply rate term of radioactive wastes separated from the spent fuel to the repository. Rrep. is also a diagonal matrix whose diagonal elements were set to be zero for radioactive nuclides for confinement in the repository and -84-

5 Vol. 29, No. 3 (Mar. 1992) 279 set to be 2.2x1-ss-' for stable nuclides for removal from the repository. 2. Accumulation of FP The nuclides with long half life and large cumulative fission yield are considered to accumulate largely in the future. Figure 3 shows the ultimate amounts of radioactive fission products calculated from Eqs. ( 1 ) and ( 2 ) in the repository in equilibrium state for both U- and Th-cycle. These radioactive fission products amounts are balanced between the charge from the reactor and natural decay in the repository. As shown in Fig. 3, the main constituents of wastes in the far future were found as, 129 I, 135 Cs, 17 Pd, 93 Zr, 99 Tc, "'Sn and 79 Se whose half lives are roughly ~..!:l b.o... Q) ~ l 5 rvl 7 yr. Each amounts in equilibrium state reached l 1 rvl 5 t/3 GWt. There are little differences between U- and Th-cycle. The next accumulated nuclides were Cs, Sr and 151 Sm whose half lives are rvlo yr. These FP cause most of the radiotoxicity which lasts a few centuries in the wastes from conventional reactors. Although these FP had large cumulative fission yields, the accumulated weights were rvlo" t at most. It may not be so large but manageable amount. The accumulations of other FP were less than 1 kg/ 3 GWt. It may be suggested that troublesome radioactive nuclides which will accumulate to huge amounts in the future are composed of the seven long-lived FP mentioned above. 112~ ~ ~ ~-... -~-... ~... ~-... -~... ~... /-3 5 Q:~Jj] -~~~-~ ~ :...;...;...:...:... ;.99T lll 93z i Pd...: c r : -- :... ; : -: 126 i...;... i...;.... -:... Snw :... :-...!... :... :...!... ;.::--... :... ~... :... : (].; - : : : --~--- : Se.--- :- ;----~ - : ;...;... -~ ~ -~... -~...:... ~-...;...:... ~-...;... - I ~ I I I 1 t t I 3- :::: :~::: :~ ~~:::: r::: :j::::: 1 :~:: :j:::: :j~::: i:::: ::::::::::::: 1 --~.. :!ctj : :! >+ : : U-cycle - ")lf~~q~.. j 6 ~--.. ~ i ~ l? ~ ~ + Th-cycle 1o -~ ~--~ :... ~ ~ m:.. ~ : ~ : I I I I I I I I I lkt It I kg Half life of nuclide (year) Fig. 3 Radioactive waste equilibrium composition in repository for actinides confining fuel cycle In order to investigate whether it is practically possible to attain the equilibrium state in the repository with this fuel cycle scheme, the required time to reach the equilibrium amount was evaluated for the long-lived FP in a simple manner. The change in nuclide number density of radioactive FP nuclide i stored in the repository was determined by the following transmutation equation : d dtnrep.. t=rreac., it nreac.. t + 2J Djj().)nrep.. j-du(j.)nrep.,i (3) jep where P denotes the set of parents of nuclide i. For long-lived FP, both first and second terms, denoted by a product term Grep.. ii can be assumed to be time independent. Then the solution of Eq. ( 3) with initial condition -85-

6 28 f. Nucl. Sci. Techno!., nrev.. ;()= can be written as nrep.. ;(t) ~;;{"i)i (1-e-D;;<.llt). ( 4) Thee quilibrium number density nrep.. ;( oo) can be simply obtained from Eq. ( 3 ) with dnrep.. t! dt=o as Grep..;t/ D;;(J.). Hence the ratio of nrep.. ;(t) to nrep.. ;( ) is nrep.. ;(t) nrep.. ;( oo) ( 5) Consequently the attained degree of equilibrium does not depend on the production rate but depends on only it's decay constant, though the equilibrium number density itself is dependent on the production rate. As it is obvious from Eq. ( 5 ), time equal to the half life is required for 5% accumulation of equilibrium, and 3.3 times of half life is required for 9 q accumulation. As for 129 I, 9% accumulation requires 5.2x 1 7 yr. It is too long since fission resources would be exhausted by that time even for perfect use of fission resources by the fuel cycle confining actinides. In other words, the long-lived seven fission product will keep on accumulating through operation of fission reactors until the end of use. However a proper waste management can make the amount of radioactive wastes saturated soon in manageable quantity. m. CONFINEMENT OF LONG LIVED FP IN REACTOR In order to reduce the amount of long-lived wastes accumulated in the reposity, the long- ACTINIDES AND lived FP were confined in the reactor. In this chapter, we discussed the feasibility of the fuel cycle confining long-lived nuclides. 1. Analytical Model The fuel cycle scheme is similar to the fuel cycle confining actinides, mentioned in Sec. 11-1, except for the treatments of some FP, described in analysis conditions (2) and (3). They are modified as (2') and (3'). (2') The spent fuel discharged from reactor were reprocessed and separated into ( i) actinides (including Ra) to be recycled, (ii) the long-lived seven FP ( 129!, 135 Cs, 17 Pd, 93 Zr, 99 Tc, 126 Sn and 79 Se) and their parents to be recycled, (iii) short-lived FP and a-decay daughters, namely Tl"' Fr, to be stored in the repository, and (iv) stable wastes to be removed from the fuel cycle. (3') The actinides, the long-lived FP and their parents were confined in the reactor. The actinides were thoroughly incinerated by fission reactions and the longlived FP were transmuted into shorter lived or stable nuclides by mainly capture reactions. The radioactive wastes were stored in the repository after isotope separation. (There were not actinides and the long-lived FP in the repository.) The nuclides flow in this model is shown in Fig. 4. The parents of long-lived FP were also confined in the reactor as described in condition (3). This is because that productions of long-lived FP in the repository due to,8-decay of the parents must be avoided. LONG-LIVED FP WITH PARENTS FAST REACTOR I... * ENERGY PRODUCTION REPOSITORY RADIOACTIVE FAB.: FUEL FABRICATION SEP.: REPROCESSING AND SEPARATION FP Fig. 4 Long-lived nuclides confining fuel cycle -86-

7 Vol. 29, No. 3 (Mar. 1992) 281 However this may not be necessary if wastes are cooled for a few years enough for completing decay of parents. The other design criteria were similar to that described in Chap. II. In this analysis, Eqs. ( 1 ) and ( 2 ) were used with some elements of diagonal Rreac. matrix modified. In accordance with condition (3), the diagonal elements for actinides, the longlived seven FP and their parents were set to be zero for confinement of these nuclides in the reactor, and the other elements were set to be 2.2 X lo-s s- 1 for removal. Compared to the actinides-confining fuel cycle, the inventory in the reactor is estimated to become larger because the more kinds of nuclides were confined. Accordingly the more 112-~ ~ I I I I I I I I t I -... :..... I I I I I I I I I I t -,, 1,, 1,, r..,, - I I I I I I I I I I I l i..., i , kt -... :... :.....!... :... :...!... :... :...!... :... : :! Cs : ~ ~ : ~ ~ : 6 ~ :. r... Sr,... =f.,... ' :... : : : : : r...., go n 1 Sin ---~--- h -~ t -...:... lid~~-...:...:... i...:.~--.:m---;---- _:_ -.: J -.-m-- ~:... j :':.:... b... ; ; -eye c rnfil : : + : : : : Tl r... m ,... li!j L u I 1kg - 4U"!. --~ ~-- oh m... ~ l-cyc e Q i!d-l,.-,' ' ' ' ' + 1 I I I I I I I I I I I Half life of nuclide (year) Fig. 5 Radioactive waste equilibrium composition in repository for long-lived nuclides confining fuel cycle Confinement of long-lived FP No Table l(a), (b) Reactor Repository ----~ ~- --~~ Yes Reactor Repository Some results for nuclear equilibrium state in future (a) U-cycle (b) Th-cycle Total weight (t/3 GWt) , Total radioactivityt (GCi/3 GWt) koo Total weight I ' (t/3 GWt) Total radioactivityt (GCi/3GWt) 1.27 I _-~ 58_o_, o_oo L_6 _ 1.17 I t a+ f3 decay amounts of short-lived and stable nuclides must be removed from the reactor to keep mass balance. 2. Reduction of Long-lived FP Waste The constituents in the repository with this fuel cycle are shown in Fig. 5. The total FP accumulation was remarkably reduced because of no long-lived seven FP stored in the repository. Then 137 Cs, 151 Sm and 9 Sr became main constituents in the stored wastes. As shown in Table l(a), (b), the total weight of wastes in the repository (containing stable FP slightly) was only a few tons per 3 GWt reactor. Some long-lived FP as 1 Be (1.6 x 1 6 yr), 81 Kr (2.1 x yr) and Nb (2. x 1 4 yr) were accumulated slightly but their accumulated weights were not more than 1kg/3GWt. For -87-

8 282 ]. Nucl. Sci. Techno!., the other nuclides, the amounts did not exceed 1 kg/3 GWt since they have half lives less than 1 yr in addition to rather small production yields. Therefore they were manageable as wastes and easier to be isolated from biosphere until their activity decays to an innocuous hazard level. The difference in compositions between U- and Th-cycle was caused by different fission yields of each main fission contributors. For Th-cycle, the amount of l61sm whose half life is 9 yr was about 1/3 of that for U-cycle. 3. Nuclide Composition and Features of Reactor The nuclide compositions in the reactor are shown in Fig. 6(a), (b). In this analysis, fuel and confined FP were assumed to be mixed homogeneously. The conspicuous FP above two-humps distribution in Fig. 6 are the confined long-lived seven FP. These recycled FP occupied about 5% of atomic density in the reactor. Their high number densities are ascribed to the recycling operations, large cumulative fission yields and small disintegra- (a) U-cydc :... (l).. s ;:l z I I t I I - -~ - - : : - - 'd :... : : : : : ;.D~. D... ;.. ;..... [j.;... : ~ : : :......;.....;.o ;....;... B.;.... ~~---a' : ~ ; ,... :... tl.:... I I I 1 [J..a p t ~cr~~~~o..n~ ~ ~ _...~.Ff.~..--.l.l...:jr'TlGI;:...!,.i;..,.;;_;;;...;;;...El~;;:m,~ l {) J 6;----T", ~. 12 Mass lluiilber (h) Th-cyclc..... ~..... :... -~ 'tl:.:...---, r---==l!,j""'-:-' ---~.... ~ +: ++ :.. ~ : : : : "of :..... : A:. A: : t: ~ +.;....;..-1: ; :-.;. - ~ ~~i.-~\--~ 1 ~ :.:: ~ ;... +t:+-.:t~l : t.:..;.... fl' ~ :.t+:+'t+:t J+ ~1-:t!+~+~!_ : -~ +" -:~:: ~ +~~- : +,;,++±,-h'l-h.t+... : ++ +: 1 lli;----r-,-~.a..;:.~~-=~~.:i:.ll:..---,--_.;.-=t.t-,--~ GO 2 2GO Fig, 6(a), (b) Mass lllllllbcr Nuclides composition in reactor for long-lived nuclides confining fuel cycle :------,- -88-

9 Vol. 29, No. 3 (Mar. 1992) 283 tion cross sections. From a view point of waste incineration, larger cross section of the confined FP seems desirable since the frequent reactions enable to hasten the disintegration and reduce the equilibrium amount. In fast reactor, capture reactions of actinide leading to higher nuclide were not so frequent such as in thermal reactor. Hence the distribution of nuclide densities in equilibrium did not shift to higher mass number so much in the actinide burn-up chain. Capability of criticality for U-cycle was superior to that for Th-cycle. It is attributed to the fact that the number of emitted neutrons per fission of the main fission contributor 239 Pu as in U-cycle is about 1.17 times as large as that of 233 U in Th-cycle. The infinite multiplication factors koo are listed in Table 1 for some fuel cycle schemes. The increased neutron absorption caused by confining long-lived FP was 9rv1 %. In the neutron balance calculation, only heavy nuclides and FP were taken into consideration. When the neutron leakage and absorption by structural materials and coolant are taken into account, the fast reactor with Thcycle in this scheme may be difficult to attain critical in the equilibrium state. Since harder spectrum fast reactor shows better neutron economy, it may be feasible even for Thcycle if the reactor is designed properly. IV. DISCUSSION AND CONCLUSIONS We studied the feasibility of fast fission system confining long-lived nuclides in the future society in nuclear quasi-equilibrium from the aspects of nuclear material balance and neutron economy. The fast fission system which confines all actinides in the reactor but discharges all FP will lead to huge accumulation of radioactive wastes such as 12ul, 13scs, J7Pd, 9 zr' 99Tc, J2 sn and 79Se whose half lives are roughly l 5 rvl 7 yr, in the far future. On the other hand, the equilibrium amounts of 137 Cs, 9 Sr and!51sm whose half lives are rv1 1 yr, were not more than a few tons per 3 GWt reactor. Although these nuclides are generally main radiotoxic wastes generated from conventional reactors, they are not considered as the object of incineration from long-term view point. Our concept to reduce the ultimate huge accumulation of the seven long-lived FP is based on confining them in the reactor after separating isotopically. It was confirmed that isotope separation is inevitable for fission reactors to perform both fuel sustainment and waste incineration. If only the chemical separation to each element is applied to this system, it is impossible to achieve the criticality due to increase of neutron absorption by many stable isotopes of Zr, Cs and Pd. As a result, the combination of fast reactor with a proper neutron spectrum and fuel cycle confining long-lived nuclides (all actinides and the seven long-lived FP) enable us to maintain the nuclear quasi-equilibrium state, where there is no increase of the radioactive wastes. The feasibility depends on many technological innovations in reprocessing, isotope separation and fuel fabrication besides advances in reactor design. (!) SEKIMOTO, H., TAKAGI, N.: ]. Nucl. Sci. Techno!., 28[1], 941 (1991). (2) ScHAPIRA,]. P.: Nucl. Instrum. Methods, A28, 568 (1989). (3) TAKAGI, N., SEKIMOTO, H.: Proc. Int. Conf. on Fast Reactors and Related Fuel Cycles, Kyoto, Japan, Oct. 28~Nov. 1, Vol. IV, (1991). (4) ICRP Pub!. 3, (1979). -REFERENCES- -89-