The CNRS Research Program on the Thorium cycle ... - Pacen - IN2P3

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Thorium Cycle – Molten Salt Reactors June 2008 accumulated over two hundred years it will dominate the final waste. With the log scale used in Fig.17, an operation of the fast TMSR-NM over 400 years instead of 200 which would induce an upward shift of the three purple curves by less than a third of a division (factor 2), does not change this conclusion. Thus, for societies which consider that the choice of a technology covering their present energy needs must take into account their legacy to future generations, there is a definite incentive to optimize global radiotoxicity including both the waste stream and the inventory. In that respect, finding ways to reduce the radiotoxicity of the TMSR-NM inventory as one enters the End-of-Game phase to the level of that of the accumulated waste stream is of major interest. For the TMSR-NM, this requires a destruction of 233 U whose decay dominates the radiotoxicity of the inventory between 10 4 and 10 5 years. Fig.17 Time evolution of several waste radio-toxicities. <strong>The</strong> purple dash curve at the top is the actinide waste radiotoxicity resulting from 200 years of operation of a GEN-III reactor; i.e. a two century open cycle. <strong>The</strong> green dash curve corresponds to the matters extracted from the spent fuel of a GEN-III reactor which are used as the initial inventory of the TMSR-NM. <strong>The</strong> green dash-dot curve corresponds to the contribution of the radioactive inventory of the fuel in a TMSR-NM core and its recycling facility when the steady state has been reached. <strong>The</strong> green solid curve corresponds to the transformed inventory after sixty additional years of incineration in a dedicated TMSR. <strong>The</strong> purple dash-dot curve gives the accumulated actinide waste stream produced over 200 years of TMSR-NM operation while the purple solid curve shows how this waste stream radiotoxicity is modified by the contribution of the additional actinide waste stream during the 60 years devoted to reduction of the inventory. If a decision to abandon fission and to move over to a new technology is taken at some date in the future 19 , apart from the decision to immediately consider the inventory as an ultimate waste to be disposed of in a geological storage, it seems that the only acceptable alternative will be to work on the destruction of this inventory. Should this choice be made, it is unlikely that it will be decided to design and built an entirely novel system relying on fission for that sole purpose. On the other hand, as Physics tells that low energy neutrons are by far the cheapest way to achieve the fission of actinides, it seems reasonable to assume that one will try to hand this task over to an already available and welltested fission-based technology albeit with a changed mission. Now, instead of the generation of energy and brreding, the priority will be given to the incineration of as much as possible of the inventory in order to reduce the burden on the storage 20 . This is a scenario that we have started to investigate as another illustration of the versatility of the TMSR-NM concept. To that aim, we consider a situation where, in fleet of TMSR-NMs which it has been decided to stop, for every seven reactors 21 an incinerator TMSR is started which, over a sixty year period, will burn the inventory of the 7 reactors. Keeping in mind that one of the criterions is the minimization of technological untested innovations as one enters the End-of-Game phase, we choose a TMSR with 19 We assume that this novel technology does not rely on nuclear fission and has no use for the energy content of actinides. 20 Of course, this incineration will also contribute to an additional production of energy 21 As a matter of fact the exact value of the ratio is 6.9 to 1 which for this preliminary study, we approximate as 7 to 1 for the sake of redaction simplicity. 24/29
Thorium Cycle – Molten Salt Reactors June 2008 the same basic design and characteristics described in Sect III and IV. Since 233 U generation is now to be avoided, the main modification is that the fuel of this TMSR will not anymore contain Th. <strong>The</strong> reactor will also not have breeding blankets. This TMSR has the same nominal power 2.5GWth (1 GWe) as those studied for energy production and will thus also produce energy (at the level 1 / 7). For the incinerating function which is now assigned to the TMSR, the salt must be changed to the socalled FLiNaK (46.5% LiF, 11.5% NaF, 42% KF) which has a low fusion temperature compatible with a small load of heavy elements (680kg) while ensuring a a not too much thermalized neutron spectrum. Element 7 TMSR-NM Inventory TMSR-NM Incinerator Inventory Reduction Factor U 60.240 5.520 10.9 Np 1.160 0.422 2.7 Pu 2.400 1.210 2.0 Am 0.055 0.039 1.4 Cm 0.039 0.078 0.5 HN 63.894 7.269 8.8 Table 2 Comparison of the inventories of 7 TMSR-NMs (see note 21) with that of the incinerator TMSR after sixty years of operation. HN stands for heavy nuclei. Masses are given in ton. <strong>The</strong> last column gives the destruction efficiency. In Fig. 17, the two solid curves correspond to the situation after sixty years of operating the incinerator TMSR 22 . <strong>The</strong> purple solid curve shows the effect of the additional contribution to the actinide waste stream. Expectedly, it barely increases the waste stream resulting from a 200 year operation of the TMSR-NMs. <strong>The</strong> green solid curve gives the evolution of the inventory of the original TMSR after 60 years of incinerating mostly 233 U but also some neptunium and plutonium (see Table 2). Although the radiotoxicity of this inventory still exceeds the total actinide waste stream over 260 years, the comparison with the green dash-dot curve shows an improvement by at least a factor 10 for the period considered critical for the dependability of geological storage civil engineering (beyond several 10 3 years). In principle, this process which already decreases the reactor fleet by a factor 7 can be repeated to complete the End-of-Game phase. In this way, after sixty years a reactor fleet of the size of that presently operating in France would be reduced to just one or two reactors terminating the process of destruction of the U and Pu elements. Once their task is completed, these few remaining reactors would be stopped. At that moment, the radio-toxicities of the accumulated waste stream and of the inventory for the entire initial TMSR-fleet would have been reduced to about the same magnitude 23 . At any rate even considering the sum of this remaining inventory and the waste stream after 260 years, the transition from the GEN-III open cycle (red dash-curve) to GEN-IV using TMSR-NM (sum of green solid curve plus purple solid curve) already leads to a decrease of radiotoxicity by more than a factor 10. Except for the fact that the waste streams of fast reactors working with the U-Pu cycle is likely to be larger, these conclusions should qualitatively hold for any other GEN-IV type of reactor. This discussion aimed to show that any optimization of the interim storage and the final disposal capacities for the global nuclear waste issue cannot restrict itself to a consideration of only the waste stream accumulated during the energy production phase. <strong>The</strong> End-of-Game inventory which may well turn out to be the dominant contribution to the waste has to be considered simultaneously even if this requires making additional assumptions as to when in the future it is decided that the game should end. <strong>The</strong> TMSR-NM offers interesting perspectives in that respect. In the framework of the <strong>CNRS</strong> program, further studies are being planned which will investigate not only radio-toxicities but also heat emission from the waste. We plan also to investigate schemes where the small amount of americium and curium produced during the exploitation of a TMSR-NM instead of being kept in the salt is regularly discarded to the waste stream. We also have to consider the modifications to the chemistry of the recycling batch process (Sect. V.B.2.b & c) which result from the elimination of Th in the incinerating phase. 22 <strong>The</strong> contribution of the incinerator to both waste stream and inventory have been rescaled (factor 1/7) to that of the original TMSR-NM. 23 Once the FP contribution to the waste stream radiotoxicity has disappeared in the background of that of the actinides 25/29
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The CNRS Research Program on the Thorium cycle ... - Pacen - IN2P3

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