Study On Neutron Properties Of Long-lived Fission Product Transmutation In A Molten Salt Thermal Reactor

Long-lived fission products?LLFPs?are the primary contributors to the radioactive hazards of spent nuclear fuel.With the rapid development of nuclear power industry,the accumulation of LLFPs impedes the sustainable development of nuclear energy.Partitioning&transmutation strategy has been proposed to reduce the mass and radiotoxicity of LLFPs.Compared with conventional solid-fuel reactor,molten salt reactor?MSR?with characteristics of online refueling and reprocessing brings the advantages of Th-U breeding and LLFPs transmutation.Because of its long half-life,high fission yield and large radiotoxicity coefficient,¹²⁹I and ¹³⁵Cs are the most important LLFPs,which have a negative impact on envirment.Based on the high solubility of ¹²⁹I and ¹³⁵Cs in MSR,we conceived the transmutation of ¹²⁹I and ¹³⁵Cs in a 2250-MWt single-fluid double-zone thorium molten salt reactor?SD-TMSR?.Different transmutation scenarios are designed and analyzed for the ¹²⁹I and ¹³⁵Cs transmutation in this thesis.A molten salt reactor reprocessing sequence is used to simulate the various influences on neutron properties of the core for different transmutation scenarios to investigate transmutation efficiencies of long-lived ¹²⁹I and¹³⁵Cs in the SD-TMSR.For the long-lived ¹²⁹I in the discharged spent nuclear fuel of light water reactor?LWR?,we assess the transmutation capability of the SD-TMSR by considering two methods.One is achieved by loading an appropriate amount of ¹²⁹I before the startup of the reactor,and the amount of ¹²⁹I during operation is kept constant by online feeding¹²⁹I.The other adopts only an initial loading of ¹²⁹I before startup,and no other ¹²⁹I is fed online during operation?Intial loading transmutation scenario?.For both the scenarios,LiI is chosen as sample form and is loaded in the fuel salt to substitute some LiF in the carrier salt.The investigation for both the scenarios first focuses on the effect of the loading of I on the Th-²³³U isobreeding performance.The results indicate that a²³³U isobreeding mode can be achieved for both the scenarios for a 60-year operation when the initial molar proportion of LiI is maintained within 0.40%and 0.87%,respectively.Then,the transmutation performances for the two scenarios are compared by changing the amount of injected iodine into the core.It is found that the scenario that adopts an initial loading of ¹²⁹I without online feed shows a slightly better transmutation performance in comparison with the scenario that adopts online feeding of ¹²⁹I when the equal net ²³³U productions for the two scenarios are kept.The initial loading of ¹²⁹I scenario with LiI=0.87%molar proportion is recommended for ¹²⁹I transmutation in the SD-TMSR,and 1.88 t ¹²⁹I can be transmuted in the ²³³U isobreeding mode over 60 years operation.Two kinds of ¹³⁵Cs from different spent nuclear fuels of reactors?LWR and SD-TMSR?are transmuted in the SD-TMSR.For the ¹³⁵Cs transmutation,CsF is chosen as sample form and is loaded in the fuel salt to substitute some LiF in the carrier salt.?1?For the ¹³⁵Cs in the discharged spent nuclear fuel of LWR,the initial loading transmutation scenario is selected to calculate the transmutation capability of the SD-TMSR.The ¹³⁵Cs transmutation efficiency in SD-TMSR is low relative to the ¹²⁹I transmutation efficiency under the ²³³U isobreeding mode over 60 years operation(the transmuted mass and fraction of ¹³⁵Cs are 0.11 t and 29%,respectively).To transmute¹³⁵Cs in the core as much as possible,the operation time of the core is prolonged to 120years.The results indicate that when the initial molar proportion of CsF is 1.0%,the transmuted mass and fraction of ¹³⁵Cs in the SD-TMSR are improved to 0.48 t and80.12%under the Th-U breeding mode(the net ²³³U production is 0.58 t),repectively.?2?¹³⁵Xe with an extremely large neutron poisoning effect is extracted by the bubbling system during the operation of SD-TMSR and decays to long-lived ¹³⁵Cs.Therefore,we proposed a cooling-transmutation method to address the issue of ¹³⁵Cs decayed from¹³⁵Xe in the off-gas processing system of SD-TMSR.The results indicate that the production rate of ¹³⁵Xe for the SD-TMSR is about 3.48 g/h during operation.To get a high mass fraction of ¹³⁵Cs in the Cs isotopies with solid form,we designed two cooling systems for accommodating the separated Xenon gas in the off-gas processing system during operation.The mass fraction of ¹³⁵Cs obtained from the two cooling systems can be improved to about 80%in the Cs isotopies with solid form,which is selected for the¹³⁵Cs transmutation.Based on the online refueling of the SD-TMSR,we proposed the online returning transmutation scenario,namely,the ¹³⁵Cs decayed from the ¹³⁵Xe in the cooling system is online injected into the core during operation(the feeding rate of¹³⁵Cs is 3.48 g/h).In other words,the SD-TMSR can transmute the ¹³⁵Cs produced by itself during the whole depletion process.The results indicate that the transmuted mass and fraction of ¹³⁵Cs are 0.7 t and 39%during the 60-year operation under Th-U breeding mode(net ²³³U production is 1.83 t),respectively.Based on the ¹²⁹I transmutation,the initial loading transmutation scenario is selected to improve the transmutation efficiency of ¹³⁵Cs in the SD-TMSR.The transmuted mass and fraction of ¹³⁵Cs are 1.7 t and 63%for a 60-years operation under the ²³³U isobreeding mode,respectively.However,the transmuted mass of ¹³⁵Cs is slightly less than the ¹³⁵Cs production of its self-creation?1.83 t?under the same operating time.In addition,the higher ¹³⁵Cs loading for the initial loading transmuting scenario shows a better transmutation performance and is also beneficial to the next stage transmutation because of the higher cooling mass fraction?CMF?of ¹³⁵Cs in the discharged spent nuclear fuel.Therefore,the high ¹³⁵Cs loading for the initial loading transmutation scenario is recommended for the transmutation of ¹³⁵Cs decayed from ¹³⁵Xe in off-gas processing system of the SD-TMSR.However,the high ¹³⁵Cs loading is not beneficial to the Th-U breeding of the SD-TMSR.So a special reactor need to be build to transmute the ¹³⁵Cs.The preliminary results indicate that when the initial molar proportion of CsF is 1.9%,the transmuted mass of ¹³⁵Cs is equal to the yield from 1.65MSRs with a power of 1 GWe for a 60-year operation.