Study On Thorium Fuel Cycle Transition And Transmutation In A Thermal Molten Salt Reactor Started With TRU

With the growing demand for nuclear energy,the sustainable supply of nuclear fuel and disposal of nuclear waste have become the important issues,which should be addressed as soon as possible.The utilization of thorium fuel with a closed nuclear fuel cycle to recycle and incinerate transuranic elements(TRU)is an effective way o solve the above two problems.As the only liquid fueled reactor among the six nuclear energy systems chosen by the Generation IV International Forum(GIF),the Thorium-based Molten Salt reactor(TMSR)nuclear energy system has potentially promising characteristics to meet the criteria of Generantion IV nuclear systems such as safety,economy,sustainability and non-proliferation.TMSR also has high flexibility and excellent neutron economy,which can achieve high thorium breeding and TRU transmutation.Therefore,TMSR is considered to be suitable for nuclear fuel supply and nuclear waste minimum,which would pave the way for sustainable nuclear energy development.It is prior to produce available U233 before a closed thorium fuel cycle is deployed in TMSR.By employing TRU as starter fuel,various transition scenarios in TMSRs are provided in this work.Moreover,two transmutation strategies that adopt pure TRU transmutation and MA transmutation,respectively,are established to minimize the nuclear waste.First,the neutronic characteristics of transition to thorium fuel cycle in TMSRs are studied.The properties of transition to thorium fuel cycle in a small modular molten salt reactor(SM-MSR)with batch reprocessing and a thorium based molten salt breeding reactor(TMSBR)with on-line reprocessing are investigated,respectively.(1)The performances of transition to thorium fuel cycle in a small modular molten salt reactor(SM-MSR)are conducted.Considering the solubility limit and the inherent safety of TRU as fissile fuel,the core parameters(the pitch of graphite lattice(P)and ratio of fuel volume fraction to graphite(VF))and four molten salt compositions(FLibe,FLi,FLiNaK,NaCl)with different TRU solubility limits are firstly analyzed and compared to obtain reasonable and feasible parameters of the SM-MSR core.Based on the design flexibility of SM-MSR,two different transition schemes(single-reactor transition and multi-reactor transition)are adopted in this work.The shortest transition time is 16.9 years for the single-reactor transition scheme,while the shortest transition time is 12 years for the multi-reactor transition scheme.(2)The performances of transition to thorium fuel cycle in a thorium based molten salt breeding reactor(TMSBR)are investigated.Benefiting from the advantages of online reprocessing,two flexible transition schemes(in-core transition and ex-core transition)can be implemented in TMSBR.Online reprocessing rate is a key parameter that affects the transition performance.Therefore,this work is mainly focused on the impact of different reprocessing rates(100 L/d,1000 L/d and 4000 L/d)on the transition performance,which can provide a proper reprocessing rate for the two transition schemes.The reprocessing rate more than 1000 L/d are suitable for the in-core transition while the reprocessing rate less than 1000 L/d preferably recommended for the the ex-core transition scheme.In conclusion,employing TRU as starter fuel in TMSRs can solve the problem of U233 absence effectively by deploying flexible and feasible transition scenarios.The paper further evaluates the performances of pure TRU utilization in TMSR.TRU contain considerable fissile fuel,which can serve as fuel directly in TMSR without additional fissile fuels and is benefical for high TRU transmutation.For the pure TRU fuel cycle,the amount of TRU transumutation is equal to the fission consumption during operation.The results show that the specific incineration consumption can reach as high as about 370 kg/GWth.y.Aiming at the issue of a positive temperature feedback coefficient in the pure TRU fuel cycle,two solutions are proposed: adding Er167 in the fuel and adjusting the fraction of MA in TRU.When the Er167 addition scheme is adopted for the VF ranging from 5% to 40%,the required mass of Er167 for the above VFs(5%~40%)ranges from 94.4 g to 178.3 g if a total temperature reactivity coefficient of-2 pcm/K is kept.When the proportion of MA in TRU is kept beyond 50%,a negative temperature feedback coefficient can be obtained.To satisfy both the TRU solubility limit and the negative temperature feedback coefficient,a MA mole fraction of 50% with VFs ranging from 5% to 25% and a MA mole fraction of 60% with the VF less than 5% are recommended,respectively.In short,the pure TRU fuel cycle in TMSR can reduce the nuclear waste significantly.Meanwhile,the above two schemes can effectively improve the positive temperature feedback coefficient caused by TRU.MA in the used fuel has a potential environmental radiotoxicity hazard in the long term.Therefore,the transmutation of MA is also necessary for realizing sustainable development of nuclear energy.Considering the very little TRU production in thorium fuel cycle,the neutronic performances of MA transmutation in a TMSR are studied with aim to further reduce the nuclear waste.Three salt compositions(FLibe,FLi,FLiNaK)with different MA solubility limits are chosen and four different schemes of MA transmutation are proposed.The critical performance(critical U233 loading,neutron spectrum),burn-up,MA transmutation rate/efficiency and radiotoxicity are evaluated.Among the four schemes,the FLibe salt can achieve the highest transmutation rate.The transmutation rate decreases with the increasing VF while the highest MA transmutation rate of 60.6% is obtained for a VF of 5%.The specific MA incineration consumption using FLiNaK molten salt without intital Th232 loading is higher than the other three schemes,and increases with the increasing VF.The specific MA incineration consumption is highest(226 kg/GWth.y)if a VF of 40% is adopted.After 50-year operation,the total MA radiotoxicity can reduce an order of magnitude.