Conceptual Design Of A Thorium-based Subcritical Chloride Reactor

As one of the six candidate reactors of the generation IV advanced nuclear power systems,molten salt reactor has the advantages of flexible fuel loading,good neutron economy and so on.It has great potential and advantage in realizing efficient utilization of thorium resources.The research and development?R&D?on the thorium-based molten salt reactors was started from the 1950s.From the 1950s to the early 1980s,ORNL led most of the TMSR projects.However,due to the high requirements of fuel salt online processing technology,the lifetime of graphite,lack of R&D funds,the R&D of the thorium-based molten salt reactors reached a low point.After 2010,interests in thorium based molten salt reactors began to revive.In 2011,the Chinese academy of sciences launched the thorium based molten salt reactors special pilot project?TMSR?,which proposed a three-step development route to finally realize thorium uranium cycle on molten salt reactors.In addition,more private companies show interest in the R&D of TMSRs,such as Terrestrial Energy Inc,ThorCon USA Inc,etc.After nearly 70 years of development,the design philosophies of the thorium-based molten salt reactors have changed a lot.High breeding ability is no longer a main pursuit of the thorium-based molten salt reactors design to avoid the complex online fuel processing.Simplified core designs are adopted to avoid the R&D of new technology and speed up the commercial deployment.Under these new design philosophies,a Thorium-based Molten Salt Fast Energy Amplifier?TMSFEA?,which is based on the subcritical chloride salt fast reactor,is proposed.It adopts a single-fluid,moderator-free simple core design.The molten salt in the core serves both as spallation target and fuel.The external neutron source provided by the accelerator is introduced to compensate for the absorption of neutrons by fission products during operation,thus simplifying the fuel processing.Based on low accelerator beam power demand and without any online fuel processing,the feasibility of thorium utilization on TMSFEA was studied.Since TMSFEA is a subcritical reactor,the neutron flux density distribution in the reactor is very different from that of a critical reactor,and the general burnup calculation program cannot be directly used for the calculation and analysis of its fuel cycle.Therefore,a burnup code?MOADS?,which is based on the existing burnup code,for the subcritical reactor was developed.MOADS can be used to calculate the evolution of effective multiplication factor,nuclide mass,neutron efficiency and external neutron source strength of subcritical reactor during operation.The accuracy of MOADS has been verified in the IAEA-ADS benchmark model proposed by IAEA.TMSFEA uses the high-energy protons from the accelerator to bombard the spallation target,to generate external neutrons to drive the entire subcritical molten salt reactor,in order to sustain the chain fission reaction as well as the continuously generate energy production.Therefore,the simulation of spallation reaction and the analysis of the neutronics properties of the spallation targets are the basis of TMSFEA design.The spallation reaction includes the excitation process of the target nucleus and the de-excitation process of the target nucleus.The excitation process can be simulated by the intra-nuclear cascade model?INC?,while the de-excitation process can be described by the evaporation and fission models.In this work,the neutronics properties of liquid lead,LBE,chlorine salt and fluoride targets were evaluated.The INC model Bertini and evaporation model of Dresner were employed to calculate the spallation neutron yield,spallation neutron energy spectrum and energy deposition distribution.The INC model INCL4 and evaporation model of ABLA were adopted to analyze the spallation products from the above spallation reaction processes.The results show that the neutron yield of liquid lead and LBE target is the largest,while the neutron yield of chlorine salt target is moderate,and the neutron yield of fluoride target is the smallest.Compared with liquid lead and LEB targets,the gradient of energy deposition distribution of high-energy protons in molten salt target is much smaller,which is beneficial for heat removal.In addition,the molten salt target does not require special structural materials to separate the target from the subcritical core,which is helpful to simplify the structure of the entire core and reduce the complexity of the system.TMSFEA adopts the simple core design based on single-flow,free-moderator and integrated target.The ternary chloride salt Nacl-Pucl₃-Thcl₄ system is selected as spallation target and fuel salt because of its high solubility of plutonium-thorium,suitable melting point and high yield of spallation neutron.Firstly,the impact on the energy gain from the proton energy and the incident position is studied.It is found that the energy gain of the system reaches the maximum value when the proton incident position is above the center of the core.Besides,1000 MeV is a more reasonable and economical incident proton energy.Secondly,through the comparative analysis of the different types of reflector,liquid lead has a smaller softening effect on the neutron spectrum of the core,which is helpful to obtain a larger conversion ratio.Then the detailed design parameters of TMSFEA were obtained through the calculation of different core sizes and molten salt compositions.Finally,with the limitation of the maximum beam current?4 mA?,the total power of the TMSFEA system is selected as 300 MW_th.Under the above design parameters,the initial conversion ratio?CR?of TMSFEA can reach 1.19 and the initial energy gain can achieve 117.The lifetime is 39 years.By end of the lifetime,57%of the plutonium and 16%of the thorium had been consumed,and the fission rate fraction of ²³³U is70.9%,indicating that TMSFEA could make efficient use of thorium resources without any online fuel processing.In order to achieve the sustainable development goals of nuclear energy system,namely,maximum utilization of nuclear fuel and minimum production of nuclear waste,this paper studies the burnup evolution,neutron physical properties and spent fuel characteristics of TMSFEA from the primary to the fourth cycle reactors under the offline fuel reprocessing scheme based on the concept of multi-cycle TMSFEA evolution.The results show that in the process of multi-cycle TMSFEA evolution,the fission rate fraction of Pu and the initial CR decrease in each cycle,whereas the fission rate fraction of ²³³U increases,from 0%as the first cycle TMSFEA starts to90.4%when the fourth cycle ends.By the end of the four cycles operation,the amount of ²³⁸Pu,²³⁹Pu,²⁴⁰Pu,²⁴¹Pu and ²⁴²Pu in the reactor decreased by 6.17%,90.43%,41.48%,91.04%and 29.22%,respectively,compared with that of the first cycle TMSFEA.The ²³³U conversion ratio of the first cycle,second cycle and third cycle reactors is always greater than 1,while that of the fourth cycle reactor is larger than 1 for most of the operation time.Therefore,Th-²³³U breeding was realized in the first to the fourth cycle TMSFEA.After the evolution of the four cycles TMSFEA,the total net production of ²³³U is 2832.7 kg.The utilization efficiency of Th increased linearly with the burnup time,and the Th utilization rate at the end of life for the four cycle TMSFEA is 11.38%,18.53%,23.42%and 26.49%,respectively.The energy contribution of thorium increased from 0%at the beginning of the first cycle to 66.9%at the end of the fourth cycle In addition,the radioactive toxicity of the discharged fuel in the fourth cycle TMSFEA is 63.7%,which is lower than that of the first one.In conclusion,the fuel cycle scenario based on multi-cycle reactor mode without any online refueling and reprocessing can significantly improve the utilization efficiency of thorium by TMSFEA and reduce the level of radioactive toxicity of nuclear waste.