Optimization Of Fuel Cycle For Once-through Molten Salt Reactor Using Enriched Uranium And Thorium

As one of the six fourth generation reactors,molten salt reactors（MSRs）have obvious advantages in sustainability,economy,safety and anti-nuclear proliferation.Since its development in the 1950 s,various types of MSRs have appeared.After 2010,Thor Con,IMSR,TAP and other once-through MSRs emerged,which can be modular.The second phase of the Chinese Academy of Sciences TMSR project is the small modular thorium based MSR,which also uses a once-through fuel cycle.The once-through MSR is not designed to breed,so the molten salt requirement is relaxed and there are many carrier salts to choose.The continuous operation of the reactor is realized by gradual feeding without online post-processing and only removal of fission gas and insoluble mental.Therefore,the technical difficulty of MSRs is reduced and commercial deployment can be carried out quickly.Enriched uranium or transuranium（TRU）is used to start the reactor,and enriched uranium is easier to obtain.The fuel utilization analysis and nuclear design of once-through MSRs need to be further studied.As one of the important indexes of power reactor design,burnup is directly related to the economy of nuclear plant.There are many ways to improve burnup.This paper discusses and analyzes the influence of various core design parameters on burnup based on the MSRs using low enriched uranium（LEU）or thorium-uranium（Th-U）startup fuel.Firstly,the cross sections and neutron absorption proportions of various carrier salts are compared using the infinite grid element model.After selecting FLi Be carrier salt,the influence of different grid element shapes and sizes on the burnup of reactor is discussed,and the corresponding analysis is carried out by neutron absorption,energy spectrum,etc.Among them,hexagonal grid element has the largest burnup,and the sensitivity of burnup to grid element size is not high,especially when the grid element pair distance is 5-25 cm,the maximum natural uranium burnup is almost the same.After selecting the hexagonal grid element with the pair distance of 20 cm,the effects of the fuel volume fraction（VF）,initial heavy nuclei concentration（HN₀）and feeding uranium enrichment on the burnup are analyzed and discussed successively.The influence of VF and HN0 on burnup will change under different conditions.When feeding uranium enrichment reaches 20 wt%,the natural uranium burnup increases to a stable value.On the basis of feeding uranium enrichment of 20 wt%,the difference of burnup between LEU startup reactor and Th-U startup reactor is compared.The results show that Th-U startup model has stronger neutron regeneration ability and is superior to LEU startup model in terms of fuel utilization.Then,by changing the diameter of the active zone of the reactor core,the influence of the volume of the reactor core on the reactor burnup is discussed.The larger the volume of the reactor core is,the less neutron leakage is and the larger the burnup is.The comparison and analysis of the situation of LEU and Th-U startup are carried out.The results show that Th-U fuel is still superior to LEU fuel with larger core volume because of less neutron leakage and greater burnup.The reactivity temperature coefficient in nuclear design represents the strength of the feedback effect brought by the temperature change of the core material to the reactor.The negative temperature coefficient is of great significance to the regulation and operation safety of the reactor.One of the basic principles of MSR design is to ensure that the temperature coefficient must be negative.According to the model of oncethrough MSR,the influence of parameters on temperature coefficient at the beginning of life is firstly studied.The influence of the shape of the grid element on the temperature coefficient is studied.The results show that the reactivity temperature coefficient of the hexagonal and square grids is similar both in trend and value,while that of the plate grid element is larger.Furthermore,the reason of temperature coefficient variation of the grid element is analyzed by using the four-factor method.Then,the variation of fuel density coefficient,fuel Doppler coefficient and graphite coefficient of MSRs with different grid element sizes are analyzed and discussed one by one.The total reactivity temperature coefficient tends to be slightly positive with the increase of the grid pair distance.Then,the influences of VF,HN0 and startup fuel type on the temperature coefficient are discussed,and the reasons for the change of temperature coefficient are analyzed in detail by calculating the neutron absorption proportion of each nuclide of different energies.For LEU startup,the total temperature coefficient is negative（-15 to-3 pcm/K）in the range of 5-30% VF and 2-12 mol% HN0,and tends to be positive with the increase of heavy metal amount.For low heavy mental amount,the total reactivity temperature coefficient of Th-U startup MSR tends to be positive with the increase of heavy metal amount.For high heavy mental amount,the total temperature coefficient tends to be negative with the increase of heavy metal amount.The total temperature coefficient varies from-4.6 to-1.7 pcm/K in the range of 5-30% VF and 2-12 mol% HN0.The core volume also affects the temperature coefficient.The temperature coefficients of MSRs with different volumes using LEU and Th-U startup are discussed in this paper.The total temperature coefficient tends to be positive with the increase of the volume using enriched uranium,and the variation trends are similar with VF and HN0.When the volume of Th-U startup MSR is small,the change rule is similar to that of LEU startup MSR.When the volume is large,it gradually changes to the infinite grid element of Th-U startup MSR.Numerically,the larger the volume is,the more positive the total temperature coefficients are under the two startup conditions,but both are negative.Finally,the change of temperature coefficients of MSRs in the process of operation with infinite grid element and large burnup of different volumes are studied.The results show that when the core diameter is larger than 4 m and the startup fuel is LEU,and when the core diameter is larger than 6 m and the startup fuel is Th-U,the total reactivity temperature coefficient might be positive.The reasons affecting the temperature coefficient are discussed by analyzing the neutron absorption proportions of different nuclides.According to the above research,two schemes are designed respectively,one is a large volume MSR scheme with high burnup and negative reactivity temperature coefficient,and the other is a small modular reactor scheme considering burnup and temperature coefficient under the condition that the core diameter can meet road transportation requirement.The physical parameters,such as energy spectrum,burnup,evolution of main actinides nuclides,fission fraction,flux density distribution and reactivity temperature coefficient,are given respectively.In this paper,the effects of various parameters on burnup and reactivity temperature coefficient of MSRs using LEU and Th-U are studied on the basis of the once-through fuel cycle,which provides theoretical analysis basis for the core design of once-through MSRs.The core parameters are optimized to improve burnup of the reactor core,and the recommended schemes are given on the basis of ensuring the negative reactivity temperature coefficient and considering the small modular MSR,which provides references for the design of the core of the once-through MSR.