Research On Assembly Homogenization Method For Pebble-bed Fluoride Salt-cooled High Temperature Reactor

The molten salt reactor(MSR)as the only liquid fuel reactor in the 4th generation nuclear reactor systems,has been widely studied recently,benefiting the advantages of the nuclear fuel on-line processing and utilization of thorium resources.Pebble Bed Fluoride molten salt-cooled High temperature Reactor(PB-FHR)is a new concept reactor system based on the high-temperature molten salt cooling technology from MSR and the coated fuel pellet technology from High Temperature gas-cooled Reactor(HTR).PB-FHR is more safety and economics and it shows high commercial feasibility.At present,the neutron transport calculation of PB-FHR is mainly based on Monte Carlo(MC)method.The calculation is very time-consuming and can not meet the needs of commercial reactor design,Therefore,it is important to carry out the research based on deterministic method as an alternative to MC method.The core of PB-FHR has the so-called typical double heterogeneity structure unlike the regular fuel assembly arrangement in a conventional Pressurized Water Reactor(PWR).The random distribution of triso fuel particles on the microscopic and the random distribution of pebble fuel on the macroscopic can not be accurately positioned,which makes it difficult to solve the neutron transport equation for a double heterogeneity structure.The traditional homogenization method for assembly of pressurized water reactor does not work and a new method is needed to be developed to treat the double heterogeneity of PB-FHR.In this dissertation,to deal with the double heterogeneity of PB-FHR,the homogenization method for assembly has been developed based on the subgroup resonance calculation and the Dancoff correction.The main contents of this dissertation include the following topics:First of all,the resonance calculation has been analyzed.The resonance calculation is important in lattice transport computation and it plays an important role as a “bridge” for connecting nuclear data library and transport calculation.Moreover,the resonance calculation directly determines the accuracy and efficiency of the following homogenization calculation.In this part,subgroup method is applied to deal with the resonance calculation of the triso fuel to obtain the self-shielding cross-section on the microscopic triso fuel level.Secondly,for the resonance calculation and transport computation,multigroup nuclear data library has been developed and subgroup parameters and energy group-dependent Goldstein-Cohen factor are added to the library.The accuracy of the subgroup parameters used for fitting the multi-group cross-section have been verified.What is more,on the macroscopic pebble level,the Dancoff factor has been adopted to modify the resonance cross-section of a triso fuel because of the shielding effect induced by other fuel particles.Considering that a valid Dancoff factor is necessary to improve the self-shielding cross-section by subgroup resonance calculation on microscopic triso fuel level,we deduce the calculation method of fuel Dancoff factor in PB-FHR and the calculation code has been developed.Meanwhile,the MCNP5 code has been modified to verify the correction of Dancoff model in PB-FHR based on a two-region model.Afterwards,the Dancoff factor for a fuel kernel in PB-FHR has been analyzed by considering the FLiBe salt effect.The sensitivity of the infinite multiplication factor to the Dancoff factor for a fuel cell of PB-FHR has been further analyzed by the neutron transport calculation.It is found that the infinite multiplication factor increases almost linearly with the increase of Dancoff factor,and a higher density of coated particles will lead to a sharper variation of kinf with Dancoff factor.Afterwards,the neutron transport calculation has been performed on the unit cell for PB-FHR with the modified resonance cross-section,and few-group homogenized cross-section of the unit cell for PB-FHR has been obtained.In addition,the macroscopic parameters including infinite multiplication factor,few-group homogenized cross-section and neutron energy spectrum have been calculated and compared with those generated by the MICROX-2 and MCNP5 codes to verify the accuracy of homogenization method of unit cell for PB-FHR.The results show that the homogenization method based on subgroup resonance method and Dancoff factor correction has a better precision than MICROX-2,and improves the efficiency of calculation compared with MCNP5.Therefore,the few group cross section generation methodology in this dissertation can be used for PB-FHR analysis and is a good reference for development of completed multigroup neutron computation package for PH-FHR.