Study On Neutronics Performance Of Pebble In Thorium-based Fluoride Salt-cooled High Temperature Reactor

As one of the six candidate reactor types of the fourth-generation advanced nuclear reactor,the fluoride salt-cooled high-temperature reactor（FHR）has a lot of inherent safety by using molten salt cooling and tristructural-siotropic（TRISO）coated fuel particle technology.In this paper,the pebble in FHR is taken as the study object,and a series of analyses on the utilization of thorium fuel are carried out from the point of view of neutron physics by optimizing the fuel composition,the structure and the size of the pebble.The design parameters of the pebble suitable for high burnup and safe operation is given.Monte Carlo method is used to simulate neutron physics in this thesis.The tool is based on the Standardized Computer Analyses for Lisensing Evaluation（SCALE 6.1version）software developed by the Oak Ridge National Laboratory（ORNL）.The calculation of critical and burnup for the FHR are carried out using the CSAS6 module and the TRITON module in the SCALE 6.1 respectively.The PANAMA model is used to calculate the failure rate of TRISO coated fuel particles.In order to study the fuel composition and the structure of pebble with using thorium on FHR.Based on the comparative analysis of six fuel combinations,²³³UO₂-²³²ThO₂,which is the most suitable for thorium utilization on FHR,is selected for research.The ²³³U to heavy metal ratio(²³³U/HM)from 5.0%to 20.0%and the graphite-to-heavy metal ratio（C/HM）from 66 to 800 were studied.The neutron physical properties of FHR with different ²³³U/HM and C/HM are compared and analyzed.The initial effective multiclication factor(k_eff),the initial temperature coefficient of reactivity（TCR）and the conversion ratio（CR）are analyzed in terms of neutron usage,core safety and burnup.The fuel composition and structure design parameters suitable for a high power FHR are given.The calculation results show that the core should be in the undermoderated zone where k_eff increases with the increase of C/HM.The maximum C/HM for ²³³U/HM with 20.0%can reach 400 as the range of discussion in the future.The neutron spectrum and neutron spectrum factor of the core and the variation of the neutron balance of the core with ²³³U/HM and C/HM are further analyzed.In order to ensure the negative feedback of the core,the range of C/HM is reduced to less than 304.The coolant void reactivity coefficient（CVRC）,the fuel temperature coefficient of reactivity（FTCR）and the total temperature coefficient of reactivity（TCR）are analyzed in detail.The variation of CR and k_eff with burnup are analyzed according to the fixed ²³³U/HM and fixed C/HM,respectively.In order to evaluate the ²³³U saving for thorium based fuel,the burnup per fissile mass（Bu）is introduced.Bu can be improved through reducing C/HM,and it increases firstly and then achieves saturation as ²³³U/HM increases.It is recommended that the pebble with C/HM=124 and ²³³U/HM=12.5%is best suitable for thorium utilization.Secondly,according to the above optimization results,the fuel composition with²³³U/HM of 12.5%is selected to optimize the structure of TRISO coated fuel particle in the pebble.The TRISO coated fuel particles in the pebble were arranged next to each others.The the radius of kernel,the thickness and density of coating layers are studied respectively.The k_eff,the initial neutron spectrum and the neutron spectrum factor of the core with the radius of kernel from 0.005 cm to 0.06 cm are compared and analyzed.In a FHR,the change of the radius of kernel still needs considering the core safety.The optimization range of the radius of kernel is reduced to 0.01 cm⁰.06 cm.The optimization of the radius of kernel should also ensure the negative feedback.The variation of the TCR with the radius of kernel is studied.The results show that TCR is negative and decreases with the increase of the radius of kernel.When the radius of kernel is larger than 0.04 cm,the TCR does not change obviously with the radius of kernel,and the optimization range of the radius of kernel is reduced to 0.01 cm⁰.04cm.The behavior of fission gas will affect the safety of TRISO coated fuel particles.The variation of xenon（Xe）,krypton（Kr）and iodine（I）vary with burnup under the radius of kernel is analyzed.Reduce the range of the radius of kernel from 0.015 cm to0.025 cm.Based on PANAMA model,the variation of damage rate of TRISO coated fuel particles with the radius of kernel from 0.015 cm to 0.025 cm is calculated.The size of the radius of kernel is determined to be 0.015 cm for the smallest damage rate.According to the result of the radius of kernel,the thickness and density of coating layers are studied respectively.The influence on the neutron physical properties of core is analyzed.The results show that the thickness of porous carbon（PrC）buffer and the inner pyrolytic carbon layer have great influence on k_eff.The increase of thickness of PrC layer is helpful to decrease the damage rate.The thickness of silicon carbide（SiC）layer decreases,the damage rate increases.The effect of density change on k_eff is very small.It is recommended that the radius of kernel is 0.015 cm,and the thickness and density of the coating are still applicable.Finally,on the basis of the above study,the size of the fuel pebble is optimized.The effects of the radius of the pebble fuel zone（named as the inner radius）,the radius of the fuel pebble（named as the outer radius）and shell thickness of the pebble on the neutron physical properties of the FHR are analyzed.In order to ensure that the core reaches deep enough burnup,the inner radius of the pebble and the outer radius of the pebble need to be designed too large,and the shell thickness of the pebble should be reduced.In this paper,the thickness of the shell should be 0.3 cm.The size design of pebble should also ensure the negative feedback and the low damage rate of the TRISO coated particles.Keeping the shell thickness of the pebble unchanged at 0.3 cm,the outer radius is optimized,and the change of damage rate of TCR and TRISO coated fuel particles with the outer diameter of pebble was analyzed.The larger the outer radius is,the lower the TCR is,and the greater the damage rate is.The recommendation size of the pebble is final given that the inner radius with 2.1 cm and the outer radius with2.4 cm is best suitable for the FHR in this thesis.The burnup is 120 GWd/tHM,the TCR is-2.935 pcm/K,the failure rate is 4.511×10^-7.The above studies can provide certain reference for the effective thorium utilization on the FHR and thorium molten salt reactor（MSR）from the point of fuel pebble design.