Study Of Multi-scale Spatial Separation In A Block-type Thorium-loaded High Temperature Gas-cooled Reactor

Since natural thorium does not contain any fissile nuclide,additional fissile nuclides must be added to provide fission neutrons to convert ²³²Th to ²³³U.The addition of fissile nuclides into thorium-based fuel will involve two fundamental problems.One is the thorium content,meaning the mixing proportion of thorium and uranium.The other is the spatial separation effect,meaning how thorium and uranium is to be mixed.According to the order of spatial separation scale increasing,there are four levels in a block-type HTR:no separation,TRISO level,channel level and block level.Focused on thorium content and spatial separation effect,this paper calculates the performance of equilibrium cycle at each spatial separation level and thorium content,reveals the mechanism of multi-scale spatial separation effect and determines the optimal separation scale and thorium content by fuel cycle cost.To obtain the performance of equilibrium cycle,a computing platform that can simulate the refueling of block-type HTR has been constructed using DRAGON V4 code and based on two-step calculation scheme of reactor physics.Since the refueling pattern and spatial separation scale can significantly affect the performance of equilibrium cycle,to study multi-scale spatial separation effect alone,firstly,an analysis of multi-scale spatial separation effect under single-batch refueling scheme has been carried out.The result shows that the origin of multi-scale spatial separation effect is the different nuclear physical properties of²³²Th and²³⁸U,and the spatial self-shielding effect enlarge these difference by enhancing the neutron absorption of ²³²Th and weakening that of ²³⁸U.The above studies have demonstrated that the multi-scale spatial separation effect is conducive to improve the performance of equilibrium cycle under the same refueling scheme.However,whether this conclusion is still valid under the different refueling scheme is unknown.Hence,the analysis of multi-scale spatial separation effect under the optimized three-batch refueling scheme which is obtained by the genetic algorithm has been made.The result indicates that the multi-scale spatial separation effect is still helpful to improve the performance of equilibrium cycle even under the optimized three-batch refueling scheme.Because the origin of multi-scale spatial separation effect is the different nuclear physical properties of ²³²Th and ²³⁸U,the change of refueling pattern will only affect the value of the equilibrium cycle performance but not change the trend.If using the fuel cycle cost as the evaluation criterion,the best separation scale is the block level.When the thorium content is about 40%,the difference of the fuel cycle cost among four levels spatial separation is the largest,up to 16.7%.The best thorium content is 90%.With no separation level as an example,when thorium content increased from 0%to 90%,the fuel cycle cost will reduce 18.4%.