Study Of Multi-scale Neutronics And Thermal-Hydraulics Coupling Method Of Pebble-bed Very High Temperature Gas-cooled Reactor

As one of the six fourth-generation nuclear energy systems,the Very High Temperature Gas-cooled Reactor(VHTR)in pebble bed design boasts inherent safety and a design outlet temperature above 750℃,resulting in advantageous safety and economic characteristics.The VHTR employs a three-dimensional structure of core,fuel pebble,and TRISO(TRistructuralISOtropic)fuel particles,which poses challenges for neutronics and thermal-hydraulics(N/TH)coupling research in VHTRs.At the fuel pebble scale,there are tens of thousands of TRISO fuel particles randomly distributed with a structurally double heterogeneity.Existing research models use a homogenized computational approach at the fuel pebble scale to reduce computation cost,but this neglects the N/TH coupling characteristics at the fuel pebble scale and the effect of fuel pebble N/TH coupling on the multi-scale N/TH coupling of the VHTR core.At the fuel pebble scale,this study proposes a multi-zone fuel pebble N/TH coupling method and compares it with the traditional method that uses the uniform internal heat sources.The influence of the spatial self-shielding effect on the power density distribution and peak temperature is quantitatively evaluated.For different fuel pebble powers and wall temperatures,the maximum fuel pebble scale temperature and TRISO scale temperature considering the spatial self-shielding effect are lower than the calculated results without considering the spatial self-shielding effect.This proves that the traditional method is conservative.In response to the uniform treatment of TRISO multilayer coating structure and its spatial random distribution by the traditional method,this paper further proposes a N/TH coupling method based on fine structure.The multi-layer coating structure of TRISO fuel particles is accurately constructed.The influence of heterogeneity on the temperature of the fuel pebble is analyzed.The results shows that the peak temperature obtained by the traditional method based on single-zone with uniform internal heat sources for fuel pebble temperature calculation is significantly higher than that obtained by the fine structure coupling method,and the maximum deviation reached 23.37 K in the 18 operating conditions studied in this paper.The average and peak temperatures of the fuel pebble were calculated using the fine structure N/TH coupling method.The effective thermal conductivity of the fuel pebble was determined under N/TH coupling conditions,and the impact of N/TH coupling and heterogeneity on the effective thermal conductivity was analyzed.Building upon this research,a fuel pebble temperature calculation method based on a coupling equivalent model was proposed.This method enables accurate prediction of temperatures obtained by the fine structure N/TH coupling method,and the number of grids required is significantly reduced.A Pebble Coupled Equivalent Model(PCEM)method is proposed for the multiscale N/TH coupling of core,fuel pebble,and TRISO fuel particles.The PCEM method considers the effects of N/TH coupling and heterogeneity at the fuel pebble scale.This method is compared with the SZM(Single-Zone Model)multi-scale N/TH coupling method which adopts the traditional single-zone uniform internal heat source temperature calculation model at the fuel pebble scale.The influence of the fuel pebble N/TH coupling on the multi-scale coupling characteristics of the whole core of the VHTR was quantitatively evaluated.The results show that,under the condition of steady-state N/TH coupling,the average and peak temperatures of the fuel pebbles calculated by the SZM method are both greater than those calculated by the PCEM method.The temperature difference between the two methods first increases and then decreases with the axial height,with the maximum values of 0.6 K and14.2 K occurring at a height of 5 m.For the coolant flow rate step change transient,the PCEM method considering the fuel pebble scale N/TH coupling effect calculates the highest fuel pebble average temperature and the peak temperature are both lower than the SZM method at the same time.Due to the Doppler effect,the transient response of the traditional calculation method is slower.In summary,the proposed multi scale N/TH coupling method significantly improves the accuracy of the calculation of the average temperature of fuel pebble and the peak temperature of TRISO fuel particles in the safety analysis of VHTR.