The neutronics and thermal-hydraulic characteristics of lead-bismuth cooled reactors are significantly affected by the geometric configuration of fuel assembly and lattice parameters. Reactor cores loaded with different geometry type fuel assemblies have different critical dimensions and fuel loadings under the same refueling cycle and thermal-hydraulic constraints.

This study aims to analyze these key factors and select a geometric structure of fuel assembly that is conducive to miniaturization and lightweight of lead-bismuth reactor.

First of all, the core model of a 4 MWt small lead-bismuth reactor was established, and simulation analysis of reactor physical characteristics was conducted using the RMC Monte Carlo program developed independently by the Reactor Engineering Calculation and Analysis Laboratory of Tsinghua University and the nuclear database ADS-2.0 released by the International Atomic Energy Agency (IAEA) in 2008. Then, three fuel assembly schemes of rod bundle type, annular type and honeycomb coal type were selected for comparison and analysis in term of fuel consumption characteristics, reactivity coefficient and steady-state thermal parameters under the same core size, fuel loading, coolant flow area, cladding and air gap volume, 10-year refueling cycle and basically consistent steady-state thermal safety margin.

The results show that compared with the rod bundle fuel assembly and the annular fuel assembly, the honeycomb coal fuel assembly has good steady-state thermal characteristics and hard neutron spectrum. The core of the honeycomb coal fuel assembly can realize smaller core size and fuel loading, and has obvious expansion negative feedback, and can effectively flatten the power distribution and reduce the core pressure drop. It is a fuel assembly solution that is conducive to the miniaturization and light weight of lead-bismuth reactors.