Multi-physics Analysis Method and Conceptual Design of Helically Metallic Fuel

Abstract

The helically metallic fuel is promising to improve the power density and safety margin of reactors core by its advantages of high thermal conductivity, large heat transfer area-to-volume ratio and continual inter-channel mixing. However, helical geometry and metallic U-Zr fuel will introduce challenges in the analysis of neutron physics, thermohydraulics and mechanics performance. Thus, in the current work, the analysis methods were developed for the characteristics of helically metallic fuel in neutron physics, thermohydraulics, mechanics and multi-physics coupling. For neutron physics, a 3D continuous-energy Monte Carlo method was employed to address the complex geometries and the complex intermediate neutron energy spectrum and generate few-group cross-sections. In core calculations, the 3D method of characteristics (MOC) with high geometric flexibility, accuracy and convergence was utilized. This method has been verified in 3D calculations of helically metallic fuel core. The results demonstrate that both cross-section generation and core calculation have achieved high accuracy and effectively improved computational efficiency. For thermohydraulics, the experimental technique including visual measurement and wire mesh sensor were used to measure the mixing characteristics of single and two-phase flow in helically metallic fuel rod bundle. CFD and subchannel analysis codes were developed to predict the boiling and critical heat flux. The inter-channel mixing was dominated by the flow sweeping mixing, while the turbulent mixing was insignificant. The vapor phase crowded at the elbow of the rods, where the boiling crisis was triggered. For mechanics, the molecular dynamics method was employed to predict the fundamental thermal-mechanical properties. The correlations were proposed for the thermal conductivity and elasticity modulus of U-Zr alloy under irradiation conditions with pores. The finite element tool Abaqus was used to analysis the thermal-mechanical performance of helically metallic fuel rod bundle. The maximum stress was found at the blade tips where adjacent rods contact with each other. The maximum stress varied from 326.7 MPa under fresh condition to about 313.3 MPa at 14.1% FEMA because of creep, which was far lower than the failure strength of the rod. The structure integrity can be ensured during the full life cycle. Based on the individual analysis method, the multi-physics coupling framework combining the neutron physics, thermohydraulics and mechanics was developed for the design and performance evaluation of the fuel assembly and reactor core with helically metallic fuel rods. Based on this multi-physics analysis tool, a novel boron-free small modular pressurized water reactor NETH-HCF175M design using helical-cruciform metal fuel was proposed. The core can achieve a cycle length of about 1 360 EFPD as the reactivity of the core is completely controlled by burnable absorbers and control rods.

Keywords

• helically metallic fuel
• heat and mass transfer
• transient safety analysis
• fuel service performance
• multi-physics coupling