(A) study on reconstruction of intra-pin information in pin-wise reactor analysis

Abstract

For a more accurate nuclear reactor design, more-detailed reactor analysis procedures are under development to replace the conventional assembly-based nodal methods. One such methodology is the pin-by-pin core calculation method, which directly provides reasonably accurate intra-assembly pin-wise average information. As such, there is actually need to obtain a higher level of details in the reactor design to couple with the pin-wise calculation. This research specifically investigates a novel intra-pin reconstruction procedure to obtain detailed power profile within a fuel rod. In this study, the intra-pin reconstruction process is based on the well-established form function method, which is a standard pin-power reconstruction method in the current coarse-mesh nodal analysis. The form function method assumes that heterogeneous intra-pin information can be approximated by the product of global homogeneous distribution and local heterogeneous one. The local form function is pre-generated as section-wise factors from a single fuel pin calculation with all reflective boundary condition. In this work, the heterogeneous form function for a single fuel lattice is determined by dividing a fuel region into 40 equi-volume small regions for detailed intra-pin power profile. The intra-pin homogeneous power distributions are approximated by using the analytical solution of the 2-gorup neutron diffusion equation with pin-wise boundary constraints. Four types of constraints are considered in this study to determine the homogeneous power shapes, which are surface-average flux, surface-average net current, corner point flux, and the volume-average cell flux. And six different combinations of the boundary constraints are separately evaluated for the homogeneous power profile. To investigate feasibility of the intra-pin reconstruction methods, all necessary information including form functions, homogenized group constants, reference power, and pin boundary constraints are obtained from a high-fidelity Monte Carlo calculation. It is demonstrated that intra-pin power can be retrieved with a reasonable accuracy even in a rodded condition. In addition, an error sensitivity analysis is also performed to assess the actual feasibility of the proposed methods when it is coupled with a pin-wise calculation