A Study on Reconstruction of Intrapin Power Distribution in Pinwise Two-Group Diffusion Analysis

Detailed pin-by-pin core calculations are under development to replace the conventional assembly-based nodal methods. This research investigates a novel intrapin reconstruction procedure coupled with these pinwise calculations to obtain a detailed power profile within a fuel rod. The reconstruction process is based on the well-established form function (FF) method. In this paper, the fuel rod is geometrically divided into 40 equi-volume subsections where the intrapin power is reconstructed with corresponding heterogeneous FF. The intrapin homogeneous flux distributions are approximated by using the analytical solution of the two-group neutron diffusion equation with pinwise boundary constraints. Four types of constraints are considered to determine the flux shapes: surface-average net current, surface-average, corner-point, and volume-average cell fluxes. Therefore, six different combinations of the boundary constraints are separately evaluated for the intrapin power profile. All necessary information, including burnup-dependent FFs, homogenized group constants, reference power distribution, and pinwise boundary constraints, are predetermined from a high-fidelity Monte Carlo calculation. The numerical results demonstrate that the intrapin power can be retrieved for enriched and Gd-loaded fuel pins with reasonable accuracy, even at rodded conditions and in highly burned conditions of 10 and 30 GWd/tonne U. In addition, a sensitivity analysis is also performed to assess the feasibility of the proposed method when it is coupled with a pinwise calculation.