Development of albedo-corrected parameterized equivalence constants for macroscopic depletion in coarse-mesh nodal analysis of nuclear reactors

Abstract

This dissertation introduces the APEC (albedo-corrected parameterized equivalence constants) method for a macroscopic depletion in coarse-mesh nodal analysis for light-water reactors. For a highly reliable macroscopic depletion analysis, three related topics are investigated in detail: 1) impacts of power density on homogenized group constants, 2) higher-order burnup gradient correction (BGC), and 3) the APEC method for macroscopic depletion in the nodal analysis. First, a fission yield correction scheme has been proposed for the accurate estimation of Xe-135 and Sm-149 number densities based on simplified decay chains. The change of homogenized two-group constants due to the FA power density is analyzed, and the power density is introduced as an independent variable in the 2-step FA homogenization to account for the reactor history effects in the macroscopic depletion analysis. A higher-order BGC method is proposed and its performance and limitations are discussed. In order to consider the burnup gradient and neighborhood effects accurately, a burnup-dependent 2x2 APEC leakage correction is proposed for the macroscopic depletion in the nodal expansion method (NEM) analysis. Both burnup-dependent and independent 2x2 APEC functions are parameterized as a function of a normalized leakage parameter, current to flux ratio (CFR), by using various color-set models. A low-order polynomial fitting is also introduced for the burnup-dependent APEC coefficient to prevent the overfitting effect due to the finite number of color-set models. To assess the general applicability of the new APEC method, both UOX-loaded and partial MOX SMRs and their small & large variants are analyzed. Results confirm that the new burnup-dependent 2x2 APEC leakage correction can improve the nodal equivalence significantly in terms of the effective multiplication factor and fuel assembly-wise power distribution.