Improved macroscopic depletion in 2-D nodal analysis by 2x2 albedo-corrected parameterized equivalence constants method

Abstract

This work introduces a 2x2 albedo-corrected parameterized equivalence constants (APEC) method for the macroscopic depletion of nuclear reactors in 2-D nodal analysis to overcome the inherent limitations of the conventional two-step method. A new 2x2 APEC leakage correction that improves fuel assembly (FA) homogenized group constants (HGCs) with a 2x2 FA subdivision is proposed as an improvement on the previous APEC method correcting the HGCs with a 1x1 FA subdivision. Both burnup-independent and -dependent APEC functions are devised by performing various color-set depletion calculations using a transport lattice code. The APEC method is implemented to an in-house macroscopic depletion nodal code based on the nodal expansion method (NEM). For macroscopic depletion, Xe-135 and Sm-149 transient models are proposed based on simplified Xe-135 and Sm-149 decay chains. In addition to the lattice burnup, power-density is newly introduced as an additional independent parameter in the lattice calculations to consider a history effect in HGCs. Both UOX-loaded and partially MOX-loaded SMR benchmarks are analyzed to assess the performance of the new APEC method. Furthermore, the general applicability of the APEC correction scheme is also evaluated using random variants of the two reference cores, including large-sized PWR problems. Results show that the new burnup-dependent APEC leakage correction can substantially improve the nodal equivalence in terms of the effective multiplication factor and fuel assembly-wise power distribution.