Woosong Kim

This paper suggests novel approaches to generate exact discontinuity factors (DFs) for transverse-integrated nodal analyses with a two-by-two node configuration in a homogenized fuel assembly (FA). We have shown that the conventional DF calculation scheme cannot generate exact DFs with the nodal expansion method (NEM) calculation when a FA is subdivided into two-by-two nodes due to the inherent discrepancy between the transport-based heterogeneous analysis and the diffusion-based NEM calculation. In order to overcome the difference, an iterative, two-node NEM sweeping method is proposed. In addition, three different formulations to define a single representative DF per assembly surface for the two-by-two NEM are suggested for convenient application for the existing nodal codes while maintaining enough accuracy. Numerical assessments with a colorset model and small modular reactor cores with 16 × by × 16 FAs show that the iterative two-node NEM sweep method successfully corrects the error caused by an inherent discrepancy between the transport and the diffusion method. Among the candidates of a single representative DF, the net current weighted average DFs are found to be the most adequate.

The Albedo-corrected Parameterized Equivalence Constants (APEC) method, a new leakage correction method for two-group nodal analysis of light water reactors, has been extended to discontinuity factor (DF) correction. First, the error of nodal calculations induced by an inaccurate assembly discontinuity factor (ADF) is evaluated using the reference two-group cross section (XS) and DF calculated from heterogeneous core transport calculations. Functionalization of DF is performed by finding relationships between surfacewise current-to-flux ratio and change of DF from ADF. The least-squares method is used to fit several candidate functions to various core calculation results. The coefficients of APEC XS and DF correction functions are determined considering several color-set models. In this work, the two-dimensional method of characteristics–based lattice code DeCART2D is used for reference core calculations and lattice calculations. The extended APEC method is implemented in an in-house NEM nodal code using the partial-current coarse mesh finite difference acceleration. A small modular reactor (SMR) initial core benchmark is analyzed to evaluate the performance of the extended APEC method. In addition, the extended APEC method is applied to several variants of the SMR core and large variants to assess its general applicability.