Spectral history model in DYN3D: Verification against coupled Monte-Carlo thermal-hydraulic code BGCore

•Pu-239 based spectral history method was tested on 3D BWR single assembly case.•Burnup of a BWR fuel assembly was performed with the nodal code DYN3D.•Reference solution was obtained by coupled Monte-Carlo thermal-hydraulic code BGCore.•The proposed method accurately reproduces moderator density history effect for BWR test case. This research focuses on the verification of a recently developed methodology accounting for spectral history effects in 3D full core nodal simulations. The traditional deterministic core simulation procedure includes two stages: (1) generation of homogenized macroscopic cross section sets and (2) application of these sets to obtain a full 3D core solution with nodal codes. The standard approach adopts the branch methodology in which the branches represent all expected combinations of operational conditions as a function of burnup (main branch). The main branch is produced for constant, usually averaged, operating conditions (e.g. coolant density). As a result, the spectral history effects that associated with coolant density variation are not taken into account properly. Number of methods to solve this problem (such as micro-depletion and spectral indexes) were developed and implemented in modern nodal codes. Recently, we proposed a new and robust method to account for history effects. The methodology was implemented in DYN3D and involves modification of the few-group cross section sets. The method utilizes the local Pu-239 concentration as an indicator of spectral history. The method was verified for PWR and VVER applications. However, the spectrum variation in BWR core is more pronounced due to the stronger coolant density change. The purpose of the current work is investigating the applicability of the method to BWR analysis. The proposed methodology was verified against recently developed BGCore system, which couples Monte Carlo neutron transport with depletion and thermal-hydraulic solvers and thus capable of providing a reference solution for 3D simulations. The results clearly show that neglecting the spectral history effects leads to a very large deviation (e.g. 1700pcm in multiplication factor) from the reference solution. Application of the Pu-correction method results in a very good agreement between DYN3D and BGCore on the order of 200pcm in kinf.