Numerical Power Tilts in Full-Core Monte Carlo CANDU Model Exacerbated by Equilibrium Xenon

Abstract

The availability of multiple Monte Carlo (MC) codes and computing hardware advances has made full-core MC simulation a tractable problem. To shift research emphasis from predicting keff to the local flux distribution, there are practical challenges when applying MC codes and models to full-core problems, such as determining key simulation parameters like inactive cycles, active cycles, number of histories (N), and initial source distribution. This paper investigates the power tilt phenomena in a 1/8 symmetry CANDU-6 full-core model developed in the Monte Carlo code MCS. Also, Central Limit Theorem (CLT) study is performed to understand the variation in bundle powers due to power tilts, with 100 low N cases initialized with different random seeds and the results show that the xenon-free 1/8 CANDU model simulation is well-converged using simulation parameters of 120 inactive cycles, 500 active cycles, and 10 million N. However, when simulated with equilibrium xenon core, there is strong autocorrelation of the CLT bundle power errors with systematically overestimated low power bundles and underestimated high power bundles. This result is indicating a power tilt contaminating the reference solution. This study highlights that the xenon effects on the spatial power distribution might be a contributing factor and more investigation is needed including testing larger N.