The Effect of Neutron Source Distribution on Subcriticality Measurement of Pressurized Water Reactors Using the Modified Neutron Source Multiplication Method

Abstract

A modified neutron source multiplication method has been investigated for application to subcriticality measurement in commercial Pressurized Water Reactors (PWRs). This method is based on the extraction of the fundamental mode from measured neutron signals and corrections for spatial effects. In commercial reactors, mainly two types of neutron sources are expected, one from burnt fuels and the other from in-core fixed neutron sources. To calculate the extraction and spatial correction factors, the fixed source problem was solved by the Finite Difference Method (FDM) to explicitly deal with spatial intensities of neutron sources. Sensitivity analysis was carried out to investigate how the accuracy of estimated subcriticality depends on approximation levels of neutron source distributions. It was found that both of the neutron sources should be taken into account. For a pin rod type of the in-core fixed neutron source, a node-wise distribution for which the localized source intensity is averaged over the radial direction in the relevant nodes is applicable, since it can estimate the subcriticality with almost the same accuracy as in the case with a rigid spatial treatment. This ensures that nodal calculations can be used in the evaluation of correction factors instead of burdensome FDM calculations.