Performance evaluation of AHWR flux mapping system during normal operational scenarios

Abstract

In large nuclear reactors, a number of appropriately distributed in-core neutron detectors are used for monitoring and control of neutron flux distribution in the core. In Advanced Heavy Water Reactor (AHWR) too, a set of 168 self-powered neutron detectors installed vertically at 6 locations in 28 in-core detector housing units has been proposed to be used. This configuration arrived at using the K-means clustering approach, has been found to reconstruct the core neutron flux distribution for a number of steady-state flux shapes. For practical application, in addition to steady-state, the optimized set of in-core detectors should provide reasonably accurate core monitoring for regular reactor operations as well. This paper investigates the accuracy characteristics of flux mapping using the proposed set of 168 in-core self-powered neutron detectors in the reconstruction of time-varying flux shapes encountered during normal reactor operations. Time-dependent flux shapes arising due to perturbations such as the movement of reactivity devices, refuelling and xenon spatial oscillations are considered for this study. Flux reconstruction is performed using the well-known flux synthesis method. From the analysis, it can be observed that the proposed set of in-core detectors reliably reconstructs the flux distribution in the reactor even during the normal operational scenarios. The maximum RMS error in mesh flux using 168 SPNDs and 35 eigenmodes is found to be 4.45% across all the operational scenarios considered.