Performance of Flux Mapping System During Spatial Xenon Induced Oscillations in PHWRs

Pressurized heavy water reactors (PHWRs) are natural uranium fueled pressure tube type reactors which use heavy water as coolant and moderator. Large PHWRs are neutronically loosely coupled and are prone to spatial xenon instabilities. In core instrumentation for monitoring and liquid zone compartments (LZCs) are provided for suppressing these local oscillations in addition to global power control. Since, the zonal instrumentation measures the local flux, they have to be corrected. Flux mapping system (FMS) is present in large PHWRs to correct the zonal detector readings. This system uses flux synthesis method and reconstructs the flux shape in the reactor with the help of several vanadium self-powered neutron detectors (SPNDs). FMS will also provide power trimming functions based on different parameters and these will be very important during the operation at full power for PHWRs which have boiling at the exit. In this paper, the comparison of the estimations of the flux mapping system is done with those obtained from the solution of the space-time dependent neutron diffusion equation using improved quasi-static approximation (IQS) is carried out for situations with limited spatial power control which demonstrates the accuracy of the FMS estimations when the regional powers are allowed to oscillate. Based on these simulations, experiments involving spatial xenon oscillations will be planned in the upcoming 700 MWe PHWR to verify the accuracy of FMS during such oscillations.