Experience of Self-powered Neutron Detectors at TAPS-3&4

Abstract

Self-powered neutron detectors (SPNDs) are mainly used in-core of a medium- or large-sized pressurized heavy water reactor (PHWR) to determine the neutron flux at a specific location in the core. These flux readings can be used to produce a 3D flux map which also helps to justify or validate the 3D diffusion theory estimation. However, the measurement of neutron flux is not an end in itself but serves to provide a measure of the local fuel power, and hence, the heat fluxes through the fuel sheath and other components of fuel bundle/channel. A knowledge of the power being transferred to the coolant is required to optimize the fuel power to avoid the conditions of central line melting and/or critical heat flux. The signals from the safety or protection system detectors are normalized to the power in the fuel, integrated over the fuel channel. Thus, the detector signal represents the channel (fuel) power. Similarly, the detectors used in the control system represent the zone power. It is also to be noted that not only the overall signal of SPND is important but also the dynamic response of the detector signal is important as it should match the heat production rate in the fuel rather than the local neutron or gamma flux. It is due to the fact that about 93% fuel power is prompt and 7% is delayed (mainly coming from fission product decay). Therefore, if the maximum benefit is to be obtained from using flux detectors in power reactors, a thorough understanding of the factors that affect the detector’s response such as sensitivity, burnout, prompt and delayed fraction of detector’s signal is required.