Quantitative analysis of signal response components in self-powered neutron detectors under in-core mixed neutron-gamma irradiation fields

This is the authors' abstract. We don't add key points for this paper.

Self-powered neutron detectors (SPNDs) enable continuous neutron flux monitoring, which is essential for ensuring the safe operation and accident prevention in nuclear power plants. However, accurately predicting their signal responses is highly complex, not only because of their inherent physical properties but also due to the harsh in-core mixed neutron-gamma irradiation. The strong gamma-ray field in reactors induces interfering currents that degrade the linear relationship between the detector signal and the reactor power. Distinguishing the effective signal response and noise components is therefore crucial for online monitoring systems, as it is critical to the accuracy of reactor neutron flux measurements. In this study, we propose a computational model based on a multi-step Monte Carlo method to quantitatively determine the signal components of SPNDs. Using this model, we numerically simulated representative SPNDs under various reactor neutron-gamma environments and experimentally validated its effectiveness and accuracy prior to its application in the HPR1000 reactor. Our results demonstrate that for inherently delayed-response SPNDs (e.g., those using rhodium as the neutron-sensitive material), the neutron-induced delayed signal constitutes over 90% of the total response in a thermal reactor, while the prompt signal accounts for approximately 9%. In contrast, in a fast reactor environment, the signal is almost all induced by gamma radiation. The model establishes the fundamental principles for in-core neutron measurements in the HPR1000, concurrently provides a versatile perspective for analyzing signal composition and predicting the performance of diverse SPNDs across different core environments.