Simulation and Analysis of the Space Charge Effect in a Self-Powered Neutron Detector

Self-powered neutron detector (SPND) plays a vital role in the Gen-III pressurized water reactors (PWR). When electrons are trapped in the insulator of SPND, an electric field can be generated in the insulator that affects the response current. It is called the 'space charge effect.' To analyze this effect quantitatively, a novel method based on the electric current continuity equation was proposed to simulate the variation of the electric field in the insulator: 1) the time-dependent response current and the recursive equation for the electric field were derived; 2) an SPND simulation code system was developed and validated. In the code system, the Bamboo-Lattice code and the Nuclear Engineering Computational Physics (NECP)-MCX code were used to calculate the neutron-photon spectrum near the SPND, while the neutron-photon-electron transport calculation was performed in Geant4; 3) the calculated sensitivities were verified by measurements on various SPNDs, among which good agreements were found; 4) the electric field and electric potential distribution of the V-SPND in the AP1000 core were simulated with the formation process and the equilibrium duration illustrated. The components of the response current were compared with the measurements. The error of the response current ranges from -6.48% to 6.2%, and the difference of the prompt current ratio ranges from 1.6% to 3.4%; and 5) the influence of neutron flux, electrical conductivity, permittivity, and time step on the electric field was analyzed, revealing that both neutron flux and electrical conductivity significantly influence the response current and electric field, whereas the influence of permittivity and time step is negligible.