Analysis of Reliability and Life of Fission Ionization Chamber of Nuclear Instrumentation System (NIS)

Abstract Fission ionization chamber of Nuclear Instrumentation System (NIS) is an essential equipment for the nuclear power plant. By measuring the neutron fluence rate in the immediate range (IR), the power level of the reactor can be determined. In order to ensure its performance to get accurate measurement results, the reliability and aging degree should be analyzed so that we can replace the fission ionization chamber before it expires. In this paper, the aging mechanism of NIS fission ionization chamber is analyzed and calculated from four aspects: moderator degradation, irradiation damage, sensitive uranium plating loss and detector working gas loss. The calculation model is the fission ionization chamber developed by our institute. As the results indicated, moderator degradation and irradiation damage of neutrons cause tiny effect of detector performance. The uranium plating loss caused by neutron reaction leads to 2% detector sensitivity decrease when irradiation time reaches 45.8 years. The working gas loss leads to less than 2% detector sensitivity decrease in 146.4 years if the gas leakage is highly controlled. Furthermore, the mean time between failure (MTBF) is analyzed and calculated. This research provides detailed analysis and methods to determine the reliability and life of NIS fission ionization chamber. And the results indicated that the detector developed by our institute has high reliability to apply in the cruel environment outside the reactor.