Sodium In-Pile Loop Experiment on Delayed Neutron Detector-, Precipitator-, and Cover-Gas Gamma-Ray Spectrometer-Type Fuel Failure Detection Systems

Performance of four different types of fuel failure detection systems has been studied using a sodium in-pile loop located in Japan Research Reactor No. 2. In the sodium in-pile loop, 8.5 liter sodium circulates through an irradiation section (500°C), a main cooler (400°C), an expansion tank (380°C), an electromagnetic pump, and a main heater at a flow rate of 3 liters/min. In the irradiation section, four metallic uranium (20% enriched) plates of a total amount of U-235 of 9.18g had been irradiated at a thermal neutron flux of 5.4×1010 n/cm2sec. A delayed neutron detector consisting of a BF3 counter system in a graphite moderator (50cm×5Ocm×8Ocm) detects delayed neutrons from fission products in the expansion tank. Helium cover gas over the sodium in the expansion tank carries gaseous fission products to a precipitator (The Plessey Co. Ltd., Mark XI) by which beta-rays from Rb-88 and Cs-138 collected on the precipitator wire are detected. The precipitator was modified to measure gamma-ray spectra from the wire using a Ge(Li) detector system for the nuclide identification. The helium gas is also guided into a gas reservoir whose gamma-ray spectra are analysed by another Ge(Li) detector system. Construction and performance of these detection systems was described in detail and their detection sensitivities and response were compared.