METHODOLOGY FOR DETECTING FAILED FUEL ASSEMBLY IN A LIQUID METAL REACTOR

Abstract

When the fuel assemblies in a liquid metal reactor are failed, the safety and operability of that reactor will be threatened. The early detection of the failed fuel assembly during a normal operation is an important issue for a liquid metal reactor. The gaseous isotopes of fission fragments and the gaseous precursors of delayed neutron are confined in the fuel assemblies and are not released into primary coolant when the integrity of the fuel assembly is maintained. If a fuel assembly is failed, they are released into the primary coolant through the failed location. So, the existence of gaseous isotopes and delayed neutrons in a primary coolant can provide some information on the fuel assembly failure. These gaseous isotopes in the primary coolant are diffused into the cover gas region between the hot pool and the reactor head and the precursors of the delayed neutrons are dissolved in the primary coolant and circulated the reactor along the coolant. For detecting the isotopes in the cover gas region, we adopted the HPGe (High purity Germanium) gamma-ray detector on the reactor head and it was found to have a sufficient resolution to identify the gamma spectrum of the specific gaseous isotopes of fission fragments like xenon and krypton. The fission chambers for detecting the delayed neutron in the primary coolant were adopted and installed in the intermediate heat exchanger. The fission chambers are not sensitive to background noises such as the high energy gamma ray and alpha particles in a reactor and can be manufactured with a small size. After measuring the gaseous isotopes of fission fragments in cover gas and the delayed neutrons in primary coolant, we can identify a failure of the fuel assembly in a liquid metal reactor by evaluating whether those levels are beyond the predefined thresholds which will be determined from the detailed plant design data.