Burst Cartridge Detection in British Gas-Cooled Reactors

Abstract

A burst cartridge remaining in a reactor may eventually release such quantities of radioactive fission products into the coolant circuit as to cause a potential health hazard or maintenance difficulty. It is important, therefore, to be able to detect and locate a burst so that the reactor operator can discharge the cartridge concerned. Of all the possible methods, only detection by the released fission products is satisfactory. Gaseous fission products diffuse through the leak in the can wall against a coolant inflow induced by oxidation of the fuel, and their presence in the coolant has to be detected against an unavoidable background count. The leak size at the threshold of detection can be estimated from the operating conditions, and the manner in which b.c.d. (burst cartridge detection) signals arise from a developing burst merits special study. To discriminate against the radioactive decay of the coolant gas itself the precipitation system of monitoring is used. From each fuel channel in the reactor a sample of gas is drawn periodically. This is passed through a precipitation chamber where particular fission product nuclides decay to give ionized daughter products which are collected on a charged wire. The wire is moved to a shielded scintillation counter and the fission product nuclides, which undergo a second radioactive transition, are measured to the exclusion of the active nuclides in the coolant which do not. The performance and sensitivity of the system can be calculated and optimized by considering the expression for the count from the wire. The mechanical design of the equipment varies in detail from station to station. In later designs the mechanical equipment is simpler and easier to maintain, and greater use is being made of automatic processing and presentation of the signal.