MEASUREMENTS ON SHORT TIME COOLED PWR FUEL WITH A FORK DETECTOR

The Fork detector has normally been used to measure gamma rays and neutrons from irradiated fuel assemblies to verify the consistency of burn up declarations. An unusual series of measurements was conducted with a Fork detector on PWR fuel in the storage pond of the Angra-1 Nuclear Power Station. This fuel had cooled only 41 days and had burn-ups in the range of 16 to 31 GWd/tU. Despite the intrinsically low sensitivity of fission chambers to gamma rays, a special adjustment and calibration was necessary to reduce the contribution of gamma-ray pile-up to the neutron count rate. This was necessary because of the high gamma flux from short-lived nuclides in the fuel, which could, otherwise, alter the neutron to gamma-ray counting ratio, the basis for the burn-up determination. The measured results, expressed as a plot of the ratio neutron and gamma-ray counting rates versus the gamma-ray counting rate showed very distinct data clusters for each burn-up. This made it possible to confirm the irradiation cycle history of the fuel assemblies.