Use of Gamma spectrometry for measuring fission product releases during a simulated PWR severe accident: Application to the VERDON experimental program

Abstract

The release of fission products (FP) from a pressurized water reactor (PWR) during a hypothetical severe accident is a major topic in nuclear reactor safety assessment, since they are the main contributors to the source term in the environment. Fission products with short half lives are of particular importance due to their potential high radiological effects. In order to precisely quantify their release rate, simulating experiments are performed on freshly irradiated fuel rods that are heated in a furnace in a dedicated shielded hot cell. Gamma spectrometry is the main tool used to quantify FP releases from such experiments. The implementation of gamma spectrometry measurements requires specific developments which are exposed in this paper: • The various and generally high activity of the samples involves special measures, such as long distance between the emission source and the detector (around one meter or more), and various aperture collimators (thicknesses ranging from less than 1 mm to several centimeters). These dimensions have to be determined by using gamma transport codes, such as MCNP, according to the different objects to measure (size and activity). This particular configuration makes it possible to use a specific methodology to obtain quantitative measurements of the FP located within the samples. • The freshly irradiated fuel leads to complex spectra containing a high number of gamma-ray lines and the presence of much interference (overlapping lines) which must be properly processed. In addition, several FPs are still in filiations, making it necessary to implement different types of corrections depending on the type of “mother” and “daughter” products. • Finally, the need to measure the FP release kinetics makes it necessary to record spectra with a frequency greater than the physical variations, typically faster than one per minute, as well as getting accurate measurements of the surfaces under the useful peaks. As a consequence, it requires choosing very fast electronic acquisition units operating at a high rate (more than 100 kc/s) without counting loss.