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

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. These specificities are illustrated by the gamma spectrometry equipment which has been implemented for the experimental VERDON program. The VERDON program is a component of the International Source Term Program (ISTP), a co-operative research program, for which the French part is being co-funded by the CEA, EDF and IRSN. This program is a continuation of the previous VERCORS program carried out by the CEA between 1989 and 2002. It will be performed in a new laboratory which is under construction at the CEA Cadarache Nuclear Research Centre. The VERDON tests will be performed in a shielded hot cell. A typical fuel sample will be composed of a short PWR section containing 2 pellets in their original cladding, extracted from a father rod irradiated for several years in an EDF nuclear power plant. Before the experiment, this sample will be re-irradiated in a CEA material testing reactor (MTR) for a few days at low power in order to re-build the inventory of FPs with short half-lives. A typical test will consist in heating this sample in a high frequency furnace, generally up to fuel melting, in conditions representative of a severe accident: under a flow mixture of steam and hydrogen, or steam and air. During the accidental sequence, the FP release kinetics will be measured by several on-line gamma spectrometers, in particular one pointed directly at the fuel sample. After the test, gamma spectrometry will be performed on the fuel sample to quantify the total released fraction of each FP (by means of a comparison with a similar gamma spectrometry measurement performed prior to the test), as well as on all the loop components, to locate and quantify the FP depositions along the circuit lines, and draw up the FP balances. This paper provides details on: The on-line gamma spectrometry equipment used to examine fuel samples and designed to measure FP release kinetics during the test. The gamma scanning bench devoted to calculating FP balances after the test. This paper will describe the main functions of this bench which has been designed to perform axial gamma scanning on various samples, as well as transversal scanning on the fuel rod to obtain images of the FPs in the fuel section by gamma emission tomography. The capabilities of all this equipment are illustrated by results from the previous VERCORS program.