Synthesis of the VERCORS experimental programme: Separate-effect experiments on Fission Product release, in support of the PHEBUS-FP programme

Abstract

The VERCORS programme is an experimental programme composed of separate-effect tests specifically focusing on determining the Source Term (i.e. quantifying the nature and the release rate of fission products (FP) and actinides) from an irradiated fuel rod sample, under conditions representative of those encountered during a severe Light Water Reactor (LWR) accident. These tests, financed jointly by the French Institute for Radiological Protection and Nuclear Safety (IRSN) and the French electric utility EDF, were carried out in a specific high-activity cell at the CEA Grenoble Centre (French Alternative Energies and Atomic Energy Commission), and were commonly considered to be complementary to the PHEBUS-FP integral programme. They are comparable with certain tests carried out abroad, such as HI/VI in the USA, VEGA in Japan and the programme conducted at the CRL in Canada. Following on from the HEVA programme, VERCORS was conducted in a similar manner over a 14-year period between 1989 and 2002, using an experimental set-up which continued to evolve over time. A total of 17 tests were performed in accordance with three main phases. A first series of six tests (VERCORS-1 to -6) was conducted between 1989 and 1994 on UO2 fuel samples up to 2600K. Next, two different test series – VERCORS HT (three tests) and RT (eight tests) – were alternately performed between 1996 and 2002 at higher temperature, up to the fuel sample melting. These tests focused on UO2 and MOX fuels with a variety of initial configurations (intact or debris beds). The instrumentation become progressively more sophisticated, thus making it possible to extend the knowledge database to include the transport of FP (HT loop), or on the contrary focus on the release objective by associating the study of actinides (RT loop).With the exception of two tests (in debris bed configuration), the fuel sample was composed of a Pressurised Water Reactor (PWR) section containing three pellets in their original cladding. This sample was obtained from a father rod irradiated for several years in one of EDF’s nuclear power plants. In most of the tests, this sample was re-irradiated for several days in a Material Testing Reactor (MTR) of the CEA (SILOE or OSIRIS) so as to rebuild the inventory of FPs with short half-lives.This programme made it possible to quantify precisely fission product releases in all the situations explored, as well as to identify similar behavioural patterns amongst some of these fission products, thus making it possible to classify them schematically into four groups with decreasing volatility:•Volatile FP including fission gases, iodine, caesium, antimony, tellurium, cadmium, rubidium and silver with very high releases (practically total release) at temperatures of around 2600K.•Semi-volatile FP, a category composed of molybdenum, rhodium, barium, palladium and technetium with releases of 50–100%, but very sensitive to oxygen potential and with marked deposits in the vicinity of their emission point.•FP that are low volatile, such as ruthenium, cerium, strontium, yttrium, europium, niobium and lanthanum, with significant releases of around 3–10% on average, but capable (for some elements and under particular conditions) of reaching 20–40%.•Non-volatile FP composed of zirconium, neodymium and praseodymium, for which no release can be measured by gamma spectrometry for the envelope conditions of the VERCORS test grids.Actinides each have their own type of behaviour. They can nevertheless be subdivided into two categories, the first including U and Np, with releases of up to 10% and behaviour similar to that of the low volatile FP, and the second (Pu) with very low releases, typically less than 1%.