Behaviour of fission products under severe PWR accident conditions VERCORS experimental programme—Part 1: General description of the programme

VERCORS is an analytical experimental programme focusing on the release of fission products (FP) and actinides from an irradiated fuel rod, under conditions representative of those encountered during a severe PWR accident. The 17 tests – financed jointly by EDF and IRSN – were conducted by the CEA on its Grenoble site in a specific high-activity cell at the Laboratory for Active Materials (LAMA) over a 14-year period (1989–2002), in accordance with three test phases. A first series of six tests (VERCORS 1–VERCORS 6) was conducted between 1989 and 1994 on UO2 fuel close to the relocation. 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 collapse. These tests focused on UO2 and MOX fuels with a variety of initial configurations (intact or debris beds). This programme made it possible to precisely quantify fission product releases in all the situations explored, as well as to identify similar behavioural patterns between some of these fission products, thus making it possible to classify them schematically into four groups with decreasing volatility: (1) volatile FP including fission gases, iodine, caesium, antimony, tellurium, cadmium, rubidium and silver with very high releases (practically total release) at temperatures of around 2350 °C; (2) 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 redeposits nearby the emission point; (3) 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%; (4) 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%.