Fission products (FP) release and transport in case of PWR severe accident (SA) is a major topic in reactor safety assessment due to the potential radiological consequences for surrounding populations and the environment. In this context, the Institute for Radiological Protection and Safety (IRSN) and Electricité de France (EDF) have supported the VERCORS analytical test program which was performed by the "Commissariat à l'Energie Atomique" (CEA). It is usually considered as complementary to the PHEBUS FP in-pile integral experimental program.
25 annealing tests were performed between 1983 and 2002 on irradiated PWR fuels under various conditions of temperature and atmospheres (oxidising or reducing conditions).The influence of the nature of the fuel (UO2 versus MOX, burn-up) and the fuel morphology (initially intact or fragmented fuel) have also been investigated. These led to an extended data base allowing on the one hand to study mechanisms which promote FP release in SA conditions, and on the other hand to enhance models implemented in SA codes.
Among all the FP investigated, ruthenium is of specific concern because of its high radiological effects due essentially to the combination of both its short and long half-life isotopes (i.e. 103Ru and106Ru respectively), but also by its ability to generate volatile gaseous oxides (RuO3, RuO4) in very oxidising conditions, in particular in the case of air ingress accidents. Important uncertainties still remain on the release and transport of this element in such situations, and investigations on this open issue are notably carried out in the SARNET European framework.
The present communication gives a general overview of the VERCORS program and presents more deeply the main findings concerning the ruthenium release.
Its global behaviour is analysed on the basis of several comparative tests :
Same UO2 sample (35 and 50 GWd/t) under hydrogen or steam conditions,
Similar MOX sample (40 GWd/t) under hydrogen or mixed (steam and hydrogen) conditions
Similar oxidising conditions for UO2 samples at different burn-up from 35 to 70 GWd/t.
Significant ruthenium release has been evidenced in pure steam conditions from high burn-up fuels. A lack of data still exist under air conditions and will be completed in a future analytical program, the VERDON program, devoted to FP release from high burn-up UO2 and MOX fuels. This new program is part of the International Source Term Program (ISTP) launched by CEA, EDF and IRSN, and open to foreign collaboration.
(1) : CEA
(2) : IRSN
(3) : EDF