The final tests of the fission-product release experiments VERCORS RT and HT were performed in 2002. These tests represent the culmination of fission-product release experiments that started in 1983 with the HEVA programme , continued with the VERCORS programme before the HT then the RT series started in the mid-1990s. HEVA and the three VERCORS series represent a total of 28 tests. The results of all these experiments (carried out by CEA with the support of IRSN and EDF) and their interpretation (by IRSN) have made a considerable contribution to the overall knowledge of fission-product release in severe-accident conditions. This can be combined with the efforts of a number of other significant experimental campaigns that have been performed, notably at ORNL, at AECL/CRL, and by JAERI. Despite this extensive knowledge base, there remain gaps and little-explored areas where IRSN intends to pursue its acquisition of the information needed for it to fulfil its objectives in relation to accident evaluation, management and mitigation.
The motivation for further experiments can be summarized as arising from the following aims:
- fill in the remaining gaps for existing fuels (e.g., TU2-type MOX) and accident conditions (e.g., air ingress);
- reduction of uncertainties and overly-conservative assumptions with respect to level 2 PSAs;
- improve the understanding of fission-product release processes by confirming or improving conclusions reached in experiment interpretation (most notably with the MFPR code);
- based on the existing, detailed-modelling capability, produce a tool capable of predictive applications in order to anticipate the consequences of evolutions in fuel design and fuel-cycle management.
This will not only improve confidence in results of detailed analyses but also in analysis of accident sequences. This will be achieved through substantiation or improvement of assumptions used in the simplified release modelling of the ASTEC code. It is concluded that, as a priority, a limited number of experiments are required in the following specific areas:
- micro-characterization of fuel (SEM, EPMA, etc.) before and after annealing in inert, oxidizing and reducing atmospheres;
- FP release (rate and amount) experiments on MOX fuel, especially TU2-type, in a variety of reducing/oxidizing conditions;
- FP release (rate and amount) experiments in air ingress conditions for UO2 and MOX fuels;
- FP release (primarily the amount) from high burn-up UO2 and MOX fuels in design-basis LOCA conditions, this not being an immediate concern but necessary in the mid-term.
IRSN is pursuing the goal of obtaining the majority of results from such experiments through its so-called “Source Term Programme” (which includes fission-product release and covers several other issues). This programme is being established as a partnership already involving IRSN, CEA and EDF.