Studies performed in IPSN and elsewhere pointed out that high burnup may induce specific effects under LOCA conditions, especially those related with fuel relocation. Uncertainties exist regarding how much these effects might affect the late evolution of the accident transient and the associated safety issues. IPSN estimates that a better knowledge of specific phenomena is required in order to resolve the pending uncertainties related to LOCA criteria.
The main issues for reactor safety in case of severe accident sequences are the containment integrity and the characteristics of the Source Term. Since the TMI2 accident, in March 79, the progress of research has made it possible to estimate the potential consequences of a rather large spectrum of severe accidents sequences. Nevertheless, there are still issues about containment integrity and Source Term which remain unaddressed and which impact on the reliability of the present estimates. Regarding the containment, large uncertainties subsist about the influence of MOX or high burn-up fuel on the amount and properties of the corium which can be involved in various phenomena, e.g. interaction with water. Considering hydrogen production during core quenching, there is practically no information available for severe accidents representative conditions. Regarding Source Term, the uncertainties are related to the release rates from MOX and high burn-up fuel and the associated large amount of fission gases. Furthermore, no information is available about the additional new releases induced by temperature escalation, the possible re-entrainment of previously deposited materials in and above the core by large steam/hydrogen flow.
This analysis leads IPSN to promote a new programme named STLOC for Source Term and LOCA programme, to be conducted in the Phébus facility after completion of the present Phébus-FP programme. An overview of this new project is given hereafter through two different papers which provide the main points of the IRSN analysis.