Rupture of LIPOSO in Superphenix

After a review of the French safety approach for the fast breeder reactors and its application to the design basis of Superphenix, we describe the scenario of a primary pipe rupture from a phenomenological viewpoint. The first physical points involved in this accident are a fast partial loss of the core flow despite the natural increase of the pumps flow, and the dissymmetry in the pressure core inlet, in the grid plate, leading to local fuel channels overheating. In the pessimistic reference assumption of a practically instantaneous double-ended guillotine break, the potential consequences, due to a quick overheating of the sodium, are: - The risk to reach a local sodium boiling in the core, starting from stable boiling conditions, but with possible propagation towards unstable conditions due to either, directly the flow reduction, or indirectly to a void reactivity effect; - The risk of a cladding rupture with possible impact on the neighbouring fuel pins by gas release. This gas release could be an initiator for other cladding ruptures and associated gas releases and then introducing some phenomena for sodium boiling propagation and/or reactivity insertion. Extensive studies of the LIPOSO accident, including experimental programmes, were performed on the understanding of the various phenomena involved in a pessimistic scenario, on the prevention and protection means and on the calculation models including all the uncertainties. The final result was that the Superphenix safety provisions were able, thanks to the reactor and the core design, to cope with this type of fast loss of core flow accident, despite pessimistic assumptions, which was initially considered as a main initiator of a core disruptive accident. (author)