Bibliographic review of the computer codes and experiments dealing with steam explosion in Europe

Abstract

Since the Three Miles Island accident in 1979, steam explosion has been considered as a potential risk for PWR nuclear power plants. Among the hypothetical severe accidents induced by a core melting, steam explosion has been intensely studied because it might lead to potentially catastrophic environmental consequences caused by an early and massive containment failure. Assuming that the loss of coolant caused a degradation of the reactor core and its melting, the principal accident sequences can be summarized as follows:- The corium relocates from the core area into the lower plenum and premix coarsely with the water.- A trigger causes a fine scale fragmentation of the corium drops which increases strongly the corium thermal exchange surface and allows the explosive vaporization of the water due to high energy transfers between the corium and the water. During a very short initial period, the pressure rises quickly in the premixing zone and expands through the lower head. The resulting dynamic forces may reach a magnitude sufficient enough to fail the lower head vessel.- At the same time, the interaction zone expands upwards what can cause the acceleration of all the materials above the explosion zone. This slug may fail part of the reactor upper head, thus creating missiles which may be hurled against the containment roof and damage it. In Europe, several research centers have been working on steam explosion for several years. The whole work has been organized so as to share out the study set necessary to fully understand the matter between the different research centers and countries and to pool the final knowledge. The European research program is organized around three main themes: the premixing phase, the steam explosion and the slug impact. For each issue, test-facilities and numerical computations have been realised. This paper comprises a review of the test-facilities and codes available at the present time in Europe concerning steam explosion.On the one hand, the research program is based on the understanding of the local phenomena governing the explosion by means of several complementary test-facilities such as KROTOS, ECO, MICRONIS and TREPAM. On the other hand, it is based on the development of numerical computer codes able to predict globally the pressure pulse propagation like IDEMO, CULDESAC and CASTEM-PLEXUS or codes specialized in the local description of the steam explosion as MC3D and COMETA