Validation Status of the ASTEC Integral Code for Severe Accident Simulation

The French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) and the German Gesellschaft für Anlagen und Reaktorsicherheit mbH (GRS) have been jointly developing for several years a system of calculation codes (or "integral" code), ASTEC (Accident Source Term Evaluation Code), to simulate the complete scenario of a hypothetical severe accident in a nuclear light water reactor from the initiating event through the possible radiological release of fission products out of the containment, the so-called "source term." Very intensive validation work has been performed in recent years by IRSN and GRS on the V1 versions by comparison of code calculations with results of more than 160 international experiments. Complementary validation was performed by 30 partners of the SARNET European Network of Excellence in the 6th Framework Programme of the European Commission, where ASTEC is considered the European reference code. The global status of validation is good for most phenomena, as shown by several examples that are described in this paper, and even very good on fission product behavior. The main need for modeling improvement concerns reflooding of a degraded core, due to the lack in ASTEC V1 of any dedicated model, and intensive efforts will focus on this topic in the next years. Molten core concrete interaction models are at the state of the art, but new experiments under way in the international frame and a better understanding of physical mechanisms are necessary to make further progress. Version V2.0 of the new ASTEC series, released mid-2009, takes benefit of the previous very intensive validation of the ICARE2 IRSN mechanistic code since its core degradation models have now been implemented. Validation will continue in the SARNET network from 2009 to 2013.