A preliminary analysis of the unit 1 accident at the Fukushima Daiichi NPP by the RELAP/SCDAPSIM code

In the framework of the activities devoted to the development of the ENEA-Casaccia 'NPP Engineering Simulator', a RELAP5/SCDAPSIM model of the unit 1 of the Fukushima Daiichi NPP was developed and applied for a severe accident analysis. In this paper the preliminary results are reported. In order to correctly describe the main phenomena, a detailed nodalization of a BWR-3 primary system and of Mark I containment were developed and coupled. BWR-3 NPP public available data of a similar unit were used for setting up the RELAP/SCDAPSIM model and for performing its steady state and transient validation. Main events reconstruction of the Fukushima scenario was based on the official Japanese data. The first 24 hours of the accident were simulated, beginning with the reactor scram as a consequence of the earthquake, and reproducing the behavior of the main engineered safety features (safety/relief valves, isolation condensers). Results showed that the core uncovering and degradation began at +2 hours after the tsunami wave hit the plant. Core melting was predicted having occurred in the subsequent 6 hours, with a fuel relocation at the bottom of reactor pressure vessel. RELAP/SCDAPSIM special models calculated the severe damage of the reactor boundary allowing to estimate the time of the consequent containment over-pressurization that resulted well beyond the design limits. Calculations ended at the time of the actuation of the containment venting procedures since they were immediately followed by a major hydrogen explosion in the reactor building. Sensitivities analyses were performed to test the different RELAP/SCDAPSIM models of core degradation and a bounding range for the main parameters involved during the core degradation was obtained. This work constitutes also an example of the present capabilities and one of the steps being performed at the ENEA-Casaccia Research Center for the reintroduction of a 'Enhanced NPP Engineering Simulator'. (orig.)