Preliminary ex-vessel sensitivity study of Fukushima Dai-Ichi unit 1 using DSA1 and CVPA coupling modules of the Sampson Code

The phenomena taking place during a severe accident are complicated and difficult to predict using integral codes, especially during the ex-vessel phase of the accident, where the experimental database is still poor (particularly with respect to the scale), and the models have more modeling parameters, are less general and not well validated. Therefore a stand-alone analysis using as boundary and initial conditions the available data of the accident seems to be a more accurate approach than an integral calculation, even if for this procedure the quality of information is essential. In the case of Fukushima Dai-ichi unit 1 (1F1) accident, available plant data provided by TEPCO during the first 24 hours of the accident are few and fragmented not sufficient to describe the initial conditions for simulating a transient. In order to overcome this issue, the only way to reduce the uncertainties on accident conditions after the Reactor Pressure Vessel (RPV) failure is to adopt different assumptions for main unknown parameters and compare the calculated results with available measurements to explain observations or to validate assumptions. In this paper, a sensitivity study on the ex-vessel accident progression in the 1F1 based on the best available information at this time were performed. The work has been carried-out coupling the Debris Spreading Analysis (DSA1) and the Containment Vessel Phenomena Analysis (CVPA) modules of SAMPSON code reproducing 10 different cases. The results obtained has provided a basis for making judgments concerning a preliminary evaluation of the debris distribution inside the PCV and concrete mass eroded following the Molten Core Concrete Interaction (MCCI). This information is vitally important in the decommissioning process and severe accident management.