IRSN, the French Institute for Nuclear Safety and Radioprotection, attempts to simulate the Fukushima accidents using the ASTEC code. The first analysis carried out concerned the reactor number 2 transient. Results were considered as satisfactory being quite consistent with measures reported by TEPCO and similar computations performed with MELCOR or MAAP. Knowledge gained from PWR practise and different lectures available in the open literature for BWR confirm the trends observed or provide some potential explanations to formulate alternative assumptions. Leakage model from the containment up to the refuelling bay through the head flange seal was very efficient to retrieve pressure evolution inside the dry well. Extension of the model to reactor number 3 gave also results quite coherent with what similar codes computed. However for both reactors some figures characteristic of the transient as hydrogen production, are liable to vary a lot if models for bottom and top nozzles are added which has not been done in reference computation due to initial lack of data. Uncertainties with simulation of accident on reactor number 1 are rather large due to scarcity of data. On the other hand as the measurement points were quasi absent for most of the first 24 hours there is no reference to compare to. Bottom head failure is predicted but due to the high number of penetrations the mechanical failure models developed for PWR may not be so relevant for BWR.
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