Les cahiers de l'ASN #03 - 10 years after Fukushima

The successive explosions in these buildings are due to the consequences of core melt. Owing to the lack of cooling, the water in the vessel turned into steam and the temperature rose to more than 1200°C. The zirconium* making up the fuel cladding then oxidised and this reaction produced hydrogen. On contact with air, this pressurised hydrogen created violent explosions. As a result of the accident, the Fukushima Daiichi NPP is to be decommissioned; The duration of this decommissioning process was initially evaluated at 40 years but, owing to the difficulties encountered, this time-frame has been extended. Réacteur n o 4 Réacteur n o 3 Réacteur n o 2 Réacteur n o 1 The Fukushima Daiichi NPP comprised 6 boiling water reactors (BWR). The fluid which passes through the core is demineralised water which, when brought to boiling point on contact with the fuel rods, turns into steam and drives the generators to produce electricity. At the time of the accident, only reactors 1, 2 and 3 were in operation (the others were shut down for maintenance). The epicentre of the earthquake was situated 130 km from the port of Sendai, in the Pacific Ocean, at a depth of 25 km below the sea. 14 march 2011 EXPLOSION IN REACTOR 3 BUILDING The roof of the reactor 3 building was blown off by a hydrogen explosion. 3 15 march 2011 EXPLOSION IN REACTOR 2 BUILDING The explosion was once again caused by the hydrogen which had built up in the reactor building. 15 march 2011 EXPLOSION IN REACTOR 4 BUILDING The roof of the spent fuel pool was blown off, probably owing to an explosion of hydrogen from reactor 3. 15 march 2011 2 4 10 years after Fukushima, what safety improvements for nuclear facilities in France? • 3