the electrical switchboards, so these were also lost. Further to the loss of the diesel generator sets and the batteries, the operators no longer had any reliable information on the emergency cooling systems. As time went by these systems stopped functioning: with no means of cooling available, core meltdown was inevitable. The meltdown of the reactor cores caused explosions due to the hydrogen concentration in the reactor buildings. In effect, owing to the lack of cooling, the water in the reactor vessel turned into steam and the fuel cladding temperature rose to more than 1,200°C. The zirconium making up the fuel cladding then oxidised and this reaction produced hydrogen. On contact with air, the hydrogen under pressure caused violent explosions. The intentional depressurisation operations undertaken by licensee to limit the pressure in the reactor containments led to the first releases of radioactive substances into the environment. The hydrogen explosions contributed to the release of massive quantities of gaseous radioactive effluents. Then, having lost containment integrity, the contaminated water present in the reactor buildings led to the release of large volumes of liquid radioactive effluents. Managing the effluents and radioactive liquid releases became a major site management challenge. 12 March 2011, explosion in the reactor 1 building The building housing this reactor collapsed following a hydrogen explosion. 14 March 2011, explosion in the reactor 3 building The roof of the reactor 3 building was blown off by a hydrogen explosion. “There can be no grounds for complacency about nuclear safety in any country. Some of the factors which contributed to the Fukushima Daiichi accident are not specific to Japan. A permanently critical approach and the ability to learn from experience is the foundation of the safety culture and essential for anyone working in the nuclear energy sector. Safety must always come first.” These words from Yukiya Amano, Director General of the IAEA from 2009 to 2019, illustrate what inspired ASN’s reflection and decisions to improve the safety of nuclear facilities in France. 15 March 2011, explosion in the reactor 2 building, then in the reactor 4 building The explosion was once again caused by the hydrogen which had built up in the reactor 2 building. For reactor 4, the roof of the spent fuel pool was blown off, probably due to a hydrogen explosion from reactor 3. Overheating of the spent fuel pools Alongside this, the pools of reactors 1 to 4, in which the spent fuel was stored, were no longer cooled due to the failed electrical power supply. As the spent fuel continued to give off heat, the temperature of the water of the reactor 3 and 4 pools continued to rise to boiling point, causing the water level to drop. Despite the explosions and the loss of cooling, the pools and the spent fuel did not suffer any significant damage. It was possible to supply the pools with makeup water. Fukushima: establishing the final outcome of the nuclear accident is complex Although no health consequences linked to radioactivity have been observed directly, the toll for the tsunami totals 18,000 deaths and more than 2,000 missing persons. UNSCEAR issues regular reports on the psychological, environmental and financial impacts of the industrial accident. 12, 14 and 15 March 2011 Successive explosions occurred in reactor buildings 1, 3, 2 and 4, caused essentially by the build-up of hydrogen in the reactors. Nuclear accidents and developments in nuclear safety and radiation protection • 27
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