A major step forward Filtration of the air in the reactor containment In the event of an accident, should an increase in pressure threaten to damage the containment, the depressurisation system would, as a last resort, enable the gases in the containment to be released after filtration. The filter is capable of retaining some of the radioactivity and thereby mitigating the environmental consequences of the accident. Valve Sand Steel shell Expanded clay Concrete deck Drain Stack Inside the stainless steel shell, the gases pass through different layers, including 80 cm of sand. Nuclear accidents and developments in nuclear safety and radiation protection • 15 Radioactive gases Reactor building Integration of the lessons learned from monitoring the operation of nuclear power plants The detection of precursor events became a major concern of the licensees and nuclear safety organisations. The organisation of operation and operating experience feedback thus developed around this new priority. Modification of certain technical systems Between 1994 and 2008, ASN sought the opinion of IRSN and the Advisory Committee for Nuclear Reactors concerning technical modifications, of which the main ones adopted are listed below ■ enhanced reliability of commanded opening of the pressuriser relief valves on the 900 MWe reactors: the aim of this modification was to limit the risks of reactor vessel melt-through, particularly in the event of core meltdown further to a total loss of the electrical power supplies; ■ installation of passive auto- catalytic recombiners on all the reactors (installation completed in 2007); ■ improvement in the closing system of the equipment hatch (TAM) for the 900 MWe reactors in order to improve the leak-tightness of the TAM, a containment weak point, to obtain a pressure of about 8 bars; ■ installation of hydrogen detection and reactor vessel corium melt-through detection sensors on the 900 MWe reactors in order to have, in the event of a severe accident, information on the how the situation is evolving.
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