Réacteur 4 CONSEQUENCES The explosion destroyed a large part of reactor 4 and of the turbine hall and intermediate constructions. 26 April 1986, between 1:03 and 1:07 am Two additional recirculation pumps were put into service. The additional flow caused the temperature in the heat exchangers to rise. At 01:19, to stabilise the water inflow to the moisture separators, the power of the pumps was further increased and exceeded the authorised limit. The system requested an emergency shutdown, but the signals were blocked and the operators ignored the request. 26 April at 1:23:04’’ The test began: the turbine steam supply valves were in closed position. The recirculation pumps slowed down and the flow rate decreased. The core temperature rose, causing – due to the design of the reactor – an increase in the reactivity. The reactor power increased uncontrollably. 26 April at 1:23:40’’ The chief operator ordered emergency shutdown. All the control bars started to descend into the core, but produced the opposite effect to that expected. The power again increased uncontrollably. 26 April at 1:23:44’’ The power peak was reached, exceeding more than 100 times the nominal power of the reactor. The high pressures in the pressure tubes caused them to rupture. An explosion raised the upper plate of the reactor, weighing 2,000 tonnes. The upper part of the reactor core was exposed to the open air. The graphite caught fire, and several fires broke out in the facility. It took the firemen three hours to put out these fires. The graphite fire restarted. It was not definitively extinguished until May 9. From 27 April to 10 May 1986 5,000 tonnes of materials (sand, boron, clay, lead, etc.) were transported by helicopter and released onto the reactor with the aim of covering it to reduce the air flow feeding the graphite fire and the release of radioactive emissions. “Chernobyl confirmed that a major nuclear accident could occur with consequences affecting several countries: in this case Ukraine, Russia and Belarus, as well as a large part of Europe. This led to the widespread realisation that it was necessary to have an international approach to nuclear safety issues.” Pierre-Franck Chevet ASN Chairman from 2012 to 2018 A radioactive cloud was released into the atmosphere. Driven by the winds, it crossed part of Europe (see box on page 20). Nuclear accidents and developments in nuclear safety and radiation protection • 17
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