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The Three Mile Island, Chernobyl, Le Blayais and Fukushima accidents: Causes and lessons learned

Three Mile Island
On March 28, 1979, at the Three Mile Island nuclear power plant in Pennsylvania (USA), a failure of the main feedwater pumps of unit 2 (TMI-2), combined with an erroneous signal indicating that an open relief valve was closed, caused the reactor to overheat. Part of the core melted down, but the reactor building’s containment vessel was not breached and no radioactivity was released into the environment.

Chernobyl
On April 26, 1986, during a safety improvement experiment at the Chernobyl power plant in the Ukraine (then part of the USSR), a series of six critical human errors caused the chain reaction in one of the site’s four nuclear reactors to spiral out of control. The temperature of the reactor core rose, generating enough heat to melt the fuel rods, whose uranium oxide pellets exploded upon contact with the surrounding water. This was followed by the explosion of hydrogen formed by the steam generated by the superheated water. With no containment vessel to protect it, the reactor core was then exposed to the open air and the graphite used to slow the neutrons caught fire. Large amounts of radioactivity were released into the environment in the following 10 days until the fire was finally contained.

Le Blayais
On December 28, 1999, after a storm that caused the Gironde estuary in western France to burst its banks, the basement levels of the Blayais power plant, in particular units 1 and 2, were flooded, incapacitating some of the safety systems. However, the emergency equipment took over and disaster was avoided.

Fukushima
On March 11, 2011, following a Richter scale 9 earthquake in the Pacific Ocean off the northeastern coast of Japan, the reactors at the Fukushima nuclear power plant were shut down and the cooling system was activated. But two hours later, a 15-meter tidal wave overwhelmed sea defenses, which had been built to withstand waves of “only” 5 meters, flooding everything in its path and destroying the plant’s cooling circuits. The temperature of the reactor cores then rose, causing the fuel cladding to melt. Hydrogen formed and exploded, destroying reactor buildings and thus releasing a large amount of radioactivity into the air and sea.
Since the risk of a nuclear plant breakdown is theoretically very slight, each major accident serves as a basis for future improvements.

The Three Mile Island failure had positive repercussions in terms of reactor safety, giving rise to a new safety-oriented culture and leading to improvements at nuclear sites across the world.
The Chernobyl catastrophe provided a real-life lesson for the management of serious nuclear accidents, including communication and public information. While it was quickly blamed on an ageing Soviet regime incapable of ensuring the maintenance of its power plants, it dramatically demonstrated the potential cross-border impact of a nuclear accident. These consequences, which can directly or indirectly affect many different countries — including the most remote — motivated the introduction of the International Nuclear Event Scale (INES ), which, like the Richter scale for earthquakes, is used to assess the severity of nuclear accidents.
The Fukushima disaster, which could have been prevented if, for example, seawalls had been higher, has refocused attention on the need to ensure the safety of nuclear installations even under the most extreme and unlikely conditions. In France, the flood risk at French nuclear sites was re-evaluated and works were initiated to raise the seawalls, following the Blayais plant incident in 1999. In the wake of Fukushima, the French government asked the ASN, its Nuclear Safety Authority, to carry out complementary safety assessments (ECS).