Level 7: major accident

1986: Chernobyl, Ukraine. The primary cooling system of unit 4 of the Lenin nuclear plant (110 kilometers from Kiev) failed as a result of design flaws compounded by a series of human errors. This caused an explosion, destroying the reactor building, followed by a meltdown. Both of these events released large quantities of radioisotopes into the atmosphere.

Level 6: serious accident

1957: Maiak nuclear complex, Kyshtym, Soviet Union. Maiak nuclear complex, Kyshtym, Soviet Union. This accident resulted in a very large off-site release of radioactive material. At least 200 people died. The Soviet regime kept the accident secret. Information was revealed only in 1976 by Soviet biologist Jaures Medvedev when he emigrated to the United Kingdom.

Level 5: accident with wider consequences

1979: Three Mile Island nuclear plant, United States. The core of one of the plant’s reactors melted after the feed water pumps for the secondary cooling system failed. The event was made possible by a series of mechanical failures, human errors, design flaws and the lack of procedures. There were no victims and the environmental impacts were very low.

Level 4: accident with local consequences

1999: Tokai Mura, Japan. The accident occurred 120 kilometers from Tokyo on September 30, 1999. A criticality¹ occurred when an abnormally large quantity of uranium, far in excess of safe levels, was introduced into the decanting tank.

Level 3: serious incident

2008: Toulouse, France. For several minutes, a worker at Onera’s Toulouse site was accidentally exposed to radiation emitted by a highly radioactive cobalt-60 source for several minutes.

Level 2: incident

2009: Melox, France. On March 3, 2009, two grams of material in excess of the 370 grams authorized (i.e. less than 1%) were used in a laboratory station during a research-related operation on a plutonium and uranium oxide sample. Corrective actions were taken as soon as the deviation was observed. The French nuclear safety authority (ASN) classified it as a Level 2 event due to the failure of alarm software.

Level 1: anomaly

2008: Tricastin, France A leak in a storage tank at the Socatri plant resulted in the release of a solution containing natural uranium (74 kilograms in all).

1. A criticality accident is a nuclear accident caused by an involuntary and uncontrolled chain reaction in fissile material such as uranium or plutonium.