On 26 April 1986 the world's most severe nuclear reactor accident occurred in Chernobyl, Ukraine (at that time part of the Soviet Union), when operators tested the capability of the plant's electrical back-up system while part of the plant was shut down for routine maintenance. Unfortunately, the test was carried out without a proper exchange of information and coordination between the team in charge of the test and the team in charge of the operation and safety of the nuclear reactor. Consequently, inadequate safety precautions were included in the test program, and the operations team was not alerted to the nuclear safety implications and the potential danger of the test. The lack of communication between the testing team and the operations team was compounded by the existence of significant drawbacks in the reactor design. A sudden and uncontrollable power surge was triggered that resulted in violent explosions and almost total destruction of the reactor. The consequences of this event were exacerbated by the burning of the graphite moderator and other material fires that broke out in the building and contributed to a widespread and prolonged release of radioactive materials to the environment.
An area of about 5 million hectares - almost the size of Nova Scotia - was contaminated and 160,000 people had to be permanently evacuated. The health consequences were devastating. Radioactive material affected not only the Ukraine but also neighbouring countries and parts of Western Europe.
The Chernobyl accident demonstrated the need to include the possibility of trans-boundary implications in national emergency plans. The concern that any country could be affected not only by nuclear accidents occurring within its own territory but also by the consequences of accidents happening abroad, stimulated the establishment of national emergency plans in several countries.
The trans-boundary nature of the contamination resulting from the Chernobyl accident prompted international organisations to promote international cooperation and communication, to harmonise actions, and to develop international emergency exercises like those organised by the Nuclear Energy Agency and its program of exercises. The international community reached agreements on early notification in the event of a radiological accident and on assistance in radiological emergencies through international Conventions developed by the International Atomic Energy Agency.