The worldwide nuclear power industry operates within a culture of continuous improvement. Whenever accidents have occurred at nuclear facilities, increased regulation and cooperation have followed, both nationally and internationally.
Windscale – UK, 1957
Windscale was an experimental facility designed to produce plutonium for nuclear weapons. In 1957, one of the facility's reactors overheated and caught fire. The International Atomic Energy Agency (IAEA) uses a seven-point rating system, the International Nuclear Event Scale (INES), to classify the significance of incidents and accidents involving nuclear material. The Windscale fire was classified at level 5, or 'accident with wider consequences'.
Windscale's air-cooled graphite core was not designed to generate electricity and would not meet today's stringent safety standards. Two years after the Windscale accident, the Government established regulation of the UK nuclear industry through the Nuclear Installations Act. This is currently the responsibility of the Office for Nuclear Regulation (ONR).
Three Mile Island – US, 1979
The core of one of the Three Mile Island nuclear power station's two pressurised water reactors (PWRs) melted in March 1979. Poorly designed instrumentation and inadequate emergency response training meant that staff failed to respond properly to a minor cooling malfunction. The situation escalated and the core overheated. The reactor core was severely damaged despite shutting down automatically, as it was designed to do. A small amount of radioactive gas leaked into the environment, but no one was killed or injured. The accident was classified at INES level 5, or 'accident with wider consequences'.
Cleaning up the mess took nearly 12 years and cost about US$973 million. After this accident, the US Nuclear Regulatory Commission (NRC) greatly expanded its inspection regimes and international knowledge-sharing activities. The US nuclear industry also formed the Institute of Nuclear Power Operations (INPO) to promote the highest possible levels of safety in commercial nuclear power stations.
Chernobyl – USSR, 1986
The Chernobyl 4 nuclear reactor, part of the Chernobyl Power Complex, was destroyed in 1986 during a poorly managed safety test. The core overheated, causing several fires and two explosions. The explosions released large amounts of radioactive material into the atmosphere. Radioactive substances continued to escape into the air for the next 10 days. Two workers were killed in the initial explosions. In 2005 fewer than 50 deaths had been directly attributed to radiation exposure from the accident, but it has been estimated that the number of premature deaths could eventually reach 4,000 as the long-term effects make themselves felt.
The event was rated at the highest INES level: 7, or 'major accident'. Chernobyl 4 was encased in concrete soon after the accident. A more permanent enclosure is scheduled to be completed in 2014, to better contain about 200 tonnes of highly radioactive waste which remains inside the facility. Analysis of the disaster said it was caused partly by the reactor technology and partly by the lack of a safety culture amongst staff. The World Association of Nuclear Operators (WANO) was formed in 1989, linking 130 operators of nuclear power stations in more than 30 countries. Aid amounting to almost US$1 billion has funded more than 700 safety-related projects in the former Soviet Union.
Since 1989, more than 1,000 nuclear engineers from the former Soviet Union have visited western nuclear power plants and more than 50 twinning arrangements between eastern and western nuclear power stations have been put in place to share expertise. The design of the Chernobyl 4 reactor was never used outside the Soviet Union. Its shortcomings and safety flaws are now well understood and it would not be used today. Other reactors built to the same design have been modified to address these issues.
Fukushima – Japan, 2011
A magnitude 9.0 earthquake and a 14 metre-high tsunami devastated the north-east coast of Japan, including the Fukushima Dai-Ichi nuclear power station, in March 2011. The station's nuclear reactors shut down automatically in response to the earthquake, but the tsunami knocked out power to its cooling systems. Overheating in the reactors and spent fuel storage ponds resulted in several hydrogen explosions. Radioactive material was released into the atmosphere. For a time, both the reactors and the ponds had to be cooled using seawater. Thousands of tonnes of used cooling water, contaminated with radioactivity, were later released back into the sea.
The event was provisionally rated at INES level 5, or 'accident with wider consequences'. It was later amended to the highest INES level: 7, or 'major accident'. No one has died as an immediate result of exposure to radiation from Fukushima Dai-Ichi, though the long-term consequences of the accident cannot yet be fully evaluated.
The accident is being carefully analysed by the international nuclear authorities. Any and all lessons that can be learned from the events will be disseminated to nuclear operators and safety bodies worldwide. All countries that have signed up to the Convention on Nuclear Safety have initiated reviews of the measures that defend their nuclear power stations against extreme events. The major nuclear regulators have agreed to share the results of these reviews.
The UK Government commissioned Dr Mike Weightman, the UK's chief nuclear inspector and head of the Office for Nuclear Regulation (ONR), to review implications for the country's nuclear energy industry in light of the situation at Fukushima Dai-Ichi. In October 2011 the Government published Dr Weightman's final report, in which he concluded that while the Fukushima accident highlights the importance of observing and strengthening existing safety procedures, it should not alter the UK's nuclear strategy.