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The nuclear power industry has improved reactor safety and performance, and has proposed new and safer reactor designs. However, perfect security cannot be guaranteed. Potential sources of problems include human errors and external events that have a greater impact than anticipated.
The designers of the Fukushima reactors in Japan had not foreseen that a tsunami generated by an earthquake would disable the auxiliary systems that were supposed to stabilize the reactor after the earthquake. Scenarios including terrorist attacks, domestic sabotage, and cyberattacks are also conceivable.
The security of nuclear weapons, like the security of military research into nuclear materials, is generally managed by bodies other than those coordinating civilian security, for various reasons, including secrecy. There are fears for the possibility of terrorist groups taking possession of nuclear material suitable for making bombs.
Internationally, the International Atomic Energy Agency works with its Member States and numerous partners around the world to promote nuclear technologies that are safe in all senses and peaceful. It is feared that a combination of human error and mechanical failure in a nuclear facility could result in significant damage to people and the environment: "Operating nuclear reactors contain large quantities of radioactive fission products which, if released, can present a direct radiation hazard, contaminate soil and vegetation, and be ingested by humans and animals.
Human exposure at high enough levels can cause both short-term illness and death, and long-term death from cancer and other diseases." The Chernobyl disaster was a nuclear accident that occurred on April 26, 1986 at a nuclear power plant in Ukraine.
An explosion accompanied by fire released large amounts of radioactive contamination into the atmosphere, which spread throughout much of western USSR and Europe. It is considered the worst nuclear power plant accident in history, and is one of two events rated as a level 7 on the INES scale.
The battle to contain the contamination and avert an even greater catastrophe ultimately involved more than 500,000 workers and cost an estimated 18 billion rubles, crippling the Soviet economy. The accident raised safety concerns for the nuclear industry, slowing its expansion for several years.
UNSCEAR has carried out 20 years of detailed scientific and epidemiological research on the effects of the accident. Apart from 57 deaths directly caused by the accident itself, UNSCEAR projected in 2005 that up to 4,000 additional accident-related cancer deaths would appear among the 600,000 most significant radiation-exposed people.
Russia, Ukraine, and Belarus have been burdened by the continuing substantial costs of disaster decontamination and medical care. A 2012 article in The Economist said: "The reactors at Fukushima were of an old design. The risks they faced had not been well analysed.
The operating company had inadequate regulations and did not know what was going on. The operators made mistakes. Safety inspectorate representatives quit. Some of the facilities did not work. The organization repeatedly underestimated the risks and withheld information on the movement of the radioactive plume, so some people were evacuated from less contaminated towards others who were more so." The reactor designers at the Fukushima I Nuclear Power Plant did not anticipate that a tsunami generated by an earthquake would inhibit the backup systems designed to stabilize the reactor after the earthquake.
Nuclear reactors are systems so inherently complex, tightly interconnected that, in rare, emergency situations, cascading interactions will develop in ways that human managers cannot predict and master.
