2007 Chūetsu offshore earthquake in the context of Iizuna, Nagano

A nuclear and radiation accident is defined by the International Atomic Energy Agency (IAEA) as "an event that has led to significant consequences to people, the environment or the facility." Examples include lethal effects to individuals, large radioactivity release to the environment, or a reactor core melt. The prime example of a "major nuclear accident" is one in which a reactor core is damaged and significant amounts of radioactive isotopes are released, such as in the Chernobyl disaster in 1986 and Fukushima nuclear accident in 2011.

The impact of nuclear accidents has been a topic of debate since the first nuclear reactors were constructed in 1954 and has been a key factor in public concern about nuclear facilities. Technical measures to reduce the risk of accidents or to minimize the amount of radioactivity released to the environment have been adopted; however, human error remains, and "there have been many accidents with varying impacts as well near misses and incidents". As of 2014, there have been more than 100 serious nuclear accidents and incidents from the use of nuclear power. Fifty-seven accidents or severe incidents have occurred since the Chernobyl disaster, and about 60% of all nuclear-related accidents/severe incidents have occurred in the USA. Serious nuclear power plant accidents include the Fukushima nuclear accident (2011), the Chernobyl disaster (1986), the Three Mile Island accident (1979), and the SL-1 accident (1961). Nuclear power accidents can involve loss of life and large monetary costs for remediation work.

The Kashiwazaki-Kariwa Nuclear Power Plant (柏崎刈羽原子力発電所, Kashiwazaki-Kariwa genshiryoku-hatsudensho; Kashiwazaki-Kariwa NPP) is a large, modern (housing the world's first advanced boiling water reactor or ABWR) nuclear power plant on a 4.2-square-kilometer (1,000-acre) site. The campus spans the towns of Kashiwazaki and Kariwa in Niigata Prefecture, Japan, on the coast of the Sea of Japan, where it gets cooling water. The plant is owned and operated by Tokyo Electric Power Company (TEPCO), and it is the largest nuclear generating station in the world by net electrical power rating.

On 16 July 2007, the Chūetsu offshore earthquake took place, with its epicenter located only 19 km (12 mi) from the plant. The earthquake registered M_w 6.6, ranking it among the strongest earthquakes to occur in the immediate range of a nuclear power plant. This shook the plant beyond design basis and initiated an extended shutdown for inspection, which indicated that greater earthquake-proofing was needed before the operation could be resumed. The plant was completely shut down for 21 months following the earthquake. Unit 7 was restarted after seismic upgrades on 19 May 2009, followed later by units 1, 5, and 6. (Units 2, 3, and 4 were not restarted by the time of the March 2011 earthquake.)