Tokaimura nuclear accident happened in 1997.

In 1999, there was a nuclear accident at the Tokaimura uranium processing facility in Japan, which exposed dozens of workers to high levels of radiation. This accident resulted from a criticality accident during the processing of nuclear fuel.

The term "Broken Arrow" is used to indicate a nuclear accident involving a nuclear weapon that does't pose the threat indicated. The term "excursion" (as in nuclear excursion) or "criticality accident" is applied to a nuclear reactor or nuclear material accident, respectively.

The Chernobyl nuclear accident.

There are many ways a nuclear accident may occur, however most nuclear power plant accident occur due to problem in an reactor.