According to Wikipedia:

"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 effect to individuals, large radioactivity release to the environment, 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 disaster in 2011.

The situation regarding Russian control of Ukrainian nuclear reactors, particularly the Zaporizhzhia Nuclear Power Plant (ZNPP), has been a focal point of concern since the onset of the Russia-Ukraine conflict. The ZNPP, Europe's largest nuclear plant with six reactors, has been under Russian occupation since the early stages of the invasion in February 2022. The plant came under armed attack in March 2022, marking the first instance of an operating civil nuclear power plant being attacked during an active conflict. This attack led to a projectile hitting a training building and resulted in a fire, but essential equipment and the reactors remained unaffected​​.

Since the conflict began, the operation of the reactors has varied, with some being taken offline or put online as needed, depending on the stability of external power supplies and conditions around the plants​​.

There are six reactors at Zaporizhzhia maintained by Energoatom personnel, according to Kotin, but Russia controls the plant. Those reactors are currently offline. Meanwhile, Ukraine is operating nine reactors generating electricity at three other locations around the country.

In June 2023, Russia and Ukraine accused each other of plotting to stage an attack on ZNPP.