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.
Russian military forces seized Chernobyl during the first day of the Ukrainian invasion as well as Zaporizhzhia, the largest nuclear plant of its kind in Europe, during the seventh day.
Fighting near nuclear power plants could possibly mean an increased risk of a serious radiation incident.