Ukraine's operating nuclear power reactors could melt down and spew radiation if they shut down and lose grid power with which to keep the fuel cool.

War could force them to shut down even if no army intends to do so. But Russian attacks towards the plant could kill the staff or drive them away. If Russian troops seize te plants, and try to force the staff to work as prisoners, there is no telling what might happen.

It is ominous that Russian troops have seized Chernobyl and imprisoned the staff that work there. The wrecked reactors are not a weapon, and provide no service to Ukraine that Putin might threaten to shut off. It would have been easy and safe to ignore the place and leave the staff alone.

I can only imagine that he plans to use the wrecked reactors to do something vicious, such as poison Ukraine with radioactive waste. But that doesn't make sense either; I can't see that it would gain him anything.