(Adds quote from energy minister)
KYIV, Nov 24 (Reuters) - Russia risked causing a "nuclear and radioactive catastrophe" by launching attacks in which all Ukraine's nuclear power plants were disconnected from the power grid for the first time in 40 years, Ukraine's nuclear energy chief said on Thursday.
Ukrainian officials said on Wednesday that three nuclear power plants on territory held by Ukrainian forces had been switched off after the latest wave of Russian missile strikes on Ukrainian energy facilities.
Petro Kotin, head of nuclear power company Energoatom, said the vast nuclear power station in the Zaporizhzhia region of southern Ukraine, which has been out of commission since September, had also been disconnected from the grid on Wednesday and became reliant on backup diesel generators.
He added that the Zaporizhzhia plant, which has been occupied by Russian forces since soon after Russia invaded Ukraine nine months ago, had been reconnected to the grid by Thursday morning and that the backup generators were turned off.
"There is a real danger of a nuclear and radiation catastrophe being caused by firing on the entire territory of Ukraine with Russian cruise and ballistic missiles, and a huge risk of damage to nuclear plants," he said in a written statement.
"Russia must answer for this shameful crime," he said.
Each side has blamed the other for shelling of the Zaporizhzhia plant complex.
Energy Minister German Galushchenko said the Rivne, Pivdennoukrainsk and Khmelnytskyi nuclear power stations were expected to back on line by Thursday evening after units there were switched off on Wednesday because of the Russian attacks.
Ukrainian officials have warned repeatedly of a new nuclear disaster in the country that suffered the world's worst nuclear accident in 1986 at the Chornobyl nuclear power station, which is no longer in operation. (Reporting by Tom Balmforth, Dan Peleschuk and Pavel Polityuk, Editing by Timothy Heritage)