On Friday night, Russian troops captured the Ukrainian Zaporizhia nuclear power plant.

Before that, the facility was targeted by rocket fire.

The fire in the training building has now been extinguished.

Increased radioactivity around the nuclear power plant was not measured, according to the Ukrainian supervisory authority.

Manfred Lindinger

Editor in the department "Nature and Science".

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Mr. Walther, how alarmed are you?

We are very concerned that there could be more attacks that damage the reactors themselves.

Is there a risk that radioactivity can subsequently escape?

This is not to be feared after the current fire, as it did not affect the nuclear parts of the plant.

What would have happened if the rockets had hit one of the six reactors?

A large inventory of radioactive materials is present in a running reactor.

The reactor pressure vessel contains water at a pressure of about 130 bar and a temperature of more than 300 degrees.

If a rocket were to destroy all safety barriers here, there would be a risk of an immediate release of significant amounts of radioactivity.

The scale could be comparable to that of the Chernobyl disaster, even if the accident sequence and reactor type were different.

How are the reactors in Zaporizhia protected, for example from missiles?

There has never been such an incident in history.

Therefore, we do not know whether the reactors at Zaporizhia would have withstood bombardment or rocket fire.

Most likely not.

Unlike those in Chernobyl, the reactors in Zaporizhia are water-cooled.

Could there be a meltdown, for example if the cooling circuit is damaged and fails?

The fuel rods have to be cooled. If cooling by water is no longer guaranteed, core meltdown can still occur days later.

The effects can be different.

This has led to overheating, a hydrogen chemical explosion, a core meltdown and ultimately the release of large amounts of radioactivity in Fukushima.

There was also a meltdown at Three Miles Island (Harrisburg) due to a lack of cooling.

The safety precautions worked there and practically no radioactivity escaped.

Is it already clear whether the reactors in Zaporizhia have been shut down?

These are pressurized water reactors of Russian design that went into operation between 1986 and 1996.

One reactor is still running at 600 megawatts.

Five of the six reactors were shut down or are running in hold mode or cool down.

The latter means that no more electricity is fed into the grid, but the fuel still produces heat.