The Wylfa power station on Anglesey could be out of action for up to month for checks on both its reactors.

Mystery marks were found in one reactor shut down for its annual overhaul.

The second reactor has now been closed as well as a precaution.

British Nuclear Fuels says there is no threat to safety.

The marks were found by a remote television camera.

There are four channels in each reactor core through which a television camera can be lowered during a reactor's annual overhaul.

Television camera

This enables a television camera to be lowered right down through the graphite core and out the other side in order to inspect the bottom of the core.

During the overhaul of reactor two, unexpected marks were seen in these channels.

A decision has been taken to shut reactor one while further assessment is undertaken.

The two reactors were shut down down earlier in the year after an element in the system became lose.