The emergency services in Scotland were called to an incident at the Torness nuclear power station south of Edinburgh last night.
The alarm was raised at the plant near Dunbar just before 9pm when staff disposing of spent fuel in the ponds at the plant became aware of a problem.
Officials described the problem as the 'anomalous behaviour of the irradiated substances'.
A spokesman for British Energy, which operates the plant, said officials were monitoring the situation, but there was no major panic.
He said none of the 38 staff had been evacuated and the plant was continuing to generate electricity.