Both reactors at the Torness nuclear power station have been shut down after huge numbers of jellyfish were found in the sea water entering the plant.
The jellyfish were found obstructing cooling water filters on Tuesday.
The East Lothian plant's operator, EDF Energy, said the shutdown was a precautionary measure and there was never any danger to the public.
A clean-up operation is under way, but it is understood it could be next week before Torness is operational again.
Torness has two Advanced Gas Cooled Reactors but also relies on supplies of sea water to ensure it operates safely.
It has filters which are designed to prevent seaweed and marine animals entering the cooling system.

If these screens become clogged, the reactors are shut down to comply with safety procedures.