British Energy has applied for permission to reopen Hinkley B nuclear power station in Somerset, after six months of repair work.
Hinkley Point B is due to be decommissioned in 2011
The station was closed in September last year for repairs to cracked pipes in the boiler system.
The Stop Hinkley Campaign Group says it is not safe and British Energy will be taking a huge chance by operating it.
The power station, which costs about £1m a day to run, is due to be decommissioned in 2011.
Campaign group spokesperson Jim Duffy said: "We're already passed the effective life of this nuclear reactor and what's happening is that Hinkley are acting on a string and a prayer."
He said the fact that it took three months longer than expected to complete the repairs meant it was not safe.
However, Nigel Cann director of Hinkley B section, said if the units were not in a position to start up safely, then they would not.
"Our responsibility to the safety of the public and our staff members is always our number one priority," he said.
The Nuclear Installations Inspectorate is expected to make its decision by the end of the month.