Part of Hinkley Point B nuclear power station was closed for several hours after radioactive dust leaked into the atmosphere.
Faulty filters were found during a routine inspection
The Environment Agency ordered the closure after discovering ventilation filters designed to remove low-level radioactive dust from a workshop were faulty.
Inspectors who visited the plant, near Bridgwater in Somerset, found the filters were not working and told the station's owners to close the area until the problem was fixed.
But the agency says no-one's health was put at risk by the leak, which came to light on 21 May.
The plant was closed for a matter of hours on Thursday 3 July, officials told BBC News Online.
"The amount of radioactivity going to the filters was low and the amount escaping was very low," said Granville Roberts, a spokesman for the Environment Agency.
"It poses no danger to members of the public or workers at the plant."
The Environment Agency served an enforcement notice requiring the area where the filters are used to be closed.
And the power station's owner, British Energy, has been told to review its filter systems.
The company says the fault has been rectified and that it will check all other filters and submit a report to the Environment Agency.