Sellafield bosses face leak charges.
Safety regulators have warned there could be criminal charges over the acid discharge, which forced production to stop at Sellafield, in Cumbria.
Work was halted at the site's thermal oxide reprocessing plant (Thorp) in April after the discovery of discharge from a pipe.
Sellafield's managing director, Barry Snelson, has confirmed the plant may remain closed for months.
He described the incident as 'a stumble, not a fall An investigation by the British Nuclear Group found the pipe may have begun to fail as early as last August.
And it found that opportunities were missed between January 2005 and April 19 that would have shown material was leaking.
The pipe fractured and discharged nitric acid on to the floor of a concrete-lined cell in the Thorp complex.
A secondary containment cell ensured there was no release of radioactivity to the environment.
The leak could not have been prevented, but the amount of liquid released could have been reduced, the regulators found.
No staff were contaminated
ACID DISCHARGE: Sellafield