The problem occurred when clean water was accidentally fed into the cell in the currently mothballed cementation plant which processes intermediate-level waste from the site's decommissioning.
The Nuclear Installations Inspectorate – part of the Health and Safety Inspectorate – is unhappy not to have been told about the incident by the UKAEA. The first it knew about the incident was in an anonymous tip-off a week or so after it happened.
An unplanned £4 million-plus clean-up was mounted after the accidental spill of a quantity of highly-active waste in the cell in September 2005. The leak of about 400 litres of the water into the sump of the cell was spotted on August 20 last year.