A statement on incidents at nuclear installations in Britain which meet Ministerial reporting criteria is reported to the Secretary of State for Trade and Industry and the Secretary of State for Scotland and is published every quarter by the Health and Safety Executive.
For the period 1 April 2005 to 30 June 2005 there was 1 incident at a nuclear licensed installation that met the reporting criteria.
On the 20 th April 2005 British Nuclear Group Sellafield Ltd, (BNGSL), the licensee, discovered a leak which had occurred within the Thorp Head End Feed Clarification Cell. The leak was from pipe-work to a vessel designated an Accountancy Tank that held dissolved fuel in nitric acid solution while accountancy calculations were made. The leak was estimated at 83 cubic metres. HSE/NII was informed of the leak later the same day.
The relevant plant and operations including fuel shearing were promptly shut down by BNGSL. Some down-stream parts of the plant were subsequently restarted, after discussion with NII, in order to drain other tanks in the same cell which had liquor held within them when the plant was shut down.
The leak was into a stainless steel clad, heavily shielded cell designed to contain such leaks, and there has been no indication that any liquid leaked from the cell. Plant radiation monitoring showed that there was neither abnormal radioactivity in air nor radioactivity released to the environment as a result of this event. BNGSL has concluded that no workers or members of the public have been affected.
Camera inspections within the cell showed that other pipe work and vessels within the cell showed no sign of leakage. BNGSL criticality assessors concluded that a criticality was not possible based on the properties of the leaked liquor.
On notification of the event, NII set up an internal review group to consider the adequacy of the actions taken to recover the situation. NII was kept fully informed of recovery plans and monitored the situation closely to ensure that safety remained assured. The cell has built-in equipment for the recovery of any leaked liquor back into primary containment tanks and all the leaked liquid has now been recovered this way. NII is satisfied that the plant remains in a safe state and that there have been no harmful effects to people or the environment. Liaison was maintained throughout with the Environment Agency.
BNGSL evaluated this event as INES category 3, because of the significant release of radioactivity into the secondary containment.
Subsequent to the completion of the make safe operations, an NII team commenced an investigation at the site on 16 th May 2005. The preliminary findings led NII to issue two Improvement Notices on 17 th June 2005 to ensure that BNGSL promptly rectified deficiencies in compliance with nuclear site licence conditions relating to the operation of the THORP Feed Clarification Cell. The Improvement Notices compel BNGSL to improve compliance in the areas of leak detection, operating instructions, record keeping and maintenance and testing of safety related equipment. The NII investigation is continuing, but it is too soon to say whether this will result in further regulatory action.
In the meantime, BNGSL is considering options for the restart of the plant and has liaised with regulators on its initial ideas, but a firm proposal has not yet been submitted.
Single copies of statements are available free from the Health and Safety Executive, Nuclear Directorate, Division 4a, Building 4 NG1, Redgrave Court, Merton Road, Bootle, L20 7HS,