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 January 2005 to 31 March 2005 there was 1 incident at a nuclear licensed installation that met the reporting criteria.
On 13 February 2005, three staff were contaminated whilst changing a thermocouple on Thorp Head End Dissolver vessel C. The risk assessment for the work did not anticipate a breach of containment, as the design intent was for the thermocouple separated from the dissolved nuclear fuel by being located inside a sealed tube called a thermowell. This intent was not achieved. After initial incident, whilst seeking reassurance monitoring, the workers spread contamination to various plant areas and were then taken to site surgery for decontamination. The workers were contaminated on the face and hands and decontamination of the hands continued over several days. BNGSL have advised NII that no statutory dose limits were exceeded, although one worker received an extremity dose greater than 10% of the annual statutory limit.
On the day of the event, the Thorp Incident Control Centre was set up and plant access restricted until decontamination was complete. Investigations indicated that there was a slight leak in the Dissolver C thermowell, which appears to have allowed contamination in-seepage. Following completion of the BNGSL investigation, a second barrier was engineered around the thermowell pocket and the dissolver was returned to normal operations.
BNGSLs analysis of the amount of contamination on the thermocouple and the smaller amount contaminating the people and spread on plant, indicated a spillage which exceeded Ministerial reporting criteria by virtue of the Ionising Radiations Regulations 1999, Schedule 8, column 4. BNGSL advised NII that they were reporting this event under these criteria.
In response to the event BNGSL set up a Board of Inquiry, which reported promptly, and NII is satisfied with the Boards conclusions. The licensees (or BNGSLs - without the; Thorp is a building) response to the recommendations will be monitored by NII the coming months.
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,