Contamination of the carbon dioxide supply system at Hunterston B Power Station, February 1997

The following is an abstract from a priced publication. The publication is available from HSE Books, P.O. Box 1999, Sudbury, Suffolk CO10 6FS, tel: 01787 881165, fax: 01787 313995.

Introduction and event summary

1. On 3 March 1998 Scottish Nuclear Ltd (SNL) reported to HM Nuclear Installations Inspectorate (NII) and the Scottish Environment Protection Agency (SEPA) an event which was first discovered by the operators at Hunterston B Nuclear Power Station on 20 February 1997. The station is located on the Ayrshire coast approximately 30 miles south west of Glasgow and comprises two Advanced Gas-cooled Reactors (AGRs) which became operational in the mid-1970s.
2. The immediate cause of the event was a number of defective valves which allowed an unintentional backflow of carbon dioxide gas. This gas is used as reactor coolant and flowed on that occasion from the reactor's high pressure circuit to the station's storage tanks used for holding liquid carbon dioxide supplies (operational storage tanks). The cause for concern was the potential for transfer of radioactively contaminated carbon dioxide to a road tanker which had made deliveries to the operational storage tanks on 21 February 1997. The road tanker subsequently left the station and connected to the gas supplier's carbon dioxide distribution network, raising the possibility of onward spread of contamination to other carbon dioxide users, which included food and drink manufacturers.
3. Upon notification, NII and SEPA instituted inquiries to ascertain the facts of the event and to determine what further actions were necessary. A joint investigation was initiated on 4 March 1997 involving specialist staff from both regulatory bodies.
4. The investigation confirmed that the immediate cause of the event was that a number of valves between the operational storage tanks and the reactor were defective as they allowed a backflow of carbon dioxide gas from the reactor into these tanks. This, together with the potential consequences, was not apparent to the station management in the initial stages of the event. Consequently, there was a delay in identifying and resolving the plant problem and notifying the regulators. In addition, the investigation identified two previous events when reactor gas had backflowed into the clean part of the carbon dioxide system. However, no evidence was found that the operational storage tanks had themselves been contaminated in the past.
5. Assessments of the maximum potential off-site release, and the results of off-site monitoring of the carbon dioxide distribution and food supply chains, indicated that there was no significant radiological risk to the public as a result of this event.
6. Following the reporting of this event, an embargo on the receipt of carbon dioxide supplies was put in place by SNL and all other licensees operating commercial nuclear reactors until they could confirm that adequate measures were in place to prevent a similar event.
7. In order to secure the wider situation these licensees carried out an extensive review of all fluid systems. These reviews have been completed and were supplied to NII on programme by the end of May 1997. These revealed no major weaknesses but identified a number of areas where improvement could be made to enhance the longer term position. A programme for implementing these enhancements has been produced by the licensees which has been reviewed by NII and is considered to be realistic and acceptable because adequate measures are in place to prevent a recurrence. This programme is continuing to be monitored by NII and is on target.
8. The event caused significant media interest and a statement on the event was made by Mr Michael Forsyth, the then Secretary of State for Scotland in the House of Commons on 5 March 1997 (HANSARD, 5 March 1997 [columns 904-910]). In his statement, Mr Forsyth requested NII and SEPA to provide him with a full report on the event which he intended to publish.

Added to the HSE web site 6/10/98