Catalogue of failures caused Sellafield leak

Poor design, sloppy maintenance and ignored alarms allowed a radioactive leak at the THORP nuclear fuel reprocessing plant at Sellafield to go unnoticed for eight months, the Health and Safety Executive reports.

The HSE’s investigation led to the site’s operator, British Nuclear Group Sellafield, being fined £500,000 in October last year (ENDS Report 382, p 57 ). Its full report, published in February, offers important lessons for the nuclear and other major hazard industries.1More than 80,000 litres of nuclear fuel liquor - 22 tonnes of uranium dissolved in nitric acid - leaked from a broken pipe at the thermal oxide reprocessing plant (THORP) between August 2004 and April 2005.

The HSE eventually granted consent for the plant to reopen in January. BNGS hopes to have it fully operational by the summer.

The leak occurred in a pipe feeding a large suspended steel flask where dissolved fuel is weighed and sampled. Poor container design, exacerbated by changes to the liquor-mixing system, caused it to swing excessively. Metal fatigue caused the pipe to leak before shearing completely in January 2005.

Reports of a banging noise around this time were explained away by staff as creaking pipework. The HSE believes the noise was caused by the pipe and tank hitting each other.

The report says the case is a reminder to major hazard industries of the importance of subjecting design changes to a risk assessment.

The HSE found the leak went unnoticed because of non-compliance with some key operating instructions, a culture that condoned the ignoring of alarms, and safety-related equipment not being kept in effective working order.

A depth indicator on the containment cell’s floor had a history of erratic output, but staff routinely ignored an alarm indicating the problem. The failure was part of a wider problem of inadequate management of the vast number of alarms at the site.

The leak could also have been detected by regular sampling for uranium in the sump at the bottom of the containment cell. But faulty equipment meant the sampling programme was irregular and positive samples were not always followed up.

The report reveals that in 1998 there was a similar, but less serious, leak which highlighted shortcomings in leak detection and monitoring. But the company failed to effectively implement any changes as a result.

The investigation uncovered other failures that contributed to the incident, including inadequate staff training in process management systems. Maintenance of monitoring devices was frequently carried out by non-specialist staff who were not aware of past problems with the equipment.

Inspectors were also concerned by the complacent attitude towards plant safety shown by senior managers who assumed the lack of incidents meant the plant was operating safely.

The HSE has published guidance on process safety indicators aimed at helping managers identify early signs of deteriorating performance (ENDS Report 384, p 18 ).

The loss of fuel was eventually discovered when a stocktake revealed the flasks contained less liquor than expected. Nevertheless, the company delayed shutting THORP down for several days.

THORP’s safety procedures assumed any leaked liquor would be recovered within a few days. The building’s foundations are permeable, so the cell’s stainless steel floor is the only line of defence between the liquor and the ground. However, borehole samples beneath the cell suggest the leak was contained.