Office for Nuclear Regulation

Second quarter 2009 statement of nuclear incidents at nuclear installations

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 2009 to 30 June 2009 there were two incidents at nuclear licensed installations that are confirmed as meeting the reporting criteria.

Sellafield Limited

On 1 April 2009, cooling water flow was lost to the Highly Active Liquor Evaporation and Storage (HALES) plant. A quantity of this water was discharged to ground because of a pre-existing rupture to an underground pipeline. No one was injured as a result of the incident and the cooling water was not radioactive.

HSE launched two separate but linked investigations to consider the reasons why cooling water flow was lost and the pipeline was ruptured. We have concluded that the immediate cause of the former aspect was the inadvertent opening of a valve during the plant reinstatement following the completion of engineering work, and of the latter that a borehole had been drilled through the underground pipeline in March 2009.

Sellafield Limited responded to the incident in an appropriate manner. The valve in question was closed and cooling water was restored promptly.
The incident was significant to nuclear safety because of the reliance that the HALES plant places on cooling water supply. The plant stores a significant quantity of highly active liquor (HAL) which can self heat to boiling temperature. Redundant and diverse cooling water supplies are consequently engineered to prevent boiling; ensuring the temperature of the HAL is kept within acceptable limits to prevent an off site release of radioactivity.
HSE is continuing its investigations to fully understand the underlying reasons for the incident and is considering appropriate enforcement options.

Dungeness B

Following a failed attempt to connect new fuel at Dungeness B Nuclear Power Station, a new fuel assembly was left suspended ten foot off the ground within the new fuel transfer route. As part of the recovery, building foam was injected in the tube below the fuel assembly to prevent the assembly dropping. Subsequent analysis showed that the foam could act as a moderator and may have challenged the margin to criticality. The foam escaped from the tube and landed on the floor of the fuel cell. A mechanical restraint was applied to prevent the fuel assembly from moving, and work subsequently done to safety delatch and recover the assembly.

HSE has carried out a preliminary investigation and the investigating inspectors are preparing to continue with a formal investigation.

Single copies of statements are available free from the Health and Safety Executive, Nuclear Directorate, Division 4, Building 4 N2, Redgrave Court, Merton Road, Bootle, L20 7HS,