Operations staff did not act appropriately to off-normal conditions at Sellafield’s Thorp plant, according to a board of enquiry report released by British Nuclear Group (BNG).

The BNFL board of enquiry launched an investigation into the leak which Thorp operators BNG passed on to Sellafield’s owners, the Nuclear Decommissioning Authority. BNG took the step of releasing it partly due to the number of requests made for it under the Freedom of Information Act.

The exceptionally frank report lists the following management systems as having shortcomings which contributed to the event: safety case; procedures and systems of work; hazard reporting systems; training; supervision; design; maintenance; and plant modification. The report states that a ‘new plant’ culture existed and Thorp was simply not expected to leak.

The report states that 83.4m3 of dissolver liquor leaked from a pipe attached to an accountancy vessel. The pipe failed through fatigue cracking at the nozzle into the vessel because of relative movement of the feed pipe and vessel, which has been known to move significantly during agitation and emptying cycles.

Accountancy vessels operate suspended from the ceiling so that their contents can be weighed. The original design allowed vertical movement, with lateral motion prevented by a restraining system. The report says that when the safety case for the basis of design was produced, the vessels were seismically uncoupled from their steel frame. To accommodate this change, it is clear from visual evidence that restraint blocks were not fitted and that some steelwork had been modified to provide additional space between the vessels and the steel frame.

It appears that the design intent was modified at some point after the original design, but before early commissioning. No evidence was found that the design was reviewed to consider fatigue following the modification. The board of enquiry found that it was “inconceivable” that, had fatigue calculations been performed it would have been thought acceptable to operate the vessels without lateral restraint.

There is evidence to suggest a staged failure of the pipe by crack development, propagation and ultimately a more significant failure. It is thought this process began as early as July 2004, with a “step change in leak rate” occurring in January 2005.

The first signs of a problem came around August 2005 when the shipper receiver difference (SRD, the difference between materials known to have entered the plant system and those accounted for at the end) for a certain campaign was above normal operating margins (0.59% instead of ±0.45%). The report says that in hindsight this could have been an indication of a problem.

The first clear indication of a problem was an SRD of 3% noted in March 2005. Although it was confirmed by an independent check, the SRD was thought to be an error due to the complexity of the campaign, and work continued.

On 13 April, an SRD of 3.9% was recorded, followed the next day by an SRD of 9%. A few days later, on 18 April it was calculated that the dissolver liquid had been lost from the primary circuit over three campaigns and the plant was shutdown.