Thorp Feed Clarification Cell Update
On 22 April, British Nuclear Group reported that a pipe had failed in one of the heavily shielded cells, known as the feed clarification cell, in the Thorp Head End plant at Sellafield resulting in a quantity of dissolved nuclear fuel being released into a sealed, contained area.
The leakage of liquor had been identified on 19 April when a CCTV camera was introduced into the cell. At the time of this announcement, Barry Snelson, Manager Director, Sellafield stated: "Let me reassure people that the plant is in a safe and stable state. Safety monitoring has confirmed no abnormal activity in air and there has been no impact on our workforce or the environment. I have asked for the front end of the plant’s reprocessing operations, including shearing, to be closed down. The plant is in a safe, quiescent state". This situation remains the case. The event has caused no harm to any individuals nor release of radioactivity to the environment.
The liquid, around 83m3, escaped from a fractured pipe (primary containment) feeding one of the Accountancy Tanks into a purpose built, thick walled concrete cell lined with stainless steel (secondary containment). The secondary containment was specifically designed for failure of primary containment releasing liquid in quantities significantly greater than that released in this event. It is fitted with engineered systems to pump liquid from the cell floor back into primary containment tanks within the cell.
Since this event was reported, recovery of the escaped liquid back into primary containment has commenced. Recovery of all liquid is planned to take a further four weeks.
In accordance with arrangements under our Nuclear Site Licence an investigation was established with the purpose of finding out exactly what happened, and to make recommendations to the Managing Director of Sellafield to ensure that any necessary learning points are acted upon.
The investigation has now been completed. It followed two lines of enquiry covering how and why the failure occurred and the response to it. Its key findings are as follows:
- The pipe failed because of metal fatigue. This was caused by stresses arising from the fact that the pipe was attached to a suspended tank (for the purposes of weighing the tank’s contents). This is the only area in Thorp (and indeed the rest of Sellafield) in which tanks are operated in this way.
- The secondary containment performed as designed. Once the pipe failed, the material collected in a purpose designed area, preventing release to the environment or harm to any personnel.
- The cause of the failure dates back to a change in design intent when changes to the restraint mechanisms on these suspended tanks were introduced that allowed greater stresses to be exerted on associated pipework than had been anticipated.
- There is some evidence to suggest that the pipe may have started to fail in August 2004. Failure of the pipe (at which point significant amounts of liquor started to be released into the cell) is believed to have occurred in mid January 2005.
- However, in the period between January 2005 (and perhaps earlier) and 19 April 2005, opportunities, such as cell sampling and level measurements, were missed which would have shown that material was escaping to secondary containment. While the failure could not have been prevented at this stage, had these opportunities been taken the quantity of liquid released could have been significantly reduced.
The investigation made recommendations that fall into three broad categories:
- Ensuring that a detailed engineering review is conducted to confirm that the potential for stress-induced fatigue is adequately addressed across Sellafield.
- Improving the maintenance, testing and reliability of cell instrumentation and other systems that give indications of plant abnormality.
- Reviewing operating practices throughout the plant to ensure that lessons learned are implemented and embedded.
As stated above we are now returning the liquid to primary containment. Barry Snelson, Managing Director at Sellafield, said: “From the information we have available at this stage we are confident that we have the capability of returning Thorp to service.”
Commenting upon the investigation report, he said: "The investigation has been extremely thorough and has identified the root causes of the event. I will personally be ensuring that recommendations are implemented not just in Thorp but across Sellafield. I am disappointed that plant indicators were not acted upon as quickly as they should have been and I shall be taking action to ensure that any complacency with respect to acting upon plant information is addressed. The maintenance of safety and environmental integrity remain our absolute priority during both the ongoing recovery of the liquor and the subsequent return of the plant to service".