The leakage of dissolved nuclear fuel from a pipe in Thorp’s Head-End plant was first discovered on 18th April after operators realised that a quantitiy of sheared and dissolved fuel had not ‘arrived’ at the next stage of the process – the accountancy tanks in the Feed Clarifdication cell. The cell, which forms part of THORP’s Head-End plant, measures 60m in length by 20m in width and, in parts, is 20m high. With the discovery, all shearing and dissolution of fuel was immediately abandoned and the Head-End plant closed down.
Investigations via remote cameras in the highly radioactive cell on 19th April showed that the liquor had escaped from what is described as a ‘clean break’ in the feed pipe leading into the top of one of the two accountancy tanks. Under normal operations the liquor’s content would be assessed in these tanks and its fissile component accounted for. It has now been revealed that as the liquor leaked from the fractured pipe it spilled onto a steel framework adjacent to the accountancy tank dissolving some of the steelwork. This dissolved steel now also forms part of the 83 cubic metres of liquor on the cell floor. CORE has been told that an assessment is currently being made as to how the rest of the steel framework – a relic of the cell’s original construction, will be treated. It is understood that the framework is not an integral part of the accountancy tank or its sensitive weighing mechanisms.
As more details of the accident emerge, the severity of the leak in terms of operational delays to THORP and also in terms of lost revenues to the Nuclear Decommissioning Authority, is becoming more clear. For the moment, with the 83 cubic metres of spilled liquor contained within the cell, the safety of those outside the plant remains a lesser issue. Without yet supplying full detail of its assessment, the Nuclear Installations Inspectorate (NII) has told CORE that the potential for a criticality accident can be completely ruled out. BNFL has said “we can categorically confirm that the material is too low an enrichment for a criticality to occur”.
CORE has been informed by a relaible source that the spilled liquor contained 22 tonnes of dissolved nuclear fuel from the German Unterweser PWR power station. Unterweser, now wholly owned by the giant E.oN company had contracted with THORP for some 225 tonnes of fuel to be reprocessed – the last of which should have been reprocessed during 2001. That it is only now being reprocessed provides an indication of the significant changes and delays to THORP’s schedule that have been forced on British Nuiclear Group (BNG – formerly BNFL) by a series of accidents and malfunctions in THORP since it started operations in 1994. Unterweser is almost ceratinly one of the German power stations contracted by E.oN in 2002 to receive MOX fuel from Sellafield’s ill-fated MOX plant.
BNFL sourced data provided to the leaked 1995 NAC International report Worldwide Reprocessing Summary shows that the final batch of Unterweser fuel that should have been put through THORP in 2001 consisted of fuel with an initial enrichment of 3.43% and an average burn-up of 37,364 MWd/MTU. From the reprocessing of 22 tonnes of this fuel, the NAC report shows that a total of 196kg of plutonium would be recovered, of which 132kg would be fissile. Still refusing to identify the nationality of the fuel in the leaked liquor BNG has only said “the material contains around 20 tonnes of uranium dissolved, and 160kg plutonium dissolved in a very dilute form”
No start has yet been made to remove the leaked material of fuel/nitric acid/dissolved steel from the cell floor. Contrary to the intial speculation that this would be accomplished by draining it through a sump in the cell floor, it is now understood that the liquor will be pumped directly from the floor into a ‘break-pot’ tank in the cell – the pumps and tank forming part of the cell’s original fittings. From the break-pot tank, the liquor will be fed back into the system via the centrifuge where, once clarified by the removal of any undissolved solids, it will be transferred to an accountancy tank.
The overall recovery strategy is being split into individual projects. The first of these – the plan to pump the liquor from the cell floor, will be put to the NII by BNG this coming week. A final project, possibly months away, will involve an assessment as as to whether or not the fractured pipe can be repaired or is worth repairing, and whether the stability of the pipework leading to the second accountancy tank (not involved in the accident) can be trusted for use when operations eventually restart. THORP could operate on one accountancy tank, but to do so would necessitate a significantly reduced spent fuel throughput.
No detailed estimate has yet been officially given for the length of time likely to be involved in salvaging the leaked liquor from the cell floor or for repairing the fractured pipe (if possible), though BNG has confirmed that it will be a matter of months. In a briefing to the media at the end of April, BNG said “given our technical experience and capability we are confidsent that we can recover this situation and get the plant back operating again”.
Following the discovery of the leak on the 19th April, THORP’s Chemical Separation plant (into which the liquor will eventually be fed for the separation of plutonium and uranium from the High Level Waste products) continued to operate until 3rd May. By this time, and as part of BNG’s spilled liquor recovery strategy, sufficient tank space in the Chemical Separation plant had been made available to accommodate all the liquor that remained in the accountancy and buffer storage tanks in the Feed Clarification cell at the time of the accident. Once this feedstock has been processed, the Chemical Separation plant must also remain closed down.
The cost of THORP’s closure has, in the past, been put at at least £1M per day. A closure of several months will therefore significantly impact on the income expected from THORP this financial year. Projected at around £560M for the year, the revenue was to have offset the costs of the urgent clean-up work required at Sellafield, and estimated at over £2Bn for the year. The work is to be undertaken by the Nuclear Decommissioning Authority (NDA) – a part of what was the UK’s Department of Trade & Industry, now renamed the Department of Productivity, Energy and Industry.
The NDA took ownership of the Sellafield site and all its commercial operations on the 1st April 2005 – with THORP classified as as an asset. Following the THORP accident, an NDA spokesperson has said “if the income from the plant is not forthcoming then obviously it will put back plans for cleaning-up”. In a more detailed statement the new Authority has said “ the NDA recognises this is a significant event that may impact on the plans and the contract for Sellafield licencee” – a referernce to BNG as the initial (3 year) contractor for THORP. The implication is that a failure by its contractor to perform safely or to target could lead the NDA to strip BNG of its contract after three years.
At a meeting this week with Greenpeace and other NGO’s the NDA has confirmed that, as plant owner it will have to pay the repair and recovery costs of the leak. To keep its clean-up programme on track, it will have to approach the Department of Productivity, Energy and Industry, and ultimately the UK Treasury, for more taxpayers’ money to cover the shortfall the NDA will face as a result of reduced THORP revenues. Questions are already being raised about ‘State-Aid’ implications for THORP being bailed out this way by the NDA.