Sellafield must be seen to be squeaky clean27 August 1998
When European environment ministers signed the latest Oslo and Paris (OSPAR) agreement on the marine environment on 23 July (See NEI, Aug p4), they committed BNFL to reducing discharges to ‘close to zero’ by 2020. Nolan Fell talked to Colin Partington and Bob Morley, principal safety advisors at Sellafield, about the challenges the plant must face.
Radioactive discharges from Sellafield peaked in the early 1970s. In 1973 the Irish Sea received around 180 TBq of alpha emitters from the site and in 1975 beta emissions were nearly 9000 TBq. Since then BNFL has reduced the quantities of radioactivity in the Sellafield waste streams by a factor of 100 and has virtually eliminated actinides, reducing discharges by 99.9%.
The Enhanced Actinide Removal Plant (EARP), which came on stream in 1994, is largely responsible for the actinide reduction. The plant removes virtually all alpha activity and about 80% of beta activity from the effluent produced through the reprocessing of spent Magnox fuel, however it does not remove the beta emitting fission product technetium-99. Before the plant started operations, BNFL had stored the waste from the reprocessing for a number of years. This meant that despite EARP’s success in removing almost all the radioactivity, when it came on stream there was an increase in radioactive emissions to the sea and in particular technetium-99 rose to measurable levels.
The Thermal Oxide Reprocessing Plant (THORP), which also came on stream in 1994, does not release technetium to the environment. The plant reprocesses spent fuel from AGR, PWR and BWR reactors during which it successfully separates the technetium into the high level waste stream which is then vitrified. Current technetium discharges are in the region of 100 TBq/y, with limits set at 200 TBq/y. This results in an exposure to the most critical groups – local inhabitants who eat a lot of seafood or live on house boats – of 40 µSv/y. Background exposure in west Cumbria is 2200 µSv/y.
“Technetium was seen as being a low hazard beta product, well below our previous discharges and of no great concern,” says Partington. “Therefore our authorisation [from the Environment Agency] allowed us to discharge technetium at these levels.”
Technetium bioaccumulates in shell fish and the isotope has been found in sediments in Ireland and Scandinavia. As a result its removal from the Sellafield waste stream became a key issue in the OSPAR talks, with the Norwegian Prime Minister Kjell Manger Bondevik raising the issue with his UK opposite number Tony Blair. As part of the OSPAR agreement Sellafield has to develop a strategy by 2000 to reduce liquid discharges to sea to near zero levels by 2020. Bob Morley is head of the team developing this strategy. Removing technetium from the EARP waste stream is vital to this long-term aim.
“The most promising option at the moment is to use tetraphenol phosphonium bromide (TPP),” says Morley. “This will precipitate the technetium allowing us to encapsulate it within an ILW. Development of this process is reliant on us obtaining approval for disposal of TPP into the environment and also for the final disposal of the technetium as ILW in a national repository. We don’t think there would be any environmental problems caused by discharging TTB into the environment at the levels we are considering.”
The TTB precipitation is an end-of-pipe solution. More elegant would be removing the technetium during the reprocessing of the Magnox waste, as takes place within the THORP plant. Morley and his team have identified that the best point to trap the technetium would be during the primary separation of plutonium and uranium from the waste products at the Magnox Reprocessing Plant (MRP). It could then be vitrified as part of the high level waste stream. In order to do this BNFL may have to insert a medium active evaporator within the MRP, or modify an existing evaporator. However this would not remove the technetium from the stocks of medium active concentrate which the EARP plant is currently processing. It is likely that both solutions will be necessary.
Long term aims
“We’re talking of a multi-million pound spend, £10 millions, maybe £100 millions” says Partington. “The time to develop the concept, to do the design, the commissioning, maybe even the pilot work and then build the plant and operate it is not six months. We’re talking about possibly several years before we see significant reductions.”
Morley and Partington are also considering how to achieve the long-term commitment to discharges ‘close to zero’. Partington points out that the agreement states that emissions should be “close to zero, bearing in mind technical feasibility and the radiation impact on people”. Interpretation of this phrase is vital and the debate is likely to focus on this.
“You could argue now that we are already at a level which is trivial,” says Partington. “The radiation exposure to the most exposed person to technetium outside this plant is a small fraction of natural background radiation and if they simply move house, they could get twice as much dose due to differences in natural background. We would argue that what we are discharging now is safe, because the limits we are set by Government are based on safety. What we are talking about is a political difficulty. The hard bit is assessing when you are close to zero. What is that point? What is safe?”
This is a point which will have to be established throughout Europe and probably the world, with governments having to listen to both the arguments of the nuclear industry and groups who oppose the industry such as Greenpeace. The level which is eventually agreed will be arrived at through a political rather than a technical process. And the political decision is likely to involve a very significant margin of error which will be incorporated so that the levels are not just safe, but are seen to be safe. This is the essence of the term ‘close to zero’.
But just as political reality will dictate the discharge limits, technical reality means the industry cannot be wished away. The reprocessing which Sellafield carries out would have to carry on for years after a decision to turn away from nuclear power. According to Partington the only alternative to reprocessing Magnox fuel would be to build a dry storage facility which would have to be maintained and monitored for centuries, something which is not a practical option. As for the AGR, PWR and BWR fuel, Partington argues that reprocessing is the most sensible way to deal with it, as it reduces the volume of waste that has to be disposed and releases uranium and plutonium which can then be burned in a reactor.
“We believe the total amount of waste generated through recycling is less than the total amount generated if you directly disposed. You can’t just dump it down a hole, you have to package it and deal with it...We think we can package our waste better than through the direct disposal route, and you extract a lot of energy that you would otherwise simply throw away.”
When the OSPAR deal was signed, Greenpeace celebrated, claiming it signalled the end of Magnox reprocessing and would lead to the closure of the UK’s Magnox stations. BNFL unsurprisingly disagrees and whilst Partington did not perhaps celebrate the OSPAR terms in the way Greenpeace did, he is clearly relieved it was not worse.
“OSPAR has given us a challenge,” he says, “and made us look very carefully at our technical ability to reduce discharges. The Government will be very challenging and the agreement will cost us a lot of money, but not so much in our view, to make the industry non-viable. Meacher and Prescott made a technically and politically right decision, one which was a win for Greenpeace, a win for the Government and not a lose for us.”
Partington expressed considerable exasperation at the difficulty of getting BNFL’s message across. Nuclear power, after all, produces no greenhouse gases. He believes that nuclear power is clean, is safe, offers society the most feasible means of supplying the energy it needs and can justify itself in a rational debate.
“We believe the debate should focus on what is the risk to people,” he argues. “And the risk from Sellafield to people outside from our routine operations is close to zero. Now clearly other people don’t think that and so there has to be a debate and a coming together of minds as to what is an acceptable level of risk. In risk terms we would like to see a level playing field.”
Unfortunately for the nuclear industry a level playing field is unlikely due to the fear which atomic issues engender. In the public mind nuclear power is associated with the atomic bomb and with the potential for an accident of Chernobyl type proportions. Shifting this perception will be a greater challenge than any posed by the terms of the OSPAR agreement.
|The EARP process|
|The Enhanced Actinide Removal Plant (EARP) handles low volume medium active concentrate streams from current and historic streams of waste from the UK’s Magnox power stations. It also treats high volume low active bulk effluent streams. The treatment process involves adding sodium hydroxide. This neutralises the waste and it also reacts with the iron within the streams to produce a ferric hydroxide floc. Most of the alpha activity co-precipitates with this floc, leaving a purified aqueous phase. Adding other specific chemicals at the precipitation stage helps remove the beta activity. The floc is separated from the aqueous permeate through an ultrafiltration process. The floc stream is recirculated through porous parallel tubes so that the permeate is forced out through the tube membranes. Once separated the floc is encapsulated within cement and the permeate is discharged to the sea on a batch basis to ensure that discharge limits are met. Any batches which fail to meet the discharge criteria are recycled for further treatment.|