Dounreay faces a century of clean-up

Built on the site of a wartime Fleet Air Arm station, which was never used, but left a legacy of three interconnecting runways still visible at the current entrance of the site, Dounreay sits impressively perched on a dramatic windswept Caithness granite clifftop. Nowadays the only other significant industrial development in the region is a fridge freezer assembly plant and an oil industry pipeline production facility.

One of Britain’s early atomic pioneers, Lord Christopher Hinton, who was head of the UKAEA’s Industrial Group in the 1950s and later the Chairman of the Central Electricity Generating Board, told a Strathclyde University seminar in 1977 how the site was chosen: “We assumed generously that there would be a one percent leakage from the Dounreay Sphere, the common name for the fast reactor, and, dividing the country around the site into sectors, we calculated the number of inhabitants.

“To our dismay, this showed the site did not comply with the safety distance specified by the health physicists. That was easy to put right: with the assumption of a 99% containment the site was unsatisfactory, so we assumed a more realistic 99.9% containment, and by doing this we established the ‘fact’ that the site was perfect.”

This type of rough and ready calculation may not be unfamiliar to other atomic energy pioneers. However, the actual contamination at the site after decades of operation is precisely known and it is a new generation of nuclear specialists that face the challenge of cleaning it up. Men such as Tony Wratten, manager of the waste management group at Dounreay, and charged with one of the most complex remediation challenges in the business. Or Sandy McWhirter, Tony Wratten’s right hand man, in charge of the clean-up of the Dounreay ILW disposal shaft (see Update in NEI, May 1998, p8) and emptying and stabilising the equally tricky – and recently contentious – ILW ‘wet’ silo nearby on the site.

Then there is George Sinclair, 17 years at Dounreay, now senior waste operations manager controlling the innovative cementation facility used to package ILW ready for final disposal. And perhaps most complex of all is the task being undertaken by Bob Matthews, a veteran of dismantling projects at Sellafield and most other nuclear sites in the UK, who is in charge of decommissioning.

Especially interesting and unique is the bespoke in situ project designed to handle the radioactively contaminated sodium and potassium (NaK) from the two fast reactors – the demonstration fast reactor (DFR) and the Prototype Fast Reactor (PFR) – on the site.

Other decontamination projects involve the on site clean up of around 1500 localised radioactively contaminated hot spots, including an intriguing restoration of the sixteenth century Dounreay castle, an historically protected (“scheduled”) ruin, known to have been a dwelling place since the thirteenth century. Several hundred cubic metres of contaminated soil have been carefully removed following sodium experiments conducted there in the 1960s. Another project involves the removal of some 150-200 ‘hot particles’ from the foreshore, arising primarily from Materials Test Reactor (MTR) fuel reprocessed at the site.

Sandy McWhirter said that there was “no area on the planet that was more monitored than the Dounreay beach.”

Critics meanwhile continue to assert that the hot particles discovered on the beach came from the explosion in the shaft in 1977, something denied by Dounreay.

DECOMMISSIONING AND RADWASTE

Since the British government decided 10 years ago to bring a halt to active fast reactor R&D at Dounreay, the management has been involved in extensive efforts to plan a decommissioning and waste management strategy. At present decommissioning takes up some 28% of the Dounreay annual budget of about £90 million ($150 million).

Radioactive waste absorbs around 22% and environmental remediation about 5%, and rising rapidly as the shaft project gets underway. The total decommissioning work currently planned is projected to cost around £85 million, and employs 80 staff at present. (The total decommissioning liabilities at the UKAEA’s eight sites will run into several billions of pounds, according to a paper presented by Dounreay’s Director Roy Nelson to a British Nuclear Energy Society conference last December).

At Dounreay, aside from the DFR, PFR and MTR reactors, there are several former research laboratories, remote handling ‘caves’, pulse column and other test rigs, and sodium coolant assemblies that are being dismantled. Most are in a state of ‘passive safety’ at present.

The DFR – closed 21 years ago – is proving the most complex if not most expensive to decommission. Its core has been fully defuelled, with the exception of one damaged fuel rod, but the 997 breeder fuel rods are still in place in the blanket. It operated with a 24 loop coolant system.

Bob Matthews said it was proving a real “learning experience” for his decommissioning team. Stage one decommissioning is set for completion by 2007.

NAK PROBLEMS

The secondary sodium coolant has been converted to common salt, and discharged to sea. But the primary coolant – 57 t of NaK – has proved more problematic, as the reactor fuel was ‘vented’ and so contaminated the NaK with considerable amounts of caesium. This is being removed in a custom-built ion exchange plant being constructed inside the reactor building, in which the caesium removal will take place in the aqueous solution downstream. The residues will thereafter be dealt with in a Water Vapour Nitrogen plant. There is also radioactively contaminated mercury waste from the seals around the reactor core to be handled, using a triple distillation process to decontaminate the material.

It is thought that the sodium plant will run to 2001, and Dounreay hopes to be able to market its successful design and project team consultancy experience abroad. There is already a government-to-government arrangement with France.

The £17 million ($27 million) two storey steel clad sodium disposal plant has been constructed by NNC inside the turbine hall in PFR, with AEA Technology and Framatome as subcontractors. The system is based in part on one developed to deal with the Rhapsodie reactor by the Commissariat à l’Energie Atomique (CEA) at Cadarache in France. It uses a ‘honeycomb’ of 500 mm thick concrete cells to house the higher active vessels. About 2.5 t will be treated per day in a 24 hour continuous process. The main chemical hazard facing the decommissioning engineers is a hydrogen explosion.

Altogether there is 900 t of sodium in the PFR at 200°C, with a further 70 t in the process caves. Only a small amount of this sodium is radioactively contaminated – about 2 TBq – from a few fuel pins which broke. Some 200 t of the sodium has solidified. A new electric heating system is being introduced to try to keep the sodium molten. A “defence in depth” philosophy is being adopted.

WASTE DISPOSAL

The on-site nuclear waste management programme has been expanded over the past few years with a number of new developments. Supercompaction and incineration technologies have now replaced the original direct disposal to LLW trenches.

These can only take wastes for a further 8 years. Dounreay is considering whether to now build interim storage facilities or go straight to a new final disposal plant.

ILW management is more complex. Before the NIREX repository research programme was scrapped in March last year when the Sellafield Rock Characterisation Facility project was rejected, Dounreay was planning for a repository to be ready by 2015; now the planning horizon is 2040. Conditioning in concrete, using a blend of blast furnace slag and cement, is the chosen method of immobilisation. The plant, which is planned to operate for 7-10 years, can handle about 14 drums a week. It takes about 2 hours to fill each drum. The Dounreay site has the space to store around 600 concreted drums over 15-20 years.

A new £6 million facility has been developed in a refurbished former apprentice training centre to provide state-of-the-art non-destructive testing combined with waste characterisation for the 200 litre drums of solid LLW. The process, dubbed WRACS (waste receipt, assay, characterisation, and supercompaction), is currently being commissioned under the direction of its designer, Martin Bundy, and will open fully by the end of the year. Bundy indicated that a key driver for the project was competition with BNFL. One of the unique features of WRACS is its real time radiography unit, giving an X-ray 360-degree image of the contents of the drums, primarily to detect high density material, data on which is recorded in hard copy, CD-ROM and video. It is expected that the number of rejected drums will be small.

The present volumes of the various waste streams at Dounreay are: solid LLW 33 800 m3; liquid LLW 72 m3; solid ILW – an estimated 850 m3 (in the shaft) and 1540 m3 in store; liquid ILW 1200 m3 in store; liquid HLW 210 m3 in interim store, probably to be transferred to BNFL. These waste figures are open to revision depending on a number of developments that have emerged in conjunction with the recently imported Georgian fuel.

THE FUTURE OF REPROCESSING & FUEL FABRICATION

Reprocessing facilities were originally established at Dounreay to service the research reactors on the site. As other countries engaged in nuclear research, the Authority developed a highly specialised fuel supply and reprocessing service to the operators of research reactors.

Although no further commercial reprocessing contracts will be sought (see Panel), there is still work to do – reprocessing about 15 t of its own fuels and another 1 t of material under several other contracts including the uranium imported from Georgia. Possible deals with Australian, German, Indian and Dutch reactor operators had been broached. This greatly limits the possibility of these countries being able to have their fuels reprocessed.

This leaves Dounreay with a small business of producing research reactor fuel and medical targets. However, with the continual problems besetting Dounreay, who knows how long this will remain a viable activity. It seems the days of upheaval are far from over for Nelson and his UKAEA colleagues.