Decontamination & decommissioning
Monolithic progress10 August 2009
No more green boxes – Dounreay’s massive new waste plant and store is bucking an architectural trend and paying tribute to the site’s 1950s civil engineering. By Corrina Thomson
A new waste plant and store at the Dounreay nuclear site, in northern Scotland, may not sound like news but for a site with waste accumulations dating back 50 years, a plant that can condition and store waste to modern standards is like the Holy Grail.
Planning permission, which has just been granted by the Highland Council, came not a moment too soon as a further backlog of waste needing to be set in drums was created by a cemented liquid waste spill at the site’s existing cement plant, which rendered it unusable from September 2005 to April 2008. In addition, the current site demolition plan indicates that the site should be closed by 2025, which means that waste should be processed and stored by then.
The existing Dounreay cementation plant was built in 1984 and became operational in 1996. But work was interrupted in 1998 when regulators stopped work in the site Fuel Cycle Area (FCA), after a contractor’s digger caused a power cut. The plant restarted in August 2003 as part of a programme of gradually opening the shutdown FCA plants with modern safety cases.
The new GBP140 million plant will consist of an intermediate-level waste (ILW) processing section (cementation plant), a waste import/export facility, a drum store and a box store.
The existing Dounreay cementation plant is the forerunner of the new plant, the latter of which is to break ground in more than a literal way by becoming the first plant in recent times to buck the trend of conspicuous, ugly dark green sheds that are commonplace at nuclear sites, including Dounreay, due to rules and regulations.
It is surprising but true to say that a grey monolith is more in keeping with the windswept landscape and grey fifties civil engineering in this area than the giant dark green sheds or mirrored office building that have formed recent additions to the site.
Whilst being no beauty, the massive proposed plant and store, which has a 5300m3 floor area, moves a small step towards acknowledging that while visual intrusion at nuclear sites may be massive, it does not have to be the wrong colour.
Highland Council granted outline planning consent for the new facility in January 2007 and detailed plans were lodged with the local authority in
February 2009. Enabling work is now taking place at the site with actual construction of the building scheduled for 2010–14.
Whereas the existing cementation plant was only able to cement the site Materials Test Reactor (MTR) liquors, the new plant will also be able to cement other waste streams – the 206m3 of high-hazard Prototype Fast Reactor (PFR) raffinates, the 214m3 of Dounreay Fast Reactor (DFR) raffinates and 116m3 of ammonium
diuranate floc (all products of reprocessing). The site currently has 300m3 of MTR raffinates due for cementation, all of which should be dealt with by the existing cementation plant. However, the new plant will have the capability to deal with this waste if necessary.
Thus the higher hazard wastes will be immobilised, changing liquid wastes into what is regarded as a “passively safe” state mixed in cement and held in drums. Dounreay also had Pu nitrate liquid waste, which was added to the PFR liquor and will be treated in the new plant.
Other liquid wastes that will not be dealt with in the new plant include: contaminated solvents and oils (treatment method under review), low-level liquid effluent treatment plant sludge that will be cemented in a mobile facility and consigned as LLW, and Th nitrate ILW that will be encapsulated into stainless steel 500L drums in a mobile cementation facility and then placed in long-term interim storage, according to Dounreay Site Restoration Ltd (DSRL).
A very small volume of dissolver liquors have already been encapsulated and placed in interim storage. Later, the waste will be transferred with other drummed remote-handled ILW to the new plant, where it will be transferred to stainless steel 500L drums and for long-term storage.
The new waste process plant has a 10-year design life and the adjoining drum and box stores, which have an import/export facility, have a 100-year design life. The drum store will hold 8000 drums and the box store 2000 boxes. Liquid waste will be piped into the plant through a purpose-built transfer line from an old active liquor store and solid waste will be imported in flasks.
Senior project manager Dave Busby said that the new plant would be able cement the PFR raffinates in 18 months, and the DFR raffinates in two years.
PFR raffinates were once classed as high-level waste (HLW) but are now classed as ILW following a consultation. This was based on the premise that the raffinates did not require forced cooling and fell within ILW criteria. The waste is also significantly less radioactive than the HLW at Sellafield, in England. Nevertheless, the PFR raffinates are one of the higher hazard wastes at the site, alongside the sodium and sodium/potassium once used as coolant in PFR and DFR, which are currently being destroyed.
The combination ILW plant, which will immobilise liquids and encapsulate solids in cement, has superseded the previous notion of having a vitrification plant at Dounreay to deal with PFR raffinates. The idea of having one of these notoriously tricky and expensive plants was shelved due to cost and the fact that it would be technically problematic to vitrify other liquid wastes at Dounreay due to their chemical composition not being compatible with setting in glass.
Busby commented: “So really we were going to spend GBP200 million on a vitrification plant to deal with about 200m3 of raffinate. It would have had a five-year operational life then the plant would have had to be knocked down. But the other thing was that we would have had to build a high-active store and that would have cost a further GBP40 million.
“In the event, we looked at modifying the current Dounreay cementation plant – it was feasible but it would have meant decommissioning and rebuilding (to include modifications), or extending it. But when you look at lifecycle costs it’s actually cheaper to build a new plant.”
All waste destined for storage will be in a 500L drum or a 3m3 box. This means that older packaged waste, such as 200L drums, will be processed and packaged in 500L drums. The cementation part of the plant will produce 500L drums.
Dounreay’s existing cement plant cost about GBP12 million in 1984 and the proposed new combination waste plant and store is set to cost GBP140 million. The previous plan in the now defunct Dounreay Site Restoration Plan was to build a vitrification plant, a conditioned waste store and an integrated waste treatment plant at a cost of GBP600 million (in year 2000 prices).
Busby explained that the current GBP140 million combination ILW plant and store has been developed from separate liquid and solid encapsulation plants, conditioned waste stores and part of the originally proposed GBP300 to GBP400 million waste treatment plant, which was being driven by three different dates – including the emptying of the site waste shaft. Along with the new plant, a separate waste treatment plant for the shaft and silo waste is also proposed. The cost of this separate plant has not yet been finalised but it is expected to be more than GBP300 million.
The desire to speed up decommissioning and reduce costs saw the termination of a contract for the new plant which had been placed with a group of contractors called the BAND Alliance: prime contractor AMEC with British Nuclear Group Project Services, NIS, DGP International and Weir Strachan & Henshaw. This alliance was awarded the contract in January 2006 but the agreement was terminated in 2007 because of a failure to deliver in terms of cost, timescales and because the project was outwith original scope.
Once the BAND Alliance contract was terminated DSRL took on the design authority role but it did not have sufficient in-house resources, so another project team was formed (what is termed an integrated project team).
“We went out to the market, brought a lot of different players on board and all the resources we need to help us design this plant to a reference design standard,” Busby said.
Through Nuvia (formerly RWE Nukem), a new team was set up in Warrington, England, which employs about 80 people. There are also around 20 Halcrow staff working on the project in Glasgow, Scotland.
In June last year, the team produced a ‘reference design’ which was different from the previous ‘concept design’ arrangement under the BAND Alliance, in that it accounted for the necessary scope changes and gave better value for money.
Following a review by the Nuclear Decommissioning Authority (NDA), it was decided that the team should continue working on the project and produce a detailed design, along with working on the necessary safety, environmental and planning approvals.
That will culminate at the end of this financial year in a detailed design with a fully substantiated safety case and what is termed a ‘best practical means’ for a detailed design. These will be used to go out to tender again for a manufactured design and build contract (capital cost GBP140 million), where the contractor develops and details the design established by the ‘integrated project team’.
Busby pointed out that the plant’s capability over its lifetime would have to be flexible due to the uncertain nature of some of the future waste streams at the site.
Busby said: “We are actually starting work on the site next year. By 2010 you should be able to see some major construction action at the site.”
Two new 500L drums have been developed through the project, resulting in them being able to hold 20% more waste with a 10% reduction in manufacturing costs. There has been interest in the drum from other nuclear sites such as Aldermaston and Hunterston but responsibility for ensuring uniformity of waste packages across the UK lies with the NDA Radioactive Waste Management Department.
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