Radwaste management: ILW
Yellow fever13 September 2011
In the UK, plans for interim intermediate level waste repositories have been turned inside out; nuclear utility Magnox Ltd has decided to store waste into self-shielded GNS Yellow Boxes, rather than building a shielded storage building.
The UK’s largest fleet of nuclear reactors are named after their fuel cladding, magnesium oxide, or magnox. Although Oldbury and Wylfa are still generating electricity, the eight other sites have been shut down. The amount of ILW varies from site to site depending on operations, with a total of 6500m3 stored at magnox sites. It includes wet and solid waste including sludge, ion exchange resin, sand, filter cartridges, nimonic springs from fuel elements and metal such as redundant control rods and charge chutes.
The original strategy for managing ILW was developed in the 1980s by the UK Nuclear Industry Radioactive Waste Executive (NIREX), which is now the Radioactive Waste Management Directorate, part of the government’s Nuclear Decommissioning Authority (NDA). The strategy focused on retrieving ILW, sorting and encapsulating it in concrete within NIREX-approved boxes. These would then be transferred to a large purpose-built shielded ILW store on site, where they would be stored until the planned final deep geological disposal facility became available. This strategy was adopted by all magnox sites and incorporated into the lifetime plan baselines.
The encapsulation strategy has high front-end costs, forecasted to be as much as £360 million, due to the fact that before any ILW can be retrieved and packaged, an expensive conditioning plant and ILW shielded store would need building and commissioning so it can receive packages. With this in mind, EnergySolutions, the parent body organisation that owns Magnox Ltd, and which runs the fleet on behalf of owner the NDA, began, in 2007, to look for an alternative. The change in ILW management is just part of a much wider new decommissioning strategy for the magnox sites, which culminated in the Magnox Optimised Decommissioning Programme (MODP). The MODP coordinates decommissioning activities across the entire magnox fleet.
Steve Templar, ILW programme director, said: “We have turned the original ILW strategy around. Where we previously relied on the building to provide radioactive shielding, we now look for self-contained boxes and only use the building to provide weather protection. That gives us more flexibility to tackle the highest hazard waste streams first in line with the NDA strategy,” he said.
The solution envisages using hollow iron cubes and cylinders from German supplier GNS. “The main advantage with the ductile cast iron containers (DCIC) is that you can prioritize the retrieval of waste at a high-hazard location for a small cost. If you are worried about the stability of a particular tank you can let a contract for that tank, without having to wait for an encapsulation facility and shielded store to be built,” he said.
In addition, adopting self-shielded boxes gives the decommissioning programme a greater degree of flexibility, through lower up-front costs and the ability to move the programme in line with budget fluctuations, where projects can be stopped and started with ease and with no significant cost implications.
About the box
For more than 20 years ductile cast iron containers by GNS have been used for ILW disposal in Germany. There are two types of cast iron containers that can be used to store ILW. The Type VI cubic containers (Type VI refers to the Konrad Repository acceptance criteria), also called GNS Yellow Boxes, weigh approximately 18 tons each when empty and have a net volume capacity of 2.9m3. They are 1600mm x 2000 x 1700mm, with a wall thickness of 150mm and will be used to store FED, sludges, resins, sand and gravels. The second type of container, the Mosaik cask, is cylindrical in design, 1060 x 1500mm, made from ductile cast iron with a wall thickness of 100-160mm and weighing between 5-10 tons depending on shielding. (Lead inserts can be added for highly-active components.) This type of container has a smaller net volume of between 0.170-0.490m3, depending on shielding, and is used to store higher-dose items. According to GNS, more than 7000 Mosaik casks have been sold.
Both GNS Yellow Boxes and Mosaik casks are made of ductile cast iron. They are cast without welding, providing high strength values even under dynamic accidental loads. Unique graphite nodules as structural elements of the casting are key to the product design. Extensive quality assurance measures after each production step lead to a total production time of approximately six months before a box is ready to use.
GNS Yellow Boxes have previously been used in Germany to package ion exchange material; Mosaik casks have been used for all types of ILW, according to a GNS spokesman. Both type of container provides its own intermediate, transport and final disposal shielding, removing the need for an expensive and technically complex remotely-operated ILW store. Lid seals have been approved by the German Federal Office for Materials Research and Testing to last at least 40 years, GNS said.
Templar says that Magnox will try to minimize the number of boxes used, as this will have commercial benefits and also benefits for the UK, with ultimately less waste destined for geological disposal. This will be achieved by conditioning (that is, drying the waste), optimizing packing factors for solids, and using some containers for decay storage. “We may find that if we store the waste for a number of years the waste will decay and can be in future be stored as low-level waste,” he said.
The move to using GNS Yellow Boxes as opposed to encapsulation has involved a great deal of technical and commercial evaluation, begun by EnergySolutions in 2007-08, with the new strategy being subjected to significant scrutiny from the NDA, regulators and local community representatives around the sites.
To substantiate the case for the Yellow Boxes, Magnox has undertaken pathfinder projects. Initially a project was set up at Dungeness A site, where ILW resins were removed from a legacy storage tank and successfully placed in three GNS Yellow Boxes (see also box, p28). The team also retrieved nimonic springs from one of the underground waste vaults. These higher-radiation items were stored in a Mosaik cask.
This project was followed by a 12-month early retrievals programme at Berkeley Site. Berkeley has one of the largest inventories of ILW in the fleet and has unique waste streams from site fuel laboratories. This in itself posed difficulties when assaying and sorting the waste currently contained within three subterranean vaults and a silo. The site early retrievals team engineered a simple solution to remove ILW using concrete shielding and a manipulator to remove FED from vault two, successfully filling a GNS Mosaik container. This project informs the ongoing, six-year, £250 million project to retrieve, package and store all site ILW in GNS Yellow Boxes, subject to final regulatory agreement.
Sean Sargent, Berkeley ILW Programme Campaign Director said: “The ILW programme activities at Berkeley represent one of the highest-hazard decommissioning activities in the magnox plan. We are working closely with the regulators to demonstrate the integrity of the solution.”
After approval from the NDA, Magnox Ltd has now changed the decommissioning baselines at all of its sites for the use of GNS Yellow Boxes for ILW management, with the exception to Hunterston A and Trawsfynydd which will continue with concrete encapsulation. This is because both sites have already built processing facilities and ILW stores in readiness to take wastes. However final approval is still required from regulators before full implementation.
One of the next stages to complete is the three-stage letter of compliance process with the NDA’s Radioactive Waste Management Directorate. During this process the RWMD will assess, among other things, whether the containers are suitable for the proposed geological disposal facility. [Although a design for the repository has not been finalized, the containers will be required to meet the generic specification for a robust shielded waste package, published in November 2010 (and also available via www.neimagazine.com/genericwastespec).]
“We are currently in the stage where we have made a submission to RWMD outlining the waste streams that we have and the processes that we wish to go through to condition the waste and store into boxes. RWMD then assess that against their requirements for the repository and then give a conceptual letter of compliance if they believe what we’re doing meets their requirements,” Templar said.
The intermediate stage will include more detail on how the waste will be processed and its condition. The final LoC will be issued once the job has been completed and the packages are inspected.
Since the LoC process needs to be carried out for all waste streams at each site, it is an ongoing process. According to Templar, Magnox expects to receive the conceptual LoC for the majority of the waste streams by July 2011. [A spokesman for the NDA’s RWMD confirmed in June that Magnox had carried out further work and supplied additional information as identified in the ‘conceptual stage assessment report’ issued in May 2010. Three of the four issues identified in the assessment have been resolved and that the final one is expected to be closed out shortly.] Submissions have also been put in for an intermediate stage letter of compliance for specific streams from Berkeley and Bradwell, but Templar says the process will be spread over a number of years.
As with the old encapsulation strategy, the new approach still has the problem of waste retrieval. Solid waste or FED can be retrieved using clamshell buckets or remote-controlled telescopic booms, for example [see also NEI October 2009, p35]. The waste also needs some degree of sorting (as with encapsulation). The main difference is that some waste streams need to be conditioned before being stored in Yellow Boxes. Typically this involves drying of wet wastes and sludges, as only low moisture levels are allowed in the boxes, to prevent corrosion.
Magnox Ltd has recently awarded framework contract packages worth more than £470 million to support the ILW programme. Three framework contract packages have been awarded to support the strategy. First, GNS Nuclear Services will supply Yellow Box containers to hold the waste in a five-year deal valued at up to £106 million. Templar says that keeping in line with current plans, on the order of 500 or 600 boxes will be needed over the next few years.
In total, six consortia—Babcock International, Nuvia, TSIF ILW (Costain, Aker Solutions and AMEC), Spencer Atkins, NSG Environmental and EnergySolutions—will support the programme with a range of services and equipment to recover ILW, process it and place the waste in the containers. The first four have agreements for processing solid ILW; all but Spencer Atkins have contracts for dealing with wet ILW.
These companies will bid against each other for work packages as they are let. EnergySolutions has also been awarded a contract for retrieval of sludges and resins at Bradwell [see box].
Finally, construction company Interserve will build weatherproof housing to store the boxes at each of the eight sites ahead of final disposal in the Geological Disposal Facility (GDF).
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