Rise of the radwaste depot21 December 2010
Increasing low-level waste disposition costs and pressure from government are forcing UK industry toward greater processing of radwaste; centralised services in this field offer economies of scale and other benefits. By David Lee
Recent management and regulatory changes in liabilities management in the UK have emphasised the importance of access to treatment processes to achieve cost reduction and for compliance with the waste hierarchy (a UK policy that expands the ‘reduce, reuse, recycle’ idea). This change is evident in the new role for low-level waste repository [LLWR] as a waste service provider and not simply a disposer of low-level waste. These are major changes in the way LLW will be managed in the UK in the future that will affect the way that both waste producers and service providers operate.
New services will lead to more emphasis on waste reduction and recycling and reduce the volume of waste disposed of to the LLWR in Cumbria. Many of these changes are brought together in the UK Strategy for the Management of Solid Low Level Radioactive Waste from the Nuclear Industry, which was published by the Nuclear Decommissioning Authority in August 2010 (www.tinyurl.com/3xlawsw).
Waste producers will need to invest in waste characterisation and segregation to take advantage of the environmental and cost benefits that will accrue from treatment to reduce volumes and to gain access to centralised processing and recategorisation.
High force compaction or ‘supercompaction’ involves crushing drums of compactible waste under a force of up to 2000 tonnes to produce ‘pucks’ that are between a half and a tenth of the volume of the raw waste. Drums are fed from a roller conveyor into the compaction chamber, which is contained in a remote-operated compaction cell. Pucks are removed from the compaction cell on a roller conveyor, monitored, measured, wrapped if required and loaded into half height ISO (HHISO) transport containers for disposal.
The average volume reduction factor achieved is between 4 and 5, and an average of 200 pucks can be loaded into a HHISO container by careful placement and arrangement of the contents. A throughput of 100 drums a day is achieved for operations at for example Inutec’s nuclear licenced facilities in Winfrith.
For asbestos and beryllium wastes, additional containment is required for the input and output materials to protect the operators and environment from asbestos fibres and particulate beryllium. Up to 300 pucks of compacted asbestos waste can be loaded into a HHISO.
In wet waste drying, water vapour is driven off and passed through a condenser to be collected as a liquid for disposal though the liquid effluent system. The dried solid is then processed through the supercompactor. For UK waste, the drums are dried to less than 5% by weight water content. Water contents well in excess of 50% by weight can be processed. The cycle time for a batch is typically 10-14 days. The resulting drums containing the dried solids are supercompacted into pucks with typical volume reduction factors of 2.
The historic route for disposal of active effluent treatment plant (AETP) sludges, which have arisen through activities such as oil separation and reactor site laundry and shower systems, has been to encapsulate the waste in 200 litre drums that were then packaged into a HHISO container for transport to the LLWR in Cumbria. This method, however, inevitably results in an inefficient use of space as cylindrical drums are packaged into rectangular containers.
Inutec has developed a mobile system that mixes the radioactive sludges with cement in a batch mixing vessel that discharges directly into a disposal container to minimise the volume of waste for disposal. This direct-pour cementation process involves mixing the contents of the sludge storage tank to produce a homogeneous pumpable slurry, retrieving of the slurry in defined batch sizes into the mixing vessel, mixing with cement, and discharging as a fluid grout into a one-third height ISO container (THISO).
Magnox North achieved savings of more than GBP 800,000 (EUR 1 million) at its Oldbury site by direct-pour cementation of a tank of sludge into third height ISO containers. The overall outcome of the work was a reduction in the total volume of cemented waste for disposal of 60% compared to the baseline programme assumption. The number of containers required for disposal was reduced from 10 half height ISO’s to six third height ISO’s, cutting processing time from twelve to five weeks, and shortening the overall project duration by four months.
Waste management is non-core business for most site licence companies, even taking account of their familiarity with handling highly radioactive materials. For general industry, the case for service provision is even more compelling, because they do not have the resources to manage radioactive waste management facilities, nor do they generate the volume of waste that would make such a proposition good business. Taking the problem away from them to discharge a liability is, however, very good for their business.
The volume of waste generated by general industry is small when compared with the nuclear industry. Small volume producers typically generate 2% of LLW on an annual basis. Quantities of naturally-occurring radioactive materials from oil, gas and mining industries can be much greater in volume; so much so that these industries may invest in treatment or partial treatment themselves. But in practice they will generally opt for a service provided by a specialist radioactive waste management contractor. Inutec’s business model for these wastes is to take title, so that the wastes can be co-treated and co-disposed, with liability transferring to LLWR once the waste has been transferred and accepted by LLWR.
The key elements of cost savings are flexibility to process waste in campaigns and to carefully pack the treated wastes into disposal containers to maximise the disposal volume reduction factor. Centralised processing of wastes also offers waste producers savings in safety case modifications and environmental permits.
Recategorisation of wastes
As a general rule, the higher the radioactive category for a waste, the higher the cost of disposal. Typically, in the UK, VLLW disposal costs hundreds of pounds per cubic metre, LLW disposal costs thousands of pounds per cubic metre and ILW costs tens of thousands of pounds per cubic metre. If waste can be recategorised, by removing radioactivity and by disposing of secondary wastes in accordance with best environmental practice (BAT), disposal costs can be cut at a stroke.
For example, steel components including ingots and gas cylinders and mixed waste materials such as cables and non-metallic items that had been exposed to high levels of tritium have been processed by thermal detritiation by Inutec so the resulting wastes could be disposed of as low-level radioactive waste.
The detritiation process entails:
Characterising and sorting waste into radioactivity and material streams
Heating the materials in a ventilated oven to controlled temperatures for each material type
Maintaining the materials at the controlled temperature for sufficient time
Collecting the gases in bubblers
Characterising the treated waste and the secondary wastes for disposal.
In addition, a washing process for removing radioactivity from tritiated desiccant from operational nuclear power plants has been introduced by Inutec as a centralised treatment offering that avoids the high costs of multiple local treatments. Historic waste from five locations, and current arisings from an additional two locations, will be treated in the plant. This represents a major change in the management of this waste.
Desiccant is received in containers and transferred into the plant where it is washed to separate tritiated water from the desiccant. The detritiated desiccant is collected, mixed with cement, and either placed into THISO containers in moulds or processed using the direct pour cement plant. Desiccants from different locations can be co-disposed for efficient plant operation and use of disposal containers.
Inutec itself benefitted from characterisation methods developed for assaying thermally detritiated wastes. They were applied to decommissioning wastes from Inutec’s facilities and from laboratory strip out to consign wastes for controlled release, saving GBP 100,000 in LLW disposal costs.
A process for removing radioactivity from tritiated mercury has also been developed to avoid disposal of this toxic chemical to shallow burial at LLWR through recycling for non-dental use.
The highly specialised process of low temperature oxidation using hydrogen peroxide (ModulOx) can be deployed to treat resins from decontamination of, for example, submarine reactors; similar thermal processes to those used for detritiation and other chemical treatments may also be deployed for solid waste treatment.
Inutec is prepared to take title for wastes generated by nuclear facility operators, using the model developed for small volume producers, and has done so for tritiated desiccants from reactor operations. Taking title is all about sharing risks and benefits. This works for the waste producer because the waste liability can be discharged to a third party prior to treatment and disposal. The waste producer will be required to provide a full description of the waste, including radioactive inventory and accept the risk of any major changes to that inventory. The waste producer will retain a responsibility to ensure that the contractor treats and disposes of the waste legally under their duty of care. The waste producer will benefit from a charge structure that takes account of processing efficiency and co-disposal. Inutec will benefit from taking title because it receives payment in advance prior to incurring any processing costs and it can manage processing of the waste in optimised processing campaigns.
Once the waste has been processed and conditioned for disposal, Inutec will pass the liability onto the disposal site operator, LLWR, under its standard contract terms.
TablesTable 1. Summary of financial benefits of centralised processing, volume reduction and recategorisation