Immobilising radioactive waste
Dr Bob Page looks at some of the optimum methods of encapsulation of nuclear waste streams, and the progress being made.
Cementation is used worldwide by the civil nuclear industry for encapsulation of higher activity wastes (HAW), but some of the waste streams generated by the industry are difficult to encapsulate using this method. In anticipation of the requirements for consignment to the UK’s planned geological disposal facility (GDF), alternatives to cementation are needed for these more challenging waste streams, often referred to as WRATs (Wastes Requiring Additional Treatment) or orphan wastes. A passively safe and stable matrix is needed, and work in the last five years has generated interest in various polymeric alternatives.
The traditional grout mix used for encapsulation, which is based on a mixture of ordinary Portland cement (OPC) with pulverised fly ash (PFA) or blast furnace slag (BFS), provides an excellent mix of chemical stability, compatibility with most wastes and well- established physical properties, and is relatively cheap with a stable supply base. This formulation is not well suited, however, for reactive metal wastes (such as those containing fines of metallic aluminium or uranium and magnox cladding), which can cause potential fracturing of the grout as a result of corrosion of the metals, or hydrogen generation from reaction with the grout. Additionally some waste streams, such as those with high boron or zinc content, directly interfere with the cement hydration process and can significantly retard or even prevent the cement curing without pre-treatment or careful grout formulation.
Polymeric encapsulation offers an alternative route, with a number of advantages for treatment of contentious waste streams. Their superior mechanical properties allow for good waste loadings (up to 75% by weight for graphite, for example), allowing the number of packages to be reduced while maintaining the integrity of the wasteforms. Their resistance to acids, alkalis and organic solvents makes them highly durable, and they exhibit excellent radionuclide retention and leach resistance. And because polymeric materials are non-aqueous systems, they minimise direct corrosion of the metals by water. Moreover, they provide a good barrier to moisture transport, they are highly efficient at infiltrating around a diverse range of shapes, they can easily entrain hydrophobic materials such as graphite and they offer good radiation resistance.
Following successful encapsulation of wet waste simulants in a polymeric system using vinyl ester styrene (VES), this method has been used for a number of years at the Trawsfynydd plant in Wales, UK, for encapsulation of ion exchange resin wastes. The first successful campaign here sentenced 656m3 of ion exchange resins contaminated with fission products and actinides, and another two campaigns to encapsulate a further 1,400m3 are in preparation. However, there are some disadvantages. Styrene polymers contain a volatile precursor material with a very low flashpoint, which may be considered too high a fire hazard for certain applications. Additionally, VES formulations are very sensitive to the ratio of ingredients to achieve an effective cure, and the presence of an indeterminate amount of water would cause problems in this respect.
Pick of the pack
Continuing research has led to the identification of epoxy resins as the polymers of choice for problem waste stream encapsulation. These have high compressive strength, can retain a good degree of integrity even after mechanical damage, are effectively impermeable to water, and display excellent leach resistance, as well as being very durable, with low flammability and high resistance to thermal damage.
Babcock has reached an advanced stage with wasteform trials on epoxy resin formulations for the immobilisation of the Windscale Piles fuels and isotope waste as part of the Windscale Piles Decommissioning Project, as well as for a range of wastes at Harwell, where polymeric encapsulation has been shown to be well-suited to the treatment of active metal wastes, such as the fuel rods from the Graphite Low Energy Experimental Pile (GLEEP) reactor (made up of uranium bars coated with aluminium).
At Windscale (where the fuels and isotopes waste includes a significant proportion of metallic uranium in finned aluminium cartridges, graphite boats, aluminium isotope cartridges containing a variety of known compounds, and fuel debris), work was undertaken to develop a polymeric encapsulation system, including inactive and active trials to assess the ability of the polymers to produce an acceptable wasteform, which has attracted significant industry interest. Research to date has been based on highdose radiation, and further research is now to be undertaken, funded by the Nuclear Decommissioning Authority (NDA) Direct Research Portfolio (DRP), to identify behaviour at a lower gamma radiation dose rate (which better represents that likely to be incurred by polymer acting as a waste encapsulant). Once proven under these conditions, the polymer will be applicable to the widest possible range of wastes within the UK inventory.
Silicone polymers offer further advantages
Babcock trials have also now expanded to include other polymeric systems, one of the most recent being siloxanes, or silicone polymers with an inorganic backbone and organic side chains. These offer a number of features making them beneficial as a waste encapsulant, including two added advantages over epoxy, in that they cure near room temperature (simplifying plant design), and long-term radiation degradation results in gradual loss of the organic side chains as low molecular weight gases, ultimately leaving a wasteform based on a silicate matrix, or quartz-like structure – essentially a lowtemperature vitrification process.
Further benefits include good flexibility and vibration resistance, an effective barrier to moisture transport, high thermal stability (greater than 360°C), good radiation resistance, and easily tailored physiochemical properties. Additionally, they show high resistance to radiation damage, and their radiative degradation mechanism, transforming to a silicate based matrix, results in an increased compressive strength over time.
Babcock was contracted by the NDA under the DRP to carry out a series of proof of concept trials, to establish the suitability of silicones as encapsulation media for orphan waste streams. This included an extensive range of trials on the physical properties of the polymers and their radiochemistry, with a view to evaluating a number of factors including radiation stability over time, the nature and rate of organic material loss, physical robustness, and efficiency of infiltration around wastes.
The trials to date have shown silicone polymers perform well as waste encapsulation materials for UK ILW, in particular where there are reactive materials present. Work is now underway to evaluate the hardening process at higher doses, and further work is planned by Babcock in conjunction with site licence companies and the NDA to establish silicones as a desirable material for orphan waste encapsulation.
The various polymeric material investigations being undertaken are at the forefront of this area of research, to identify optimum methods of encapsulation for a range of waste streams that are unsuitable for other approaches, such as cementation. Significant progress has been made to date and the work is continuing, with NDA DRP funding, to establish these polymeric media as viable alternatives for the management of problematic waste streams.
Dr Bob Page, Technical Director - Nuclear, Babcock International Group PLC. www.babcock.co.uk