Dr Steven Black looks at some of the latest developments to identify optimum methods of encapsulation for a range of waste streams, and the progress being made.
Cementation is widely used within the civil nuclear industry worldwide for encapsulation of higher activity wastes (HAW), but some of the waste streams generated by the industry are difficult to encapsulate using this method. For those waste types, often referred to as WRATS (Wastes Requiring Additional Treatment) or orphan wastes, important strides forward are being made with research by Babcock into various alternative encapsulation materials.
In anticipation of the expected requirements for consignment to the UK's planned geological disposal facility (GDF), immobilisation of nuclear waste in a passively safe and stable form is sought, to prevent the mobilisation of radionuclides in the event of container failure. 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 a good mix of chemical stability, compatibility with most wastes and well established physical properties, and is relatively cheap with a stable supply base.
Cementation process
This formulation is limited in scope, however, for reactive metal wastes and wet wastes where unknown water content and the presence of mobile ions can interfere with the curing process. The stability of grout-based wasteforms for ion exchange resins, for example, can be affected by leaching during the initial cementation process, resulting in the presence of mobile radionuclides in the grout matrix. Wastes containing metallic aluminium, uranium and magnox cladding may also prove problematic due to potential fracturing of the grout as a result of corrosion of the metals, or hydrogen generation from reaction with the grout, while some waste streams interfere with the cement hydration process and can significantly retard or even prevent curing of the cement. Additionally, absorbents designed to mop-up residual organic liquors from the reprocessing process will readily leach organic materials from grouted wasteforms.
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Polymeric alternatives
Polymeric encapsulation offers an alternative route, and extensive testing around the world has shown polymers to provide a number of advantages for treatment of contentious waste streams. Their superior mechanical properties allow for good waste loadings (up to 75 per cent 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. Polymeric materials are also non aqueous systems so direct corrosion of the metals by water is minimised. Moreover, they provide a good barrier to moisture transport; hydrogen can diffuse within the polymeric matrix preventing pressure build up; they are highly efficient at infiltrating round a range of shapes; they can entrain hydrophobic materials such as graphite easily; and they offer good radiation resistance.
Following successful encapsulation of wet waste simulants in a polymeric system using vinyl ester styrene (VES), this method is being used at the Trawsfynydd plant in the UK for encapsulation of ion exchange resin wastes. The first successful campaign here sentenced 656 m3 of ion exchange resins contaminated with fission products and actinides, and another two campaigns to encapsulate a further 1400m3 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.
Epoxy resins
Continuing research has led to the identification of epoxy resins as the polymers of choice for intermediate level waste (ILW) encapsulation. These have high compressive strengths (up to 175 MPa, where grouts typically yield in the region of 10-15 MPa after a 28 day cure), and can retain a good degree of integrity even after mechanical damage. Further, epoxy-based wasteforms are effectively impermeable to water and display excellent leach resistance.
Babcock, on behalf of Sellafield Ltd, 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 (this is made up of uranium bars coated with aluminium which are not compatible with grout encapsulant formulations).
Formulation and encapsulation
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. This included polymer desktop selection and desk study of polymer and material interactions; formulation and encapsulation performance; suspension and infiltration studies; short and long term wasteform performance studies for a range of simulated materials including uranium and a lithium alloy; polymer/grout interaction trials; gamma irradiation testing and alpha irradiation testing; full scale exotherm trials; full scale wasteform demonstration trials; and a range of other trials to demonstrate safety and geological disposal compliance. The various properties of the polymer have now been tested or demonstrated as a result, leading to determination and demonstration of behaviour and requirements for full scale operation, and formal Radioactive Waste Management Directorate (RWMD) recognition has been achieved.
This work has attracted significant industry interest, as use of polymers in the UK had previously been limited to a single case. Research to date has been based on high dose 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.[Page Break]
Silicone polymers
Meanwhile, 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 attractive 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 low-temperature 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. Being inorganic in nature silicone polymers should be more acceptable for disposal than organic polymers, thus presenting less of a challenge to the established ILW geological repository concept. 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.
Concept trials
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, meeting RWMD requirements for a repository. The work 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.
Infiltration tests showed that the waste material was encapsulated in a highly efficient manner and the cure rate allowed entrainment of a range of complex shapes with widely varying surface characteristics in terms of hydrophilicity and chemistry. The drop tests demonstrated that the polymers were highly capable of providing a sufficiently stable matrix to retain the waste during impact (displaying exceptional performance compared to a grouted drum). The silicones showed no significant generation of gas under irradiation, and progressively stiffened towards a glass material with increased doses, as anticipated. And importantly, heat generation on curing was found to be insignificant compared to large scale grout pours or epoxy resins.
The trials to date have shown silicone polymers to perform well as waste encapsulation materials for UK ILW, in particular where there are reactive materials present. Work is now on-going 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.
Conclusion
The various polymeric material investigations being undertaken by Babcock are currently 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.
Dr Steven Black is a technical and assurance services manager with Babcock International Group's nuclear business, headquartered in Wigmore Street, London, UK. He is based at Westlakes Science & Technology Park, Cumbria, UK. www.babcock.co.uk
