Nuclear Power
of the cement. Additionally, absorbents designed to mop-up residual organic liquors from the reprocessing process will readily leach organic materials from grouted wasteforms.
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. Teir 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. Teir 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. Te 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. Tese 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
www.engineerlive.com 53
Low Energy Experimental Pile (GLEEP) reactor (this is made up of uranium bars coated with aluminium which are not compatible with grout encapsulant formulations).
Fig. 2. Trawsfynydd power station, North Wales, now being decommissioned, where a VES polymeric system is being used for encapsulation of ion exchange resin wastes.
Fig 3. Trials on epoxy resin formulations are advanced at Windscale for the immobilisation of the piles fuels and isotope waste.
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