Nuclear Power
Management Directorate (RWMD) recognition has been achieved. Tis 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.
Fig. 4. Full scale sectioned Windscale liner showing full waste encapsulation.
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. Tis 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. Te 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
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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. Tese 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. Te 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. Te drop tests demonstrated that the polymers were highly capable of
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