IRRADIATED GRAPHITE | COVER STORY


Left:


Now in decommissioning, Ignalina in Lithuania contains two graphite moderated RBMK- 1500 nuclear reactors Photo credit: Ingrid Pakats/ Shutterstock.com


may include methods other than the recovery of intact components. The processing of irradiated graphite material now includes discussions of various possible pathways. For instance, processing may involve partial or complete oxidation of irradiated graphite by incineration or plasma heating or chemically using molten salt oxidation in order to enhance safety during interim storage and near-surface or deep disposal. There may also be the potential to recover and reutilise


certain graphite materials for further use within the nuclear industry; to reduce the waste category of the irradiated graphite for disposal and/or to recover specific isotopes for industrial applications. “The reason why we are looking for ways to treat


irradiated graphite is to reduce the waste classification of bulk graphite by removing radionuclides and conditioning the removed radionuclides into another waste form, explained Willie Meyer, Waste Predisposal Specialist with the Division of Nuclear Fuel Cycle and Waste Technology in the IAEA’s Department of Nuclear Energy. He noted that most of the current irradiated graphite is classed as intermediate-level waste and must be disposed deep underground. “Deep or medium-depth repositories are expensive, and few countries have such facilities… so the aim is to reduce the radioactivity levels in graphite to the low-level waste classification so that it can be disposed of in near surface disposal repositories,” he told NEI. He explained further that radioactivity in irradiated


graphite materials is due to the neutron activation of impurities present in the graphite structure itself with some contribution of air (nitrogen) found in the pore structure that could result in the increased presence of 14


C. “Research


in the past 10-20 years was aimed at the removal of the outer surface layer of the graphite – to decontaminate graphite by removing leachable radionuclides embedded on surface – without damaging the bulk graphite. In some cases, the resulting material can be re-classified as low- level waste for surface disposal. Recovered 14C and other radionuclides during this process can be reused in the industry, although the market is very small.” Not all irradiated graphite waste can be surface


decontaminated for reclassification as in some cases the contribution of embedded neutron activated impurities inside the graphite structure, and not the activity found on the surface, determines the waste class as intermediate level. For these graphite types, research concentrated on processing to reduce the disposal volume and isolate


radionuclides released during these waste processing technologies by, for example, bulk oxidation with incineration or plasma processes using sophisticated ventilation treatment systems. The current dilemma is that graphite itself is an excellent waste form for the embedded neutron activated impurities and so the cost for processing needs to be evaluated against the cost of establishing deep or medium depth repositories for these wastes. Countries pursue different strategies for the


management of graphite wastes from reactor stacks. This ranges from the lengthy ‘care-and-maintenance’ followed by ‘safe-storage’ regimes, as favoured in the UK, the US and Russia, to the immediate dismantling strategies preferred in France and Lithuania. The long-term destiny of such graphite is generally regarded as a deep or medium-depth repository, yet few countries have such facilities. As a result, in many cases available options are temporary or interim storage. Moreover, the depth of disposal impacts waste acceptance criteria, which in turn impacts the waste form, treatment and conditioning methodologies, including package selection and specifications. Kilochytska noted that deferred dismantling was the choice for most graphite reactors because of the need to find appropriate treatment and processing technology to manage the graphite and “the unclear situation on possibilities for its disposal”.


Characterising graphite Currently, the characterisation of irradiated graphite materials (modelling as well as analytical procedures) to determine the relative position and amount of the radioactivity inside is well established. This information can be used to determine management options, for instance, dismantling, decontamination, processing, or disposal as a waste form. Available dismantling, decontamination and processing methodologies have already been demonstrated successfully. In Spain, for example, the irradiated graphite waste complies with the waste acceptance criteria for their El Cabril facility for disposal. However, in the absence of available deep or medium depth repositories and appropriate processing facilities, some countries have opted for deferred dismantling or safe storage until more information becomes available as there is no ‘single solution’, not least because irradiated graphite waste differs in every country due to different reactor operational histories that affect the radioisotope content, chemical and physical characteristics.


www.neimagazine.com | February 2024 | 17


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