Nuclear waste reprocessing to remove irradiated graphite involves a catalytic treatment using specially designed quartz glass assemblies.
Typical British graphite moderated high-temperature nuclear reactors each contain about 2,000 tonnes of graphite in large blocks that becomes irradiated with the carbon-14 isotope. The purpose of the graphite is to act as a moderate and a reflector in the nuclear fission process as well as a fuel matrix. The half-life of the carbon-14 is 5,730 years, and this poses a serious problem for its long-term disposal once the reactors reach the end of their operational lifetime. At present estimates, nuclear power plants in Britain will require the disposal of some 100,000 tonnes of irradiated graphite. Scientists estimate that at present there is over one quarter of a million tonnes of such irradiated graphite in waste storage facilities worldwide.
This type of nuclear waste is classified as intermediate-level waste, and it needs to be stored in very deep burial sites in a secure – i.e. tectonically stable – geological facility. Such a procedure is not just extremely time-consuming but also highly costly.
Reprocessing of nuclear waste extracts the carbon-14 isotope from the graphite, as it usually accumulates close to the surface of the blocks. The procedure consists of heating the blocks at a high temperature and allowing the surface carbon-14 to react with oxygen to create carbon dioxide and carbon monoxide. These gases may be further reacted with calcium and magnesium oxides to form salts or be further processed for safe disposal.
Scientists at the University of Manchester’s Nuclear Graphite Research group are working on new techniques for nuclear waste disposal. This involves a specially designed catalytic process that uses a series of quartz glass tube assemblies or about 38mm in diameter that are contained within an intricately designed manifold. Irradiated graphite is crushed into particulate size and heated to 1,200 degrees C over a distance of 850mm in the presence of a catalyst.
The Manchester University experiment has managed to extract both carbon-14 and tritium from the irradiated graphite without any contamination of the surrounding vessels. The challenge now is to make this process work efficiently on a larger industrial scale.
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