Waste oils

Waste oils arising from the processing of alpha-emitting materials are prob­lematic as there is limited potential for disposal as solid waste. In the UK, small quantities of some of these wastes containing very low levels of radio­activity have been allowed to be disposed of via incineration at a commer­cial site (Environment Agency, 2004). However, the incinerator’s discharge consent of 80 MBq alpha means this route is incapable of handling all the waste generated. Three methods of treating the wastes have been investi­gated: biodegradation, acid extraction and electrochemical oxidation. Bio­degradation was shown to work, but the amount of LLW generated to dispose of the residual biomass was greater than would have been produced if the oil had been absorbed onto clay granules and cemented (Taylor and Freestone, 2001). The authors consider that with further development, there are potential improvements to the process which would make it a viable process. Using the acid extraction method developed by BNFL Tech­nology Group (now the UK National Nuclear Laboratory) on uranium- contaminated oils, it was shown that repeated washings with sulphuric acid reduced the uranium concentration sufficiently for the oils to be disposed of by controlled exempt release (Environment Agency, 2008). Whilst this process successfully treated uranium-contaminated oils, it was not considered suitable for the corresponding plutonium-contaminated oils because of plutonium’s very much higher specific activity and the greater degree of decontamination which is therefore required. More recently, elec­trochemical oxidation using boron-doped diamond electrodes (Taylor et al., 2009) has been investigated for these wastes and, although it demonstrated potential, it has not been used to date with actinide-contaminated oils.

Fluidized bed steam reforming, in which superheated steam is used as the fluidizing medium, has also been suggested for treating wastes contain­ing organic species (e. g., Williams et al., 2010; Jantzen, 2006). Pyrolysis in the absence of air converts the organics to carbon dioxide.

Management of RAW from nuclear weapons programmes 793 Highly enriched uranium (HEU)

HEU is another metal of which there is a large stockpile, estimated at 1600 ± 300 tons held globally in 2009 (IPFM, 2009) but not all of which is weapons material, some being present in spent naval and research reactor fuel. The disposal method of choice for unirradiated HEU is blending down to a low enrichment and converting to reactor fuel. It is reported that between 1995 and mid-2009 Russia treated some 367 tons of weapons grade material by this method, and that the US Enrichment Corporation is purchasing 30 tons of blended material from Russia annually (IPFM, 2009).

Tritium

Being a short-lived radionuclide (half-life 12.3 years), tritium gas does not present a disposal problem as it is possible to store it in metal containers until the activity has decayed. Metals have to be carefully selected as they need to be able to cope with the effects of embrittlement by both the tritium and the 3He decay product in addition to a doubling of pressure caused by the formation of 3 He. It is also possible to immobilize the gas as a metal hydride and this can have advantages over the gaseous form for long-term storage (Holtslander and Yaraskavich, 1981; IAEA, 2004). Titanium and zirconium are two metals whose hydrides are suitable because they have low dissolution pressures and are reasonably stable to air and water.

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