An Investigation into the Decontamination of Carbon-14 from Irradiated Graphite

Gill, James (2014) An Investigation into the Decontamination of Carbon-14 from Irradiated Graphite. Doctoral thesis, University of Central Lancashire.

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The decommissioning of nuclear power plants around the world will produce a major waste stream of irradiated graphite. Graphite has been used extensively as a reactor moderator and reflector material that becomes irradiated and contaminated over time. In the coming years ~250,000 tonnes of irradiated graphite will require management making this a significant waste management issue worldwide. Irradiated graphite is categorised as Intermediate Level Waste mostly due to its content of Carbon-14 (C-14) which is a long-lived radioisotope which could be released into the biosphere. In addition the Low Level Waste (LLW) repository at Drigg has very strict guidelines regarding C-14 authorisation and there is currently no deep geological repository available in the UK. Varying amounts of carbonaceous deposits have been identified on irradiated graphite samples removed from reactor cores. If these deposits are rich in C-14, treatment of the waste graphite by oxidation could reduce the C-14 inventory of the remaining graphite. This is the primary focus of this research.
In order to investigate a technique that would decontaminate graphite from the carbonaceous deposits it was necessary to produce a range of carbonaceous deposits on virgin graphite material to act as a simulant for the deposits present on reactor graphite. Two deposition techniques were investigated: microwave plasma assisted chemical vapour deposition and a combination of solution deposition and charring. C-13 precursors were used as they facilitate the study of the selective removal of the deposit by mass spectrometry and spectroscopy. Using C-13 analogues instead of C-14 prevents the need to work in active laboratories and allows higher concentrations of deposit to be used which is beneficial when developing a technique for selective removal. A thermal treatment which utilised the application of a vacuum was investigated to determine whether the carbonaceous deposits could be selectively removed with minimal oxidation to the underlying graphite. As carbon deposits were more amorphous than crystalline graphite it was thought that they would oxidise quicker at lower temperatures than graphite. Virgin graphite and samples with deposits were characterised before and after thermal treatment using Scanning Electron Microscopy, Raman Spectroscopy, Thermal Gravimetric Analysis and Mass Spectrometry.
An additional area of investigation was conducted using thermogravimetric studies of the oxidation of irradiated graphite which was carried out at the National Nuclear Laboratory’s Preston Lab. This would determine the distribution of C-14 in the carbonaceous deposits and underlying irradiated graphite which could be a key factor in the determination of possible treatments and eventual storage/disposal routes of the waste graphite.

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