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Title: Understanding the distribution of carbon-14 in irradiated reactor graphite in relation to geological waste disposal
Author: Payne, Liam
ISNI:       0000 0004 5916 8887
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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The decommissioning of the UK fleet of Magnox nuclear power stations will require the disposal of some 57,000 tonnes of irradiated graphite from the reactor cores. Currently, this graphite is classified as intermediate level waste due to the incorporation of long half-life radioisotopes such as carbon-14 and chlorine-36. The plan for disposal of such waste is to place it in an engineered geological disposal facility, and currently the graphite accounts for approximately 15% of the total volume of the UK intermediate level waste inventory. An understanding of the concentration, distribution and potential release of carbon-14 from the graphite is critical for ensuring the safe disposal of this ,vaste. In this study 49 irradiated PGA graphite samples trepanned from the fuel and interstitial channels of Oldbury Reactor One were examined. They were compared with virgin PGA graphite used to fabricate the core. It was observed that the graphite exposed to the service environment had a significantly different structure to the virgin PGA. A distinct surface deposit of up to a few tens of micrometre thickness was observed on both the fuel and interstitial channel wall faces. In subsequent experiments the carbon- 14 concentration and distribution were determined using two separate techniques, one based on mass spectrometry and the other using thermal oxidation and subsequent . scintillation counting. The main observations from this work were that the surface deposit was relatively enriched in carbon-14 compared to the underlying graphite, with a higher concentration measured on samples originating lower in the fuel channels. The carbon-14 content measured was not correlated with the lifetime neutron dose and was therefore likely to have arisen from transmutation of the precursor species present in the coolant gas. The remaining carbon-14 was determined to be split between two fractions, firstly, a fraction originating from nitrogen surface complexes formed on the surface and near sub-surface, that may be released slowly in a geological disposal facility, and a second fraction from nitrogen and carbon-13 precursors located in the graphite lattice, that may never be released. These results correlate well with previous reports in the literature on the leaching of irradiated graphite and results from this work may provide useful inputs into radioactive release models as well as developing analytical techniques that can be used in the future examination of graphite arising from different Magnox reactors to determine the total fraction of carbon-14 that may be released from a geological disposal facility.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available