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Conference Article: Derivation of a radionuclide inventory for irradiated graphite-chlorine-36 inventory determination

F.J. Brown, J.D. Palmer, P. Wood

Abstract

The irradiation of materials in nuclear reactors results in neutron activation of component elements. Irradiated graphite wastes arise from their use in UK gas-cooled research and commercial reactor cores, and in fuel element components, where the graphite has acted as the neutron moderator. During irradiation the residual chlorine, which was used to purify the graphite during manufacture, is activated to chlorine-36. This isotope is long-lived and poorly retarded by geological barriers, and may therefore be a key radionuclide with respect to post-closure disposal facilities performance. United Kingdom Nirex Limited, currently responsible for the development of a disposal route for intermediate-level radioactive wastes in the UK, carried out a major research programme to support an overall assessment of the chlorine-36 activity of all wastes including graphite reactor components. The various UK gas cooled reactors reactors have used a range of graphite components made from diverse graphite types; this has necessitated a systematic programme to cover the wide range of graphite and production processes. The programme consisted of:

precursor measurements - on the surface and/or bulk of representative samples of relevant materials, using specially developed methods;
transfer studies - to quantify the potential for transfer of Cl-36 into and between waste streams during irradiation of graphite;
theoretical assessments - to support the calculational methodology;
actual measurements - to confirm the modelling.

For graphite, a total of 458 measurements on samples from 57 batches were performed, to provide a detailed understanding of the composition of nuclear graphite. The work has resulted in the generation of probability density functions (PDF) for the mean chlorine concentration of three classes of graphite:

fuel element graphite;
Magnox moderator and reflector graphite and AGR reflector graphite;
AGR moderator graphite.

Transfer studies have shown that a significant fraction of the chlorine is released from the graphite during irradiation, both in natural precursor and activated form. The release rate of inactive chlorine and of chlorine-36 was modelled, to allow the calculation of a residual chlorine-36 inventory in operational and decommissioning wastes; actual measurements on operational wastes have confirmed this release. This has subsequently been used to assess the effect on disposal risks. This paper provides a description of the work undertaken by Nirex to establish the residual inventory of chlorine-36 in irradiated nuclear graphite, and outline the implications for disposal of these wastes.

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key words: Gas Cooled Reactor, Nuclear Technology

Reference:: IAEA Technical Committee Meeting on "Nuclear Graphite Waste Management", held from 18-20 October 1999 in Manchester, United Kingdom; International Atomic Energy Agency, Vienna (Austria); TCM-Manchester99, pp:143-152