Method to assess the radionuclide inventory of irradiated graphite waste from gas-cooled reactors
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Method to assess the radionuclide inventory of irradiated graphite waste from gas-cooled reactors Bernard Poncet • Laurent Petit
Received: 20 January 2013 Akade´miai Kiado´, Budapest, Hungary 2013
Abstract About 17,000 tons of irradiated graphite waste will be produced from the decommissioning of the six gas-cooled nuclear reactors operated by Electricite´ De France Limited company (EDF), an energetic utility (http://www.edf.com/the-edf-group-42667.html) in France. Determining the radionuclide content of this waste is an important legal commitment for both safety reasons and in order to determine the best suited management strategy. As evidenced by numerous studies nuclear graphite is a very pure material, however, it cannot be considered from an analytical viewpoint as a usual homogeneous material. Radionuclide measurements in irradiated graphite exhibit very high discrepancies especially when corresponding to precursors at trace level. This huge discrepancy cannot be avoided and can be easily explained by Pierre Gy’s theory of sampling of finely divided materials. The assessment of a radionuclide inventory only based on few number of radiochemical measurements leads in most of cases to a gross over or under-estimation that can be detrimental to graphite waste management. A method using an identification calculation–measurement process is proposed in order to assess the radionuclide inventory as precisely as possible whilst guaranteeing an upper margin corresponding to a 2.5 % risk of under-assessment. This method closely reflects the reality of the main phenomenon at radionuclide origin in a reactor, while also incorporating
B. Poncet (&) EDF CIDEN, 154 Avenue Thiers, CS 60018, 69458 Lyon cedex 06, France e-mail: [email protected] L. Petit EDF R&D, Site des Renardie`res, 77818 Moret Sur Loing cedex, France
the secondary effects that can alter this result such as radionuclide or precursor release during reactor operation. Keywords Nuclear graphite Radioactive waste Decommissioning Radionuclide inventory
Introduction Graphite waste management and radionuclide inventory Electricite´ De France (EDF) operated six gas-cooled reactors in France, all now shutdown for at least 15 years. These reactors are called in French, ‘‘uranium naturel graphite gaz’’ reactor type (UNGG). They were graphite moderated, cooled by carbon dioxide and fuelled with metallic natural uranium. UNGG reactors will be dismantled as soon as possible (beginning in 2025 with the removal of the first graphite blocks from the Bugey 1 pile according to the most recent planning). The design of UNGG reactors is, in principle, very close to that of the British Magnox reactors that were developed independently at the same period in United Kingdom. In the absence of uranium enrichment, graphite was used as a moderating material with a very high level of purity due to the necessity of the highest neutron transparency. Graphite was also chosen as a mechanical support of the fuel cartridges (graphite sleeves) and as a biological shielding in
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