Milled graphite as a pertinent analogue of French UNGG reactor graphite waste for a CO2 gasification-based treatment

Graphite has been used in gas-cooled nuclear reactors as a neutron moderator. The dismantling of nuclear reactors will generate significant amounts of graphite waste. Neutron irradiation is responsible for C-14 formation in graphite, and it leads to severe structural and nanostructural degradations. At high neutron fluence, nanoporous turbostratic carbon is formed from original lamellar graphite. This phase is supposed to be especially C-14 enriched. An original C-14 extraction process was proposed: to decontaminate graphite waste from C-14 by selectively gasifying such degraded areas, without entirely consuming the graphite waste. To specify the operating conditions, milled graphite was used as a non-radioactive analogue. Raman microspectrometry and transmission electron microscopy techniques show that neutron irradiation and milling lead to similar multiscale organization, and especially nanoporous carbon formation. Thermogravimetry experiments were then carried out between 800 and 1100 degrees C, at a CO2 pressure of 0.1 MPa. To determine the best temperature range allowing a nanoporous component selective gasification, Raman microspectrometry analysis was coupled with transmission electron microscopy observations on the residues obtained for each gasification temperature. The 950-1000 degrees C temperature range is the most efficient allowing a complete elimination of degraded areas supposed to be representative of nuclear graphite waste C-14-rich areas. (C) 2015 Elsevier Ltd. All rights reserved.