Thermal oxidation of nuclear graphite: A large scale waste treatment option

Using molybdenum carbiding to induce digestion of carbon in H2O2: A sustainable approach to eliminate radioactivity for hazardous graphite waste inherited from nuclear enterprise

To properly manage nuclear wastes is critical to sustainable utilization of nuclear power and environment health. Here, we show an innovative carbiding strategy for sustainable management of radioactive graphite through digestion of carbon in H₂O₂. The combined action of intermolecular oxidation of graphite by MoO₃ and molybdenum carbiding demonstrates success in gasifying graphite and sequestrating uranium for a simulated uranium-contaminated graphite waste. The carbiding process plays a triple role: (1) converting graphite into atomic carbon digestible in H₂O₂, (2) generating oxalic ligands in the presence of H₂O₂ to favor U-precipitation, and (3) delivering oxalic ligands to coordinate to Mo^VI-oxo anionic species to improve sample batching capacity. We demonstrate > 99% of uranium to be sequestrated for the simulated waste with graphite matrix completely gasifying while no detectable U-migration occurred during operation. This method has further been extended to removal of surface carbon layers for graphite monolith and thus can be used to decontaminate monolithic graphite waste with emission of a minimal amount of secondary waste. We believe this work not only provides a sustainable approach to tackle the managing issue of heavily metal contaminated graphite waste, but also indicates a promising methodology toward surface decontamination for irradiated graphite in general.

Single-Phase Flow Model of a Screw Reactor for Decontamination of Radioactive Graphite Waste Using Surface Gasification

A screw reactor is a promising apparatus for decontaminating radioactive graphite waste by uniform gasification under ambient air. However, developing the design equation for a screw reactor is difficult due to the reactor’s fundamentally intricate gas and solid interactions. In this study, we performed three-dimensional computational fluid dynamics simulations to predict and characterize the graphite particles that flow through the screw reactor and are thermally gasified. This was done using the Eulerian single-fluid approach coupled with the experimentally established kinetic model for graphite gasification. The numerical results show that the counter-rotating flow, generated along the rotating screw of the reactor by the relative motion of the reactor wall to the rotating screw, mixes particles spatially and reduces their axial velocity. The diameter of the feed graphite particles can be reduced by as much as 28% depending on the screw rotating velocity and the temperature of the reactor shell, according to the conducted numerical calculations. These numerical simulations can be used to provide proper operating parameters for the laboratory-scale screw reactor by which to decontaminate radioactive graphite waste by gasifying the radiocarbons, together with a part of the graphite matrix, on the surface of the graphite particles.

Characterizing thermal-oxidation behaviors of nuclear graphite by combining O2 supply and micro surface area of graphite

The effects of different parameters on oxidation rate are non-linear, interactive and diversified in which the change of adequacy of O₂ supply is an important indicator. The influence of microstructure on oxidation rate became stronger worsening the fitting linearity to calculate the activation energy based on present method with the decreased adequacy of O₂ supply due to the increase of temperature, the decrease of gas flow rate, etc. Here, we proposed a method to characterize thermal-oxidation behaviors of nuclear graphite by combining O₂ supply and micro surface area of graphite. The proposed method improved the linearity and reduced the standard error of Arrhenius plots of oxidized graphite IG-110 (10 L/min reactant gas) and ET-10 (0.2 L/min reactant gas). The value of activation energy of graphite IG-110 oxidized under ASTM D7542 condition is calculated as 220 kJ/mol by this method echoing the results of previous studies with sufficient O₂ supply. For the conditions with less O₂ supply at low gas flow rate and/or high temperature, the change of microstructure of oxidized graphite should be obtained as an important factor influencing oxidation rate of graphite.