Abstract
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.
Highlights
Graphite Waste Using SurfaceThroughout the world, first-generation nuclear power plants are approaching the end of their designed service life, and some of them have already ended their service
The intrinsically complex interaction among solid particles, atmospheric gas, and the physical structure of a screw reactor raises obstacles to developing an analytical tool for predicting the thermal gasification of the pretreated radioactive graphite waste flowing through the screw reactor
When using the Arrhenius equation to convert the dynamic weight loss of graphite powder measured by a thermogravimetric analyzer (TGA) to fractional conversion α, the instantaneous conversion rate is represented as a function of temperature T [28]: dα
Summary
Throughout the world, first-generation nuclear power plants are approaching the end of their designed service life, and some of them have already ended their service. The simple design and easy operation of a screw reactor offer effective control for removing radioactive carbon selectively (using thermal gasification in an oxidizing atmosphere) from the surface of pretreated graphite particles. The intrinsically complex interaction among solid particles, atmospheric gas, and the physical structure of a screw reactor raises obstacles to developing an analytical tool for predicting the thermal gasification of the pretreated radioactive graphite waste flowing through the screw reactor. The granular graphite was handled as a single-phase fluid in order to compute effectively the granular graphite flow in the screw reactor This method is capable of accurately describing complex flow and a progressive gasification reaction that cannot be examined in physical experiments.
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