Mathematical modeling of ozone assisted cerium redox process for the surface removal of stainless steel components using a static mixer as gas-liquid contactor

Abstract

This paper deals with a cerium based redox chemical process which is used for the surface removal and decontamination of stainless steel components arising from the operation and decommissioning of nuclear facilities. The process involves the corrosion of top contaminated metal surface (∼10–50 μm) by highly oxidizing Ce(IV) ions, in order to remove the entrapped radionuclides. The reduced Ce(III) ions thus generated after the oxidation of metal surface are continuously oxidized back to Ce(IV) by ozone. In this work, a lumped parameter model, accounting for the reduction of Ce(IV) to Ce(III) by steel components and the simultaneous oxidation of Ce(III) to Ce(IV) by ozone in a static mixer, has been developed. The model is experimentally validated by carrying out a corrosion experiment at 5 L scale in ∼0.4 M solution of Ce(IV) in 4 M HNO₃ at room temperature, by using non-radioactive AISI SS304L stainless steel components of different geometries viz. plate, pipe, elbow and T-Joint with a total surface area of 348 cm². Using the validated model, design simulations of a pilot metal decontamination facility are carried out, to illustrate the effect of process parameters on the equilibrium Ce(IV) concentration in the loop, which is critical to component corrosion and decontamination. Simulation results show that, for a given initial cerium salt concentration, increasing the gas/liquid flow rate, ozone concentration, initial nitric acid concentration, and solution volume reduces the rate of fall of equilibrium Ce(IV) concentration with time. However, an increase in the temperature and surface area of the components enhances the rate of fall of equilibrium concentration of Ce(IV) with time. Additionally, reducing acidity of nitric acid has been found to limit the treatment time of components. Furthermore, the choice of a Ce(III) or Ce(IV) salt, as a source of cerium ions, has been shown to have no effect on the corrosion of metal components in a long run, when ozone regeneration of Ce(IV) is employed. Among the components of various geometries, relatively higher corrosion rates have been observed for the components with a curved geometry or a weld joint. SEM images of the welded and non-welded components show the occurrence of intergranular corrosion due to ozonated Ce(IV) solution, which is the likely mechanism for the removal of radionuclides from the metal surface.