Nuclear Reactor Decommissioning Process
A major technical challenge in the decommissioning of a Prototype Fast Reactor (PFR) is the safe removal of residual sodium, which is widely distributed in the primary reactor vessel after removal of the bulk sodium. A Water Vapour Nitrogen (WVN) process was proposed for cleaning the residual sodium from the reactor vessel. This is achieved by reacting the sodium with water vapour in a chemically inert (nitrogen) environment. Application of the process to the complex reactor geometry (Fig. 1) may result in localised conditions within the reactor which are outside the range of safe operating conditions determined by laboratory and pilot plant studies. Hence there is a need to gain a detailed understanding the process at a local level within the reactor.
CFD Modelling
A Computational Fluid Dynamics (CFD) study of the cleaning of the reactor using the WVN process was undertaken by Sinclair Knight Merz (SKM) on behalf of the UKAEA. The CFD model shown in Fig. 2 (note that the fuel rod assembly has been removed) consisted of 1.8 million cells and included the major reactor components and the locations of residual sodium. A multi-species and chemical reaction model was used to model the sodium-water reaction and the evolution and dispersion of gaseous components. The reaction model was extensively validated against laboratory and pilot plant tests over a wide range of operating conditions. Good agreement was obtained with the experimental measurements.
Outcomes of CFD Modelling
The flow of reactants and products inside the reactor was calculated for different WVN injection locations, injection flow-rates and other operating conditions. The CFD model allowed the distribution of flow, temperature, water vapour concentration, hydrogen concentration and sodium consumption to be determined. Examination of the results allowed the identification of such potential problems as thermal hot-spots, hydrogen-rich regions, localised pressure build-up and locations where WVN penetration is difficult. A number of design recommendations were made including: localised delivery of cooling nitrogen to reduce thermal hot-spots and dilute hydrogen, as well as multiple WVN delivery for greater penetration.