Modelling the physico-chemical speciation of plutonium in the eastern Irish Sea

A numerical model to simulate the speciation of 239,240Pu in the eastern Irish Sea is presented. The model solves the three dimensional hydrodynamic equations, using normalized σ coordinates in the vertical direction. Simultaneously, the equation for suspended matter, that includes advection, diffusion, settling, deposition and erosion of the sediment, is solved too. Thus, the tidal dispersion of non-conservative radionuclides can be simulated. Reduction and oxidation reactions are included in the model; they are described in terms of reaction velocities. It is considered that Pu can be present in solution, suspended matter and bottom sediments. Also, Pu can be in reduced and oxidized forms in each of these three phases. Thus, six equations are solved, whose solutions give the temporal evolution of Pu concentration in each phase for both reduced and oxidized forms. The model has been applied to the eastern Irish sea, where Pu is released from the Sellafield nuclear fuel reprocessing plant. Observed and computed Pu distributions in water and bottom sediments have been compared. Also, the model gives the percentage of Pu that is reduced and oxidized in each phase. Finally, distribution coefficients for total Pu (reduced plus oxidized), oxidized and reduced Pu have been calculated over the sea. Results are, in general, in agreement with observations.