A new model to simulate the dispersion of plutonium in the eastern Irish Sea is presented. The model solves the 3D hydrodynamic equations using normalized coordinates in the vertical simultaneously with the suspended matter equation. Pu can be present in three phases: water, suspended matter and bottom sediments. Reduction and oxidation reactions are also
included in the model, in terms of reaction rates, since Pu can be present in the marine environment in principally two different oxidation states. Two kinetic models are presented to describe the transfers of radionuclides between the liquid and solid phases: a one-step model consisting of a single reversible reaction and a two-step model consisting of two consecutive reversible reactions. It has been found that both models can properly simulate the contamination of the waters and sediments from the eastern Irish Sea due to the releases from the BNFL Sellafield nuclear fuel processing plant, since the outputs from both models are very similar and in agreement with observations. Also, both models can simulate the speciation of Pu between the reduced and oxidized forms. However, if the dominant source of radionuclides to the water column is redissolution from a contaminated sediment, a process that is actually occurring in the Irish Sea, it has been found that a two-step model must be used. Indeed, a one-step model predicts the redissolution from the sediment to be unrealistically rapid.

Introduction

Over the past years, several models to simulate the dispersion of particle reactive, or nonconservative, radionuclides in the marine environment have been developed (Perianez et al., 1996a; Margvelashvily et al., 1997; Aldridge, 1998; Piasecki, 1998; Cheng et al., 2000; Perianez, 2000). These models are all based upon kinetic transfer coefficients instead of the less
appropriate equilibrium distribution coefficient, kd. Kinetic transfer coefficients, or rate constants, control forward and backward reactions for the sorption/desorption processes. Thus, it is assumed that a single reversible reaction governs the transfers of radionuclides between the dissolved and solid phases (suspended matter and bottom sediments).