The impact of tidal and mesoscale eddy advection on the long term dispersion of 99Tc from Sellafield

In the present work, numerical models are used to study the fate of the 99Tc discharges from Sellafield with a specific focus on the role of mesoscale eddy and tidal advection on the transport and dispersion of this radionuclide. Transport estimates are made with an ocean model that resolves a large part of the ocean mesoscale eddy field and also includes tides. Equivalent estimates are also computed with another model in which these processes are either absent or parametrized. Comparison with field observations shows that the coarse-resolution model can reproduce the general features of the observed time-space 99Tc distribution if the diffusivity in its eddy parametrization scheme is suitably chosen. However, the eddy-permitting simulations capture regional details better and show an overall higher prediction skill, with the model predictions agreeing with the observations within a factor of two to four. The importance of tidal advection is investigated by comparing transport in the eddy-permitting model when this is run either with tides included or with tides filtered out. The results point to systematic Lagrangian tidal drift in the Irish Sea and the North Sea that eventually impacts the 99Tc activity concentration levels also far downstream.