Diagnosing ocean tracer transport from Sellafield and Dounreay by equivalent diffusion and age

simple approach for estimating the equivalent diffusion for diagnosing tracer transport is proposed.Two difierent expressions are derived;one iS based directly on an analytical solution of the two-dimensionaladvection-diffnsion equation.the other uses the variance of the tracer distribution.To illustrate some featuresof the equivalent diffusion and possible applications thereof,idealized releases of passive tracers from thenuclear ruel reprocessing plants at Sellafield in the Irish Sea and Dounrea.v on the northern coast of Scotlandhave been simulated with a regionalisopycnic co-ordinate Ocean General Circulation Model.Both continuousand pulse releases are considered;the former being representative of the actual historical discharges from thereprocessing plants,the latter resembling an accidental scenario.Age tracers are included to calculate themean time elapsed since the tracers left their source regions.It iS found that in the Nordic Seas the age oftracers from Dounreay is approximately 2 years younger than the age from Sellafield.Although tracers fromboth sources eventually end up along the same transport routes,signifcant qualitative differences regardingthe dispersion properties are found.It is argued that one single parameter,the equivalent horizontal diffusion,which iS estimated to be in the range of 20-56 m2 s-1 from Sellafield and 170-485 m2 s-1 from Dounreay,determines these difierences.