Dissolved radionuclide measurements used for qualitative and quantitative calibration of hydrodynamic models in the English Channel and the North Sea ; validation of “TRANSMER” model.

BAILLY DU BOIS P. DUMAS F. "Tracer methods in geophysical fluid dynamics" 34th International Liège Colloquium on Ocean Dynamics - Liège, May 6-10, 2002.

Dissolved radionuclides released by nuclear fuel reprocessing plants are very powerful tools to study the movements of water masses on a scale of European continental shelf and calibration of hydrodynamic models. Some radionuclides as antimony 125 have only one significant source terms with well known characteristics, a conservative behaviour during several years, and it is possible to measure it from the English Channel to the Norwegian Sea. Use of this tracers for qualitative and quantitative determination of water masses circulation, fluxes and dilution factors in the English Channel and North Sea have been already presented [Bailly du Bois et al, 1993 – 1999, Guéguéniat et al, 1994]. It has also contributed to the validation of long-term hydrodynamic models based on the “lagrangian barycentric method” [Salomon et al, 1988 – 1995, Breton and Salomon, 1995]. A two-dimensional hydrodynamic model called “TRANSMER” has been developed to simulate the dispersion of the seawater and transfers to the living species and sediments. This model is used for research purposes as studies of transfer functions between seawater and living species, and for evaluation of consequences of releases of radionuclides in marine ecosystem in normal or accidental conditions. It extends from the Loire estuary (Gascogne Gulf) to Denmark, including the English Channel, South of the North Sea and the Irish Sea with a mesh size of one kilometre. TRANSMER benefits of a very large dataset for its validation, including 1400 measurements performed in this area between 1987 and 1994. These data allow qualitative and quantitative exploitation in order to compare the measured and calculated fluxes and quantities of tracers in the English Channel and the North Sea. The main radionuclide usable for this calibration is antimony 125. Tritium, technetium 99 and caesium 137 data have also been used. The only adjustment parameter of this model is the calculation of the wind stress at sea surface. The best fitting between measurements and calculations has been obtained by multiplying wind speed by a factor 1.5. Such correction reveals that some important phenomena, associated with meteorological events, are not reproduced by this kind of model. Works are in progress in order to solve this question. At this state of calibration, the fluxes of radionuclides and water masses in the English Channel and the North Sea are balanced for the whole period of field measurements (1987 – 1994); the correlation factor between the 1400 individual measurements in seawater and calculation results is 0.88 with an average error of 54%. The error attributable to the measurement process only is 20% in average.