Analyse et prédiction de la dispersion des radioéléments dans l'environnement marin par la méthode de la réponse impulsionnelle

This work presents analysis and prediction of the dispersion of radioelements in the marine environment based on the pulse response theory. The analysis is made on three data sets over the period 1984-1988 : the amounts of gamma-emitting radioelements contained in controlled releases from the nuclear reprocessing plant of La Hague (=signal emission point), and gamma-emitting radionuclide concentrations in sea water at two stations (=signal reception points), situated at 8 and 35 km distance from the release point. Using this data base, the pulse response is determined for each station and for each element. This allows us to estimate the mean delay and the amplitude of the reception signal (i.e. the concentrations of radioelements) and its sensitivity to wind forcing. We then demonstrate that the convolution of the pulse response and released radionuclide time-series is capable of yielding a fairly good prediction of the concentrations at each station. Some particularities of the behaviour of Cs-137, Ru-106 and Co-60 are evidenced by comparison with that of Sb-125, assumed to be a conservative element in sea water: adsorption of Ru-106 and Co-60 on to particles, additional sources of Cs-137. The mean delays calculated for Sb-125 at each station are close to each other (28-30 days) despite their different distances from the release point, and are explained by the specific hydrodynamics of the studied area.