Simulation of radionuclide dispersion in the Pacific Ocean from Mururoa atoll

A series of numerical simulations was carried out to evaluate, on a Tropical South Pacific scale, the consequences over 10 yr of radionuclide releases to the ocean from French Polynesian nuclear test sites. Tracer advection and diffusion terms were computed based on a dispersion model using ocean dynamics and turbulent mixing coefficients derived from an Ocean General Circulation Model on the basis of a yearly climatological average. Preliminary experiments demonstrated sensitivity of the results to injection depth and type. Two instant unit releases were then carried out at the model characteristic depths of 5 and 364 m: maximum concentrations decrease by factors of more than 104 and 103, respectively, over 10 yr. Temporal evolution tables for concentrations are given and can be applied to any given quantity of tracer introduced from Mururoa atoll into the Pacific Ocean. Taken as a whole, our results present, from qualitative and quantitative viewpoints, different dispersion scenarios as a function of release characteristics. Finally, a comparison with the surface release study by Ribbe and Tomczak (1990) shows that, while the order of magnitude of tracer concentrations is comparable, the direction of propagation is significantly different.