Radionuclide release and transport from nuclear underground tests performed at Mururoa and Fangataufa - predictions under uncertainty

In the context of a study by the International Geomechanical Commission (IG C) and the International Atomic Energy Agency (IAEA) on the effects of nucl ear tests at the: atolls of Mururoa and Fangataufa, release to the biospher e is estimated for 35 radionuclides originating from 147 nuclear undergroun d tests. Based on a qualitatively characterised hydrogeological situation o f atolls and relatively scarce site-specific data, a model chain was develo ped to conservatively estimate the radionuclide fluxes via groundwater, fro m their sources, the explosion cavities. towards the biosphere, the ocean o r lagoon. Finite element hydro-thermal modelling was used to describe water flow. Par ameters were calibrated by a very few measured pre-test temperature profile s in bore holes. The impact of the tests on groundwater flow and mechanical impact on rock was considered. Estimates were made to quantify spatial ext ensions and temporal evolution of impact by using measurements on refilling rate of the cavities. Tests were categorised according to their specific y ield and location although detailed data were missing. A base case paramete r set was defined for the hydraulic conditions and for the initial radionuc lide inventory of individual tests. Models were used to describe the concen tration of radionuclides in the cavities as a function of time. Radionuclid e transport from the cavities to the biosphere was represented by two diffe rent approaches: a double porosity model for the fractured volcanic rock an d a single porosity model for the overlaying, highly porous carbonates. Res ults consist of conservative estimates on radionuclide release into the env ironment, or concentration in the lagoon or ocean water. Their sensitivity was investigated using different models and parameters. A few measured data (concentrations in a few cavities, in the deep carbonates and in the lagoo ns for selected radionuclides, such as H-3, C-14, Cl-36, Sr-90, I-129, Cs-1 37(239), Pu-240 and Am-241) were available for a comparison with the calcul ations. In view of the lack and uncertainty of site-specific data, the agre ement is of acceptable quality. (C) 2001 Elsevier Science B.V. All rights r eserved.