9OSr dispersion in the geosphere from a waste burial in the Chernobyl exclusion zone.

L.Dewiere, D. Bugai, V. Kashparov, N.Ahamdach International Conference in the Radioactivity in the Environment - 2/5 sept 2002 - Monaco.

A field study is being conducted of hydrologic and geochemical processes and parameters governing 90Sr migration from one trench of waste to the sandy aquifer at the Chernobyl Pilot Site located at "Red Forest", 2.5 km westward the Chernobyl NPP. Radionuclides are leached by meteoric water from the trench containing reactor fuel hot particles, and infiltrate the sandy aquifer. Experimental results for the years 1999-2001 are presented from multidisciplinary radiological and hydrogeological studies of processes and parameters controlling 90Sr releases from the trench containing nuclear fuel particles and subsequent radionuclide transport in the underlying eolian and alluvial sandy aquifer at Chernobyl Pilot Site. Microscopic analyses of fuel particles separated from waste have identified two families of particles: U-O and Zr-U-O. The Zr-containing particles exhibit low dissolution rate, therefore radionuclide inventory in source term available for migration is significantly less than estimated before. The 90Sr migration velocity in the eolian sand layer is estimated at 17% of the groundwater averaged microscopic velocity (Kd 3 ml/g). Alluvial sediments comprising the middle part of the aquifer have notably higher sorption capacity (Kd 20 ml/g), and may represent essential natural sorption barrier to geo-migration. Radioactivity balance calculations show that 4-7% of initial trench inventory of 90Sr has migrated by now to the geological environment. Substantial results from modeling and experimental validation business have been produced since the last year. Modeling has been focused on the source term of radionuclides from the fuel particles in conjunction with transport modeling in groundwater. Also, a technique of stochastic model analyses for transport parameters estimation from the permeability field has been faced to the results of a tracer test in natural flow field conditions. This set of experiments and modeling has to be seen as the development of the experimental validation approach in the frame of the Chernobyl Pilot Site project. The final product will be an increase of our understanding of radionuclides behavior in contaminated area. It is in the concern of both the parameters identification and measurement, and the associated modeling.