GEOCHEMISTRY OF ACTINIDES DURING THE LONG-TERM STORAGE AND DISPOSAL OF SPENT NUCLEAR FUEL

Abstract

The spent nuclear fuel (SNF) stored in underground repositories should be isolated from groundwater attack by engineered barriers for 500-1000 years. The short- and medium-lived radionuclides will decay in this period, and 98% of the residual radioactivity of SNF will be related to Pu and Am. The SNF contains 95-98% UO2 with isomorphous impurities of Pu and Am. Thus, uraninite stability in the geological medium is the main factor of repository safety. The analysis of data on stability and decomposition of uranium ores and experiments on uraninite dissolution in groundwater have demonstrated that this mineral is stable in uranium-saturated groundwater with reducing, almost neutral to slightly alkaline, properties. The groundwater attains such properties in the crystalline massifs at depths <500 m from the surface, which is caused by their long-term interaction with rocks under conditions of very low water exchange. The Pu and Am behavior at the oxidizing conditions, when the UO2 matrix is highly soluble and cannot retain these elements, was also considered in this paper. The oxidizing conditions could be the result of radiolysis or some other process. The extensive sorption of Pu and Am by rocks precludes the migration of these elements from a repository to the biosphere as dissolved species. However, their transportation in a colloidal form is probable. The distance and scale of transportation of radioactive colloidal particles depend on many features of the geological medium and can only be estimated experimentally in an underground laboratory. The absence of open fractures, low water permeability, the distant area of the groundwater discharge, and isolation of SNF from the host rock with bentonite backfill are efficient factors counteracting the actinide migration in colloidal form.