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DISCLAIMERThis document was pnpard as M account of work sponsored by the U n i d States Govcmmmr While this document is believed to contain coma information. natha the United States G o v a n m a t nor any agency thamf. AbstractFor the successful solution of the high-level waste (HLW) problem in Russia one must take into account such factors as the existence of the great volume of accumulated HLW, the large size and variety of geological conditions in the country, and the difficult economic conditions. The most efficient method of HLW disposal consists in the maximum use of protective capacities of the geological environment and in using inexpensive natural minerals for engineered barrier construction. In this paper, the principal trends of geological investigation directed toward the solution of HLW disposal are considered. One urgent practical aim is the selection of sites in deep wells in regions where the HLW is now held in temporary storage. The a i m of long-term investigations into HLW disposal is to evaluate geological prerequisites for regional HLW repositories.

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The production of enriched uranium used in nuclear weapons and fuel for atomic power plants is accompanied by the formation of depleted uranium (DU), the amount of which annually increases by 35-40 kt. To date, more than 1.6 Mt DU has accumulated in the world. The main DU mass is stored as environ mentally hazardous uranium hexafluoride (UF 6 ), which is highly volatile and soluble in water with the for mation of hydrofluoric acid. To ensure safe UF 6 storage, it is necessary to convert this compound in chemi cally stable phases. The industrial reprocessing of UF 6 into U 3 O 8 and HF implemented in France is highly expensive. We substantiate the expediency of long term storage of depleted uranium hexafluoride in under ground repositories localized in limestone. On the basis of geochemical data and thermodynamic calcula tions, we show that interaction in the steel container-UF 6 -limestone-groundwater system gives rise to the development of a slightly alkaline reductive medium favorable for chemical reaction with formation of ura ninite (UO 2 ) and fluorite (CaF 2 ). The proposed engineering solution not only ensures safe DU storage but also makes it possible to produce uraninite, which can be utilized, if necessary, in fast neutron reactors. In the course of further investigations aimed at safe maintenance of DU, it is necessary to study the kinetics of conversion of UF 6 into stable phases, involving laboratory and field experiments.