ASPECTS OF THE PERMANENT STORAGE OF URANIUM TAILINGS

Uranium tailings are generated as solid and liquid wastes in uranium m ining/milling operations. Since most of the uranium deposits in the wo rld have low grades, millions of tonnes of such wastes are produced an nually. Often, the uranium tailings are locally disposed of, using sit es with suitable conditions to construct tailings basins. The main con cern during the operation of a disposal site is the presence of radium in the liquid phase which overflows from dewatered tailings. This bar ium is precipitated by adding proper chemical reagent. The potential e nvironmental hazards of uranium tailings arise when the disposal site is abandoned after the decommissioning of the uranium mill. Huge amoun ts of solid waste as small particles of depleted ore remain in place. Top soils are usually for soil stabilization and for controlling radon emissions. Hence water infiltration through uranium tailings presents a potential hazard to underground aquifers. Water plays a dual role b y triggering a sequence of reactions and by carrying contaminants away from the wastes sites. The situation is more serious in a country lik e Canada, where the precipitation rate is higher than the evaporation rate and the water table is about one metre beneath the ground surface . For permanent disposal of uranium tailings, either the hazardous con stituents of these wastes have to be isolated from the percolating wat er, or the subsoils must have acceptable sealing and sorptive characte ristics. The first approach is effected by blending proper solidifying reagents with the tailings in order to totally block them from the le aching effects of aggressive pore water. Solid waste treatment may pro ve costly in additive usage and the blending operation. The leaching o f toxic constituents, however, becomes effective so long as the tailin gs are in a loose form. This is because of the acid-generating propert ies of the pyrite, a substantial constituent (up to 8% wt) of the tail ings. Two types of contaminants tend to leave the solid and migrate to the pore water. These are the heavy metals and the radioactive elemen ts, both of which are hazardous for their toxicity and persistence. Th e sorptive and neutralizing capacities of the underlying soils have to be carefully examined and measures have to be taken to stop the progr ession of acidic interstitial water. This paper focuses on three aspec ts of crucial importance to the permanent disposal of uranium tailings . These are: leaching in uranium tailings, sorption on natural geologi cal barriers, and radium immobilization.