Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA EARTH SCIENCES DIVISION Weathering Products of Basic Rocks as Sorptive Materials of Natural Radionuclides

The principal requirements for employing natural minerals as buffer and backfill material in high-level waste (~W) repositories are high sorptive properties, low water penneability, relatively high thennal conductivity, and thennostability. The major task of the buffer is to prevent the penetration of radionuclides into groundwater. In most waste-isolation technologies montmorillonite with a mixture of quartz sand is considered for use as a buffer material. advantages are its low water penneability and high sorptive characteristics relative to Cs and Sr. At the same time, the results of radiogeochemical investigations indicate that montmorillonite is not an optimum sorptive barrier.

The principal requirements for employing natural minerals as buffer and backfill material in high-level waste (~W) repositories are high sorptive properties, low water penneability, relatively high thennal conductivity, and thennostability. The major task of the buffer is to prevent the penetration of radionuclides into groundwater.
In most waste-isolation technologies montmorillonite with a mixture of quartz sand is considered for use as a buffer material. Montmorillonite's advantages are its low water penneability and high sorptive characteristics relative to Cs and Sr. At the same time, the results of radiogeochemical investigations indicate that montmorillonite is not an optimum sorptive barrier. Montmorillonite is less effective than many other abundant minerals, especially those fonned in the weathering processes of basic igneous rocks. ~ W contains a wide range of radionuclides that are characterized by various geochemical properties.
Therefore, one mineral cannot provide sorption for all radionuclides. Some minerals can restrain radionuclides thanks to their cation exchange capability; others retard radionuclides because of their reductive properties or large specific surfaces. Along with montmorillonite, the weathering products of basic rocks also contain various mixed-layered minerals; hydromicas; oxides and hydroxides of Fe, Mn, and TI; and many others. All secondary minerals that are fonned at the expense of olivine, pyroxene, hornblende, and biotite have high sorptive capacity. Effective buffer materials have particularly great importance for Russia, where the existence of a great volume of HL W and a very difficult economic situation do not allow the use of expensive engineered barriers. The most efficient method of HL W disposal consists in the maximum use of natural inexpensive minerals for engineered barriers and the inherent protective capacities of host rocks.

Radiogeological prerequisites for selection of sorptive natural materials
Wide application of the fission-track radiography method for studying uranium distribution in rocks and minerals allowed us to accumulate substantial infonnation on uranium behavior in various geological processes. The behavior of uranium under granitization, regional and contact metamorphism, magmatic melt crystallization, and hydrothennal-metasomatic and hypergene processes was studied [Gavshin et al., 1975;Eliseeva, Omelianenko, 1976;Omelianenko et al., 1983]. The following results bearing on the problem of interest were obtained: ( 1) all geological processes are accompanied by -1-supply, loss, or redistribution of uranium; (2) in more than 95% of rocks uranium is ftxed in discrete minerals, while the other minerals contain practically no uranium; (3) in magmatic and high-temperature hydrothermal-metasomatic rocks uranium concentrates either in uranium minerals or, in its isomorphic state, enters into the internal crystal framework of accessory minerals; ( 4) as the temperature of geological processes falls, uranium is increasingly observed in the sorptive state; (5) all geological processes involving the formation of secondary minerals containing titanium and iron are always accompanied by uranium accumulation in these minerals; (6) the greatest amounts of such minerals form in the weathering processes of basic rocks; (7) consequently, the material from weathered basic rocks is characterized by high sorptive properties for uranium.
It is important to emphasize that uranium sorption takes place both in reductive and in oxidative conditions; thus uranium accumulation is not limited to its reductive state.
One can conclude that the other radionuclides of the transuranic group will also be intensely sorbed by the residuum of weathered basic rocks. But to verify such an assumption the properties of buffer and backfill material must be characterized for their high sorptive capacities of both Sr and Cs as well as U.
We conducted a study of Cs and Sr sorption by natural minerals and their mixtures under static conditions. To provide homogeneous samples, each specimen was broken into pieces measuring less than 0.5 mm.   (Table 2). Oxides and hydroxides of Fe, Ti, and Mn in all zones and chlorite in the middle and lower zones are present as an admixture. The data in Table 2 indicate that the montmorillonite-kaolinite zone is the most sorptive. The study of weathered rocks of the "Mayak" region is very important since the search for eff~tive sorptive material here is a particularly urgent task.
The thickness of weathered volcanic rocks varies from less than a meter to 120 meters.
The weathered part of the fractured metavolcanic rocks is the basic aquifer that determines the regional hydrological conditions in the volcanites.
Three zones in the weathering profile can be distinguished: clayey, transitional, and a zone of disintegration. The clayey zone consists of two parts: kaolinite prevails in the upper part and montmorillonite in the lower one. Mixed-layered chlorite-vermiculite is present in all zones of the weathered profile. In the transitional zone, along with supergene minerals, fragments of unaltered volcanites are present. The supergene mineral content in this zone varies from 5% to 20%. The zone of disintegration is intensively fractured and composed of fragmental volcanites that contain less than 5% supergene minerals. Besides the minerals shown in Table 3, an admixture of oxides and hydroxides of Fe, Ti, and Mn is present in every zone. The quantity of supergene minerals depends on the degree of schistosity and structure of volcanites. The intensity of supergene processes in tuffs is in general much higher than in lavas and scoriaceous lavas. It should be noted that the values of Kd for different samples of weathered volcanites vary over a wide range, and at present time we cannot explain the reasons for these variations. The quantitative mineral compositions of many samples have not yet been determined; that work is in progress. Nevertheless, we can conclude that the sorptive properties of weathered volcanites are rather high and in some places very high.
For comparison, commercial sorptive materials were also investigated (Table 4).