The Columbia River basalts, which underlie a large portion of the Pacific Northwest of the United States of America, are being investigated as one of the candidate media for a nuclear waste repository. The Basalt Waste Isolation Project (BWIP) of Rockwell Hanford Operations (Rockwell) is conducting these investigations for the U.S. Department of Energy (DOE). Since the inception of the program in 1976, a number of studies have led to the selection of a reference repository location and the start of construction of an exploratory shaft.1-3

Geologic waste isolation systems currently under consideration for long term containment of high-level radioactive waste is based on a set of sequential barriers to release of radionuclides. Recently, Klingsberg and Duguid and Pigford have reviewed in detail the multiple-barrier disposal concept. These barriers are the waste form, canister, buffer, overpack, backfill and geologic media. Each of these barriers acts to retard ground water penetration to the repository as well as migration of radionuclides to the biosphere.

In response to draft radioactive waste disposal standards, R&D programs have been initiated in the United States which are aimed at developing and ultimately using radionuclide transport-delaying (e.g., long-lived waste containers) and radionuclide transport-controlling (e.g., very low release rate waste forms) engineered components as part of the isolation system. Before these programs proceed significantly, it seems prudent to evaluate the technical justification for development and use of sophisticated engineered components in radioactive waste isolation.

One long-term solution proposed for isolating radioactive waste from the biosphere is containment in underground geologic repositories. A requirement for this procedure is that most penetrations into the repository region be sealed following emplacement of the waste. The general requirements of repository seal materials and the seal system are that these potential pathways to the biosphere be sealed adequately to prevent a) excess fluid flow into the repository and b) radionuclides in the waste from reaching the biosphere in quantities in excess of acceptable levels. For boreholes, shafts and tunnels it is generally recommended that in the post-closure phase, the permeability of sealed boreholes and shafts be approximately the same as the host rock or the aquitards in the zones where aquitards are intersected.

Sodium and calcium bentonites, pressed to densities between 1.9 and 2.2 g/cm3, have hydraulic conductivities in the range of 10−11 to 10−13 cm/s. Batch sorption distribution ratios (Rd) indicate that Sr, Cs, and Am are strongly sorbed on bentonites and zeolites, that Np and U are moderately sorbed on bentonites and zeolites, and that Am, Np, U, I, and Tc are strongly sorbed on charcoal. Sorption results with basalt and tuff ground waters are similar; however, iodine in tuff ground water sorbs more strongly on bentonites Thermal diffusivity measurements for dry, compacted (p ∼ 2.1 g/cm3) sodium bentonite indicate that the thermal conductivity of a high density bentonite backfill should be roughly similar to that of silicate host rocks (basalt, granite, tuff). These results indicate that a bentonite backfill can significantly delay the first release of many radionuclides into the host rock and that by forming a diffusion barrier a bentonite backfill can significantly decrease the longterm release rate of radionuclides from the waste package.

The development of a suitable technique for isolating unreprocessed nuclear reactor wastes from the biosphere has led to the Swedish multibarrier concept KBS 2 with two engineered ccmponents, a thick-walled copper canister and a clay body which confines the canister, Fig. 1. The clay consists of well fitting blocks of highly compacted Na bentonite made by “isostatic” compression of bentonite powder. They are not water saturated when placed in the deposition holes but take up water from the surrounding rock, swell and ultimately form a tight contact with the rock and the canisters. When the bentonite is in physical equilibrium with the surroundings it forms a medium with a number of valuable properties, such as self-healing and ion exchange capacities. The healing means that initial joints between blocks and voids, or local passages in the clay cased by slight rock or canister displacements, will be sealed by the swelling potential of the clay.

Compacted bentonite has been proposed as a suitable backfill material in the Swedish concept for underground storage of high-level waste. The backfill barrier will serve both as a physical barrier, preventing convective water flow and allowing radionuclide penetration only by diffusion, and as a chemical barrier capable of chemically interacting with the radionuclides.

This is a preliminary report dealing with the surface analysis of reaction products formed on Ti and a Ti-Pd alloy during their exposure in hot water. The compositions of the aqueous media were varied with respect to the dissolved oxygen and the content of chloride ions. The temperature was 60°C and the exposure times were 10 min. and 6 months. Work is in progress in which samples are exposed at 80°C and 95°C in the aqueous solutions. Surface analysis was also performed on a sample which had been exposed in water-saturated bentonite.

It appears from the ESCA spectra that the oxide products formed on the surface consist of TiO The results also indicate that the thickness of the film formed at 60°C in water is in the range 50 Å to 100 Å. This is somewhat more than that obtained after exposure in water at room temperature. Exposure for 6 months increases the thickness of the oxide two to three times compared to that obtained during the short exposure at 60°C. The analyses of the samples that had been embedded in bentonite indicate that the surface reaction products are thinner than those found on the surface after exposure in an open vessel.

Groundwater circulation is in the long term probably the most important mechanism for the transport of radionuclides from deep underground repositories to the biosphere. The fate of leached long-lived nuclides, such as the actinides, is of particular interest to repository safety analysis. Both experimental and theoretical studies should lead to a comprehensive description of their behaviour in the geosphere.

In Switzerland the host rock formations currently considered for disposal of high-level wastes lie in the northern part of the country. They form the crystalline basement and are covered by substantial layers of sedimentary rocks.

A series of in-situ radionuclide migration tests are in progress in the Studsvik area on the Swedish east coast. Three well defined flowpaths have been located and characterized using non-sorbing tracers (I-131 and H-3), and one of these pathways have been used for a study of the migration of sorbing elements (Sr-85).

Laboratory sorption studies with Sr-85 have been performed on materials (rock-water) from the same location as the field tests and with variation of parameters not easily varied in-situ (e.g. pH and nuclide concentration).

A fairly good correlation was obtained between retention in natural fractures (field experiment) and laboratory column studies, although the latter were performed on crushed samples of whole rock (retention factors 15 – 30).

The concept of emplacing nuclear waste in the argillaceous sediments covering extensive regions of the ocean floor is being given serious consideration by a group of interested nations.1 For such a subseabed repository to be acceptable the effectiveness in isolating the radioactive material must be demonstrated. Since the waste form cannot be relied upon to retain its integrity in the suboceanic environment, the subseabed repository relies principally on the containment qualities of the sediment that forms the geologic barrier impeding the spread of radionuclides.

A detailed knowledge of the geochemical environment of a site for the disposal of radioactive waste is of fundamental importance. To evaluate the migration behaviour of radionuclides in geological media a series of data are needed, amongst others a number of physico-chemical properties of the media, such as permeability, porosity, dispersion-, diffusion-, and sorption characteristics. In this connection liquid chromatography appears to be advantageous as a facile experimental technique to obtain relevant data for these physico-chemical properties.

Information concerning the migration of thorium in natural environments is of interest in studies related to impact evaluations of mining and milling operations, to geochemical prospecting and at some point, to predict the long-term behavior of critical waste nuclides.

The Morro do Ferro1,2,3 is a hill on the Pocos de Caldas plateau in the state of of Minas Gerais, Brazil which, except for a few monazite beaches, may have the highest levels of natural radioactivity of any place on the surface of the earth (1–3 mK/hr). The radioactivity originates from an ore body located on the upper slopes of the hill (Fig. 1), which rises about 140 m above its surroundings to a maximum altitude of 1540 m. The ore body is estimated to contain about 20,000 metric tons of Th and a somewhat greater quantity of rare earths. The Morro do Ferro has been the site of a number of radiobiological studies conducted during the past 20 years.4,5,6,7

The mobility of radionuclides in groundwater flow systems can be significantly altered by processes such as complexation, sorption on particulates, or hydrolysis, precipitation and the formation of colloids. Such processes provide potentially significant pathways for transport of radionuclides in the biosphere. Previous soil column studies demonstrated that radiocesium could be transported by particulates and that the process was likely mediated by micro-organisms. That observation provided a plausible mechanism to explain the anomalously rapid transport of small amounts of cesium-137 released from glass blocks buried below the water table at the Chalk River Nuclear Laboratories.

The interaction between radionuclides in solution and exposed geologic media, e.g. in connection with underground storage of radioactive waste, is largely determined by the chemical properties of the system. The mobility of radionuclides in contact with low-capacity minerals would depend on the chemical state of the element (e.g. degree of hydrolysis, formation of organic and inorganic complexes, etc.). Particularly for the actinides in their lower oxidation states (III, IV), however, would a formation of colloid particles be feasible in the groundwater environment. These colloids could be either true radiocolloids, i.e. aggregates of the radionuclide itself, or pseudocolloids, i.e. colloid material already present in the groundwater, onto which the radionuclide has sorbed. These colloidal particles may be very poorly sorbed on water exposed geologic media in comparison with radionuclides in true solution. Studies on the formation and properties of some actinide colloids (Am, Np, Pu) are discussed in this paper.

The plans for the disposal of radioactive waste leave very little time for testing long term performance of a repository so the evaluation of the hazards involved in the operation relies heavily on model calculations. It is therefore of utmost importance that these model calculations take all important parameters into account and are based on a thorough understanding of the possible physical and chemical processes in which the migrating species take part.

The long-lived actinides and their daughter products largely dominate the biological hazards from spent nuclear fuel already from some 300 years after the discharge from the reactor and onwards . Therefore it is essential to make reliable assessments of the geochemistry of these elements in any concept for long-term storage of spent fuel or reprocessing waste, etc.

The migration behaviour with respect to retardation by sorption of radioactive Ce, Cs, I, Nb, Ru, Sm, Sr, Tc and Zr in fluviatile and aeolian sand aquifers has been examined in laboratory tests. In these tests it was attempted to simulate the natural conditions of the field. The tests were performed as batch and column experiments. The results of both types of experimental procedures agree within the error limits and show

- small sorption for I− and TcO4−

- medium sorption for Sr2+

- high sorption for Cs+, Ce3+, Sm3+, Ru4+, Zr4+ and Nb5+

The investigations will be continued with other geologic materials from the same site.