The materials science of radioactive waste forms and containment materials has long been a subject of interest to the Materials Research Society. One of the earliest (and continuing) MRS symposia, the Scientific Basis for Nuclear Waste Management, has been held 18 times since 1978. This symposium rotates abroad every third year: Berlin in 1982, Stockholm in 1985, Berlin in 1988, Strasbourg in 1991, and Kyoto this past October. Nearly 170 papers were presented at the Kyoto meeting.

Materials science issues for nuclear waste disposal are unique in their scale and consequences. The wastes include an extremely wide variety of materials: spent nuclear fuel from commercial and research reactors; high-level liquid waste produced at West Valley, New York, during the reprocessing of commercial spent nuclear fuel; high-level waste (HLW) generated by the nuclear weapons program; nearly pure plutonium from the dismantling of nuclear weapons; highly enriched uranium from weapons; low-level, medium-level, and mixed waste from laboratories and medical facilities; and, finally, mill tailings from uranium mines and the residues from chemical processing, such as the radium-bearing filtrate presently in storage at Fernald, Ohio, and Niagara Falls, New York. Some material can be simply stabilized and monitored in situ, as is done for most uranium mill tailings and residues, but other materials require retrieval, processing, immobilization, and permanent disposal. The volumes of material that will require handling, immobilization, and disposal are enormous. In the United States, much of the weapons program waste is stored in tanks at Hanford, Washington and Savannah River, South Carolina.

Borosilicate glass is the principal solid matrix for immobilizing 99% of the highly radioactive, heat-generating nuclides extracted by reprocessing spent nuclear fuel. Production of the glass has begun in several countries, yet no final disposal site is available anywhere in the world. This is due partly to political issues and partly to the difficulties of credibly demonstrating that nuclear waste can be safely isolated in deep underground repositories for hundreds of generations. The release of hazardous quantities of radionuclides from a repository is prevented by a multiple barrier containment system, including a central engineered system consisting of the canistered glass, an overpack, and backfill materials. If the glass could retain all radionuclides upon contact with groundwater, it would not be necessary to demonstrate that geological isolation is safe. However, the glass corrodes slowly in water and humid air, and inevitably, certain quantities of radionuclides are mobilized. The glass is not inherently corrosion-resistant, but rather depends on the waste package and on surrounding geochemical and hydrological constraints. The difficulty is predicting the release/retention of radionuclides for long time periods while considering the interactions with other engineered and natural barriers.

The geologic disposal of spent nuclear fuel is currently under consideration in many countries. Most of this fuel is in the form of assemblies of zirconium-alloy-clad rods containing enriched (1–4% 235U) or natural (0.71% 235U) uranium oxide pellets. Approximately 135,000 Mg are presently in temporary storage facilities throughout the world in nations with commercial nuclear power stations.

Safe geologic disposal of nuclear waste could be achieved using a combination of a natural barrier (the host rock of the repository) and engineered barriers, which would include a low-solubility waste form, long-lived containers, and clay- and cement-based barriers surrounding the waste containers and sealing the excavations.

A requirement in evaluating the safety of disposal of nuclear waste is a knowledge of the kinetics and mechanism of dissolution of the waste form in groundwater and the solubility of the waste form constituents. In the case of spent nuclear fuel, this means developing an understanding of fuel microstructure, its impact on release of contained fission products, and the dissolution behavior of spent fuel and of UO2, the principal constituent of the fuel.

Synroc, a ceramic made from a reactive mixture of Al, Ba, Ca, Ti, and Zr oxides, is proving to be a suitable and effective medium for immobilizing nuclear wastes.

Synroc-C, a titanate-based ceramic variant, was initially developed in 1978 by Ringwood et al. for immobilizing high-level nuclear waste (HLW) from nuclear power reactor fuel reprocessing. HLW is essentially a solution of radioactive fission products, actinides, and process contaminants in ~3 mol/L nitric acid. The developers of Synroc-C aimed to immobilize radioactive waste ions by incorporating them in a ceramic. They accomplished this by mixing the HLW solution (liquid waste) with a ceramic precursor, then forming the ceramic by drying, calcining, and hot-pressing the mixture in a metal container for two hours at 1200°C/20 MPa. The result, Synroc-C, is composed of hol-landite, zirconolite, perovskite, and rutile, together with a few percent of minor phases and metal alloys. The Synroc-C precursor has the following composition (wt%): Al203(5.4); BaO(5.6); CaO(11); TiO2(71.4); and ZrO2(6.6). Since 1984, it has been prepared by hydrolyzing a mixture of Al, Ti, and Zr alkoxides with an aqueous slurry of Ba and Ca hydroxide. The abundances of the phases, and the radionuclides contained in them in dilute solid solution, are identified in Table I.

Depending on their radioactive content and abundance of long-lived radionuclides, radioactive wastes are often described as low, intermediate or high-level. Cements play a major role in the engineered structures, existing and planned, of most national programs for low- and intermediate-level (ILW) radioactive wastes. Final disposal of ILW is usually by burial at considerable depth (>250 meters), e.g., in planned repositories in clay at Mol (Belgium), in salt at Gorleben (Germany), and in volcanic tuffs at Sellafield (United Kingdom). A sample disposal concept is shown in Figure 1. Shallow land burial is also employed, mostly for low-level wastes (LLW) and often in concrete-lined vaults, e.g., at Drigg (UK) and Center de la Manche (France). Cements are likely to be used in a waste repository as structural elements in the engineered structure and for encapsulation of the actual waste itself (see Figure 1). In the UK, cements are also the favored material for infilling of the vault space (backfilling), and sealing.

LLW and ILW are characterized by considerable heterogeneity, and comprise liquids, solids, floes, sludges, exchange resins, and contaminated laboratory equipment. Cements are capable of converting most of these waste streams into solid and stable monoliths that can be further encapsulated in a steel or concrete container. Such a product is ideal for interim storage, transportation, and final emplacement in a repository. The technology of cementation is well-established, and suited to automation and remote handling (thereby reducing the radiation dose to workers).

Selecting materials to contain high-level nuclear waste presents an unparalleled challenge to materials scientists and engineers because of the very long times required for containing the waste and the high degree of certainty required when predicting a container's long-term performance. Material endurance without severe degradation is the primary property sought in container materials. There are, of course, additional factors related to mechanical properties, fabricability and weldability, and economics, but these factors are usually secondary to the chemical properties of the material. Metals and alloys have been given prime consideration as container materials because metal materials are ductile and tough, they can be easily fabricated in mass quantities and large container sizes, and they can be readily joined to create intact gas-tight seals between parts.

In selecting a suitable container material, the container's environment is the most important consideration. Several geological formations and resulting geochemical environments are under consideration as disposal sites in different countries. This short review will discuss the principles governing corrosion of the primary container materials being considered in various national nuclear waste disposal programs and will show how this background can guide materials selection. Important candidate materials are then considered along with examples of how these materials will be expected to perform under repository conditions existing in a range of national programs.

The concept for the disposal of high-level radioactive waste (HLW) in Switzerland is illustrated in Figure 1: the waste, solidified in a borosilicate glass matrix, is encapsulated in a 25-cm-thick steel canister which has a minimum life expectancy of 1,000 years. After this time, the heat produced by 90Sr and 137Cs will have dissipated and the repository will have adjusted to the host rock ambient temperature of around 60°C.

The steel canisters will be placed in horizontal tunnels, 3.7 m in diameter, at a depth of around 1,000 m in the granite of northern Switzerland, and the tunnels then backfilled. The backfill material, an important link in the chain of engineered and natural safety barriers, satisfies both physical/mechanical and chemical requirements. The most important of these are:

• Low hydraulic conductivity compared to the host rock;

• Good swelling properties to seal construction-caused joints and rock fractures;

• Plasticity to absorb rock movements and to distribute pressure homogeneously;

• Good retention of radionuclides; and

• Stability over a period of at least 106 years.

Economic aspects and availability should also be considered: each canister requires 88 tons of backfill material, or of the order of 250,000 metric tons for the planned repository.

Immobilization and geological isolation are the general goals of nuclear waste disposal. The appropriate isolation of immobilized long-lived radionuclides, mostly actinide elements, from interaction with water for a period of many thousands of years in any given geological formation is a scientific and technological challenge. This challenge can be met by considering a multibarrier concept that appraises engineered, geoengineered, and geological barriers designed to confine the migration of long-lived actinides. Once interaction with water occurs, one may expect many complex geochemical reactions of immobilized actinides; actinides will therefore be distributed into two kinds of chemical states, mobile and immobile species. Reflecting results in laboratory systems, the two species are expected to be chemical partners in equilibrium reactions. In multicomponent geochemical systems, however, the chemistry of the mobile and immobile species of actinides in trace concentration is, in general, masked by geochemical reactions of natural aquatic constituents, which are found in much higher concentrations. The mobile species are either complexed anions or colloidal species, whereas the immobile species are new solid phases of very low solubility, which are thermodynamically better stabilized than in the original host phases.

A question sometimes posed to researchers in silicon is: Why are you still working on silicon? How much more do we need to learn? Not only is Si a relatively simple element, it is one of the most common in the earth's crust. This element has been very extensively studied. One reason we keep studying silicon is because it is not just an element, it is also an industry. The microelectronics business has a massive economic impact on our lives. From cars to washing machines, almost all consumer goods contain silicon microprocessors in some form. Our daily lives depend on the stuff. Figure 1 shows the predominance of Si in a breakdown of the microelectronics industry. In 1994 microelectronics has been a $1 × 1011-per year industry worldwide, and it is still growing rapidly. CMOS, or complementary metal-oxide-semiconductor transistors, dominate this business, some being bipolar. The entire compound semiconductor industry is a small part of this picture, composing 2% of the business (although $2 billion is not bad for a “niche” market). The $100 billion business built around silicon makes it the most important element in world economics, although there is some evidence that C and O may also be important for the support of our daily lives. One thing, however, is clear-the economic need to understand silicon and everything it does is tremendous.