Waste form stability

It will be convenient to examine the underlying chemistry in terms of interactions of pairs of constituents of the repository and, finally, the waste-form stability itself ... 

Waste-Form Stability. If they occur at all, solid state transformations in dry glass and UO2 matrices will be too slow under the temperature conditions of service to be observable in the laboratory at the same temperature. Here we need to extrapolate from high temperature laboratory conditions to low temperature service conditions. It will be desirable to develop an intimate knowledge of the processes of phase separation and devitrification of sodium borosilicate glasses at temperatures below the softening point by meticulous application of electron microscopic. X-ray crystallographic and other techniques. The glasses will contain inactive elements representative of the fission product... 

Shade, J.W., "An Approach to the Prediction of Waste Form Stability", Atlantic Richfield Hanford Company Report, ARH-ST-105 (August 1974). 

Lapina, M. I. Yudintsev, S. V. 1999. Study of natural zircon-xenotime assemblages for estimation of the actinide waste forms stability. Materials Research Society Symposium Proceedings, 556, 785-792. 

The treatment of spent nuclear fuel by the IFR process results in two HLW forms ceramic and metal. The ceramic waste form stabilizes the active FPs (alkali, alkaline earth, and rare earths) and the metal waste consists of stainless steel cladding and, fuel matrix material... 

Sulfur polymer cement shows promise as an encapsulation and stabilization agent for use with low level radioactive and mixed wastes. Use of SPC allows accommodation of larger percentages of waste than PCC. As of this writing (1997), SPC-treated waste forms have met requirements of both the Nuclear Regulatory Commission (NRC) and the Environmental Protection Agency (EPA). 

For the cement stabilization option, a facility would be constmcted to dewater and treat the wastes. There would be a 7- to 21-day staging period of wastes for quality assurance operations. An online rate of 250 days a year was assumed. It is estimated that cement stabilization would result in a volume factor increase roughly 3.75 times the total volume of waste treated. This increase in volume is necessary to immobilize technetium present in the wastes and to achieve a final waste form that could withstand pressures of 500 pounds per square inch (psi). The cement would be placed in 4-ft by 4-ft by 8-ft steel containers that would serve as a mold and to facilitate the handling of the finished blocks (D114432, Appendix A). 

Chemical stability of final waste form under aerobic and anaerobic conditions. 

Perma-Fix Environmental Services, Inc. (Perma-Fix), has developed the Perma-Fix Process for the neutralization and stabilization of hazardons, radioactive, and mixed wastes. The Perma-Fix Process is a two-step treatment involving proprietary chemical treatment of wastes followed by the addition of stabilization chemicals to create a final waste form with the hazardous component of the wastes neutrahzed. The technology has been used commercially for several years. 

WASTECH, Inc., has developed a solidification and stabilization technology to produce a final waste form for soils, sludges, and liquid wastes. First, a proprietary reagent chemically bonds... 

The successful production of a nonleachable glass waste form from mixed low-level waste allows land disposal of the waste at a lower cost than the baseline technology (cementation) if it were used to stabilize the waste. This savings is primarily due to the large waste volume reductions realized during vitrification. In addition, glass final waste forms have been shown to have decreased teachability and increased structural stability as compared to the baseline waste form. 

The radiation stability of waste forms is determined by changes in their structure, chemical,... 

Malow, G., Lutze, W. Ewing, R. C. 1984. Alteration effects and leach rates of basaltic glasses implications for the long-term stability of nuclear waste form borosilicate glasses. Journal of Non-Crystalline Solids, 67, 305-321. 

Cocke, D. L. Molla, M. Y. A. 1993. The chemistry and leaching mechanisms of hazardous substances in cementitious solidification/stabilization systems. In Spence, R. D. (ed) Chemistry and Microstructure of Solidified Waste Forms. Lewis, Boca Raton, 187-242. 

Any solid waste form should serve to reduce waste volume and provide short term stabilization under conditions such as fracture, fire, or water immersion which could result from a transportation mishap. In the geologic storage scenarios currently receiving the most scrutiny, the most likely path to the biosphere has been identified as aqueous transport of nuclides via groundwater. Thus an acceptable waste form would also resist dissolution under ambient repository conditions, with the obvious benefit of assuring a sufficiently low nuclide release rate into an aquifer to preclude a significant threat to health and safety. 

The characterization work has shown that, although the titanate ceramic appears to be homogeneous on a macro scale, it is actually quite heterogeneous on an atomic scale and contains at least l4 different phases and possibly more. Since interactions of the waste form with any environment surrounding it will also occur on an atomic scale, it is important that this type of information be available when speculating on the long term stability of any waste form. 

The use of inorganic ion exchangers to solidify liquid radioactive waste followed by pressure sintering to produce a ceramic waste form appears to be a viable alternative to calcina-tion/vitrification processes. Both the process and waste form are relatively insensitive to changes in the composition of the waste feed. The stability of the ceramic waste form has been shown to be superior to vitrified wastes in leaching studies at elevated temperatures. Further studies on the effects of radiation and associated transmutation and the influence of temperature regimes associated with potential geologic repositories are needed for a more definitive comparison of crystalline and amorphous waste forms. 

Solid state chemistry of potentially important waste forms is covered in the fifth section. Solid state reactions can determine the oxidation state and physical and chemical stability of radionuclides in various host waste forms. This information can be used to evaluate the utility of crystalline materials as potential hosts for radioactive wastes. 

Zircon, complete solid-solution behavior is observed, and a plot of the unit cell volume against x shows that Vdgard s Law is followed. When the end members are not is structural, a systematic change in the solubility range in both structures is found as A is varied, and the data have been systematized in terms of a simple, potentially predictive, structure-field map. The pervasive polymorphism of these ABO4 compounds, involving both reconstructive and displacive transformations and metastable structures produced by different sample preparation methods, indicates that the crystal structural stability of substituted compounds needs to be carefully evaluated as a function of temperature to assess the structural integrity of waste-form materials. 

If the waste is isolated in a geologic repository, the iodine form should be stable to at least 100°C and possibly at 250°C depending on the repository site. If the waste form satisfies the thermal stability requirement, the most likely release mechanism then becomes leaching in the event that groundwater contacts the immobilization form. Allard et al. (11) report log Kd values for silicate minerals ranging from -0.5 to -3.5. Fried et al. (12) found little retention of iodine (as iodide or iodate) by Los Alamos Tuff. Thus, once the Iodine has been removed by leaching, it will potentially move at the same velocity as the groundwater. 

Cheng KY, Bishop P (1992) Metals distribution in solidified/stabilized waste forms after leaching. Hazard Waste Hazard Mater 9 163-171... 

Poon CS, Lio KW (1997) The limitation of the toxicity characteristic leaching procedure for evaluating, cement-based stabilized/solidified waste forms. Waste Manage 17 ... 

In most applications, a small amount of binder powders is mixed with a large volume of inexpensive hllers and then the entire mixture is stirred in water to form the reaction slurry. For example, if the phosphate binders are used for manufacturing construction products, invariably the hllers are sand, gravel, ash, soil, or some mineral waste. The phosphate binders provide adhesion between the particles of these hllers and bind them into a solid object. Thus, these mixtures mimic conventional concrete mixmres in which Portland cement binder is mixed with large volume of sand and gravel to produce cement concrete. When phosphate binders are used, the products may be termed as phosphate concrete . In waste stabilization, the waste itself becomes the hller and the hnal product is termed as a waste form . 

---