Radioactive wastes stabilization

Over the years, several technologies, inspired by differing ideas, have been developed to treat radioactive waste streams. Some of them are based on efficient separation methods that remove radioactive contaminants from the waste streams to reduce radiological 

Each method has its merits and drawbacks. For example, separation technologies remove some of the high-activity products from the streams and convert them into low-activity streams that are easier to stabilize, but generate separated components with high activity that also require stabilization. This two-phase approach adds to the total cost and may turn out to be very expensive. 

As in the case of hazardous contaminants discussed in Chapter 16, CBPC treatment converts radioactive constituents of waste streams into their nonleachable phosphate mineral forms. It follows the philosophy [7] that, if nature can store radioactive minerals as phosphates (apatite, monozites, etc.) without leaching them into the environment, researchers should be capable of doing the same by converting radioactive and hazardous 

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Applicability Most hazardous waste slurried in water can be mixed directly with cement, and the suspended solids will be incorporated into the rigid matrices of the hardened concrete. This process is especially effective for waste with high levels of toxic metals since at the pH of the cement mixture, most multivalent cations are converted into insoluble hydroxides or carbonates. Metal ions also may be incorporated into the crystalline structure of the cement minerals that form. Materials in the waste (such as sulfides, asbestos, latex and solid plastic wastes) may actually increase the strength and stability of the waste concrete. It is also effective for high-volume, low-toxic, radioactive wastes. 

T0088 Battelle Pacific Northwest National Laboratory, Terra-VIT Vitrification Technology T0151 Ceramic Immobilization of Radioactive Wastes—General T0169 Clemson University, Sintered Ceramic Stabilization T0178 Constructed Wetlands—General... 

Ceramicrete is an ex situ stabilization technology that uses chemically bonded phosphate ceramics to stabilize low-level radioactive waste and hazardous waste containing radionuclides and heavy metals. The technology mixes phosphates with acidic solution, causing an exothermic reaction similar to that used in forming concrete. But while concrete is based on relatively weak hydrogen and van der Waals bonding, Ceramicrete uses a combination of ionic, covalenf and van der Waals bonds to stabilize contaminants. 

The vendor claims that this technology will have four basic applications (1) on-site stabilization of high-level liquid radioactive waste (2) development of cheaper thermal and electrical energy sources (3) creation of scarce elements from more plentiful elements and (4) design and fabrication of table-top particle accelerators. 

OTD estimated that cement stabilization would produce 2,080,600 yd of stabilized wastes that could not be delisted and would have to be stored as mixed waste. The Duratek vitrification system would generate 417,000 yd of waste that may meet criteria for delisting as hazardous wastes and could be stored as only radioactive wastes (D114432, Appendix A). 

Oversby, V. M. Ringwood, A. E. 1981. Lead isotopic studies of zirconolite and perovskite and their implications for long range synroc stability. Radioactive Waste Management, 1, 289-307. 

Plodinec, M. J. 1984. Stability of radioactive waste glasses assessed from hydration thermodynamics. In McVay, G. L. (ed) Scientific Basis for Nuclear Waste Management VII. Materials Research Society Symposia Proceedings, 26, 755-762. 

Missana, T., Alonso, U. Turrero, M. J. 2002. Generation and stability of bentonite colloids at the bentonite/granite interface of a deep geological radioactive waste repository. Journal of Contaminant Hydrology, 61, 17-31. 

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. 

Recently there has been interest in the sorptive behavior of natural clays toward metal ions potentially present in radioactive wastes. Initial studies of the transplutonium elements have been carried out to define their sorption behavior with such materials ( ). However, it is also important to understand the stability of the clay-actinide product with regard to radiation damage and to be able to predict what changes in behavior may occur after exposure to radiation, so that accurate transport models may be constructed. 

The disposition of the radioactive waste resulting from spent nuclear-fuel reprocessing is one of the major problems of nuclear technology. Many of these wastes are high-level (HLW) and present a potential environmental hazard. Because of the expense associated with long-term storage, it is desirable to minimize the volume of radioactive waste and store it as material with maximal chemical stability to avoid the dissipation of radionuclides in the environment. 

The incorporation of fly ash into Portland cement has been identified as one of the treatment parameters of cement composition to be evaluated. There is already an extensive experience database on the performance of fly-ash-modified Portland cement for heavy-metal immobilization and the solidification/stabilization (S/S) of radioactive waste. The United Kingdom (Wilding, 1992) and the United States (Huang et al., 1994) have used these materials, in the form of cement grouts, for the S/S of low- and intermediate-level radioactive wastes. In this section, we will review the known benefits of fly-ash-modified Portland cement over unmodified Portland cement, along with the anticipated improvements expected by the supercritical C02 treatment of modified Portland. 

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. 

To evaluate the factors affecting the structural stability of some crystalline materials that are potential hosts for radioactive wastes, the crystal structures of a series of 3+p5 xv5+o compounds, where A is lanthanum or a member of the rare-earth series, were determined. The end-member phosphates (APO4) have the monoclinic Monazite structure (P2 /n) for A La, Ce-Gd, and the tetragonal Zircon structure (l4]/amd) for A Tb - Lu. The corresponding vanadates have the Monazite structure only for LaVO, and the Zircon structure for A = Ce - Lu. When the end members are iso-structural, e.g., LaPO /LaVO, Monazite, YbPC /YbVOA,... 

A.S. Wagh, D. Singh, and S.Y. Jeong, Chemically bonded phosphate ceramics for stabilization and solidification of mixed wastes, Hazardous and Radioactive Waste Treatment Technologies Handbook (CRC Press, Boca Raton, FL, 2001), pp. 6.3.1-6.3.18. 

In addition to the utility plant fly ash, one may also use volcanic fly ash, ash produced from burning municipal solid waste or any other combustion product that contains ash. The role of ash is also important in management of hazardous and radioactive waste because often such waste, if combustible, is incinerated to reduce its volume. The incinerated ash now is richer in inorganic hazardous components and needs to be stabilized. CBPC processes are ideal for stabilizing such ash because, phosphates are ideal materials to stabilize hazardous and radioactive contaminants, but as mentioned before, ash improves the physical and mechanical properties of the end products. Stabilization of such ashes is discussed in Chapters 16 and 17. 

D. Singh and A. Wagh, A novel low-temperature ceramic binder for fabricating value-added products from ordinary wastes and stabilizing hazardous aud radioactive wastes, Mater. TechnoL, 12 [5/6] (1997) 143-157. 

Inorganic contaminants are immobilized by washing the waste with soluble phosphates. This treatment uses a very small amount of phosphate, does not change other characteristics of the waste such as its granular nature or volume, and is relatively inexpensive. If the waste contains radioactive contaminants, phosphate washing is not sufficient because the dispersibility of the radioactive contaminant powders needs to be reduced, and hence, the waste needs to be solidified. Solidification requires generating phosphate ceramics of the waste in the form of a CBPC. In the case of radioactive waste, both stabilization and solidification are needed because they not only immobilize the contaminants, but also solidify the entire waste. As we will see in this and the next chapter, whether phosphate treatment is used only for stabilization or for both stabilization and solidification, it is very effective for a wide range of waste streams. 

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