Phosphate ceramic waste forms

The samples were stored for 3 weeks for curing. Each sample was then crushed and was subjected to the TCLP test. The TCLP test results on both the waste stream and the treated CBPC waste form are given in Table 16.6. The results on the untreated waste streams show that the leaching levels far exceed the regulatory limits. The results for the waste forms, on the other hand, are an order of magnitude below the EPA limit. These results indicate superior stabilization of Hg in the phosphate ceramic waste forms coupled with sulfide immobilization. 

Dacheux N, Clavier N, Le Coustumer P, Podor R (in press) Immobilization of tetravalent actinides in the TPD structrrre. Proc 10th Inti Ceramics Congress. Vincenzini P (ed) Techna Publishers, Florence, Italy Davis DD, Vance ER, McCarthy GJ (1981) Crystal chemistry and phase relations in the synthetic miner s of ceramic waste forms. II. Studies of uranirrm-containing monazites. In Scientific Basis for Nuclear Waste Management, vol. 3. Moore JG (ed) Plentrm Press, New York, p 197-200 Day DE, Wu Z, Ray CS, Hrma P (1998) Chemically durable iron phosphate glass waste forms. J Non-Crystalline Solids 241 1-12... 

Yang LJ, Komameni S, Roy R (1984) Titanium phosphate (NTP) waste form. In Nuclear Waste Management. Wicks GG, Ross WA (eds) Adv Ceramics 8 255-262 Zhao DG, Li LY, Davis LL, Weber WJ, Ewing RC (2001) Gadolinium borosilicate glass-bonded Gd-silicate apatite A glass-ceramic nuclear waste form for actinides. Hart KP, Lumpkin GR (eds) Mater Res Soc Proc 663 199-206... 

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. 

Ceramicrete cures to create final waste forms that are analogs of naturally occurring phosphate minerals. These minerals have been shown to be relatively insoluble over geologic time scales. The final waste form is stronger than typical room temperature, hydraulic cements and performs in the manner of high-temperature fused ceramics. The technology has been evaluated in bench-and operational-scale tests on contaminated wastewater, sedimenL ash, and mixed wastes. 

Low-temperature treatment of low-level mixed wastes has also been accomplished by solidification/stabilization with chemically bonded phosphate ceramics (CBPC). These are made by hydrothermal chemical reaction rather than by sintering. Chemical bonding develops when acid phosphates react with oxides to form crystalline orthophosphate (Singh et al. 1997). The ceramic matrix stabilizes the wastes by microencapsulation. The low temperature of the reaction allows volatile radionuclides to be treated (Singh et al. 1997). 

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. 

In the phosphate washing discussed above, only a small amount of acid phosphate is used to convert contaminants into their insoluble phosphate forms. To fabricate CBPC waste forms, however, a larger amount of binder is needed, so compared to the phosphate washing, the cost of the binder is high. This condition does not mean that the volume of the stabilized waste will increase. Typically, the washed waste is loosely packed, but the fully stabilized ceramic matrix is dense. As a result, the volume does not increase and, hence, the disposal cost will remain the same. Depending on the nature of the waste and amount of the phosphate binder used, the binder cost may be the only higher cost in the CBPC treatment compared to simple acid washing. 

Vitrification technology for production of waste forms is the most developed and is only presently utilized at an industrial scale. Currently actual HLW from SNF reprocessing is being vitrified with production of borosilicate glass in France and UK using an inductively-heated (200-300 kHz) metallic (Inconel-690) melter [44,45]. Replacement of the induction-heated metallic melter by a cold-crucible melter is being considered [46]. In the USA and Russia Joule-heated ceramic melters are implemented for HLW vitrification in borosilicate or phosphate [24,47-50] glasses. By the end of 2000 the total amount of vitrified radioactive waste in the world was about 10 000 MT [43]. 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

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