Hazardous or toxic waste destruction or containment – Destruction or containment of radioactive waste – By fixation in stable solid media
Reexamination Certificate
2000-11-27
2002-10-29
Silverman, Stanley S. (Department: 1754)
Hazardous or toxic waste destruction or containment
Destruction or containment of radioactive waste
By fixation in stable solid media
C588S002000, C588S252000
Reexamination Certificate
active
06472579
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is related to processing liquid radioactive waste and other hazardous liquid waste by means of its incorporation into a porous glass-ceramic block matrix. The invention can be applied, for example, in nuclear power engineering and defense production activities for solidification of liquid radioactive waste of various levels of radioactivity, including homogenous process solutions and heterogeneous finely dispersed sludge. It also can be used in the chemical and metallurgical industries for immobilization, transportation, decontamination and disposal of extremely hazardous liquid waste, containing heavy metals (Pb, As, Be, Ni, Cd, etc.)
The most common method for handling high-level radioactive waste (radwaste) is disposal of its solidified forms in deep geological formations with application of the multi-barrier principle for protection of the biosphere, in accordance with which, the confinement of the waste should be provided by several barriers. The primary barrier that ensures integrity of the high-level waste disposal system is provided by a high stability of the solidified waste form in groundwater, at high temperatures, and in radiation fields.
A promising method for production of stable solidified forms of high-level radwaste is considered to be incorporation of radionuclides into stable ceramic matrices with generation of man-made mineral-like compounds that have a low leach rate of radioactive components in water (10
−6
−10
−7
g/cm
2
×day). To accomplish this, liquid radioactive concentrates are exposed to evaporation and calcination, which produces thermal decomposition of the waste components subject to radiolysis (e.g., nitrates), followed by sintering or hot pressing of the calcined products with various additives.
It is a challenge to solidify liquid radwaste because the radioactive components have to be evenly distributed in the matrix structure. One solution to this problem is to use porous ceramic materials with a homogenous porous structure and a large share of free available volume. The loading of such porous material with radioactive waste solutions and further steps of moisture removal by evaporation and salt calcination in the pores make it possible to achieve homogenous distribution and immobilization of radionuclides in the matrix volume. The known radioactive waste solidification methods, using such porous materials, primarily include the use of microporous adsorbents, for example zeolites and silica gel, as well as foam corundum, porous fireclay (chamotte), diatomite clays, and porous silicate glasses which, after being loaded with radionuclides, are mixed as sludge with various solidifiers or are exposed to hot pressing, melting, or calcination. The weakness of those methods is that their applications are limited since they cannot be used for all liquid radioactive waste compositions and activity levels. Beyond that, these microporous materials are not capable of functioning over a long period of time with actual liquid waste compositions containing suspensions and non-radioactive salt macrocomponents because the suspensions and salt macrocomponents tend to clog the pores, thereby decreasing the loading capability of the solid adsorbents. It is also important to note that it is very difficult to achieve complete dehydration of these microporous materials because it would require temperatures in the range of 500° C. and most microporous materials are unstable at these temperatures in the acid media which is often typical of liquid radwaste. Also, these matrices are saturated with the waste components only at solution boiling temperatures, and heating at the boiling point is necessary to dry the solid. This is undesirable because of the potential for radionuclide aerosol formation during the drying process.
One known method for treatment of radwaste to remove radionuclides requires dripping liquid radioactive waste through a sorbent installed in a filter. The filter is filled with a porous matrix to be treated to generate the sorbent in the form of a porous molded block. The liquid radioactive waste first goes through the central zone of the filter and then through the periphery zone, and, ultimately, the spent sorbent is sent to disposal with the filter. Granulated silica gel has been proposed to be used as a material for the porous molded block, mixed with an inorganic pseudoboehmite-based binder, porous sodium silicate glass and porous iron. One of the disadvantages of this process is a high leach rate of radionuclides from the porous molded block (≧10
−3
g/cm
2
×days), which makes it unsuitable for long-term disposal. Furthermore, this process does not completely solve the problem of improving the environmental situation at a nuclear facility since it implies additional treatment of the filtrates with a high salt content (up to 320 g/l). Therefore, there is a need for an effective processing method of removal and solidification of radwaste and other hazardous material, wherein the stability of the solidified form is increased.
SUMMARY OF THE INVENTION
Solidification of liquid radioactive waste, and other hazardous wastes, is accomplished by the method of the invention whereby the waste is incorporated into a porous molded glass crystalline block which is first loaded with liquid waste and then dehydrated and exposed to thermal treatment at 50-1,000° C. The porous molded glass crystalline block consists of glass crystalline hollow microspheres separated from fly ash (cenospheres), resulting from incineration of fossil plant coals. In one embodiment, porous glass crystalline blocks are formed from cenospheres of a particular desired composition, wherein the selected cenospheres are consolidated into a porous molded block with a binder, such as liquid silicate glass. The porous blocks are then subjected to repeated cycles of saturating with liquid radioactive wastes, and drying, and after the last cycle the blocks are subjected to calcination to transform the dried salts to more stable oxides.
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Aloy Albert S.
Anshits Alexander G.
Knecht Dieter A.
Kostin Eduard M.
Macheret Yevgeny
Gottlieb Paul A.
Nave Eileen E.
Silverman Stanley S.
The United States of America as represented by the Department of
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