Patent Number: 058754075
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for immobilizing waste chloride salts containing radionuclides and hazardous nuclear material for permanent disposal, and, in particular, a method for immobilizing waste chloride salts containing cesium, in a synthetic form of pollucite. Zeolite is a tectosilicate mineral comprised of SiO.sub.4 and AlO.sub.4, with large vacant spaces within its crystalline structure for cations. Chabazite (NaAlSi.sub.2 O.sub.6), a mineral within the zeolite group, is ideal for pollucite formation because the silicon to aluminum ratio of chabazite is identical to that of pollucite (CsAlSi.sub.2 O.sub.6), that is, the ratio of silicon to aluminum in chabazite and pollucite is (Si:Al=2). Another advantage is the commercial availability of the chabazite. The method is a one-step, direct thermal conversion at low temperatures of cesium, in the form of dry, non-aqueous cesium chloride, to pollucite by mixing and heating the cesium chloride with chabazite. The cesium is ion-exchanged and a reaction occurs to form pollucite in a single heat treatment step. The combination of cesium chloride and chabazite is heated to above the melting point of CsCl, or above about 700 .degree. C., forming pollucite. For example, a mixture of CsCl and chabazite was combined at ambient temperature, in a ratio of about 0.6 g of CsCl to 1 g of chabazite, the mixture having a cesium to aluminum ratio of 0.5. The mixture was heated in an alumina crucible to a temperature above 700.degree. C. to form the pollucite product. Analysis of the pollucite revealed a complete conversion of the CsCl to pollucite, with no NaCl or CsCl evident as by-products of the conversion process. FIG. 1 shows an x-ray diffraction pattern of the pollucite produced. Surprisingly, pollucite forms in the presence of the chloride ion. In particular, chloride ion appears to remain in the structure of the pollucite, apparently occluded as sodium chloride. Importantly, the method does not involve an aqueous ion exchange step, elevated temperatures (about 1000.degree. C.) and pressures, or complex starting materials and processing steps, as required by prior art methods. An advantage of the invention, therefore, is the ability to convert dry, solid cesium chloride without dissolving the cesium chloride in an aqueous solution to perform an ion exchange step. The solid cesium chloride is substantially dry, meaning dry to a great extent or degree, or being largely but not necessarily wholly without moisture. In other words, the cesium chloride is to a great extent free or relatively free of liquid, especially water. Thus, the method can be used with processes that require very dry environments, such as pyrochemical processes. Table 1 provides composition data for chabazite prior to mixing with cesium chloride, and the final, synthesized pollucite. The values are presented as mole/mole of Al. In particular, the retention of cesium and sodium in the pollucite is 0.922 and 0.587, respectively. TABLE 1 ______________________________________ Sample Al Si Na Ca Cs ______________________________________ Pollucite 1.000 3.663 0.587 0.058 0.922 Chabazite 1.000 3.633 1.393* 0.019 0.002 ______________________________________ *Chabazite was ion exchanged in 1 M NaCl at 90.degree. C. Importantly, mixtures of sodium chloride and chabazite do not result in conversion to analcime (NaAlSi.sub.2 O.sub.6), which has the same structure of pollucite (CsAlSi.sub.2 O.sub.6), indicating that the presence of cesium is required for the structural conversion to pollucite to occur. FIG. 2 shows that by mixing sodium chloride with chabazite and heating the mixture to a temperature above about 700.degree. C., without cesium present, albite or anorthite feldspars are produced. In the preferred embodiment, the pollucite is further cooled, and, next, heated with glass, including glass frit, and hot pressed at a temperature up to about 725.degree. C. to form a solid pollucite and glass product. Alternatively, glass frit may be added to the CsCl and chabazite prior to heating, which results in lowering the temperature required for conversion to pollucite to about 700.degree. C., by apparently speeding up the reaction time, with no apparent affect on the formation of the pollucite. The reduction in temperature reduces the risk of volatilizing the cesium. The radionuclide cesium is thus encapsulated and immobilized in the solid pollucite and glass product, which is leach resistant and suitable for long term storage. In an alternate embodiment, zeolite A is heated with cesium chloride to a temperature above 750.degree. C. Zeolite A shows a partial reaction to pollucite and has a silicon to aluminum ratio of 1. The reaction product is a mixed pollucite (CsAlSi.sub.2 O.sub.6)-sodalite (Na.sub.4 Al.sub.3 Si.sub.3 O.sub.12 Cl) phase. FIG. 3 shows an x-ray diffraction pattern for the sodalite/pollucite product. In another embodiment, the retention of cesium in ceramic forms comprised of zeolite A, or zeolite A converted to sodalite, is surprisingly improved by adding up to 10% chabazite by weight to the waste form compositions (either zeolite A containing the radionuclides or zeolite A converted to sodalite containing the radionuclides) prior to their conversion to the final product. This embodiment is particularly applicable to mixed chloride salts having a low concentration of cesium (&lt;2 wt. %). For example, in a typical electrometallurgical treatment process, the primary component of the salt is an LiCl--KCl eutectic (melting point of 355.degree. C.), containing cesium, strontium, barium, rare earth fission products, and transuranics. One method of producing ceramic waste forms containing the radioactive wastes is to blend zeolite A powder with the waste salt and heat the mixture to trap the fission products within the zeolite A structure in an ion exchange step. The salt-loaded zeolite powder can serve as the waste product, or it can be further heated to a temperature sufficient to convert the salt-loaded zeolite powder to sodalite. Next, the salt-loaded zeolite powder or the resulting sodalite is mixed with glass frit and consolidated by hot isostatic pressing (HIP) at elevated temperatures and pressures to form a composite ceramic. By adding 10% chabazite by weight to the waste products prior to the HIP step, in accordance with this embodiment of the method, the cesium retention in the composite ceramic is significantly improved. To demonstrate the significant improvement in the cesium retention rates according to this embodiment of the method, a baseline composition was established for the zeolite A and the sodalite waste products, without the addition of chabazite. A 50:50 mixture of a borosilicate glass and zeolite A and a 50:50 mixture of borosilicate glass and sodalite powder were hot pressed under conditions appropriate to retain the crystalline structure of each material. Both the zeolite A and sodalite ceramic compositions showed good retention of fission products based on standard leach tests. However, for both the zeolite A and sodalite ceramic compositions, cesium was the fission product most readily released. Next, up to 10% chabazite by weight was added to both the zeolite A and the sodalite waste products prior to the HIP step. The chabazite tended to sorb the lower charged alkali and alkaline earth cations to a greater extent than zeolite A during blending, which is, in part, attributable to the higher ratio of silicon to aluminum in chabazite relative to zeolite A, which favors sorption of lower charge species. When the chabazite is added to the powder mixture prior to the HIP cycle but after blending, cesium chloride that is expelled by zeolite A or sodalite during the HIP step can react with chabazite to form pollucite. However, pollucite has not been detected in the diffraction patterns for either the zeolite A or the sodalite waste products. Thus, it is preferable to add the chabazite during the blending step. Table II below provides leach test data for the baseline ceramic waste forms and ceramic waste forms containing chabazite, for three day and twenty-eight day MCC-1 tests in deionized water at 90.degree. C. TABLE II ______________________________________ Leach Rate Test Results for Ceramic Waste Forms Containing Chabazite % Cesium Release* % Chloride Release* 28 28 Sample 3 day day 3 day day ______________________________________ Zeolite Waste Form 10% Chabazite added 0.1 0.3 0.21 0.37 Baseline WF Average, without Chabazite 0.28 NM 0.54 NM Sodalite Waste Form Chabazite added to Zeolite A prior to in blend 0.025 0.16 0.16 0.39 10% Chabazite added after conversion to Sodalite 0.02 0.13 0.09 0.32 Baseline WF Average, without Chabazite 0.08 0.65 0.63 0.57 Other Compositions Chabazite blended with salt, then with Zeolite A 0.047 0.19 0.04 0.13 ______________________________________ *% Release = 100*Mass of Species Lost/total Mass of species in Waste Form The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical applications and should enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention, rather the scope of the invention is to be defined by the claims appended hereto.