Patent Number: 048470082
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention and Contract Statement The invention relates to primary containment media for the disposal of high-level radioactive nuclear waste. The United States Government has rights in this invention pursuant to Contract No. DE-W-7405-eng-26 between the U.S. Department of Energy and Union Carbide Corporation. 2. Discussion of Background and Prior Art In the past, nuclear waste has been temporarily stored, frequently as a liquid or as a sludge in conjunction with a liquid. The art has recognized that means must be provided for permanent disposal of the waste, preferably as highly stable solids. Such solids must have certain characteristics which make such solids safe and economical for the long-term (10.sup.3 to 10.sup.5 years) retention of radioactive waste isotopes. Because of the long half-lives of some radionuclides (e.g., certain actinide isotopes), it is necessary that the selected storage medium exhibit certain properties in order to achieve the desired long-term stability. Some of the factors which must be considered in the selection of a storage medium include: high chemical stability, i.e., low corrosion rates; structural stability; simple to manufacture; acceptable preparation temperature; ability to store a high proportion of waste to insure minimum storage volume; and availability to components making up the storage medium. Various glass compositions have been suggested and tested for suitability as a storage medium. The borosilicate glasses have been considered among the more promising compositions. However, the borosilicate glasses have demonstrated significant instability under hydrothermal conditions, i.e., exposure to water at temperatures greater than 100.degree. C. Such hydrothermal conditions can be encountered in deep geological repositories. Two highly desirable properties of any potential nuclear waste glass are a low preparation temperature and a low melt viscosity at the glass processing temperature. Pure lead phosphate glasses exhibit both of these properties [see: Argyle, J. F., and F. A. Hummel, J. Amer. Ceram. Soc. 43 (1960) 452; Osterheld, R. K. and R. P. Langguth, J. Amer. Chem. Soc. 59 (1955) 76; Ray, N. H., Glass Tech. 16 (1975) 107; Klonkowski, A., Phys. and Chem. Glasses 22 (1981) 163; and Furdanowicz, H. and L. C. Klein, Glass Tech. 24 (1983) 198]. Unfortunately, it is well known that these substances are susceptable to aqueous corrosion and that they tend to devitrify at temperatures as low as 300.degree. C. [see: Furdanowicz, H., et al., ibid.; Ray, N. H., C.J. Lewis, J.N.C. Laycock and W.D. Robinson, Glass Tech. 14 (1973) 50; and Longman, G.W., and G. D. Wignall, J. Mat. Sci. 8 (1973) 212]. Scientific Basis for Nuclear Waste Management, Vol. 1, Edited by G. J. McCarthy, Plenum Press (1979), pp. 43-50, 69 to 81 and 195 to 200. The phosphate glasses described in the reference include sodium aluminum phosphate glasses, or very complicated combinations of metal oxides and P.sub.2 O.sub.5. Of those phosphate glasses in the reference, only p. 74, Table 2, shows a composition containing lead oxide along with phosphorus pentoxide and nuclear waste oxides. The ratio of the phosphorus to lead content is very high and the phosphate glasses discussed therein are a multicomponent mixture of up to eight oxides. In addition, the composition ranges given for each oxide in the reference on page 74 covers such a broad spectrum of possible phosphate glasses that the table has no significance due to the lack of specificity resulting from an effectively infinite array of permutations and combinations of glass constituents and concentrations. See also: Scientific Basis for Nuclear Waste Management, Vol. 2, Edited by C. J. M. Northrup, Jr., Plenum Press (1980), p. 109 to 116; Report BNL-50130, Development of the Phosphate Glass Process for Ultimate Disposal of High-Level Radioactive Waste, R. F. Drager, et al., January 1968; and Symposium on Management of Radioactive Wastes from Fuel Reprocessing, Nov. 27 to Dec. 1, 1972, pp. 593-612. No non-patent reference was found that indicated that lead-iron phosphate glasses have ever been seriously considered as a viable potential storage medium for the immobilization of nuclear wastes. The glass and ceramic fields include the following domestic patents. U.S. Pat. No. 3,365,578 (Grover et al.) discloses placing radioactive waste in a Na-Pb-Fe-phosphate/silicate glass, within a steel vessel. (Other Na-Pb-phosphate systems are disclosed in the examples of Grover et al.) To recap, Grover et al. teaches the use of a glass containing both Pb and phosphate for nuclear waste containment. U.S. Pat. No. 4,314,909 (Beall et al.) teaches glass-ceramic which is used for waste storage and which consists of monazite, pollucite and ZrO.sub.2 and/or mullite. The glass-ceramic can contain up to 20 percent of P.sub.2 O.sub.5. Beall et al. does not mention the presence of Pb. U.S. Pat. No. 4,351,749 (Ropp I) teaches nuclear waste storage blocks which include a polymeric phosphate glass from a trivalent metal selected from Al, In or Ga. U.S. Pat. No. 4,382,974 (Yannopoulos) discloses a glass containing nuclear waste which is stabilized by the application of synthetic monazite by means of chemical vapor deposition or detonation gun. The monazite contains 27 to 35 weight percent of P.sub.2 O.sub.5. No Pb is mentioned in Yannopoulos. U.S. Pat. No. 3,161,600 (Barton I) and U.S. Pat. No. 3,161,601 (Barton II), respectively, show Sr and Cs sequestrated in phosphate glasses. U.S. Pat. No. 3,120,493 (Clark et al.) teaches a process wherein ruthenium volatilization is suppressed during the evaporation and calcination of nuclear waste solutions by the addition of phosphite or hypophosphite. A glass-like solid is obtained. U.S. Pat. No. 4,049,779 (Ropp II) teaches stable phosphate glasses of formula M(H.sub.2 PO.sub.4)n, wherein M may be Pb and n is 2 or 3 (for divalent or trivalent M), which are prepared via H.sub.3 PO.sub.4 and a metal compound by adding a precipitant, crystallizing from solution and then melting the material. While Ropp II discloses lead phosphate glasses, it is not directed to nuclear waste disposal, although it does not mention stability to leaching. U.S. Pat. No. 3,994,823 (Ainger et al.) discloses lead zirconate ceramic, which may also contain Bi. U may be added to reduce electrical resistivity. The ceramic of Ainger et al. is not aimed at nuclear waste storage. SUMMARY OF THE INVENTION An object of the invention is to provide an improved glass composition and a method of making same for the primary containment of high-level radioactive nuclear waste. Another object of the invention is to provide a wasteform less subject to corrosion or ionic release than the prior art waste forms. A further object of the invention is to provide a stable wasteform which can be processed (i.e., that will dissolve the waste constituents) at a temperature lower than borosilicate glasses. A still further object of the invention is to provide a stable wasteform that exhibits a lower viscosity than borosilicate glass in the temperature range between 825.degree. and 1050.degree. C. A yet further object of the invention is to provide a stable wasteform for high-level radioactive nuclear wastes which is adaptable for use with existing glass fabrication technology. Other objects and advantages of the invention are set out herein or are obvious herefrom to one ordinarily skilled in the art. The objects and advantages of the invention are achieved by the composition and process of the invention. To achieve the foregoing and other objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention involves a glass composition for the immobilization and disposal of high-level radioactive nuclear waste. Lead-iron phosphate glasses with several different compositions can be used as hosts for high level radioactive wastes. The lead-iron phosphate glass frit that is combined with the nuclear waste and melted to form radioactive waste monoliths can be prepared using either of two simple processes. In one process, the appropriate amounts of PbO and Fe.sub.2 O.sub.3 are combined with (NH.sub.4)H.sub.2 PO.sub.4 and the glass is formed by heating the mixture to about 850.degree. C. A second procedure for forming the lead-iron phosphate glass frit involves simply mixing PbO and Fe.sub.2 O.sub.3 with the appropriate amount of P.sub.2 O.sub.5. The formation of the lead-iron phosphate glass frit can be accomplished in standard chemical processing facilities, since radioactive material is not involved at this stage of the production of a nuclear glass waste form. The most economical process would be used, but for the purposes of discussing the formation of the frit and its characteristics, the discussion is temporarily limited to the second process involving only the simple oxides. In this case, the practical concentration limits for the three oxide constituents of the host glass (i.e., PbO, Fe.sub.2 O.sub.3, and P.sub.2 O.sub.5) are listed in Table I. Pure lead phosphate glass (i.e., a glass that does not contain either iron or nuclear waste) can be prepared by fusing PbO (lead oxide) with P.sub.2 O.sub.5 (phosphorous pentoxide) between 800.degree. and 900.degree. C. The composition of the resulting glass frit can be continuously varied by adjusting the ratio of lead oxide to phosphorus oxide. If the weight percentage of lead oxide exceeds about 66 percent, however, a crystalline form of lead phosphate and not a glass is formed. Hence, the composition (66 wt. percent of PbO) represents a critical limit in the sense that compositions which contain larger amounts of lead oxide which can be melted together with P.sub.2 O.sub.5 to form a suitable host glass for nuclear waste is not as well as defined. The composition consisting of about 45 wt. percent of PbO and 55 wt. percent of P.sub.2 O.sub.5 was taken to represent the practical lower limit for the amount of lead oxide, since the viscosity of the molten glass increased rapidly as the PbO content was reduced below 45 wt. percent. The higher the melt viscosity, the harder the glass is to pour and the higher the processing temperature becomes. High processing temperatures for nuclear waste and undesirable since volatile radioactive species may be lost through vaporization, and the operation and maintenance of high temperature equipment in a remote processing facility are not economical. The amount of iron oxide which must be added to form the lead-iron phosphate waste glass depends on the iron concentration already present in the nuclear waste. High-level defense waste typically contains about 50 wt. percent of Fe.sub.2 O.sub.3 (see Table II, first simulated nuclear waste composition), and, for this type of nuclear waste, no additional iron is added to the pure lead phosphate frit for the formation of a very stable nuclear waste glass. For most high-level commercial waste of the type generated by light water nuclear power reactors (see Table II, second simulated nuclear waste composition), however, additional iron oxide must be added to the pure lead phosphate glass in order to form a sufficiently stable, corrosion resistant nuclear waste glass. The effects of iron oxide on the properties of pure lead phosphate glasses are critical. The addition of iron oxide to these glasses improves the corrosion resistance by a factor of more than 10,000 (see FIG. 3) and results in the formation of glasses that do not exhibit any evidence of devitrification after being heated in air at 575.degree. C. for 100 h. Perhaps most significantly, extremely stable lead-iron phosphate glasses can be prepared and poured easily at temperatures between 800.degree. and 900.degree. C. The results illustrated in FIG. 3 can be used in tailoring the composition of the lead-iron phosphate glass frit depending upon the iron concentration of a given type of nuclear waste. The highly stable waste form is realized when the iron concentration is adjusted to correspond to a content of about 9.0 wt. weight of Fe.sub.2 O.sub.3 relative to the total weight of the glass composition. Preferably the final nuclear waste glass composition contains about 9 weight percent of Fe.sub.2 O.sub.3. Also preferably the Fe.sub.2 O.sub.3 can be added to the glass composition in the form of one of the metal oxides present in the radioactive nuclear waste material. The Fe.sub.2 O.sub.3 can also be added to the glass composition as a separate component. Preferably the radioactive nuclear waste material is present in an amount of about 15 weight percent, based on the total weight of the glass composition, in the glass composition. The invention can also generally be described as a stable primary containment medium for disposal of high-level radioactive nuclear wastes comprising lead-iron phosphate glass having a composition in the ranges indicated in Table I plus preferably about 15 weight percent of a mixture of metal oxide nuclear waste material. Such nuclear waste material can be, for example, of the type in interim storage at nuclear facilities or a combination of such interim storage type and the type of high level waste generated by commercial power reactors. The advantages of lead-iron phosphate nuclear waste glasses as compared to borosilicate nuclear waste glass are: 1. Corrosion resistance at elevated temperature between 90.degree. and 135.degree. C. is at least 100 to 1000 times better; 2. Lower processing temperature for the wasteform (260.degree. to 110.degree. C. lower); 3. Lower melt viscosity in the 800.degree. to 1050.degree. C. temperature range; 4. Waste loading per unit volume which is at least as good as the practical waste loadings per unit volume achievable by using borosilicate glasses; 5. The ability to use a relatively inexpensive aluminum, aluminum alloy or stainless steel cannister for the glass casting step in processing; and 6. The lead-iron phosphate nuclear waste glass can be prepared using basically the same technology that has been developed to produce large monoliths of borosilicate nuclear waste glass. The lead-iron phosphate glass wasteform of the invention provides an excellent containment medium for wastes such as those in interim storage at government nuclear facilities and high-level wastes generated by commercial nuclear power reactors. The invention also includes a process for preparing the glass composition of the invention. The process includes admixing about 34 to about 55 weight percent, based on the total weight of the glass composition, of phosphorus oxide, about 45 to about 66 weight percent, based on the total weight of the glass composition, of lead oxide, and about 0 to about 9 weight percent of Fe.sub.2 O.sub.3, based on the total of the glass composition and the amount of iron content in the nuclear waste to be processed. The admixture, to which about 15 weight percent of nuclear waste oxides have been added, is then melted to provide a liquid melt of a lead-iron phosphate glass. Usually the melt is heated to and kept at 800.degree. to 1050.degree. C. About 10 to about 20 weight percent, based on the total weight of the glass composition, of radioactive nuclear waste material containing at least one metal oxide is added to the liquid melt of lead phosphate glass. Preferably the radioactive nuclear waste material contains sufficient Fe.sub.2 O.sub.3 to provide preferably about 9 weight percent, baesd on the total weight of the glass composition, of Fe.sub.2 O.sub. 3 in the glass composition. The liquid melt is then solidified to provide the glass composition for the immobilization and disposal of the radioactive nuclear waste material. Preferably the phosphorus oxide is used in the form of ammonium orthophosphate monohydrogen, i.e., (NH.sub.4).sub.2 HPO.sub.4. The addition steps for the nuclear waste and the Fe.sub.2 O.sub.3 can be be conducted simultaneously or in any desired sequence. In an alternative to the preferred embodiment of the invention, all or part of the Fe.sub.2 O.sub.3 used in the glass composition can be added as a separate component. In such case the Fe.sub.2 O.sub.3 can be added directly to a liquid melt of lead phosphate glass and/or added to the radioactive nuclear waste material before such is added to the liquid melt of the lead phosphate glass. The lead-iron phosphate nuclear waste glass of the invention is a very stable, easily prepared storage medium for some important classes of nuclear waste. Relative to borosilicate nuclear waste glass, the lead-iron phosphate nuclear waste glass has several distinct advantages. These advantages of the invention glass compositions include: (1) a corrosion resistance at 90.degree. C. that is about 1000 times higher than a comparable borosilicate glass in the pH range between 5 and 9, which is mostly due to the presence of iron in the phosphate glass composition, (2) a processing temperature that is 100.degree. to 250.degree. C. lower than that currently required to process borosilicate glass, and (3) a lower melt viscosity in the 800.degree. to 1050.degree. C. range. The presence of iron is primarily responsible for the very high corrosion resistance of the lead-iron phosphate nuclear waste glass of the invention relative to that of the pure lead phosphate glass. The lead-iron phosphate glass of the invention is an excellent storage medium for high-level radioactive nuclear waste. Reference will now be made in detail to the present preferred embodiment of the invention, some of the advantages of which are illustrated in the accompanying drawings.