Patent Number: 048470082
Section: description

DETAILED DESCRIPTION OF THE INVENTION All parts, percentages, ratios and proportions are on a weight basis unless otherwise stated herein or obvious herefrom to one ordinarily skilled in the art. Pure lead phosphate glass (i.e., a glass that does not contain any nuclear waste or iron) can be prepared by melting together PbO (lead oxide) and P.sub.2 O.sub.5 (phosphorus oxide) at elevated temperatures. The composition of the resulting glass can be varied by varying the ratio of the weight of PbO to the weight of P.sub.2 O.sub.5. However, if the weight percent of lead oxide exceeds about 66 weight percent a crystalline form of lead phosphate, not a glass, is formed. Hence, this composition (66 weight percent of PbO, and 34 weight percent of P.sub.2 O.sub.5) represents a critical limit in the sense that compositions which contain larger amounts of lead oxide no longer form a glass. The lower limit on the minimum amount of PbO which can be melted together with P.sub.2 O.sub.5 to form a suitable host glass for nuclear waste is important, although not as clear cut. The composition consisting of about 45 weight percent PbO and 55 weight percent of P.sub.2 O.sub.5 was taken to be a practical lower limit on the amount of lead oxide needed, since the viscosity of the molten glass became much larger as the PbO content was reduced further. The higher the viscosity, the harder the glass is to pour and the higher the required processing temperature becomes. The addition of simulated nuclear waste to the pure lead phosphate host glass does not substantially modify the lead oxide and phosphorous oxide limits discussed above. It was found, however, that the addition of simulated radioactive nuclear waste containing Fe.sub.2 O.sub.3 to the lead phosphate host glass produced a dramatic decrease in the corrosion rate. That is, pure lead phosphate glasses (i.e., with no Fe.sub.2 O.sub.3 containing nuclear waste added) are quite susceptable to aqueous corrosion. When the simulated radioactive nuclear waste containing Fe.sub.2 O.sub.3 was added to the lead phosphate host glass, however, a highly corrosion resistant and stable nuclear waste glass was formed. The lead phosphate glass appears to be insensitive to the details of the preparation procedure and can be made with a very large variation in the molar ratio of PbO to P.sub.2 O.sub.5 as indicated. For example, specimens of homogeneous lead-iron phosphate nuclear waste glasses loaded with 15 weight percent of simulated radioactive nuclear waste material have been prepared with the amount of PbO in the lead-iron phosphate host glass varied from 45 to 66 weight percent, the amount of P.sub.2 O.sub.5 varied from 34 to 55 weight percent, and the amount of Fe.sub.2 O.sub.3 varied from 0 to 10 weight percent depending on the iron content of the simulated nuclear waste. Most of the decrease in the corrosion rate of the lead-iron phosphate nuclear waste glass is due to the large amounts of iron oxide present in the radioactive nuclear waste material (for example, at one nuclear facility about 50 weight percent of the radioactive nuclear waste material is iron oxide Fe.sub.2 O.sub.3). The effects of various amounts of iron oxide on the corrosion rate of lead phosphate are shown in FIG. 3. As can be easily seen from FIG. 3, the addition of 9 weight percent Fe.sub.2 O.sub.3 to a pure lead phosphate glass improves the corrosion resistance by a factor of about 10,000. Hence, by purposely adding about 9 weight percent iron oxide to pure lead phosphate glass, one can produce a very stable and easily prepared glass, which can then be used to immobilize other types of radioactive nuclear waste material which do not contain large amounts of iron oxide. These wastes include reprocessed commercial nuclear power reactor wastes. FIG. 3 shows that the corrosion rate is substantially reduced by including at least about 9 weight percent of iron oxide (Fe.sub.2 O.sub.3) in the lead phosphate glass composition. This application has been successfully demonstrated in experiments where both simulated high-level defense nuclear waste and simulated radioactive nuclear-power reactor wastes were added to the lead-iron phosphate host glass. The resulting nuclear waste glass was a highly corrosion resistant and stable wasteform. The combining of radioactive nuclear waste with lead-iron phosphate glass forms a nuclear waste glass that is highly corrosion resistant, not susceptible to devitrification, and that can be prepared at a relatively low temperature. The presence of a high level of Fe.sub.2 O.sub.3 is critical. This type of synergistic effect, in which the corrosion resistance of the combined material is enhanced, also occurs in the case of borosilicate glass waste forms in that the waste loaded glass is significantly more stable than a glass formed from the pure borosilicate glass frit. In fact, the pure borosilicate host glass typically corrodes about 10 times faster than the glass in combination with simulated nuclear waste. [See: Sales, B.C., L.A. Boatner, H. Naramoto and C.W. White, J. Non-Cryst. Solids 53 (1982) 201; and Clark, D.E., C.A. Mauer, A. R. Jurgenson and L. Urwongse in Scientific bases for Nuclear Waste Management, Vol. 11, ed. W. Lutze (Elsevier North Holland, New York, 1982) pp. 1 to 14.] For the lead-iron phosphate waste form, however, the improvement in corrosion resistance following addition of the simulated iron-containing waste is much greater. Lead-iron phosphate glass is quite suitable as a long-term storage medium for high-level nuclear waste. The properties of lead-iron phosphate nuclear waste glass are superior to a borosilicate nuclear waste glass, which was recently selected for the long-term storage of some high level nuclear defense wastes. The borosilicate nuclear waste glass therefore is used herein as a standard to which new wasteform of the invention is compared. The invention provides a stable primary containment medium for disposal of high-level radioactive nuclear waste. The invention wasteform typically comprises a lead-iron phosphate glass containing up to 20 weight percent of nuclear waste of the type typically consisting of 50 weight percent of Fe.sub.2 O.sub.3, 9.8 weight percent of Al.sub.2 O.sub.3, 13.8 weight percent of MnO.sub.2, 4.5 weight percent of U.sub.3 O.sub.8, 3.7 weight percent of CaO, 6.2 weight percent of NiO, 1.2 weight percent of SiO.sub.2, 7.1 weight percent of Na.sub.2 O, 1 weight percent of Cs.sub.2 O, 1 weight percent of SrO and 1.3 weight percent of Na.sub.2 SO.sub.4 (or other nuclear waste mixtures with similar compositions). Such compositions with varying amounts of iron and aluminum represents a class of nuclear defense wastes. In addition, the lead-iron phosphate nuclear waste glass can typically be prepared containing 10 weight percent, of the above composition plus 5 weight percent of a composition that is representative of the waste generated by nuclear power reactors. In distilled water at 90.degree. C., the net release of all elements from both types of lead-iron phosphate nuclear waste glasses are 100 to 1000 times smaller (depending on the specific element) than the corresponding amounts released by a comparably loaded borosilicate glass wasteform. EXAMPLE Several lead-iron phosphate glasses were prepared incorporating either simulated radioactive defense nuclear waste or simulated reprocessed commercial waste combined with simulated radioactive defense waste to demonstrate the invention. Appropriate amounts of PbO and (NH.sub.4).sub.2 HPO.sub.4 powders were thoroughly mixed with 15 weight percent of a powdered form of a simulated metal oxide nuclear waste and melted in a platinum crucible at temperatures between 800.degree. and 1050.degree. C. for 3 hours. See Table II for the compositions. The compositions of the lead-iron phosphate host glass studied are given in Table I. The molten glass was then poured into a heated mold of spectroscopically pure carbon, annealed at 450.degree. C. for 2 hours and cooled to room temperature over the space of a few hours. All of the components of the waste were readily dissolved in a short time at 1050.degree. C., and all of the components except Al.sub.2 O.sub.3 and ZrO.sub.2 were dissolved at temperatures between 800.degree. and 900.degree. C. The lead-iron phosphate glass wasteforms prepared at 800.degree. to 900.degree. C. in which Al.sub.2 O.sub.3 and ZrO.sub.2 were not completely dissolved, however, were as corrosion resistant as the lead-iron phosphate wasteforms prepared at 1050.degree. C. All of the lead-iron phosphate glasses loaded with the simulated nuclear waste had a black appearance that resembled that of waste-loaded borosilicate glass. The lead-iron phosphate glasses that were heated to between 1000.degree. and 1050.degree. C. were very homogeneous. Corrosion tests of the type (MCC-1) developed by the Materials Characteristics Center located at Battelle Northwest Laboratories were used to compare the corrosion behavior of the lead-iron phosphate nuclear wasteform with that of an identically loaded borosilicate glass nuclear wasteform. Each wasteform was corroded for one month in distilled water at 90.degree. C. The results are shown in FIG. 1. The data shows that the net release of all of the elements from the lead-iron phosphate wasteform was at least 100 to 1000 times smaller than the corresponding amounts released by the borosilicate wasteform (that is, Frit 131 plus 29 percent of the first simulated nuclear waste composition--see Table II for the exact compositions). The concentrations of all of the elements present in the lead-iron phosphate leachate were below the detectable limits of the standard analytical chemical techniques employed (in this case, inductively coupled plasma emission analysis--ICP, atomic absorpotion, and flourimetry). The presence of iron (a component of the nuclear waste material) is primarily responsible for the very high corrosion resistance of the nuclear waste glass relative to that of pure lead phosphate glass. In more detail, FIG. 1 shows the 30-day corrosion rates at 90.degree. C. in distilled H.sub.2 O for lead-iron phosphate [Pb(PO.sub.3).sub.2 plus 15 weight percent of the first simulated nuclear waste] and borosilicate (Frit 131 plus 29 weight percent of the first simulated nuclear waste) nuclear waste glasses. The lead-iron phosphate and borosilicate nuclear waste glasses had the same waste per volume loading. The effects of the pH of the corroding solution on the corrosion rate of the lead-iron phosphate wasteform was also investigated (see FIG. 2) and compared to the behavior of a borosilicate glass wasteform. The lead-iron phosphate wasteform was comprised of 50 weight percent of PbO and 50 weight percent of P.sub.2 O.sub.5 plus 15 weight percent of the first simulated nuclear waste (see Table II). The waste weight percentages for the lead-iron phosphate glass versus borosilicate glass yield comparable waste per volume factors due to the higher density of the lead-iron phosphate glass (i.e., 5.+-.0.1 g/cm.sup.3) relative to borosilicate glass (2.6 g/cm.sup.3). The borosilicate glass was comprised of Frit 131 plus 9 weight percent of the first simulated nuclear waste (see Table II). In the neutral pH regions (i.e., for pH values between 5 and 9) which encompass the pH range of most natural ground waters, the corrosion rate of the lead-iron phosphate wasteform was 100 to 1000 times smaller than the corresponding corrosion rates of the borosilicate glass wasteform. At the pH extremes of 2 to 12, the corrosion rate of the lead-iron phosphate wasteform approaches but does not exceed that of the borosilicate glass wasteform (see FIG. 2). TABLE I ______________________________________ Lead-iron phosphate host glass compositions. The nuclear waste glass is formed by melting the lead-iron phosphate host glass together with a powdered form of the nuclear waste. Compound Weight % ______________________________________ PbO 40-66 P.sub.2 O.sub.5 30-55 Fe.sub.2 O.sub.3 *.sup.1 0-10 ______________________________________ Note: .sup.1 Amount of iron oxide added depends on type of highlevel nuclear waste. TABLE II __________________________________________________________________________ Lead-Iron Phospate And Borosilicate Nuclear Waste Glass Compositions Typical Lead-Iron Frit 131 Composition First Simulated Nuclear Second Simulated Nuclear Phosphate Compositions (Borosilicate Glass) Waste Composition Waste Composition (weight percent) (weight percent) (weight percent) (weight percent) __________________________________________________________________________ PbO 40-66 P.sub.2 O.sub.5 30-55 Fe.sub.2 O.sub.3 0-10 SiO.sub.2 57.9 Fe.sub.2 O.sub.3 50.0 ZrO.sub.2 12.10 B.sub.2 O.sub.3 14.7 Al.sub.2 O.sub.3 9.8 MoO.sub.3 12.67 Na.sub.2 O 17.7 MnO.sub.2 13.8 Nd.sub.2 O.sub.3 11.6 Li.sub.2 O 5.7 U.sub.3 O 4.5 CeO.sub.2 8.13 MgO 2.0 CaO.sup.8 3.7 RuO.sub.2 10.27 TiO.sub.6 1.0 NiO 6.3 Cs.sub.2 O 7.05 ZrO.sub.2 0.5 SiO.sub.2 1.3 U.sub.5 O.sub.8 5.54 La.sub.2 O.sub.3 0.5 Na.sub.2 O 7.2 La.sub.2 O.sub.3 3.6 Na.sub.2 SO.sub.4 1.3 Pr.sub.2 O.sub.3 3.6 Cs.sup.2 O 1.0 Sm.sub.2 O.sub.3 2.2 SrO.sup.2 1.0 Fe.sub.2 O.sub.3 3.7 P.sub.2 O 1.64 SrO.sup.5 2.59 BaO 3.83 PdO 3.65 TeO.sub.2 1.44 Y.sub.2 O.sub.3 1.46 Other Oxides 5.0 __________________________________________________________________________ Other tests indicated that the corrosion behavior of the lead-iron phosphate wasteform was not affected by large doses of gamma radiation, nor was the material unusually susceptible to corrosion at a higher temperature, e.g., 135.degree. C. In the 800.degree. to 1050.degree. C. temperature range, the viscosity of the molten lead-iron phosphate wasteform is much less than the prototype borosilicate glass wasteform, as evidenced by the fact that the lead-iron phosphate could be easily poured at 800.degree. C. In spite of the low viscosity for the lead-iron phosphate between 800.degree. to 1000.degree. C., the phosphate glass wasteform softened at 600.degree. C. which was about 25.degree. C. higher than the softening point of the borosilicate glass wasteform. A lead-iron phosphate wasteform was exposed to air at 550.degree. C. for 60 hours in order to determine if there was any rapid tendency to devitrify. No obvious devitrification of the wasteform was detected using X-ray diffraction analysis, and a subsequent corrosion test on the sample showed no degradation in corrosion resistance. A similar test of borosilicate glass treated at 500.degree. C. for 60 hours indicated that the borosilicate glass corrosion rate was measurably higher following the heat treatment. Since higher temperatures are needed to completely dissolve the Al.sub.2 O.sub.3 present in some radioactive nuclear wastes, an aluminum base alloy can be employed as a cannister material. A lead-iron phosphate glass wasteform of the invention was melted at 800.degree. C. in accordance with this invention and poured into a pure aluminum container (aluminum melts at 660.degree. C.). The aluminum container did not melt. The foregoing prescription of preferred embodiments 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 teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.