Characterization of Lead-Iron Phosphate Nuclear Waste Glasses

Abstract

Lead-iron phosphate glasses have recently been proposed as a new primary disposal medium for both commercial nuclear reactor wastes and some types of high-level radioactive U.S. defense wastes [1–41. The initial work on the characterization of lead-iron phosphate nuclear waste glasses, hereafter named phosphate glasses, concentrated on the preparation, thermal properties, and devitrification behavior (both during and after preparation) of these glasses. Aqueous corrosion rates were measured at 90°C in distilled water, acidic and basic solutions, low eH distilled water, and in the “reference’ natural ground water appropriate to disposal in a tuffaceous formation (i.e., J-13 well water [5]). The results of these initial investigations were extremely encouraging and indicated that homogeneous, highly leach resistant (at 90° C) phosphate glasses loaded with either commercial or high-level U.S. defense wastes could be prepared at relatively low temperatures (process temp.:≤ 1050°C). The chemical stability of the phosphate glasses was found to be primarily due to the stabilizing effect of iron on the structure of lead phosphate glass. This effect is illustrated in Fig. 1 where the leachate conductivity at 90°C is plotted versus time for phosphate glasses with varying iron concentrations. The addition of 9 wt.% iron oxide to lead metaphosphate glass [Pb(PO3)2] increases the chemical durability of the glass in water by a factor of about 104. The tendency of pure lead phosphate glasses to crystallize is also greatly suppressed by the addition of iron oxide [3,4]. Lead metaphosphate glass can be completely crystallized by heating the glass in air at 30O°C for several hours. Lead-iron phosphate glasses, however, can be heated in air for several hundred hours at a temperature as high as 500°C without exhibiting any evidence of crystallization.