Characterization on the high-level waste glasses. Annual report

The waste compo itions PW-7c and PW-9 were defined and glass development was completed. Major variations in major oxide concentration would not grossly affect the leach rates of the glass. Impact and strength tests on nonradioactive glass showed that the waste glasses produced slightly less fine particulate than commercial glass. Waste glass had 60% of the strength of the soda-lime glass. A water-quench reduced thermal conductivity about 20%, and a 24-h hold at devitrification temperatures did not produce a significant change. Densities of waste glass at process temperature were 6.6 to 9.3% lower than at room temperature. The effects of glass composition on volatility were measured. Leach tests of highly devitrified samples of 72-68 have shown that leach rates of Cs, Sr and U are increased up to 10X and that Zn leach rates are reduced by nearly 200X. In glass 76-68, where devitrification is much slower, elemental differencesbetween as-formed and thermally-treated samples have not been significant. Average Cs leach rates from the 76-68 glass in an IAEA type long-term test have decreased to 3.3 x 10-8 g/cm2/day. High temperature (250 and 3500C) leach tests showed that glass is comparable to other ceramic materials. In salt brine the glass is rapidly depleted of Cs, Rb and Mo; in water the glass structure is rapidly rearranged to a crystalline structure, and Cs and Rb tend to remain bound in the solid. 76-68 glass (low ZnO) has slow devitrification kinetics compared to 72-68 glass (high ZnO). After equivalent radiation exposures of 300,000 years, the glass buttons still retain their original physical appearance. Stored energy is not a problem for HLW glasses. Density changes are small and do not affect the integrity of the samples