Research and development activities waste fixation program. Quarterly progress report, July--September 1975

Engineering-scale in-canister melting tests were made to evaluate the use of silicon to prevent formation of water-soluble molybdate phases in melts. The tests demonstrated that silicon metal powder added to the feed stock effectively prevents formation of such phases. A test also showed that coatings of ZrO$sub 2$ or chromium carbide on stainless steel canisters prevent oxidation during the in-can melting process. Thermal analysis of canister design concepts examined effects of type of storage, use of fins, emissivity, canister size and cracks in glass. Pressurization tests show that all types of calcine can produce very high pressures within the canister unless the calcine is post-treated at temperatures greater than 850$sup 0$C. Initial studies on the effects of alpha and recoil bombardment (from $sup 244$Cm doping) on devitrification of waste- containing glass were completed. The devitrification behavior of a waste-bearing zinc orthosilicate glass is being studied using optical microscopy, electron microprobe, and x-ray diffraction examination. Fine calcine particles were agglomerated into larger particles suitable for coating via chemical vapor deposition. Thermal stability tests of nickel and Cr$sub 7$C$sub 3$ coatings on waste calcine have been conducted, and deterioration has occurred at temperatures as low as 500$sup 0$C. Waste-containing pellets were successfully coated by plasma spraying Al$sub 2$O$sub 3$ powder. Calculations indicate that plasma spraying of large pellets is feasible on a production scale. Seven scoping runs made with a disc pelletizer indicate that it can provide a simple, inexpensive process for making high-quality pellets from calcine-frit powders. Risk assessment was used to systematically identify dominant sequences for the accidental release of radionuclides during the solidification, basin storage, and rail transport activities of high-level waste management. (JGB