SCALED MELTER TESTING OF NOBLE METALS BEHAVIOR WITH JAPANESE HLW STREAMS

摘要

The Rokkasho Reprocessing Plant, located in northern Japan, includes facilities for reprocessing spent nuclear fuel and conversion of the high-level waste (HLW) into a stable glass waste form by vitrification. The vitrification process centers on two 2.6 m~2 joule heated ceramic melters. A major consideration in the design of these melters is the large concentrations of noble metal fission products (Ru, Rh, Pd) in the projected HLW streams. These streams also contain significant amounts of molybdenum, which can lead to the formation of a molybdate salt phase ("yellow phase") during vitrification. In addition to the operational issues that can be caused by settling and accumulation of noble metals, noble metals can also influence yellow phase formation. In this study, an extensive series of tests was conducted at two melter scales (～1/100 (DM10) and ～1/20 (DM100)) to investigate various factors affecting noble metals settling and their influence on yellow phase formation. In particular, tests in the DM100 melter were able to replicate the "cold-bottom" mode of operation that is planned for the Rokkasho melters to reduce noble metals settling. Samples collected throughout the tests were used to monitor the noble metals distribution over time. Melt pool bubbling was found to be reasonably effective in suspending noble metals and the presence of noble metals tended to suppress yellow phase formation.