Mitigation of Yellow Phase Formation at the Rokkasho HLW Vitrification Facility - 10107

The Rokkasho Reprocessing Plant, located in northern Japan, includes facilities for reprocessing spent nuclear fuel and conversion of the high-level waste into a stable glass waste form by vitrification. The vitrification process centers on two, 2.6 m 2 joule heated ceramic melters. The presence of significant amounts of molybdenum in the HLW stream can lead to the formation of a molybdate salt phase (“yellow phase”) during vitrification. These salts are undesirable from both melter operations and product quality perspectives. Earlier full-scale mock-up testing with waste simulants suggested that operating protocols that had been developed would be adequate to manage yellow phase formation. However, during inactive and active test campaigns at the Rokkasho plant, yellow phase formation was found to be much more prevalent than was expected on the basis of the mock-up test results. Thus, the objective of the present work was to investigate potential strategies to suppress the formation of the molybdate yellow phase. This work was specifically constrained such that the composition of the final glass product must remain unchanged since it is likely that such a solution would involve the least impact to the design and operation of the facility and therefore could be implemented more easily and more quickly. The principal mitigation strategy that was investigated in the present work employed a reformulation of the glass frit combined with redistribution of some of the components in the frit to the liquid waste stream such that the composition of the final vitrified product remains unchanged. The melting behavior of the reformulated frit and waste simulant with chemical additives was assessed in both gradient furnace and isothermal melting tests. These screening tests were used to down-select preferred modified feed blends that were then subjected to confirmatory testing on a small-scale continuously fed joule heated ceramic melter system. The test results showed that the new strategy successfully prevented yellow phase formation. Based on these results, further testing is ongoing to support the potential implementation of this mitigation strategy.