PNNL

Abstract

It has been a commonly held assumption that constraining liquidus temperature (TL) prevents the accumulation of crystalline phases in the high-level waste (HLW) glass melter because crystals, if they form at all, should dissolve easily in the melt at tempera-tures above liquidus. This, as the model calculation showed, is not the case in melters with fast circulation flow. If the melt circulates rapidly between cool and hot regions, crystals do not have a sufficient time to dissolve while in the hot zone. As a result, a steady-state size and concentration of crystals is established throughout most of the melter during normal operation. A consequence of this result is that the rate of crystal ac-cumulation in the melter only slightly increases with increasing TL, but strongly increases with increasing crystal size. For the melter simulated by the model, the TL could be 100°C above the accepted constraint without a serious impact on melter performance. Nucleation agents that keep crystals small abound in most HLWs but are often absent in simulated wastes for experimental melter runs. The weak impact of TL on melter per-formance is an important finding because without the current TL constraint, the HLW glass volume at Hanford can significantly decrease