PNNL

Abstract

To produce the conditions for settling of spinel crystals in a quiescent high-level waste glass melt, we used a double crucible assembly that eliminated Marangoni convection and limited bubble generation in a portion of melt volume. We observed the movement of the settling front as a function of time at temperatures 900, 950 and 1000?C. The shape of the settling front was approximately parabolic with a flap tip indicating that the settling crystals drove a convective cell within the melt. The rate of settling was close to that predicted by the Stokes' law when the growth rate of spinel crystals was taken into account. The calculated settling velocity was modified by a semi-empirical settling function providing an agreement with experimental results within 5%. In addition, spinel settling was simulated by the mathematical model that predicted the concentration distribution of spinel in glass melt and the accumulation of particles at the bottom of the crucible.