The Nuclear Regulatory Commission uses the ASHPLUME model in its evaluation of the basaltic volcanism scenario at the possible Yucca Mountain repository. The mixing of magma with the spent-fuel waste form is tied to a reasonable but unverified model that predicts that no dense tephra/fuel particles would form. An alternative model uses a mixing rule that allows the formation of dense tephra/fuel particles that would be transported in the volcanic plume differently. The alternative model shows significant sensitivity to the spent-fuel particle size distribution. However, differences in results between the two models are on average less than a factor of 2.