Multiphase magmatic flows at Yucca Mountain, Nevada

The proposed Yucca Mountain radioactive waste repository is sited in southern Nevada in a region that has experienced sporadic basaltic volcanism since the late Miocene. Volcanic risk assessment for the proposed repository requires estimating the consequences of a new monogenetic volcano intersecting the underground facility during its 10⁴-10⁶ year performance period. We report numerical studies aimed at understanding the range of processes and dynamic parameter values that could accompany intersection of an open repository drift by a volatile-rich trachybasaltic magma as it ascends in a dike. We focus on one end-member type of magmatic behavior, namely, a fragmented magmatic mixture under pressure interacting with an underground cavity. Initial and boundary conditions are based upon field data and previous modeling studies of the interaction between vertically propagating dikes and a repository opening. The calculations are two-dimensional and time-dependent and are conducted with the multiphase hydrodynamics code GMFIX. Calculations indicate that gas-particle mixtures, as they rise from below and interact with horizontal openings, form complex flow patterns involving varying degrees of recirculation and deposition of pyroclasts. Dynamic pressures are up to 10⁶ Pa but are more typically on the order of 10³ to 10⁴ Pa. The geometry and number of outlets play a key role in determining the types of flow patterns, as do volatile contents and the degree of fragmentation. The detailed numerical simulations provide information that will be used to confirm the adequacy of simplified probabilistic consequence models used in risk assessments.