Aspects of Potential Magmatic Disruption of a High-Level Radioactive Waste Repository in Southern Nevada

Abstract

Volcanic hazard studies, combining standard techniques of hazard appraisal and risk assessment (probability X's consequences), are being undertaken with respect to storage of high-level, radioactive waste in southern Nevada. Consequence studies, the emphasis of this work, are evaluated by tracing the steps of ascent of basaltic magma including intersection and disruption of a repository followed by surface eruption. outhern Nevada is cut obliquely by a N-NE trending belt of basaltic volcanism that contains a number of major volcanic fields separated by areas with scattered basalt centers. The basalts of the belt range in age from 8 m.y. to recent. They are primarily of hawaiite composition, and a number of the fields exhibit the straddle-type associations of the alkalic subsuite. The basalts are characterized by high $$^{87}Sr/^{86}Sr$$ isotopic ratios (~0.707) and high Sr contents and are evolved from parental compositions by crystal fractionation. Theoretical considerations suggest basalt magma ascends rapidly from mantle depth (10's of cm/sec in the bubble-free regime) but may be trapped temporarily and fractionate at the mantle/crust interface. Basalt centers are fed from narrow linear dikes. Local sheet-like intrusions formed at depths of 200 to 300 m probably due to a combination of extensional faulting during emplacement and trapping within low-density tuff country rock, aided in part by a low magma-volatile content. Basalt centers comprise scoria cones of moderate size and associated lava flows formed during Strombolian eruptions. Local hydromagmatic eruptions occurred at three centers but are unlikely in future eruptions due to the considerable depth to the ground water table and the low flux of moisture in the unsaturated zone. Incorporation of radioactive waste in basalt magma is controlled by the dimensions of basalt dikes at repository depths and the depth of magma fragmentation. Dispersal pathways of waste should follow the pyroclastic component of a Strombolian eruption. The maximum volume of waste deposited with basaltic tephra can be traced approximately by assuming waste material is dispersed in the same patterns as country rock lithic fragments. Based on a basalt magma cycle that is similar to typical Strombolian centers, $$180 m^{3}$$ of a repository inventory will be deposited in a scoria cone (of which $$~1 m^{3}$$ will be exposed to the surface in a 10,000-year period), $$320 to 900 m^{3}$$ will be deposited in a scoria-fall sheet (up to 12-km dispersal), and $$21 m^{3}$$ will be dispersed regionally with a fine-grained particle component.