Geological complexity and performance assessment: Volcanic hazards modeling at Yucca Mountain

Quantitative performance assessments (PA) are often used by regulatory agencies to evaluate the ability of a designed structure to accomplish its objectives. For various reasons, the physics of failure is sometimes abstracted' in these analyses. Because there is no general model for magmatic processes, this approach has some appeal for volcanic hazards assessment. Alternatively, the author has used a simple physical model loosely coupled to a consequence simulator to study the intrusion of a basaltic dike into to a presumed high-level radioactive waste repository at Yucca Mountain, NV. The model uses a simplified dike-intrusion model and Monte Carlo-style parameter variation with distributions taken from analogous volcanic systems. Dike contamination follows (1) hydrologic transport of radionuclides and entrainment of lithic fragments of wall rock and (2) direct interaction of waste and magma. The physical model gave average dikes of 0.25 to 1.5 m width, mean = 0.5 m, and 750 to 3,000 m length, mean = 1,500 m. The volume of contaminated dike rock is 100 m[sup 3]. Under current assumptions, releases due to magmatic activity should not exceed the regulatory limit set by EPA. This work shows that magma physics can constrain volcanic hazards in PA modeling. Stochastically varied inputmore » parameters, however, may violate the physical plausibility of the model. The parameter distributions should be categorically biased by the geological and tectonic context, detailed in the PA scenarios.« less