Seismic modeling and analysis of the prototype heated nuclear waste storage tunnel, Yucca Mountain, Nevada

Abstract

The 1997-2002 Yucca Mt. heated drift scale test (DST) used a specialized system of heaters to simulate thermal effects from stored nuclear waste on a tunnel, surrounding volcanic tuff, and groundwater flow. Seismic calibration shots were recorded on a receiver array installed around the tunnel while temperatures inside were elevated to approximately 200◦C. Receiver gathers show classic wave propagation behavior as given in the literature for pulses incident upon cylinders/tunnels. However, a combination thermal/groundwater process causes direct and reflected P-S arrival changes in the surrounding tuff as temperatures increase. Data also show these waveforms effectively do not change once the target temperature is obtained at the tunnel wall. Laboratory data for granite was used as a physical analog to develop velocity versus temperature models for Yucca Mt. tuff. Barring 3-D effects/out-of-plane reflections, 2D spectral element waveform simulation coupled with well-constrained thermal models has consistently replicated receiver data in the plane of the calibration source. Field/model waveform agreement is a function of velocity gradient for Vp,s(x, z, t, T ) models derived by fitting and adjusting said granite data. Waveforms using velocity model sets with a V −1dV/dT gradient of −0.5% per 100◦C show improved agreement with field data over those with a −2.5% per 100◦C gradient. This optimal velocity change is lower than values stated in literature. Because the velocity gradient is small, velocity changes may be controlled by fracturing due to thermal expansion and fragment compression, not large bulk modulus changes associated with groundwater phase transitions. In addition, our numerical modeling shows a diffracted/“Franz” wave propagating perpendicular to the tunnel wall in the thermal transition region. This wave is present in field and model data propagating at speeds that vary with conditions near the tunnel. P-S separation and Franz wave velocity are therefore potential tools for seismic measurement and monitoring of conditions near the tunnel.