Transport of radon gas into a tunnel at Yucca Mountain—estimating large-scale fractured tuff hydraulic properties and implications for the operation of the ventilation system

Abstract

Radon gas concentrations have been monitored as part of the operation of a tunnel (the Exploratory Studies Facility-ESF) at Yucca Mountain to ensure worker safety. The objective of this study was to examine the potential use of the radon data to estimate large-scale formation properties of fractured tuffs. This objective was examined by developing a numerical model, based upon the characteristics of the ESF and the Topopah Spring welded (TSw) tuff unit, capable of predicting radon concentrations for prescribed ventilation conditions. The model was used to address two specific issues. First, it was used to estimate the permeability and porosity of the fractures in the TSw at the length scale of the ESF and extending tens of meters into the TSw, which surrounds the ESF. Second, the model was used to understand the mechanism leading to radon concentrations exceeding a specified level within the ESF. The mechanism controlling radon concentrations in the ESF is a function of atmospheric barometric fluctuations being propagated down the ESF along with ventilated air flow and the slight suction induced by the ventilation exhaust fans at the South Portal of the ESF. These pressure fluctuations are dampened in the TSw fracture continuum according to its permeability and porosity. Consequently, as the barometric pressure in the ESF drops rapidly, formation gases from the TSw are pulled into the ESF, resulting in an increase in radon concentrations. Model calibration to both radon concentrations measured in the ESF and gas-phase pressure fluctuations in the TSw yielded concurrent estimates of TSw fracture permeability and porosity of l x lo-" m2 and 0.00034, respectively. The calibrated model was then used as a design tool to predict the effect of adjusting the current ventilation-system operation strategy for reducing the probability of radon gas concentrations exceeding a specified level.

Spatiotemporal variation of radon and carbon dioxide concentrations in an underground quarry: coupled processes of natural ventilation, barometric pumping and internal mixing

“…For example, transient changes in radon concentration can be used to assess the efficiency of mechanical ventilation (Unger et al, 2004). Transient changes of carbon dioxide concentration associated with anthropogenic sources can be another method to estimate natural ventilation and to model air exchanges patterns in underground cavities (Faimon et al, 2006).…”

Natural radionuclide mobility and its influence on U–Th–Pb dating of secondary minerals from the unsaturated zone at Yucca Mountain, Nevada

“…222 Rn concentrations in cave air vary from hundreds to thousands of Bq m À3 (Papachristodoulou et al, 2004 and references therein). Elevated concentrations of 222 Rn (up to 65,000 Bq m À3 ) were measured in the ESF air during periods without ventilation (Unger et al, 2004).…”

Estimating Large‐Scale Fracture Permeability of Unsaturated Rock Using Barometric Pressure Data

“…Both types of data have been applied to characterizing the Yucca Mountain site, with the air‐ k tests extremely valuable for calculating fracture permeability within a small‐scale (∼1 m) spatial domain (Huang et al, 1999). In addition, Unger et al (2004) reported on an indirect approach of using radon gas concentration data, measured in an underground tunnel at Yucca Mountain, to estimate large‐scale fracture properties. It should be mentioned, however, that the application of subsurface pneumatic responses to surface barometric signals is in most cases limited to one‐dimensional vertical flow scenarios (Weeks, 1987; Ahlers et al, 1999; Neeper, 2002).…”