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The thermal energy balance for emplacement drift ventilation is solved analytically by using “well-mixed” volume elements that discretize the domain down the length of a drift. The solution technique is based on the use of a lumped parameter quasi-steady-state approximation, and the principle of superposition. The lumped parameters are convective and linearized radiation heat transfer coefficients. The quasi-stead-state approximation allows the energy balance equations to be written without time derivatives and solved algebraically for a single time step. The progress of the heat transfer analysis through time is like that of integrating a function using Euler’s method. The principle of superposition is used to calculate the temperature response of the drift wall due to an arbitrary heat flux and a given set of thermophysical rock properties. The results of this calculation are used as a “multiplier” on the drift wall heat flux in the algebraic solution of the four energy balances, and eliminates the need to solve the conduction heat transfer in the rock mass at every time step. The results of the analysis are compared to a similar numerical model and include time and location dependent waste package, in-drift air, and drift wall temperatures, and ventilation efficiencies.

Predictions of waste canister and repository driftwall temperatures as functions of space and time are important to evaluate pre-closure performance of the proposed repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. Variations in the lithostratigraphic features in densely welded and crystallized rocks of the 12.8-million-year-old Topopah Spring Tuff, especially the porosity resulting from lithophysal cavities, affect thermal properties. A simulated emplacement drift is based on projecting lithophysal cavity porosity values 50 to 800 m from the Enhanced Characterization of the Repository Block cross drift. Lithophysal cavity porosity varies from 0.00 to 0.05 cm3/cm3 in the middle nonlithophysal zone and from 0.03 to 0.28 cm3/cm3 in the lower lithophysal zone. A ventilation model and computer program titled “Monte Carlo Simulation of Ventilation” (MCSIMVENT), which is based on a composite thermal-pulse calculation, simulates statistical variability and uncertainty of rock-mass thermal properties and ventilation performance along a simulated emplacement drift for a preclosure period of 50 years. Although ventilation efficiency is relatively insensitive to thermal properties, variations in lithophysal porosity along the drift can result in a range of peak driftwall temperatures can range from 40 to 85 °C for the preclosure period.

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Temperature distributions from the Single Heater Test of the Yucca Mountain Project were used to determine rock-mass thermal conductivity. The Single Heater Test, located in a densely welded tuff in Alcove 5 of the Exploratory Studies Facility at Yucca Mountain, is nominally 13-m wide, 10-m deep and 5.5-m high. A centrally located, 5-m long, 4 kW electrical heater was activated for 9 months. During the heating phase and subsequent cooling phase of a similar duration, temperatures were measured hourly from more than 300 thermocouples emplaced in boreholes strategically drilled into the test block. An inverse method, that assumes a linearized system, was applied. This method minimized the sum of residuals between temperature measurements and simulations. The simulations were based on temporal and spatial superposition of a series of point sources that represented a linear heat source akin to the line-source heater in the Single Heater Test. Also the method accounted for fluctuations in the power of the central heater through the use of convolution methods. Subsequently, the derived value for rock-mass thermal conductivity was compared to values determined from several laboratory and field techniques that accounted for both matrix and lithophysal porosity. In general, agreement between the various methods was good.