Numerical analysis of thermal-hydrological conditions in thesingle heater test at Yucca Mountain Page: 3 of 6
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Introducing a heat source in the unsaturated fractured tuff at Yucca Mountain gives rise to strong
two-phase flow effects, typically characterized as follows:
(1) drying of the rock and vaporization of pore water close to the heater,
(2) vapor transport away from the heated area due to gas pressure build-up,
(3) condensation of the vapor in cooler regions outside of the drying zone,
(4) reflux of condensate to the vicinity of the heating due to capillary suction, and
(5) drainage of water away from the heated area due to gravity.
These processes are reflected in the spatial variation and temporal evolution of the liquid satura-
tion in the rock mass. They also contribute to heat transfer in the near-field environment, as
heat-induced gas and liquid fluxes may give rise to significant convective heat transport. For
example, strong vapor-liquid counterflow may be reflected in a distinct "heat pipe" temperature
signal, i.e., the temperature values remain at the nominal boiling point for some time before they
continue to increase. Thus, close analysis of the numerous temperature measurements in the SHT
can help to identify and constrain moisture redistribution characteristics, while the comparison of
measured and modeled temperatures may serve to determine the suitability of different modeling
conceptualizations in describing heat-induced flow and transport processes.
As a typical example for the temperatures observed in the SHT, we present the time evolution of
measured temperature at one particular sensor located approximately 0.7 m away from the heater
borehole in Figure 1. The down spikes register incidences of power outages. Temperature
increases to nominal boiling within about 50 days, and continues to increase without evidencing
a significant heat pipe signal. Since the other sensor observations show similar trends, the
temperature data indicate that the SHT hydrogeological and thermal properties allow for only
limited liquid reflux from the condensation zone back to the heater; i.e., heat conduction appears
to account for most of the temperature rise. Figure 1 also gives the simulated results, obtained
using the ECM, DKM, and MINC method, respectively. Note that the ECM results display only
one temperature curve due to the local equilibrium assumption, while DKM and MINC have
separate curves for fracture and matrix temperatures. In the latter case, the measured tempera-
ture values should be compared with the simulated matrix results, as the sensor is placed in a
grouted borehole. Generally, the agreement between the measured and simulated data is very
good for the three models, indicating that the thermal-hydrological response of the SHT is well
represented. However, the ECM results display a subtle heat pipe signal, which retards the
temperature increase at nominal boiling for some time and gives rise to a slight underestimation
of temperature for the remaining heating period. The temperature curves obtained with the
DKM and MINC method are almost identical: Both simulated matrix curves match the measured
data curve exactly without showing a heat pipe signature. The fracture temperatures, on the
other hand, display a distinct plateau at nominal boiling, indicative of substantial vapor-liquid
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Birkholzer, Jens T. & Tsang, Yvonne W. Numerical analysis of thermal-hydrological conditions in thesingle heater test at Yucca Mountain, report, August 8, 1998; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc897719/m1/3/: accessed June 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.