Numerical analysis of thermal-hydrological conditions in thesingle heater test at Yucca Mountain Page: 1 of 6
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Numerical Analysis of Thermal-Hydrological Conditions in
the Single Heater Test at Yucca Mountain
Jens T. Birkholzer and Yvonne W. Tsang
Earth Sciences Division, Lawrence Berkeley National Laboratory
Berkeley, CA, 94720
The Single Heater Test (SHT) is one of two in-situ thermal tests included in the site characteri-
zation program for the potential underground nuclear waste repository at Yucca Mountain. The
heating phase of the SHT started in August 1996, and was completed in May 1997 after 9
months of heating. The coupled processes in the unsaturated fractured rock mass around the
heater were monitored by numerous sensors for thermal, hydrological, mechanical and chemical
data. In addition to passive monitoring, active testing of the rock mass moisture content was
performed using geophysical methods and air injection testing. The extensive data set available
from this test gives a unique opportunity to improve our understanding of the thermal-hydro-
logical situation in the natural setting of the repository rocks. The present paper focuses on the
3-D numerical simulation of the thermal-hydrological processes in the SHT using TOUGH2. In
our comparative analysis, we are particularly interested in the accuracy of different fracture-
matrix-interaction concepts such as the Effective Continuum (ECM), the Dual Continuum
(DKM), and the Multiple Interacting Continua (MINC) method.
The SHT consists of a 5 m long, nominal 4 kW heating element, horizontally placed among 30
instrumented boreholes, which span a block of approximately 13 m x 10 m x 13 m in a side
alcove of the underground Exploratory Studies facility (ESF) at Yucca Mountain. The test block
resides in the potential repository formation of the Topopah Spring welded tuff approximately
200 m above the groundwater table. Though the welded tuff has very low matrix permeability, it
is intensely fractured with the fracture permeability several orders of magnitude higher than the
matrix permeability. At ambient conditions, the fractures are essentially drained and not very
conductive to water. However, strong capillary forces hold a significant amount of water in the
matrix pores. This water can be mobilized due to the heating of the rock mass, raising the liquid
saturation in the fractures, so that the water flux in the fractures can be enhanced by several
orders of magnitude from its ambient values.
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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/m1/1/: accessed July 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.