Studies of altered vitrophyre for the prediction of nuclear waste repository - induced thermal alteration at Yucca Mountain, Nevada Page: 2 of 9
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STUDIES OF ALTERED VITROPHYRE FOR THE PREDICTION OF NUCLEAR WASTE
REPOSITORY-INDUCED THERMAL ALTERATION AT YUCCA MOUNTAIN, NEVADA
SCHON S. LEVY
Los Alamos National Laboratory, MS J978, P.O. Box 1663, Los Alamos, New
Mexico, USA 87545
ABSTRACT
Nuclear waste emplacement in devitrified volcanic tuff at
Yucca Mountain will raise the temperature of surrounding rock
for a geologically significant period of time. This study
evaluates the susceptibility of an underlyitig 50 ft-thick
vitrophyre to thermal alteration by examining alteration that
occurred in the rock as it cooled after deposition. A 10*C
temperature rise should have no mineralogical effects or the
vitrophyre, but an increase of 60* or more is likely. to
result in alteration. Expected mineralogic changes in the
vitrophyre caused by this amoun+ of thermal loading include
crystallization of zeolites and smectite. Alteration will be
concentrated in a thin interval near the top of the vitro-
phyre and along fractures. Adsorbed water and water in pre-
existing hydrous minerals and in glass may contribute to
hydrothermal alteration of underlying vitrophyre. Bulk
porosity change would be slight and local porosity increase
would probably be restricted to the upper part of the vitro-
phyre. Although some fracture filling could occur, such a
minor sealing effect would be balanced by development of sec-
ondary porosity. Zeolites and smectite, newly-crystallized
along fluid flow paths below the waste repository, could
provide an enhanced sorptive barrier to radionuclide migra-
tion.
INTRODUCTION
Nuclear waste emplacement in an underground repository will raise the
temperature of surrounding rock for a geologically significant period of
time [1,2), making it necessary to establish that thermal loading will not
reduce the capacity of the geologic medium to isolate the nuclear waste.
This study, as part of the Nevada Nuclear Waste Storage Investigations
managed by the Nevada Operations Office, US Department of Energy, is
concerned with thermal effects at Yucca Mountain in southern Nevada (Fig.
1). The geologic setting is a thick sequence of silicic volcaniclastic
rocks, with the candidate host rock for repository construction in densely
welded denitrified tuff [3]. A densely to moderately welded glassy tuff
(vitrophyre) beneath the devitritied tuff may be especially susceptible to
thermal alteration. The alteration that originally occurred in this rock
as it cooled following deposition can be used to predict repository-related
thermal alteration. This approach eliminates the necessity for extrapola-
tion of conditions and effects from different li-hologies and locations, a
problem common to many natural analogue studies.
METHODS OF STUDY
Core samples for this study come from drill holes at Yucca Mountain and
vicinity (Fig. 1). The area of coverage is large enough to permit investi-
gation of possible effects of lateral variation in present subsurface1
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Levy, S.S. Studies of altered vitrophyre for the prediction of nuclear waste repository - induced thermal alteration at Yucca Mountain, Nevada, article, December 31, 1983; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc690895/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.