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A
unique tunnel in YUCCA MOUNTAIN
At the site which could
become the USA's first nuclear waste repository
Yucca
Mountain, in the Nevada desert some 160 km north-west of Las Vegas,
is being studied for its suitability as the USA's first permanent
high-level nuclear waste repository. And Atlas Copco Swellex rock
bolts by the thousand are finding a home inside the 2, 043 metre-high
peak.
The Yucca Mountain Project
involves the construction of an underground laboratory called
the Exploratory Studies Facility (ESF).
It will be used
by engineers and scientists to help determine the site's suitability
as a repository for the safe storage of high-level radioactive
waste.
A major part of
the laboratory consists of a 7.8 km long tunnel, 7.6 m in diameter,
which has been excavated by a tunnel boring machine. Much of the
tunnel design has been done according to a Nuclear Quality Assurance
programme, similar to that used for nuclear power plants.
By examining the
surface, plus the underground space that will be accessed via
the ESF, the scientists will be able to thoroughly investigate
rock strength and movement, groundwater, and earthquake and volcanic
activity.
Rock
support choice
Ground support
measures, based on the assumption that a rock fall can result
in the release of radiation, must have an effective life of at
least 100 years. In competition with other rock reinforcement
products, Atlas Copco's Super Swellex rock bolts were chosen together
with welded wire fabric and rolled steel channels to provide permanent
support.
| Setting
the Super Swellex bolts: These bolts, which give immediate
support, play a major role in ground reinforcement measures
at the Yucca Mountain project. |
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So far, approximately 20,000
Super Swellex bolts have been installed in the course of this
highly-prestigious tunnel project, the first part of which is
now completed. The performance of the bolts has fulfilled requirements
and the same ground support solution for a further 200 km of tunnels
is under discussion.
Just one example
of the demands the project places on ground support is this: In
an alcove for thermal experiments, the temperature will be raised
to 300°C over one year and will be kept at that level for
a further two years. This test will simulate the effects on the
rock of heat generated by radioactive waste and show how different
types of ground support will behave when subjected to increased
temperatures and stresses.
In addition to
the Super Swellex rock bolts, cast-in-place monolithic concrete
lining will be tested in the thermal alcove.
Tough
demands
Atlas Copco's
representative Ross Tilson explains how the unparalleled reinforcement
requirements on the project led to the choice of Super Swellex
rock bolts.
"If Yucca
Mountain becomes a permanent repository, parts of the ESF tunnel
will be used for access," he says. "This means the tunnel
must be reinforced in such a way that stored nuclear waste can
be retrieved 100 years after it is put into place - so the ground
support method must guarantee long-term stability and maintainability.
"Atlas Copco
Super Swellex rock bolts met all the requirements laid down by
the US Department of Energy.
"Cement-grouted
rebar bolts were rejected because the grout in rock fractures
interferes with scientific investigation of the rock. Furthermore,
it is not possible to immediately verify the safety of the bolt,
due to the curing time of the grout.
 Atlas
Copco's Pether Jonsson, Product Manager Rock Reinforcement:"
The exceptional quality demands on suppliers of products for
the Yucca Mountain project resulted in very close co-operation
between the customer, Atlas Copco North America and Atlas
Copco Rock Drills in Sweden."
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"Resin-grouted bolting
systems and timber lagging for the steel sets were also banned
because the amount of organic material in the tunnel had to be
minimal to avoid microbial action affecting the stored nuclear
waste. The use of Swellex bolts to reinforce the rock mass involves
no organic matter.
"Finally,
the use of shotcrete is limited because it can interfere with
geological mapping and geochemical tests."
What
lies ahead
The contractor
for the ESF tunnel and a number of alcoves is Kiewit/PB. Tunnel
design is by TRW Environmental Safety Systems Inc.
USD 339 million
was requested for the 1997 operations at Yucca Mountain, named
after the attractive, white-flowered plant with sword-shaped leaves
found in southern parts of the USA. The ESF tunnel and most major
alcoves were recently completed.
Viability
assessment
In 1998, a viability
assessment of the site will be made. This will include design
and system performance factors and cost estimates for construction
and operation of the repository. Later this year, the next phase
of the Yucca Mountain project - involving a 4.8 km-long tunnel
of 3.3 m diameter - will start. Looking even further ahead, a
repository site will be recommended to the US President in 2001
and a respository license application will be submitted to the
Nuclear Regulatory Commission in 2002.
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