Years of scientific study have been devoted to designing a proposed
underground nuclear waste
repository at Yucca Mountain in the Nevada
LAWRENCE Livermore researchers thrive
on challenging assignments. Few assignments have been as demanding
a waste package
system to keep high-level radioactive waste packages essentially
intact for at least 10,000 years. A team of Livermore researchers—engineers,
metallurgists, chemists, microbiologists, and computer scientists—are
testing and refining the design and materials for what will eventually
be 12,000 waste packages. These efforts are an integral part of
a national program to design, license, and build an underground
nuclear waste repository in Yucca Mountain, Nevada.
Mountain was selected by the Department of Energy (DOE) as a highly
promising repository site. In
1987, Congress directed the DOE to focus on Yucca Mountain as the
candidate location to safely store about 70,000 tons of waste from
civilian nuclear power plants and highly radioactive waste from
defense-related activities at DOE facilities. As part of the DOE’s
Yucca Mountain Project, Livermore scientists have made major contributions
in characterizing the proposed underground site, determining the
effects on the site from storing high-temperature radioactive wastes,
and selecting and characterizing corrosion-resistant materials.
Livermore’s largest effort is developing Yucca Mountain’s
engineered barrier system, which consists of a waste package, drip
shield, and supporting structures. The engineered barrier system
is designed to work with the natural barriers of Yucca Mountain
to contain the repository’s radioactive wastes and prevent
them from seeping into the water table which lies about 300 meters
below the planned repository.
need to show that our design will substantially contain the waste
inside the canisters for at least 10,000 years under extreme
and varying conditions of temperature, radiation, and corrosion,” says
Dan McCright, Livermore metallurgist and Yucca Mountain Program
senior scientist. According to McCright extensive analyses have
shown that even if waste were to eventually leak from the canisters,
additional barriers, both natural and engineered, are expected
to keep the waste far from the water table and humans.
direct information exists about how modern materials will behave
over thousands of years under a range of conditions. The Livermore
research is based on accelerated aging tests of materials that
are proposed to make up the engineered barrier system and on computer
models that simulate how a repository built at Yucca Mountain would
perform over thousands of years.
Yucca Mountain Project
1982, Congress passed the Nuclear Waste Policy Act, which
made the Department of Energy (DOE) responsible for finding
a suitable site and designing, building, and operating
a permanent underground radioactive-waste disposal facility,
called a geologic repository. The search identified several
possible locations for the nation’s first long-term
waste repository. In 1987, Congress amended its earlier
act to focus solely on the Yucca Mountain site in Nevada,
about 145 kilometers northwest of Las Vegas. On July
23, 2002, President George W. Bush signed House Joint
Resolution 87, allowing DOE to proceed in establishing
a safe repository in which to store nuclear waste.
Mountain is located in a remote desert on federally
protected land within the secure boundaries
of DOE’s Nevada Test Site. Hundreds of
scientists and engineers have studied Yucca
Mountain’s geology, hydrology, chemistry,
climate, and other physical aspects that could
affect a repository’s safety. The U.S.
is not the only country facing the disposal
issue. Around the globe, virtually all nations
that use nuclear power are exploring approaches
to safely dispose of radioactive waste.
is preparing an application to obtain a Nuclear
Regulatory Commission license to proceed with
construction of the repository. DOE has set
2010 as Yucca Mountain’s opening date,
that is, when the first waste will be placed
in canisters and moved inside.
Yucca Mountain repository would house more
than 70,000 metric tons of spent nuclear fuel
from civilian nuclear power plants and highly
radioactive waste from defense-related activities
at DOE facilities across the U.S. Currently,
spent nuclear fuel is stored in ponds or silos
near operating commercial nuclear plants, while
high-level nuclear waste from defense programs
and experimental reactors is stored as liquid
in tanks or as glass logs at several DOE facilities.
Homeland Security Concerns
vulnerability of this waste, dispersed in so many locations,
to potential terrorists is of concern to homeland security
experts. The government’s plan is to dispose
of this waste in a centralized, well-monitored, and
highly secure repository.
90 percent of the waste will be spent fuel, which
consists of solid pellets of enriched uranium oxide
sealed in a cladding of corrosion-and heat-resistant
zirconium alloy. The spent nuclear fuel from power
plants would be delivered to the Yucca Mountain
site “as is.” The waste from DOE defense
programs would first be vitrified, that is, converted
into a borosilicate glass, before delivery to the
Yucca Mountain repository would be constructed in a layer
of rock called tuff about 300 meters below the surface
and about 300 meters above the permanent water table.
Yucca Mountain is unique among potential sites under
consideration in the waste disposal programs throughout
the world because waste would be emplaced above the water
table in an environment that is oxidizing in nature (that
is, has plenty of oxygen).
the principle of “defense in depth,” the
repository would incorporate multiple protective
barriers, both natural and engineered. The
engineered barriers include a canister and
an overhanging drip shield. Emplaced waste
would be monitored for the first 100 years
of operation, and then the repository would
be permanently closed. Scientists believe the
dry conditions within the tuff will minimize
the prospects for water to contact the canisters
and that the waste will be sufficiently isolated
for thousands of years.
Yucca Mountain Project falls under the purview
of the DOE Office of Civilian Radioactive Waste
Management. The prime contractor is Bechtel
SAIC Company, LLC, which is a joint venture
between Bechtel National and Science Applications
International Corporation. The project is one
of the most closely reviewed programs ever
undertaken by the federal government. Reviewing
organizations include Congress, the General
Accounting Office, the Nuclear Regulatory Commission,
the State of Nevada’s Nuclear Waste Project
Office, Nye County Nuclear Waste Repository
Office, the Nuclear Waste Technical Review
Board, and the National Academy of Sciences.
at Yucca Mountain’s
Drift Scale Test Facility are acquiring a better
understanding of the thermal, mechanical, hydrological,
and chemical processes that occur deep underground
at the site.
current repository design calls for waste to be stored in a package
consisting of a set of two nested canisters—an outer
canister made of a highly corrosion-resistant metal (Alloy 22)
and an inner canister made of a tough, nuclear-grade stainless
steel (316NG). An overhanging drip shield made of titanium will
provide additional protection to the waste package from dripping
water and any falling rocks from the repository ceiling. “Because
the waste package and the drip shield are made of different corrosion-resistant
materials, they form corrosion defense in depth,” says McCright.
is developing Yucca Mountain’s engineered barrier system,
which consists of a waste package, drip shield, and supporting
structures. This artist’s concept shows how the canisters
will be placed in tunnels about 300 meters underground. The
waste packages will rest on a strong corrosion-resistant pallet
and supporting steel frame.
the waste packages horizontally and commingling the different kinds
of waste packages will create a relatively uniform temperature
in each underground drift, or tunnel, carved inside the mountain.
The waste packages have a common diameter (1.8 meters), but their
lengths vary according to the type of waste—from about 3.6
meters for the defense waste to 5.7 meters for the spent nuclear
most critical element of the engineered barrier system is the 20-millimeter-thick
outer canister made of Alloy 22, which consists
of about 60 percent nickel, 22 percent chromium, 13 percent molybdenum,
and 3 percent tungsten. Alloy 22 is highly resistant to fractures
and is easier to weld than alternative materials such as titanium.
It is also extremely corrosion resistant under the conditions of
high temperature and low humidity expected to prevail for hundreds
to thousands of years in a repository. In addition, it is resistant
under conditions of either low or high humidity at the lower temperatures
expected in the repository when radiation levels decrease. Hence,
the selection of Alloy 22 would provide containment over a range
of environmental conditions. “It’s the best engineered
material available for the job,” says McCright.
stainless steel (316NG) was chosen for the 50-millimeter-thick
inner canister to add strength and bulk to the waste package. It
is corrosion resistant, more compatible with Alloy 22 than carbon
steel, and more economical than more complex steel alloys.
titanium drip shield, which McCright compares to a sturdy awning,
would be fabricated from grade 7 titanium. This material contains
a small amount of palladium to provide greater corrosion resistance.
The drip shield, however, is not considered essential to containing
the wastes. Earlier projections of Alloy 22’s corrosion performance
assumed that there would be no drip shields and that drips from
the repository walls would fall directly on the canisters.
Livermore scientists are testing
the design of the waste packages to be used in the Yucca
Mountain repository. The waste packages will have a common
diameter (1.8 meters), but their lengths will vary according
to the type of waste—from about 3.6 meters for defense
waste to 5.7 meters for spent nuclear fuel. The scientists
work on prototypes like the one shown here that have the
full-scale diameter but shortened lengths.
waste packages will rest on a pallet fabricated from Alloy 22 clad
onto steel. The pallet, in turn, will sit on a steel frame
and crushed gravel. The waste packages will be placed close together
(about a meter apart) so that by design their surfaces will reach
a maximum surface temperature of 160°C (caused by radiation
levels of up to 180 rads per hour) once the repository is sealed. “It
may take hundreds of years before surface temperatures cool below
boiling because of the slow decay of radioactive components in
the waste,” says McCright. Keeping the canister surfaces
above the boiling point will ensure they are dry, with the intention
to prevent corrosion.
the repository is sealed, it will take hundreds of years before
waste-package surface temperatures drop below boiling because
of the slow decay of the radioactive waste. This graph shows
the projected decline of temperatures on waste canisters’ surfaces
over 1 million years. The range of temperatures corresponds
to uncertainty about heat-transfer modes, variation among the
heat output of individual waste packages, and the location
of waste packages in the repository. The dotted transition
region marks where temperatures fall below boiling. At this
point, water may come into contact with the containers.
Getting a Close-up View
effort is under way to understand and characterize the environments
closest to the drip shield and the waste package because
these environments will determine the potential for corrosion and
how fast it could proceed. Surface conditions will be characterized
by the temperature, humidity, and composition of gases in the repository;
the contaminants in the dripping water from repository walls; and
the mixture of minerals and salts that may eventually be deposited
on the drip shield and canisters. As temperatures cool, for example,
moisture and dust in the atmosphere will settle on the canisters’ surfaces
despite the presence of the drip shield. If a drip shield is eventually
breached, water seeping through rock fissures could contact the
canisters directly and cause more minerals or salts to precipitate
on their surfaces, thereby increasing the potential for corrosion.
corrosion is the paramount objective. Corrosion can be general,
occurring more or less uniformly over the entire surface,
or localized, occurring in specific areas such as in pits or crevices
on a metal’s surface. Corrosion can also be assisted by cracking
from stresses in a metal or weld, a phenomenon called stress corrosion
materials chosen for the waste package are among the most corrosion
resistant of engineering materials. They are used routinely under
harsh conditions in the chemical process industry and at nuclear
power plants and are expected to perform well in the expectedly
more benign conditions within Yucca Mountain. Both titanium and
Alloy 22 gain their corrosion protection from the natural, extremely
fast growth of thin films (about 3.5 nanometers or 10 atomic layers
thick) of metal oxides caused by oxygen in the environment. When
these stable, chemically unreactive films consolidate, the corrosion
rate decreases. One Livermore research effort is studying the growth
of metal-oxide thin films on Alloy 22 and titanium under the expected
environmental scenarios at Yucca Mountain. The observed compositions
and structures of the films are compared with model predictions
of film growth.
is essential to demonstrate that Alloy 22 will survive all anticipated
repository conditions. In particular, scientists must show that
corrosion rates, both general and localized, are extremely low
and that welds will not crack over time. Materials performance
tests are conducted at Livermore’s Long-Term Corrosion Test
Facility (LTCTF) to provide assurance that the waste packages will
maintain their integrity and corrosion resistance for thousands
Corrosion tests of metal samples,
called coupons, are carried out at Livermore’s Long-Term
Corrosion Test Facility. Four types of coupons are kept
in 24 tanks, each filled with about 1,000 liters of one
of the three aqueous solutions that are likely to be found
in the underground Yucca Mountain environment. In this
photograph, a rack containing several hundred coupons is
pulled out of solution for inspection.
Aging 18,000 Metal Coupons
corrosion tests at the LTCTF are designed to rapidly “age” metal
samples, called coupons, by subjecting them to much harsher conditions
than would be expected in the repository. More than 18,000 alloy
coupons are being tested, each of which measures about 5 centimeters
square or less. Fourteen alloys are being tested to compare the
corrosion resistance of Alloy 22, stainless steel, and titanium
with other materials.
kinds of coupons are used to test the various forms of corrosion.
Crevice coupons consist of metals tightly pressed against Teflon
washers to determine the extent of corrosion from liquid trapped
between the metal and washer. Weight-loss coupons measure general
corrosion. Galvanic coupons measure corrosion that occurs when
two dissimilar alloys are pressed against each other. Finally,
U-bend coupons are metals bent under continuous stress to try to
induce stress corrosion cracking. Many of the coupons are welded
to determine the effects, if any, of welds on corrosion.
coupons are kept in 24 tanks, each filled with about 1,000 liters
of one of three different solutions derived from those likely
to be found in the Yucca Mountain environment. One solution is
a concentrated mixture of salts and minerals common to Yucca Mountain.
The second solution is a diluted version of this mixture. The third
solution is an acidified version of the concentrated mixture. Solutions
are heated to either 60°C or 90°C. The coupons are mounted
on vertical racks and are either submerged in solution, suspended
over the solution, or partially submerged.
were removed from the tanks six months, one year, two years, and
five years after mounting. (Most of the coupons are still in
the tanks awaiting longer-term tests.) When a coupon is removed,
it is analyzed to determine whether corrosion has occurred, and
if so, where it is, and how much damage it caused. Corrosion
activity is evaluated by weighing the coupons after they are cleaned
of compounds that have precipitated on their surfaces
and by using an electron microscope and an atomic force microscope
to scrutinize their surfaces.
manager Dave Fix notes that the corrosion detected in the coupons
in the various solutions is generally so slight that it
resides at the limit of what is measurable. The average corrosion
rate is about 20 nanometers per year. At this rate, a 20-millimeter-thick
barrier of Alloy 22 would be effective for more than 100,000 years
before general corrosion would provide a means for water to contact
the underlying stainless-steel layer. In addition, the extremely
low corrosion rates appear to be nearly the same for all the water
chemistries and temperatures tested.
corrosion is measured only when coupons are subjected to extreme,
unrealistic conditions. For example, the basic metallurgical
structure of Alloy 22 is transformed over long periods of time
at temperatures of more than 500°C (100°C is the boiling
point of water at sea level). Several hundred millivolts of electrochemical
potential are necessary to make the test solution extremely corrosive. “These
extreme testing conditions are totally unrealistic for the Yucca
Mountain repository setting,” says McCright, “but our
models consider them.”
U-bend coupons are one of the four
types of coupons being tested for corrosion resistance.
U-bends are metals kept under continuous stress to try
to induce stress corrosion cracking.
Assessing Microbes’ Effects
microbiologist Joanne Horn leads a team assessing the potential
damage microbes can cause to the engineered barrier system.
Some bacteria and fungi, both those indigenous to Yucca Mountain
and those introduced by construction activities, could cause corrosion
of the engineered barriers. Horn notes that an abundance of microbes
exists in the Yucca Mountain repository setting. Microbial activity
is expected on the canisters when adequate moisture is present.
Some bacteria, for example, are expected to form patchy, thin films
over the metal-oxide films covering the waste packages and drip
notes that microbes have been found in the most inhospitable environments
on Earth, such as the scalding vents at the ocean’s
floor. Some bacteria have very efficient DNA repair mechanisms
that might enable them to survive high radiation levels.
team has identified more than 65 species and subspecies of bacteria
living in the Yucca Mountain rock. The team has also
identified the different growth requirements for these bacteria.
One set of laboratory experiments analyzes the extent of corrosion
on metal coupons caused by bacteria contained in crushed Yucca
Mountain rock and fed with simulated groundwater. Another set of
experiments determines the extent of corrosion caused by specific
species of bacteria that have potentially corrosive activities,
such as a species that oxidizes sulfur compounds. The results of
these experiments are compared with the corrosion that occurs under
identical conditions but in environments that have been presterilized
to kill all microbes. The team also analyzes the solutions to determine
if bacterial metabolic products could change the repository chemistry.
“The findings of the bacterial experiments parallel those of the long-term
corrosion facility,” says Horn. “To date, our results show that over
10,000 years, the corrosion rate from bacteria would not penetrate beyond 1 millimeter.
Alloy 22 is a very tough metal.”
Coupons of (a) Alloy 22 and (b) Alloy
825 (which contains less nickel and molybdenum than Alloy
22) show many precipitates when subjected to high concentrations
of minerals and salts. However, Alloy 22 has no localized
corrosion whereas Alloy 825 has permanent pits caused by
Overcoming Weld Stresses
the waste packages are manufactured, especially the required welds,
can affect their resistance to corrosion as well as their structural
stresses are a common by-product of manufacturing, especially welding, and if
left untreated could lead to stress corrosion cracking. Livermore metallurgists
plan to treat the canister welds in the repository with an annealing process
that reduces the residual tensile stress and produces instead a compressive stress
on the canisters’ surface. Stress corrosion cracking does not occur under
The annealing process
involves subjecting the welds to 1,100°C and then quenching
the metal in a water bath to produce a small overall compressive
stress on the exterior surface. The canisters would then be shipped
from the factory to the repository for storage until they are ready
to be filled and sealed.
canister lids would be fabricated offsite and then welded on after
the canister was filled at the repository. Annealing would
not work as a technique to lessen the stresses that will unavoidably
occur in the final closure welds because the high heat generated
during the process would damage the contents. “The nuclear
waste will be in a relatively inert form when it is placed in the
canisters,” says McCright. “Subjecting the waste to
the temperatures required by annealing might compromise that inertness.”
promising alternative to annealing is laser peening, a process
developed at Livermore, in which a laser produces a shock wave
on a weld to form a compressive stress. (See S&TR,
March 2001, Shocked
and Stressed, Metals Get Stronger.) Laser peening
can produce a compressive layer about 3 millimeters deep in the
welds. Livermore experts are characterizing structural changes
of peened Alloy 22 samples using transmission electron microscopy,
x-ray diffraction, and other techniques.
Thin films of bacteria form on the
surface of metal coupons tested in a solution containing
nonsterile crushed Yucca Mountain rock. Bacteria that have
colonized on an Alloy 22 coupon are shown magnified
(a) 1,200 times and
(b) 5,000 times.
Favorable Testing Results
micrographs (8,000 times magnification) show the extremely
limited corrosion on an Alloy 22 coupon caused by bacteria
after (a) 57 months in a solution containing nonsterile crushed
Yucca Mountain rock compared to (b) a coupon that has been
in a solution with presterilized rock (to kill all microbes)
for 43 months, and (c) the same coupon at the start of the
to accelerate corrosion with solutions representing the waters
that could eventually contact the metal canisters have thus
far indicated an extremely low general corrosion rate. The tests
also have shown that the canister metals have extremely high resistance
to all forms of localized corrosion and stress corrosion cracking
in environments relevant to the repository. Also, no appreciable
differences have been noted in corrosion rates obtained from the
various water compositions and temperatures. The testing results
support Livermore’s models for long-term prediction of the
waste packages’ performance and strongly confirm the selection
of Alloy 22 for the outer canister.
and modeling at Livermore will proceed for several more years.
In the meantime, McCright and others are refining the engineered
barrier design to make components more efficient and economical
to manufacture. “The plan is to manufacture 12,000 waste
packages over 25 years, the equivalent of manufacturing more than
one canister a day,” he says. “We want every one to
be as corrosion resistant as we can practically make them.”
Key Words: Alloy 22, engineered barrier system, laser peening,
Long-Term Corrosion Test Facility (LTCTF), Nevada Test Site,
Nuclear Regulatory Commission, nuclear waste repository, Yucca
For further information contact Dan McCright
(925) 422-7051 (firstname.lastname@example.org).
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