ONE of the most daunting scientific
and engineering challenges today is ensuring the safety and reliability
of the nation’s nuclear arsenal. To effectively meet that
challenge, scientists need better data showing how plutonium, a
key component of nuclear warheads, behaves under extreme pressures
and temperatures. On July 8, 2003, Lawrence Livermore researchers
performed the inaugural experiment of a 30-meter-long, two-stage
gas gun designed to obtain those data. The results from a continuing
stream of successful experiments on the gas gun are strengthening
scientists’ ability to ensure that the nation’s nuclear
stockpile is safe and reliable.
JASPER (Joint Actinide Shock Physics Experimental Research) Facility
at the Department of Energy’s (DOE’s) Nevada
Test Site (NTS) is home to the two-stage gas gun. In the gun’s
first test, an unqualified success, Livermore scientists fired
a projectile weighing 28.6 grams and traveling about 5.21 kilometers
per second when it impacted an extremely small (about 30-gram)
plutonium target. This experiment marked the culmination of years
of effort in facility construction, gun installation, system integration,
design reviews, and federal authorizations required to bring the
experimental facility online.
experiments have drawn enthusiastic praise from throughout DOE,
the National Nuclear Security Administration (NNSA), and the
scientific community. NNSA Administrator Linton Brooks said, “Our
national laboratories now have at their disposal a valuable asset
that enhances our due diligence to certify the nuclear weapons
stockpile in the absence of underground nuclear weapons testing.”
Goodwin, associate director of Livermore’s Defense
and Nuclear Technologies Directorate, said, “I am proud of
the team effort that has produced the successful JASPER shots.
I have personal appreciation for the extraordinarily challenging
nature of plutonium. The precise data generated by these gas-gun
experiments will open up our scientific understanding of plutonium.”
Martinez, Livermore’s JASPER test director, notes that
the experimental results have been so good they are generating
significant interest in accelerating the test schedule. “The
JASPER data are demonstrating superb quality and indicate that
JASPER will meet its intended goal of generating high-precision
plutonium data,” he says.
JASPER was built at a total cost of about $20 million and sited
in existing aboveground buildings at NTS. The facility was developed
by personnel from Lawrence Livermore, Los Alamos, and Sandia national
laboratories and Bechtel Nevada, the NTS prime contractor.
JASPER Facility is located at the Department of Energy’s
Nevada Test Site, about 105 kilometers northwest of Las Vegas.
The facility is housed in three buildings previously used to
support Los Alamos National Laboratory’s nuclear test
Gas Guns Well Established
experimental technique for determining the properties of materials
at high pressures, temperatures, and strain rates
is to use a gas gun to shock a small sample of material with a
projectile traveling at high velocity and then diagnose the material’s
response. Lawrence Livermore’s three two-stage gas guns have
made important contributions to solving scientific puzzles in condensed-matter
physics, geophysics, and planetary science. For example, in 1996,
Livermore’s largest two-stage gas gun produced metallic hydrogen
for the first time. Recently, with experimental techniques that
will be used at JASPER, this gas gun was also used to determine
the melting point of iron at Earth’s core.
Holmes, chief JASPER scientist and head of Livermore’s
shock physics program, says that two advantages of a gas gun are
its proven dependability and scientists’ extensive experience
with it. Lawrence Livermore has more than 40 years experience shocking
materials with gas guns. “When the projectile hits the target,
the pressure wave is as steady as it can be,” says Holmes. “As
a result, researchers can focus on the target and diagnostics rather
than the gun’s performance.”
fire projectiles from the JASPER gas gun into plutonium targets
equipped with instruments for measuring and recording data.
(See the QuickTime movie.) The projectile’s
impact produces a shock wave that passes through the target in
a millionth of a second or less, creating
pressures of more than 600 gigapascals (6 million times the pressure
of air at Earth’s surface), temperatures to thousands of
kelvins, and densities several times that of plutonium’s
original solid state.
JASPER team’s role in the Stockpile Stewardship Program
is to measure the fundamental properties of plutonium. Data from
the experiments are used to determine material equations of state,
which express the relationship between pressure, density, and temperature.
The equation of state is essential for generating reliable computational
models of plutonium’s behavior under weapons-related conditions.
Knowledge of these properties is required to assess, without nuclear
testing, the performance, safety, and reliability of nuclear weapons.
two-stage gas gun, seen in this artist’s depiction, measures
30 meters long and includes a secondary confinement chamber
that encloses the primary target chamber.
Long Qualification Phase
to the construction of JASPER, the only facility available for
performing shock tests on plutonium was the 40-millimeter,
single-stage gas gun built at Livermore and currently located at
Los Alamos. This gun can achieve a maximum projectile velocity
of about 2 kilometers per second and up to 30 gigapascals of pressure.
determined that much higher projectile velocities were needed to
achieve the desired conditions for plutonium research.
Two-stage, light gas guns similar to the JASPER gun have been operational
at Lawrence Livermore, Los Alamos, and Sandia national laboratories
for many years, but they are not licensed to perform experiments
the late 1990s, it was recognized that a new two-stage gas-gun
facility dedicated to plutonium research, and located in a remote
location, could provide valuable data on plutonium’s equations
of state. Ideally, the facility would operate with a short turnaround
time between shots and at a modest cost per shot. In early 1998,
a study conducted by a team of scientists and engineers from several
national laboratories identified the Able Site at NTS as the best
location. The site’s three main buildings had previously
been used by Los Alamos’s nuclear test program.
and facility modifications at the Able Site started in April 1999
and were completed in September 1999. The JASPER
gas gun was installed in early 2000, and the first system-integration
demonstrations were completed in September 2000. From February
2001 to April 2003, Livermore staff verified the gun performance
and containment systems, validated the diagnostics and operating
procedures, and fulfilled the regulatory and compliance requirements.
As part of the validation process, researchers fired a series of
20 shots using nonnuclear materials to qualify the gun for use
with nuclear materials. At the conclusion of the installation project,
JASPER managers received NNSA Defense Programs Excellence and DOE
Project Management awards.
operates the facility for NNSA, and Bechtel Nevada supplies resources
for facility maintenance and operation, and diagnostic
design and operation.
gas gun at the JASPER Facility in Nevada fired its first
shot in July 2003. Livermore operates the facility for
the National Nuclear Security Administration. Bechtel Nevada
supplies resources for facility maintenance and operation,
and diagnostic design and operation.
project engineer John Warhus monitors preparations for
a gas-gun experiment from the JASPER control room.
JASPER Gun Matches Livermore’s
JASPER gas gun was designed to match the internal dimensions of
the large two-stage gas gun at Livermore, which has been operational
since 1972. By copying that design, researchers took advantage
of the extensive database and experience that exists from using
the Livermore gun, thereby minimizing the effort required to characterize
the JASPER gun at start-up. Although the internal dimensions are
the same, JASPER’s containment system is significantly more
complex because the Laboratory’s gas gun is not used with
hazardous materials such as plutonium and, hence, does not require
a special material-confinement system. The
Livermore gas gun serves as a test bed for developing techniques
and training personnel for future experiments at JASPER. “We
work out JASPER experiments first on our two-stage gun at Livermore
with nonnuclear materials,” says Martinez.
gas gun is driven first with gunpowder and then with a light gas.
In the first stage, hot gases from the gunpowder
propellant drive a 4.5-kilogram plastic deformable piston down
a pump tube. The piston compresses a light gas, typically hydrogen,
as it travels down the narrowing tube. This gas, which is the second-stage
driving medium, is compressed until it builds up enough pressure
to burst a valve. The explosive gas accelerates a 15- to 30-gram
projectile down the launch tube toward the target at a velocity
of up to 8 kilometers per second.
projectile is made of plastic with a flat, metal plate embedded
in its face to directly impact the plutonium target. Depending
on the desired shock pressure, the metal plate is made of aluminum,
tantalum, or copper. A typical projectile measures 28 millimeters
in diameter and weighs 25 grams.
speeding projectile enters the primary target chamber (PTC), which
houses the plutonium target. Just prior to entering the PTC,
the passing projectile is sensed by a continuous x-ray source and
detector, which trips a switch that triggers the detonation of
the ultrafast closure valve. This valve effectively traps radioactive
debris within the PTC following the projectile’s impact on
the plutonium target.
the projectile hits the plutonium target, the impact produces a
high-pressure shock wave of about 600 gigapascals. The temperature,
a critical variable in a material’s equation of state, can
reach as high as 7,000 kelvins. By comparison, the surface of the
Sun is about 5,800 kelvins. The destroyed plutonium target is contained
within the PTC. Following the experiment, the PTC is discarded
and sent to the federal Waste Isolation Pilot Plant in New Mexico.
velocities are precisely determined by experimental parameters
such as the type and amount of gunpowder, the driving
gas, the diameter of the barrel, and the mass of the projectile.
JASPER facility manager Ben Garcia notes that as a precaution,
all shots are first simulated using gun performance codes on computers. “We
want to make sure we don’t produce any pressures that could
exceed the design limits of the gun,” he says.
the major diagnostic instruments are two flash x-ray units, which
measure projectile velocity to within 0.1 percent
accuracy, and electrical pins, which measure the speed of the shock
wave from the impact of the projectile. The facility also has the
capability to use a Velocity Interferometer System for Any Reflector
(VISAR), a tool that measures the velocity of the exploding target
by recording Doppler-shifted reflected light. These data are essential
to understanding plutonium’s material properties. Additional
diagnostic instruments are planned that will measure the temperature,
electrical conductivity, and other characteristics of the target
Is Central to JASPER Gas-Gun Design
engineers adopted a dual-layered approach for JASPER’s
two-stage gas gun to ensure that plutonium dust or
fragments are not released into the building or the
environment after each experiment. The two layers are
the primary target chamber (PTC) and the secondary
confinement chamber. The PTC, which houses the plutonium
target, is designed to contain target material under
worst-case conditions following impact with the speeding
projectile. The PTC is discarded after every shot and
shipped to the federal Waste Isolation Pilot Plant
in New Mexico.
PTC engineer Matt Cowan notes that
designing the PTC has been a challenge
because of the dynamic loading of the
PTC during a shot. “We anticipated
two potential failure modes in the
PTC: loads that cause a rupture in
the pressure vessel and loads that
cause a dynamic gap at the sealing
engineers conducted extensive modeling
to determine where plutonium debris
would be distributed inside the PTC
following impact with a projectile.
In addition, experiments using plutonium
surrogates provided valuable experience
in refining the design of the PTC.
For example, researchers applied a
layer of phosphorous-32, which has
a two-week half-life, to a gold target
because radioactive materials are easier
to detect if they escape from the PTC.
Debris shields were added to absorb
some of the momentum of high-velocity
impacts and to protect critical O-rings
that seal the PTC’s interior. “JASPER
experiments cause particulates to fly
everywhere at extremely high speeds,
so we need to protect O-rings from
the sandblasting effect,” explains
Cowan. Engineers also expanded the
volume of space around the target impact
High Explosives Applications Facility
(HEAF) was used to demonstrate the
PTC’s design limits. The testing
at HEAF created explosive forces about
150 percent of the predicted dynamic
loads that the PTC would experience
with plutonium targets. The data from
HEAF agreed with results from simulations
and strengthened the engineers’ confidence
that plutonium would be contained.
PTC’s ultrafast closure-valve system at the chamber’s
entrance was designed and manufactured by Ktech Inc.
(Albuquerque, New Mexico), and adapted for use on JASPER
by Livermore engineers. The valve closes a 1.3-centimeter-diameter
aluminum tube in about 60 microseconds by detonating
90 grams of high explosives wrapped around the tube.
The valve then traps plutonium debris within the PTC. “A
splash-back of plutonium travels at the same speed
as the projectile, so we need to close the tube extremely
quickly,” says Cowan.
PTC is located in the secondary confinement
chamber, which has a large circular
door to access the PTC. The secondary
chamber ensures that any material that
might escape from the PTC will not
migrate into the building. “The
secondary chamber is not expended after
a test,” says Cowan, “and
it is not significantly challenged
during a shot.”
a final precaution, radiation-control
technicians, fully suited with respirators
and radiation detectors, enter the
gas-gun building following every shot
to make sure the plutonium debris has
been fully contained within the PTC.
The primary target chamber,
which houses the plutonium target, is designed
to contain target material under worst-case conditions.
It is located inside the secondary confinement
chamber. Other key features inside the secondary
confinement chamber include the flash x ray for
measuring projectile velocity, the continuous x
ray for tripping the ultrafast closure valve, and
the high-explosive gas accumulator for trapping
gases after the ultrafast closure valve has been
schematic showing how a two-stage gas gun operates. In the
first stage, hot gases from the powder propellant drive a
piston, which compresses the hydrogen gas in the pump tube.
In the second stage, the high-pressure gas ruptures the valve,
accelerating the projectile down the launch tube toward the
Targets Made at Livermore
first series of JASPER experiments used plutonium targets nicknamed “top
hats,” which consist of a plutonium disk the size of a half
dollar bonded to a smaller, nickel-size disk of plutonium. The
top hat design was first proven on Livermore’s two-stage
gas gun with copper, aluminum, and tantalum disks.
Randy Thomas, who is responsible for the production and machining
of JASPER targets at Livermore, notes the top hat targets
must meet extremely precise requirements: flat to within 2.5-millionths
of a meter with the two faces of each disk parallel to each other
within 2.5-millionths of a meter. Meeting such tight tolerances
requires a complex and time-consuming production and machining
process. Plutonium is first cast into a cylinder using a graphite
mold. The resulting cylinder is sliced into disks and then heated
to eliminate internal stress. The disks are rolled with specific
orientations to obtain correct metallurgical properties, heated
again, and machined until they are within less than 1 percent of
their final dimensions. Then the disks are checked for the correct
density and radiographed to detect any voids and inclusions. Even
slight imperfections result in the plutonium target being unusable.
The disks undergo final machining and inspection to ensure they
are flat and parallel. Then they are bonded together.
final measurement and characterization, the plutonium is loaded
into the target assembly. The assembly is aligned beforehand
so that the projectile will impact the target at its exact center.
The target assembly is leak tested, backfilled with an inert atmosphere,
placed in a federally approved shipping container, and trucked
to NTS. Holmes describes the final product as, “The highest
quality plutonium samples we’ve ever seen. That quality reflects
the superb plutonium fabrication and machining capabilities at
top hat plutonium target uses 13 diagnostic electrical-shorting
pins mounted on its surface: 6 on the large disk, 6 on the small
disk, and 1 that fits through a hole in the middle of the smaller
disk. On impact from the projectile, a shock wave travels through
the base plate and electrically shorts the pins. The velocity of
the shock front passing through the target is calculated using
the measured shock arrival times from the shorting pins and the
known target thickness. The pins’ orientations allow for
correcting the effects of projectile tilt during target impact.
extrudable piston is shown before and after firing. The piston
compresses hydrogen gas in the first stage of the two-stage
An x ray and (b) a photograph show the ultrafast closure
valve after it detonates to prevent any plutonium debris
from escaping from the primary target chamber.
Data for Equations of State
scientists are excited about the experimental results. “The
JASPER gas gun has validated itself as an important tool for plutonium
shock physics. Everything has worked as planned,” says Holmes. “We’re
thrilled with the quality of data. These experiments have never
before been done on plutonium with this accuracy.”
JASPER experiment provides one data point on plutonium’s
Hugoniot curve. The Hugoniot is derived from conservation of mass,
momentum, and energy equations using experimental values of projectile
velocity (flash x-ray data) and shock velocity (electrical pin
data). Hugoniot curves are then used to develop material equation-of-state
models used in weapon performance calculations.
“Equation of state is one of the most important elements in building a
robust capability for predicting weapon performance,” says Holmes. “We
mainly use theoretical equations of state for our simulations. That’s not
sufficiently accurate for stockpile stewardship purposes. We need data that will
either validate our theories or force changes in them.”
complement the subcritical nuclear materials experiments that Livermore scientists
have conducted underground at NTS since 1997. (See S&TR,
July/August 2000, Underground Explosions
Are Music to Their Ears.) Those experiments
blow apart tiny amounts of plutonium but stop short of nuclear chain reactions.
complex hydrodynamic experiments provide vital information on the behavior and
performance of aging nuclear materials.
The gas gun allows
scientists to study plutonium over a broader range of conditions than is the
case with subcritical experiments. Moreover, gas-gun technology eliminates
uncertainties introduced by high-explosive-driven experiments. Holmes points
out that gas-gun experiments can generate distortions in projectiles, but the
distortions are always the same shape and are readily accounted for.
Electrical pins on the target measure
the velocity of the shock front as it passes through
the target material. Velocity is determined by dividing
the difference in pin position by the difference
in shock arrival time.
JASPER Facility team from Lawrence Livermore and Bechtel
early experimental successes have generated significant discussion
regarding how to schedule more
experiments and how to extract more data from each experiment.
To meet the increasing demand for experiments, the JASPER team is exploring ways
to increase the number of experiments scheduled from the current 12 per year.
For example, a glove box (required for safe handling of plutonium) has been commissioned
at the Device Assembly Facility (DAF) at NTS, located about 15 kilometers from
JASPER. Livermore managers are planning to ship plutonium samples from Livermore
to DAF for final bonding and placement in the target assembly to support a busier
schedule. Using DAF would also decrease the risk of damage from transporting
finished plutonium targets and diagnostics over a long distance.
New diagnostics are
being considered to generate additional information about the physical processes
occurring in shocked plutonium. For example, plans are
under way to measure electrical and thermal conductivity as well as sound speeds
of shocked plutonium targets. The optical properties of the shocked target—the
light emitted during an experiment—will also be studied using lasers.
Another set of experiments
being planned would test aged plutonium to determine if its shocked properties
are different from newly cast material. At the same
time, physicists and engineers are looking at new projectile designs, such as
those made of different densities, to obtain specific shock pressures. Martinez
recalls how Livermore personnel once predicted, “If we build it (JASPER),
they will come.” He notes that physicists at Los Alamos are designing a
series of experiments, as are colleagues from Britain’s Atomic Weapons
Establishment. In fact, about 10 years of shots are already in the planning stages.
Martinez says, “People are getting new ideas all the time to find out more
about plutonium with JASPER.”
Key Words: Device Assembly Facility, equation of state, gas gun,
Hugoniot curve, Joint Actinide Shock Physics Experimental Research
(JASPER) Facility, Nevada Test Site (NTS), plutonium, stockpile
stewardship, Velocity Interferometer System for Any Reflector
For further information contact Mark Martinez
(925) 423-7572 (email@example.com).
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