The Superblock—An Essential Facility for Nuclear Operations

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man operating machinery through thick gloves (glovebox) and behind a metal and glass wall to separate him from the materials inside
Andy Cose of Materials Engineering Division’s Mechanical Manufacturing Group machines a plutonium sample using a lathe.

An essential part of the nation’s nuclear warheads are plutonium  “pits”—hollow shells of plutonium. Teams across Lawrence Livermore’s Superblock facility work together to understand the fundamental nature of plutonium, its behavior in different environments, and how it interacts with other materials. 

Established in 1961, the Superblock is one of only two defense plutonium research and development facilities in the country, providing state-of-the-art capabilities to conduct fundamental physics and engineering experiments and measure the physical, chemical, and metallurgical properties of weapons-grade materials. Superblock operations serve to validate the computational models that underpin simulations crucial to stockpile stewardship in the absence of underground nuclear testing. Research at the facility also leads to materials and manufacturing innovations and sustains expertise to effectively respond to the needs of the nation’s nuclear weapons stockpile.

Characterizing Plutonium 

The Superblock’s Plutonium Facility is a vital asset in conducting nonnuclear testing of weapons components, including support for surveying pits in the existing stockpile and certifying newly produced pits from Los Alamos National Laboratory. After the disassembly of a weapon and the removal of its pit, researchers in the Superblock subject the materials to destructive analyses to ensure the safety and functionality of the overall pit population. 

Prior to undergoing any type of testing, however, the plutonium must be properly prepared—a complex task. Because plutonium cannot be physically touched, certified material handlers at the Superblock manipulate the materials through a stainless-steel workstation called a glovebox. The outside of each glovebox has a set of protective polymer gloves that connects to the inside, permitting the safe manipulation and processing of radioactive elements. 

Doug Van Slyke, associate program leader for the Nuclear Materials Surveillance, Fabrication, and Offsite Operations team, describes the process: “We take a sample, called a ‘coupon,’ from a plutonium pit and divide it into smaller pieces that are machined down anywhere from the size of a pencil tip to a large coin.” Once machined to precise specifications using a lathe or a mill, the samples either remain in the Superblock and undergo materials characterization testing or are sent off site to the Nevada National Security Site (NNSS).

Plutonium samples delivered to NNSS are assembled into experimental targets for the Joint Actinide Shock Physics Experimental Research (JASPER) facility’s gas gun. JASPER’s two-stage gas gun is used to understand the properties and behaviors of special nuclear materials, subjecting them to high shock pressures, temperatures, and strain rates. Livermore engineers, machinists, and technologists regularly travel to NNSS to support these experimental campaigns.

Plutonium pits aren’t the only expression of plutonium subjected to analyses and testing in the Superblock. Some plutonium comes from legacy material that must undergo electrorefining, an electrochemical process that separates plutonium from impurities based on differences in electrical potential. During this process, the Superblock’s Nuclear Materials Chemical Operations team applies a voltage to an electrorefining crucible, transferring the plutonium to the crucible’s outer layer and leaving impurities behind. In the past, the team could not monitor the plutonium transfer in real time, and impurities could be carried over into the clean plutonium if the voltage was too high. Experimental parameters had to be set at a slower rate to eliminate this risk, resulting in a four-to-five-day turnaround time. To improve the process, the team designed a new electrical feedback system enabling them to monitor the plutonium transfer in real time, varying the parameters according to impurity carryover. Livermore scientist Chao Zhang led development of this system, which has sped up production time by 40 percent. Once purified, the plutonium can be further processed and fabricated into targets for materials characterization experiments. 

man accessing equipment through thick gloves (glovebox) and a wall of metal and glass that separate him from the equipment inside
Uday Mehta of Materials Science Division’s Plutonium Science and Technology Group conducts a fluids compatibility study to understand how plutonium reacts with new, environmentally friendly machining and cleaning fluids.

The Nuclear Materials Chemistry and Metallurgy groups lead materials characterization efforts in the Superblock, subjecting samples to a range of tests that generate essential data verifying stockpile reliability. Almost every project that takes place in the Plutonium Facility requires compositional data from the Analytical Chemistry team to confirm product quality and help guide experimental processes. Analytical chemistry provides a compositional breakdown of the sample, quantifying the total plutonium content and any tracing impurities that are present.

To remove soils or contaminants left behind during manufacturing without corroding or altering the plutonium’s surface, researchers conduct fluids compatibility studies to understand how plutonium reacts with new machining and cleaning fluids. Other materials characterization analyses include mechanical and microstructural tests. Mechanical testing measures the physical strength, stiffness, and hardness of the metal to confirm that it will function as designed. To study the plutonium’s microstructure, metallography and advanced microscopy methods identify defects within the sample that could affect its stability or performance.

Diamond to the Rescue 

The Laboratory has undertaken significant investments in Superblock capabilities to facilitate studies necessary to advance understanding of plutonium used in the nation’s stockpile and to enable the rapid development of special nuclear test articles. In particular, Livermore has invested in a staff of experts who possess a detailed understanding of nuclear operations, nuclear facilities, and regulatory requirements. This expertise includes the Nuclear Materials Engineering team, which supports the decontamination and decommissioning of outdated equipment for space recapitalization, design and installation of new capabilities, and research and development for new nuclear materials processing equipment such as the next-generation 5-axis mill designed for glovebox operations.

Current modernization and life extension efforts require a deeper understanding of plutonium’s characteristics—specifically its equation of state (EOS), strength, and diffraction rate—to extend the service life of aging nuclear warheads at least
30 years. To gather this data, the Superblock’s Plutonium Target Fabrication Facility creates plutonium targets for experimental testing at Livermore’s National Ignition Facility (NIF). 

Target fabrication takes place in two large gloveboxes—one for machining and another for sample preparation and assembly. In the first glovebox, the plutonium is machined with a diamond turning machine (DTM)—a computer-operated, single-point, diamond-cutting tool that removes material with the utmost precision. NIF Plutonium Target Fabrication Manager Jeff Stanford says, “The DTM is the heart of our operation and a game-changing tool to ramp up the production of these targets and help us reach our production goal of one target a month.” 

close-up view of a ridged, metallic surface
Plutonium targets to test a material’s strength are imprinted with twodimensional sine-wave patterns, or “ripples,” using a diamond turning machine.

Depending on the experiment—EOS, strength, or diffraction—the DTM shaves down a plutonium sample anywhere from
2–3 millimeters laterally by 9–50 micrometers (µm) thick. “For context, a human hair measures between 50 and 90 micrometers, so these targets are extremely small,” says Stanford. Primary machinist and senior fissile material handler Todd Matz adds, “Imagine moving dust particles with nothing but a single eyelash attached to a toothpick, that is how delicate and small-scale this work is.” Compared to EOS or strength targets, diffraction targets are extremely thin—9 µm—and may go into different structural phases when put under high pressures.

Since its introduction to the Superblock in 2021, the DTM has enabled the first EOS experiments at NIF and demonstrated the ability to machine strength targets with increased accuracy. Targets for plutonium EOS experiments provide information about how the material will behave at very high pressures. These targets are layered, with a middle that is higher than the edge of the part, creating a stepped surface that generates more data when shot with NIF’s lasers. Strength targets are imprinted with two-dimensional sine-wave patterns, or “ripples,” to test how a material deforms when it is stretched or compressed. In these experiments, the ripples grow when they experience compressive pressure—the higher the material’s strength, the slower the ripples grow. Because the ripples grow across time similar to how ocean wave amplitudes approach the coast, the team names each strength test after a famous beach.

Anticipating Future Needs

The Superblock also handles tritium in support of stockpile stewardship. The Tritium Facility leads the recovery and removal of tritium for the Department of Defense, delivers the tritium fuel required for inertial confinement fusion experiments at NIF, and provides tritium gas mixes to external facilities within the greater fusion research community. 

analytic equipment within a glovebox, which enables safe access to the equipment by way of protective gloves encased in a glass wall
The tritium processing station stores, pressurizes, and purifies the hydrogen isotopes protium, deuterium, and tritium to create custom gas mixes for research.

Tritium research is conducted inside a glovebox that houses the tritium processing station, an intricate system for the storage, pressurization, and purification of the hydrogen isotopes protium, deuterium, and tritium (HDT). During the mixing process, gases from separate storage vessels are mixed into a smaller vessel as temperature cycling gradually increases the pressure to experimental parameters. Before the HDT gas mix is ready, researchers use mass spectrometry to measure the exact isotopic ratios, chemical composition, and potential impurities within the gas sample. Once delivered to NIF, the fuel is placed in a target capsule centered inside a hohlraum and shot with high-intensity lasers. “To ramp up tritium research and development, we are building a new tritium processing system,” says the Associate Program Leader for Tritium Operations Brandon Chung. “This improved capability will have double the throughput, a modular design for streamlined maintenance, and increased safety features.” 

While the majority of the Superblock workforce supports plutonium testing programs and NIF activities, other important work occurs in the Superblock’s Hardened Engineering Test and Radiography Facilities, adjacent to the larger Plutonium Facility. In a critical step of the weapon certification process, engineering tests, called shock tests, simulate weapon environments by shaking, dropping, heating, and cooling samples of special nuclear materials. Materials standing in for explosives prevent potentially dangerous interactions with fissile materials during these tests. The test article with fissile material is radiographed before each test, and the full test apparatus is radiographed after testing to identify any damage. Following disassembly of the test apparatus, the test article with fissile material is radiographed alone so scientists can study what, if any, changes occurred as a result of the shocks. Such testing has always been incorporated in the weapons testing program. The Superblock has performed this service for Livermore designs and, on occasion, for designs from other national laboratories over the years.

All told, Superblock facilities provide the capabilities to handle all phases of new, often cross-directorate, programs involving plutonium or uranium from start to finish. Such projects begin with an initial analysis, design, and research and follow with an in-depth review of potential hazards resulting from the project and development of worker and public safety measures. The Superblock further supports new projects during performance analysis and demonstration of the project’s product, which can ultimately result in a new process shared across the national security enterprise (NSE).

With its integrated team of highly skilled physicists, materials scientists, chemists, engineers, technicians, and machinists, the Superblock is at the forefront in addressing the current and future needs of the NSE and is a testament to the confrontational geopolitical situation in which powers threaten the use of nuclear weapons. Program Director Jacqueline Meeker says, “Advancing our understanding of stockpile performance is paramount to national security. With recent technological innovations, a profound evolution of processes and capabilities has been initiated across every aspect of the Superblock. Whether the work is in programs or in managing the facility, every single person in the Superblock contributes to certifying our nation’s stockpile.” 

 —Shelby Conn 

For further information contact Jacqueline Meeker (925) 423-6419 (meeker7 [at] llnl.gov (meeker7[at]llnl[dot]gov)).