Since 1981, one or more of the U.S. Navy’s Ohio-class submarines (SSBNs) has patrolled the world’s oceans, forming the sea-based component of America’s nuclear triad. Today, each of 14 SSBNs carries 20 D5 submarine-launched ballistic missiles. These missiles are topped with either W76 or W88 nuclear warheads.
These warheads were designed by Los Alamos National Laboratory—the W76 in the 1970s and the W88 in the 1980s. Over the past several decades, each has been refurbished to extend its service and remains highly effective today.
But in late 2020, United States Strategic Command (aka STRATCOM, a Department of Defense [DOD] combatant command that plans and executes military missions involving nuclear weapons) raised a question: What would an effective Navy warhead look like in the 2030s and beyond?
To answer this question, STRATCOM, the Navy, and the Department of Energy’s (DOE) National Nuclear Security Administration (NNSA) are conducting what’s called a Phase 1—or concept assessment—study of a potential new warhead they’re calling the W93.
The key word here is potential. The purpose of a Phase 1 study is to make a preliminary assessment of nuclear weapon design options or concepts, not to nail down details on design, production, manufacturing, or cost. Those things will come later, if the W93 makes it past the Phase 1 and other phase studies. For context, between 1966 and 1985—the height of U.S. nuclear weapons production—59 Phase 1 studies were conducted. Of those, only 19 weapons (32 percent) passed through the next five phases of the Joint DOD and DOE Nuclear Weapons Life Cycle Process and entered into the stockpile.
Nuclear weapons with “W” in their names are warheads, which are launched on missiles. Weapons with “B” in their names are bombs, which are dropped from aircraft. The numbers in weapons’ names reflect the order in which they were conceived. The W93, for example, is the 93rd weapons design being considered for the stockpile. Since 1945, when the United States first developed nuclear weapons, only 63 weapons designs have made it into the stockpile. Los Alamos designed 46 of those 63 (and 29 of the first 30).
The last nuclear weapon to enter the stockpile was the W88 in 1988. Around that same time, four other weapons designs—the W89, B90, W91, and W92—were being explored in phase studies. But when the Cold War ended, so did U.S. nuclear weapons development. “To really show the Cold War was over, the United States also retired many weapons,” remembers Michael Bernardin, recently retired associate Laboratory director for Weapons Physics at Los Alamos.
In 1996, President Bill Clinton signed the Comprehensive Nuclear Test Ban Treaty, which prohibits nuclear testing of any kind. The treaty “will help to prevent the nuclear powers from developing more advanced and more dangerous weapons,” Clinton told the United Nations General Assembly in 1996. Though never ratified by the Senate, “it points us toward a century in which the roles and risks of nuclear weapons can be further reduced and ultimately eliminated.”
This posture continued for the next couple decades. “The United States will take concrete steps towards a world without nuclear weapons,” President Barack Obama told an audience in the Czech Republic in 2009. “To put an end to Cold War thinking, we will reduce the role of nuclear weapons in our national security strategy and urge others to do the same.”
At its height in 1967, the stockpile contained 26 types of weapons for a total of 31,225 weapons. In 2017, the year that DOD declassified and released stockpile numbers, seven types of nuclear weapons were in the stockpile for a total of 3,822 weapons.
In recent years, however, the geopolitical landscape has shifted, causing many to reevaluate the nuclear deterrent. According to the Nuclear Posture Review (NPR) published by the DOD in 2018:
“While the United States has continued to reduce the number and salience of nuclear weapons, others, including Russia and China, have moved in the opposite direction. Russia has expanded and improved its strategic and non-strategic nuclear forces. China’s military modernization has resulted in an expanded nuclear force, with little to no transparency into its intentions. North Korea continues its illicit pursuit of nuclear weapons and missile capabilities in direct violation of United Nations (U.N.) Security Council resolutions. Russia and North Korea have increased the salience of nuclear forces in their strategies and plans and have engaged in increasingly explicit nuclear threats. Along with China, they have also engaged in increasingly aggressive behavior in outer space and cyber space.”
And it’s not just other countries bolstering their nuclear arsenals that America has to worry about. Some countries are developing technology—including advanced projectiles and beams of various forms of energy to shoot down missiles in flight—that may have implications for the resilience of America’s nuclear deterrent.
That’s why, according to the 2018 NPR, the United States must have “modern, flexible, and resilient nuclear capabilities that are safe, secure, and effective until such a time as nuclear weapons can prudently be eliminated from the world.”
What STRATCOM wants
STRATCOM has been anticipating the current and future threat environment. “STRATCOM has the big picture,” Bernardin explains. “STRATCOM tells the Navy that the United States must be prepared for X. And then the Navy says, OK, to be prepared, we need Y.”
In this case, Y is a new reentry body—the conical tip of a missile that carries nuclear warheads, also called an aeroshell—that must be capable of reaching a variety of targets, some of which didn’t exist 30 years ago. Targets could be high-value (a specific location or facility), time-sensitive (struck at specific time), or in hard-to-hit places (in the air, underground, underwater, or in rugged terrain).
“Without a coordinated, joint effort to develop and field the W93/Mk7 as a system, the bulk of our day-to-day deterrent force will be at increased risk in the early 2040s due to aging legacy systems.” —Admiral Charles Richard
Although it hasn’t been designed yet, the new reentry body is being called the Mark 7 (Mk7). The Mk7 could differ in size from the current Mark 4 and Mark 5 reentry vehicles, which house the W76 and W88, respectively. “A possible size difference requires NNSA to explore warhead options broader than a life extension of any existing stockpiled warhead type,” explains Bob Webster, deputy Laboratory director for Weapons at Los Alamos.
These “warhead options” are what’s collectively being called the W93. If (and that’s a big if) one of the options moves forward through the entire six-part phase study process, the option will provide STRATCOM and the Navy “a means to address evolving ballistic missile warhead modernization requirements, improve operational effectiveness, and mitigate technical, operational, and programmatic risk in the sea-leg of the triad,” according to a statement by STRATCOM Commander Admiral Charles Richard to the Senate Committee on Armed Services.
“Without a coordinated, joint effort to develop and field the W93/Mk7 as a system, the bulk of our day-to-day deterrent force will be at increased risk in the early 2040s due to aging legacy systems,” Richard continued. “Research and development efforts … must begin immediately to deliver a capability in the 2030s that maintains a credible at-sea deterrent through the 2050s and beyond.”
“This work is critical to the future of the nuclear deterrent,” agrees Mark Suriano, deputy assistant deputy administrator for the NNSA Defense Programs Office of Research, Development, Test, and Evaluation. NNSA—via its national laboratories, plants, and sites—has the sole responsibility to design, develop, certify, and produce nuclear weapons for the United States.
As conversations about the W93 begin, plans to replace the Navy’s aging Ohio-class submarines are also underway. The 2018 NPR ensures Ohio-class SSBNs will remain “operationally effective and survivable” until they can be replaced, one per year, by a minimum of 12 Columbia-class submarines. These next-generation subs are in development, and the first one is scheduled to be deployed by 2031. All 12 are expected to be operational by 2042. So, because the W93 (if it goes forward) wouldn’t enter the stockpile until the mid-2030s, it would have to be compatible with two different boats.
“Our workforce is full of highly dedicated and well-trained problem solvers who are motivated by the cutting-edge science and engineering required for this study.” —Mark Suriano
“The [Ohio-class] submarines that we have today have 20 [missile] tubes,” Richard elaborated before the House Armed Services Committee in February 2020. “The Columbia has 16. So, I will need capabilities that will address the fact that we don’t have as many tubes in the new class of submarines, and the overall number of warheads [that can be carried on each submarine] is going down.”
What is a Phase 1 study?
With all of the above in mind, DOD and NNSA are embarking on a Phase 1 study. “The nuclear security enterprise [NNSA and its national laboratories—including Los Alamos—plants, and sites] is preparing to start the Phase 1 concept assessment on the W93,” Suriano says. “Our workforce is full of highly dedicated and well-trained problem solvers who are motivated by the cutting-edge science and engineering required for this study. We have confidence in our ability to meet the mission needs as laid out by the Department of Defense.”
According to Bernardin, who was involved in America’s last Phase 1 study back in 1990, Phase 1 studies have very broad parameters. “It’s like saying, ‘I have a requirement to land Americans on the moon and collect new information, but I’m not sure about the size or precise capabilities of my lunar module. The Phase 1 study says the lunar module needs to fit in this size envelope and have these types of capabilities to meet its mission requirements. If the lunar module makes it past the Phase 1 study, the Phase 2 study would provide something like three to six options for lunar module design.”
Phase 1 studies typically take 1 to 2 years. During this time, “NNSA provides federal oversight and guidance,” Suriano explains. “NNSA ensures the design agencies—Los Alamos, Lawrence Livermore, and Sandia national laboratories—and production agencies are considering all requirements.”
As the lead physics design agency—the organization responsible for the design of the nuclear warhead package and some of the nonnuclear components—Los Alamos must consider everything from the number of warheads required per missile to the size, weight, shape, and yield of the warhead. Target sets, target-kill effectiveness, and warhead survivability will also be addressed. Additionally, the Los Alamos design options identified must be able to be produced, delivered, and fielded without any underground nuclear explosive testing.
“The opportunity to be the design agency for the W93 will be leveraged on the great competencies we have developed in conducting modernization work and in supporting the legacy stockpile,” says James Owen, associate Laboratory director for Weapons Engineering at Los Alamos. “The work we’ve done in the past, the work we do each and every day, and the work that stands in front of us are all vital elements in ensuring that we have a safe, secure, and effective nuclear deterrent.”
“Working with others across the nuclear enterprise and the military, we will develop plausible design concepts,” Webster adds. “These will be evaluated for technology and manufacturing readiness, producibility, impacts to the NNSA complex, and other ongoing warhead acquisitions, development, production schedule, risks, and cost.”
In addition to thinking about concepts for the W93 and Mk7, Los Alamos must take into consideration its current and future work. If the W93 moves forward, it would use the Lab’s plutonium, detonator, and other facilities, which are already plenty busy. “We’ll also need to evaluate the impact across the NNSA complex,” Webster says. “Components would be produced and assembled at various DOE labs, plants, and sites and would have to be handled and shipped between facilities.”
“At the end of the Phase 1 study, a report on identified designs—warhead and reentry body—and an assessment of whether one or more designs can potentially meet mission requirements is provided to the Nuclear Weapons Council, along with a recommendation to either proceed to Phase 2 or terminate at Phase 1,” Webster explains. (The six-person Nuclear Weapons Council is composed of DOE and DOD senior leaders who direct interagency activities to maintain the U.S. nuclear weapons stockpile.) “In the event it is determined we should proceed, the W93 will have to be programmed in the budget, and that will be passed on to Congress for authorization and appropriation of funds.”
For the past 30 years, instead of producing new nuclear weapons, the United States has maintained a subset of its Cold War–era weapons through the stockpile stewardship program. In this program, Los Alamos works in conjunction with other labs, plants, and sites in the NNSA complex to assess and ensure the safety, security, and effectiveness of each type of nuclear weapon in the stockpile. This is done through constantly advancing a combination of surveillance studies and applied research consisting of nonnuclear experiments, computer simulations, and incorporation of data from historical nuclear tests.
For example, the Laboratory’s Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility, established in 2000, can take radiographs of materials that implode at more than 2.5 miles per second. These radiographs allow scientists to “see” inside a mock-nuclear weapon as it detonates.
At somewhat smaller scales, the proton radiography (or pRad) capability at the Los Alamos Neutron Science Center (LANSCE) permits 24 high-energy radiographic images of a dynamically imploding or exploding experimental package. High-energy-density facilities, such as the Z pulsed-power facility at Sandia National Laboratories and the National Ignition Facility at Lawrence Livermore National Laboratory, enable experiments using x-ray radiation and thermonuclear-burn or fusion reactions.
Today, data acquired at these and other experimental facilities can be analyzed and visualized using supercomputers that operate at tens to hundreds of petaflops—more than one-million times as fast as computers 30 years ago.
With every technological advance and improved facility comes more exquisite data and a deeper understanding of what’s going on inside nuclear weapons. A deeper understanding leads to higher-fidelity experiments, which leads to more exquisite data, which leads to a deeper understanding, and so on.
“The current state of our understanding of the performance of nuclear explosives compared to 25 years ago is simply remarkable,” says Charlie Nakhleh, associate Laboratory director for Weapons Physics at Los Alamos. “As we embark on this Phase 1 study, we intend to exploit the very large investments made over the past 25 years or so by the stockpile stewardship program.”
The W93’s Phase 1 study is often called a clean sheet study because—although it will incorporate decades of knowledge gained from past nuclear weapons work—the concepts for the warhead and reentry vehicle don’t necessarily hinge on any existing weapons system (however, they may incorporate existing weapons components). High-performance, multi-physics simulations will be underwritten by theory and checked extensively against experimental data from focused experiments to more integrated experiments up through archival nuclear testing data, Nakhleh explains. Using this solid foundation, scientists can be confident that no new nuclear testing would be necessary to certify the W93 for use in the stockpile.
The next generation
Most of the people who designed the weapons in the current stockpile have either retired or died, taking with them decades of knowledge. “As the people with test experience and design experience age themselves, it becomes imperative to train the next generation of stewards using all the tools we can develop to better understand and to better assess the safety and reliability of the stockpile,” Laboratory Director Siegfried Hecker wrote in his 1996 annual assessment letter. (This letter informs the president of the United States of the Laboratory’s confidence that the stockpile has evolved and that it is safe, secure, and effective.)
Closing the knowledge gap has been a concern appearing in subsequent annual assessment letters. In 2014, Director Charles McMillan raised a point about the workforce and the stockpile of the future. “Attracting, training, retaining, and establishing confidence in the stewards of the future stockpile is of principal importance,” he wrote. “Nurturing the intellectual capital to design a modern nuclear weapon, if needed for the future stockpile, is the greatest concern.” In other words, if the Lab ever got the opportunity to participate in another Phase 1 study, McMillan wanted to make sure his workforce was prepared.
In 2018, Director Terry Wallace agreed. “Creation of the stockpile of the future will require the vision and will to unleash the creative energy of the workforce,” he wrote. “Sustainment of the existing stockpile through [life extension programs] has been necessary but not sufficient … I have seen what Los Alamos scientists and engineers can accomplish, and it has been impressive. Nurturing and empowering the intellectual capital to design and certify a modern nuclear weapon, when needed for the future stockpile, is the next step.”
The W93 Phase 1 study marks the very early stages of that next step, which will introduce a new generation of scientists and engineers to the process of nuclear weapons development. These men and women have a strong foundation, thanks to their work in stockpile stewardship. “The capabilities in computing, theory, and experimentation that have been put in place as a result of this stockpile stewardship program are world leading and will enable us to field the W93 without needing any additional nuclear testing,” Nakhleh says.
A team effort
The Los Alamos scientists involved in the Phase 1 study will help all parties involved better understand the pros and cons of potential design choices. “While it has been a long time since Los Alamos was tasked with a Phase 1 study, a number of focused efforts over the past several years have enabled us to update and modernize our physics design tools as well as work through the process of quickly designing integrated and more easily manufacturable hypothetical systems tailored to the capabilities of the NNSA complex,” explains one physicist. “Thanks to these efforts, we are prepared to start the Phase 1.”
Some of the creative energy that goes into Phase 1 studies is fueled by the partnerships forged during the process. During a Phase 1 study, working groups are formed to address certain areas—such as requirements, design, surety (safety and security), target vulnerability, and mission effectiveness. Many Los Alamos scientists and engineers, as well as representatives from other NNSA sites and the military, make up these 8- to 10-person working groups.
“We’ll partner with STRATCOM, the Navy, Sandia, Livermore— in fact the entire nuclear weapons enterprise—to develop a suite of compelling and executable options for the nation that meet the STRATCOM requirements for the W93,” Webster says. “The next year or two is going to be very intense.”