Bridging Divider

preparations for a nuclear test

Bridging Divider

By Virginia Grant| February 19, 2021

Four Los Alamos women share their memories of working at the Laboratory during the early 1990s—when, to maintain its nuclear weapons stockpile, the Lab pivoted from underground nuclear testing to small-scale, non‑nuclear experiments.

On September 23, 1992, at the Nevada Test Site (now the Nevada National Security Site), an underground nuclear test named Divider was conducted by Los Alamos National Laboratory. The test was the 1,054th nuclear test executed by the United States; it was also the last.

From July 16, 1945, when the world’s first atomic device was detonated at the Trinity site, to a period of test preparation that continued after Divider, Los Alamos was the world’s top producer of nuclear weapons tests. Among the many Los Alamos scientists, engineers, and workers who made those 1,054 tests possible were Merri Wood-Schultz, Wendee Brunish, Lynne Kroggel, and Joyce Guzik—experts not only in nuclear testing, but also in the development of methods, tests, strategies, and ideas for how to maintain weapons superiority and national security in the decades since testing ended.

Thawing the Cold War

Between 1945 and 1992, the Soviet Union and the United States combined conducted 1,769 nuclear tests. Nuclear devices, sometimes more than one per test, were detonated from towers (as in the Trinity test), underwater, in the air after being dropped by aircraft, and even in outer space. Although the Limited Test Ban Treaty of 1963 banned nuclear weapons testing in space, the atmosphere, and underwater, underground nuclear tests continued for almost three decades afterward.

The end of the Cold War, solidified by the fall of the Soviet Union in 1991, gave a sense of relief to Americans, who had been wrought with anxiety since the launch of the Soviet satellite Sputnik in 1957. The end of the Cold War also enabled the long-sought ban on nuclear weapons testing that had been a back-and-forth between the United States and the Soviet Union for decades.

Preparations for the Divider test. National Archives

In 1991, the Soviet Union announced a moratorium on all nuclear testing. The United States made its own similar announcement soon thereafter, and each country began to ramp down its testing schedule. The Soviet Union conducted its last nuclear test on October 24, 1990, the United States ended its testing with Divider.

The United States and 70 other countries signed the Comprehensive Nuclear-Test-Ban Treaty (CTBT) on September 24, 1996; currently, the CTBT has the signatures of 185 states. Of those 185, 16—including the United States—have not ratified the treaty.

Amid all of this political back and forth, Los Alamos scientists and engineers worked tirelessly to maintain American superiority in nuclear weapons development and to make sure the U.S. nuclear stockpile was safe, secure, and effective, both during and after testing. Los Alamos scientists from many different backgrounds, including women such as Wood‑Schultz, Guzik, Brunish, and Kroggel, conducted a great deal of this work at Los Alamos and Nevada.

Testing nuclear weapons

“I was recruited as part of the affirmative action program,” says Wood-Schultz, a physicist and Laboratory Fellow who was lead designer for six nuclear tests and co‑leader on a seventh. Wood‑Schultz was a doctoral student in physics at Georgia Tech in 1977 when a recruiter began to talk with her about jobs at Los Alamos. After an initial visit, she was hired before she even completed her doctorate. “Affirmative action got me the interview, but I don’t believe it got me the job,” she says. She was thrilled both for the scientific opportunities and the opportunity to work at the laboratory that had, she believed, saved her father and other soldiers in World War II from being killed in Japan. Wood‑Schultz’s career at Los Alamos began in the theoretical design of weapons, which quickly became her specialty.

Merri Wood-Schultz

In April 1984, Brunish arrived in Los Alamos with a Ph.D. in astrophysics. She went to work on the containment of underground tests, making sure that no radiation would leak from underground when the tests were detonated. Radiation leakage, known as venting, was prevented by studying the geophysical properties of the test site and determining the best method of stemming—the process of filling in the hole around the test rack (a giant metal structure that held the test device and its diagnostics).

Work in containment wasn’t the stretch from astrophysics that it might seem; the Los Alamos containment program was officially started in 1970 by astrophysicist Bob Brownlee, whom Brunish describes as “a mentor to all of the Los Alamos containment scientists who came after him.” From that point forward, it was common for astrophysicists to work in the program. “In astrophysics,” Brunish explains, “you’re basically looking at nuclear processes in stars and how that affects what’s going on around them; in underground testing, you’re looking at nuclear processes in the weapon and what’s going on around that.”

After graduating from the New Mexico Institute of Mining and Technology, Kroggel began her career at Los Alamos in 1986 as a contractor with a subsidiary of Pan Am Corporation that provided a number of support services to the Laboratory, such as engineering and maintenance. Working as a quality engineer, Kroggel traveled often to the Nevada Test Site to inspect items used in underground testing.

In 1987, Guzik, another astrophysicist, began working at Los Alamos as a doctoral student under the mentorship of others in testing, including Wood-Schultz. After being taken on tours of the Nevada Test Site and being apprenticed in nuclear testing, Guzik became a staff scientist in 1989 and was part of the last generation of scientists to work on nuclear tests.

Lynne Kroggel

Firing a shot

An underground nuclear test has a particular sequence of events. First, the nuclear device detonates, creating a shock wave that emanates from the device. In less than a second, the container of the device and all the rock around it are vaporized, creating a cavity where the detonation occurred. The shockwave travels beyond that cavity, crushing rock until its force decreases and it dies out. A few seconds later, the molten rock around the detonation cavity settles in the bottom of cavity, where it begins to cool and solidify. Finally, the gas created by the pressure of the detonation begins to decline, and the rock around the cavity collapses, then a chimney is created by the progression of a rubble column up through the hole and the surrounding geologic material. This last event usually only takes a few hours, but it can take up to several months. Once the test site has stabilized, scientists can retrieve their diagnostic equipment to study the data of the explosion.

But before any of this could happen, there were months of careful planning, negotiation, and collaboration. Each test was a huge cooperative effort across disciplines at the Laboratory.

“The rack drove the schedule,” Wood-Schultz remembers. “Without the rack, nothing happened.” The rack, which could be as big as 10 feet in diameter and 100 feet tall, was loaded with the nuclear device itself and all the diagnostic equipment used to study the explosion and its effects.

“I was on top of one” for the test named Amarillo, Kroggel says. “They were preparing it to go down-hole, and we stood on top of it.” The racks were lowered far into the ground, sometimes as deep at 4,000 feet below the surface—that’s a hole more than twice as tall as One World Trade Center in New York City.

An Atomic Energy Commission documentary discusses techniques for successfully drilling holes for underground nuclear tests.

“From a designer standpoint,” Wood-Schultz says, “you do the test to get the diagnostics.” For an underground test, those diagnostics could vary widely, from seismic readings to measurements of gamma-ray output. “Part of those diagnostics is having the yield [size] of the explosion, which is certainly necessary, but the diagnostics give you a peek inside of what is happening. There are different ways of looking at things, depending on how you design your diagnostics.”

For underground nuclear tests, the designs could be radically different, but the processes were similar. There was a detailed approvals process involving a number of government agencies and committees. Then a schedule would be made, and the names of the tests would be assigned.

For security and secrecy, tests were purposefully named to distract from anything actually related to the tests themselves, and they were usually grouped together by subject. Divider, for example, was a name chosen from a list of tools. At one point, Los Alamos chose names of wine grapes while Lawrence Livermore National Laboratory, which also conducted underground nuclear tests, paired those names with a series of tests named after cheeses. Wood‑Schultz requested that the name Amarillo, one of a series of Texas city names, was assigned to a shot she was working on because, she says, “every time we drove through Amarillo, our car would break down.”

Each test had its own logo, which was often printed on patches.

Once the names were assigned, the specific designs of each test would begin. Different teams had different agendas, and there were many moving parts before the shot—the test—was fired. The different diagnostics on the rack changed frequently depending on time, budget, and priority. “We couldn’t put everything on every shot,” Wood‑Schultz explains. “So if there was something that was really needed, someone could make the case for why  that diagnostic would be particularly valuable in this particular shot.” Management would make tradeoffs for what some shots could or could not have, and all the diagnostics had to be determined and designed before the engineers could design and build actual racks.

For Brunish, the focus was on containment—making sure no radiation whatsoever leaked above ground from the test. “The Nevada Test Site was a big, dreary, barren area with crappy dorms,” she says, “but I loved going out there. The work was so fun, and it was really exciting.” As a containment physicist, Brunish was responsible for telling the other scientists when it was safe to retrieve their data from the equipment after the shot. Part of containment was studying the data transmitted through cables embedded in the material packed into the test hole. Brunish studied that data to determine whether the cavity around the rack, which expanded at detonation, had collapsed, making the area safe for scientists to retrieve their diagnostic equipment. “We’d be in this big war room,” she recalls, “with all these controllers and designers and diagnostics physicists, and the shot would go off and we would wait about five or ten minutes, and then everyone would turn to me and ask, ‘When can we go back in?’” To their dismay, the wait was usually two to six more hours.

Wendee Brunish

“We would typically be about 10–20 miles away when the shot went off,” Brunish says, “but you could feel the ground move.” To scare away animals, such as antelope, that might walk over the site as the ground started to move, very loud horns would sound right before the shot.

Guzik was present for one particularly dangerous incident. During the 1990 test named Houston, she says, “we saw a guy walking out to ground zero right before we were going to fire the test. Some protesters had shown up at the test site, hiked in from somewhere, and were going to stand on ground zero and try to stop the test.” The test had to be placed on hold for a few hours while police came to remove the protesters.

Guzik was an apprentice designer in training for much of her work in testing, and she became a staff scientist in the last few years of testing. She  modeled data from previous and related tests and then compared that data to tests happening at the time. Guzik also helped decide some specific testing features, such as the masses and compositions of particular materials used in the tests.

As a quality engineer, Kroggel examined testing equipment, checking the components used to build the tests and making sure they were ready before the shot was fired. “I would look specifically at orders” of materials for the tests, she says, “to ensure we had the correct engineering standards to meet the requirements put forth by the rack engineers.” For example, for a specific type of rope needed for testing, she says, “I would get on a flatbed and actually look at that wire rope.”

Wood-Schultz worked in design, determining how to achieve the desired data from a given nuclear test. Some tests were performed to determine something specific about the stockpile; others were performed to learn something about physics. “A test that was associated with a stockpile device,” Wood-Schultz explains, “could serve as a partial demonstration that it would satisfy the military requirements placed on it, and the resulting data would contribute to an assessment of the reliability of the available simulation capabilities to predict the operation of that device.”

Wood-Schultz preferred the physics tests, which gave her the opportunity, she says, “to try to look at something from a different angle so that I can either get new information or an unrelated look at the sorts of things that we don’t understand.” So the products of testing were often related to weapons development or national security, but they also often produced advances in more fundamental science.

Joyce Guzik

The end of an era—and a career path

“When I interviewed to become a staff scientist, back in 1977,” Wood-Schultz says, “they brought up the issue that not everybody was in favor of testing and the ability to test might go away. So that was there from day one.”

No one was shocked when testing ended; some were, however, surprised when it didn’t resume. “Most of us thought the moratorium would last a year,” says Brunish. “We didn’t think it would be forever.”

For some time after Divider, Los Alamos continued to plan for future tests. In fact, one of the last tests Guzik worked on remained on the schedule for years after Divider. “A lot of people thought I should leave” the Laboratory when testing ended, she remembers, “because I was early in my career. But I didn’t want to leave.”

She recalls learning that, of the many different types of weapons in the stockpile, after testing, all were to be discarded except for about seven. “That was going to be our work,” she remembers, “to maintain those seven or so systems” without underground testing.

This work became known as the Stockpile Stewardship Program, the program that continues today at Los Alamos as the Laboratory’s primary mission. “In conjunction with historical nuclear test data,” Wood‑Schultz explains, “this program develops and utilizes enhancements in research, non-nuclear explosives tests, and computer simulations to support and certify the nation’s nuclear stockpile without nuclear explosive testing.” The primary techniques of stockpile stewardship were already used alongside nuclear testing long before the moratorium. Now they are the primary means of determining the validity of the weapons in the stockpile.

Many of the scientists who worked on nuclear testing transitioned to work in stockpile stewardship. That transition was natural for many employees, including Wood-Schultz. She began working on quantifying uncertainty—looking at the small components of a larger issue and determining the degree of uncertainty in those individual components, plus the uncertainty that results from a comprehensive study of those parts combined. “That became even more important when we didn’t have a final integrated test,” she says. “Determining whether a bomb will work without testing it is a much harder job because every little thing has its own uncertainty, and they all add up.”

Divider nuclear test.
The Divider test rack is hoisted into position for lowering down-hole at the Nevada Test Site in September 1992. Divider was the last full-scale underground nuclear test conducted by the United States. Los Alamos National Laboratory

For Brunish, the transition from testing to other national security work was mostly organic. “When I was working on underground nuclear tests, I had to understand how the rock would be affected, what the phenomena of the test would look like,” she says. “When we stopped testing, we flipped that and said, ‘If China or India is testing, what signals will come out of that nuclear test? How can we detect what another country is doing?’” Brunish used data and knowledge of American nuclear tests to examine what other countries were doing to learn about whether they were testing and, if so, what kinds of tests they were conducting.

Guzik’s current work is similar. “We are trying to figure out what countries could be doing or what their stockpiles look like,” she explains. Her team looks at data from old U.S. nuclear tests of the 1950s, 1960s, and even 1970s as models of the earliest nuclear test data. “Those are predecessors to modern, more advanced designs,” she says, “so another country might go through that same phase.” Looking for data that resembles later American tests might cause intelligence agencies to miss nuclear testing in countries with nascent nuclear programs. Guzik and her colleagues also hypothesize what countries might be able to accomplish by looking at what the United States was able to do in the earlier years of nuclear weapons. “What if a country has X amount of plutonium? What could they make with it? I’ve been having a great time working in that more creative area.”

For Kroggel, the end of testing marked a significant transition in her career—she moved from contractor with Pan Am to a quality engineer working directly for the Laboratory. “I became the Department of Energy technical quality standards manager for the Lab at that time,” she says.

When testing was no longer an option, ensuring the efficacy of the nuclear stockpile without testing became of the utmost importance. “Anything not in its lowest energy state,” such as a nuclear warhead, Wood-Schultz explains, “is not going to stay stable. That doesn’t mean it will blow up,” she says, but the molecules of it will naturally change. “That aspect of the work kept going after testing.”

In support of stockpile stewardship, Laboratory scientists and engineers develop modern methods, such as plutonium aging models, and experiments to determine the efficacy of the current stockpile. Based on the data collected from those methods and experiments, together with data from past nuclear tests, the weapons in the stockpile may undergo life-extension programs to address aging and performance issues, enhance safety features, and improve security. Los Alamos may also conduct alterations (changes to a weapon’s systems, sub-systems, or components) and modifications (changes to a weapon’s operational capabilities).

“The stockpile will eventually be composed of updated systems that have never been fully tested,” Guzik says. But through stockpile stewardship, Los Alamos rigorously examines these updated weapons without nuclear testing to ensure they are as safe, secure, and effective as the older, tested weapons.

Woman with bombs.
Merri Wood-Schultz gave these posters to the women working in underground nuclear testing, many of whom posted them on their office doors; this is the one she gave to Joyce Guzik.


Wood-Schultz, Brunish, Kroggel, and Guzik have all paved the way for the many women who currently work in weapons at Los Alamos. For example, “One of my goals was to be the first female chair of the Containment Evaluation Panel,” says Brunish of the position she currently holds. The panel, which used to assess and review the containment plans of an underground nuclear test, is now called the Containment Evaluation Review Panel and continues to evaluate experiments. “We evaluate any activities at the Nevada National Security Site that involve explosives and special nuclear materials,” Brunish says. “There is currently a very active subcritical experiment program at the site, and we rigorously review all of those experiments prior to execution.”

On being a woman in a male-dominated field, Wood‑Schultz looks back, “It was easier for me than it might have been for other people because there weren’t many women at Georgia Tech; I started just a couple of years after they started admitting women.”

The later generation of women in testing had mentors to emulate. “I didn’t really think of it as a male-dominated field because there were always women around me,” Guzik says. “Merri Wood-Schultz was definitely a mentor to me.” Guzik once observed Wood-Schultz overseeing the assembly of a device before a test. “She modeled what you need to do, how you can’t stand around and be passive when observing an assembly,” Guzik recalls. “I hung on every word. I did the same thing later myself.” Wood‑Schultz told Guzik and other young scientists stories of times that she asked what she thought was a “dumb” question only for it to lead to an engineer realizing a mistake needed to be corrected. “I now have some of these stories of my own to pass on,” Guzik says.

The future of national security

From all four women, there is an overwhelming sense of pride and joy in their work in nuclear testing. Working in testing, Brunish says, “was the best thing I’ve ever done. The most fun, the most interesting, the most impactful, and also extremely collaborative and team-driven—not just with the team I worked on, but with people all across the Lab. Engineers, physicists, designers, everyone worked together and had a shared fate. It was really an amazing thing to be part of because everybody wanted to do the job, get it done as well as they could. It was an amazing thing that I haven’t seen since we stopped testing.”

For Kroggel, working on the testing racks was the best part of her job because she viewed the testing as so critical to national security at that time in history.

For Guzik, it was a time when she was able to learn a great deal about weapons—knowledge that she carries with her now as a Lab Fellow.

Brunish agrees. “We were pushing cutting-edge physics and design, and we were really contributing to making our nation and the world safer,” she says. She is also proud of the shutout record her team holds against radiation leaks. “The Los Alamos containment program mission was to never have an underground test leak radiation into the atmosphere, and we were able to fulfill that mission 100 percent over more than 20 years, and we are still proud of that record.”

For Wood-Schultz, working in testing was incredibly rewarding with results that other projects couldn’t match. “There was a solid bottom line,” she says, “and that’s what testing provided. It wasn’t unclear whether you did a good job or not, and it wasn’t something for your boss to decide. It was about doing good quality work. If you just got lucky, and everything behaved better than you could have expected, that’s always fun, of course, but it’s not the same thing as doing a good job.”

There are very few people still working at the Laboratory who worked directly on nuclear testing. Wood‑Schultz and Brunish in particular find ways to share their knowledge of nuclear testing with others at the Laboratory and beyond. Wood-Schultz serves on multiple committees, including the Nuclear Forensics Science Panel for the Department of Homeland Security.

In addition to chairing the Containment Evaluation Review Panel, Brunish is a lecturer for the Underground Nuclear Weapons Testing Operations Program, a sort of summer school for people from the Intelligence Community, the Department of Energy, and the Department of Defense. The program teaches how underground tests were conducted, how other countries have conducted them, and what to look for when analyzing intelligence on underground test programs.

“I tell people,” Brunish says, “that if in 30 years you want to return to testing, I’ll come in with my walker and tell you how we used to do it in the old days.”