Oak Ridge National Laboratory physicist Elizabeth “Libby” Johnson (1921-1996), one of the world’s first nuclear reactor operators, standardized the field of criticality safety with peers from ORNL and Los Alamos National Laboratory. Her work came on the heels of two incidents involving nuclear materials that took the lives of two government researchers at the end of the Manhattan Project.
Johnson was part of a team addressing the need to hasten safety in modern nuclear science. As proof of their successes developing criticality standards, there hasn’t been a criticality accident on United States soil since 1978.
“The experiments that you and your colleagues conducted over these years were crucial to the whole business of nuclear energy,” former ORNL Director Alvin Weinberg told Johnson’s colleague and mentor, Dixon Callihan, in 1996. “All of us owe you a great debt for your having pioneered in this ever-so-important enterprise and shown the way.” Callihan directed ORNL’s Critical Experiments Facility, where Johnson worked as a senior experimenter for more than a decade.
A golden age for criticality experiments lasted from the 1940s to the mid-1970s. During this era, criticality experiments and work at research reactors primarily focused on reactor design, creating the stepping-stones for today’s commercial reactors. Johnson and her cohorts conducted hundreds of experiments, setting the groundwork for developing the first nuclear safety standards for nuclear researchers and future reactor operators.
The Oak Ridge Critical Experiments Facility was established in Building 9213 in 1950. This isolated facility, now slated for demolition, had been constructed for uranium storage and was remodeled to support the studies underpinning the safety standards. Years later, Callihan would recall his unsuccessful efforts to get a women’s restroom included in the remodeling plans – a request that was later granted. His advocacy for this amenity contrasted sharply with the overall scientific environment of that time, which was organized by and optimized for men. “Johnson trod the silent path of women’s history in the nuclear industry,” said Nuclear Regulatory Commission historian Thomas Wellock.
Johnson’s influence on criticality safety and the nuclear field has gone mostly unnoticed except for her listings in American Men and Women of Science, a biographical reference work on leading scientists in the United States and Canada, and the membership roster of the New York Academy of Sciences. However, within the nuclear field, she was recognized in 1975 by the American National Standards Institute’s Nuclear Standards Management Board, citing Johnson’s contributions to the development of standards. In 1982, the American Nuclear Society, or ANS, made her a fellow. In 1985, ANS recognized Johnson with an achievement award through its Nuclear Criticality Safety Division. And in 2013, ANS posthumously awarded Johnson with its Standards Service Award. Both organizations are major proponents of nuclear standards in the U.S.
In 1944 at age 23, Johnson (née Briggs) was a graduate student in physics at Vanderbilt University when she was recruited to work on the Manhattan Project at Columbia University. At Columbia, she met and married another young physicist, Ned Johnson. Libby Johnson returned to Vanderbilt to finish her master’s degree, and Ned Johnson began working at ORNL. Then in 1948, Libby Johnson was hired as a research assistant at the K-25 uranium-enrichment plant.
From 1950 to 1961, Johnson worked as a supervisor and senior reactor operator at ORNL’s Bulk Shielding Facility, which housed the Bulk Shielding Reactor, the first swimming pool reactor, and the Pool Critical Assembly, completed in 1958. Both the reactor and the assembly featured a core and control rods submerged in an open pool of water that provided cooling and radiation shielding. Johnson oversaw fuel loadings and reactor maintenance, trained reactor operators and coordinated experimental programs. She also taught students from ORNL’s pioneering Oak Ridge School of Reactor Technology how to assemble reactor fuel safely. Johnson also wrote the standard operating procedure for the Pool Critical Assembly issued in 1960.
“To me, she was the gospel in performing experiments safely with aqueous solutions containing enriched uranium,” said John Mihalczo, who joined ORNL in 1958 and, although retired, continues documenting early experiments for future generations.
In 1954, Johnson became a charter member of ANS, which was established to provide education about nuclear energy and a publication outlet for peer-reviewed articles and standards. Over the course of her membership, she served in leadership roles, chairing ANS’s Nuclear Criticality Safety Division, speaking at numerous national standards meetings, forming standards subcommittees, contributing immeasurably to standards writing groups as both member and secretary and mentoring incoming professionals. Johnson also helped found Oak Ridge’s ANS chapter.
“Just watching her at standards meetings was a great education,” said Ron Knief of Sandia National Laboratories. “I witnessed this diminutive woman frequently going toe-to-toe with the [criticality safety] giants of the time.”
When the first commercial nuclear power plants began operations in the late 1950s and early 1960s, the U.S. Congress recognized the need to involve the public in decisions that affected local communities. It charged the Atomic Energy Commission to form the Atomic Safety Licensing Board, which hosted open hearings that included legal and technical experts to answer questions from the public and discuss concerns before construction licenses were issued. The Nuclear Regulatory Commission appointed Johnson to the board in 1975 as its first full-time female technical expert. When Johnson retired in 1994 as the longest serving female in the board’s history, she remained its sole female technical expert.
Today, mostly because of rigorous regulations and a shift in funding priorities, scientists rarely perform the type of criticality experiments Johnson led. As a result, criticality safety professionals now obtain data mostly from simulations and prior experiments, such as the ones Johnson and her colleagues performed that led to their foundational discoveries.
The current International Criticality Safety Benchmark Evaluation Project Handbook, a reference used by criticality safety professionals, documents 91 of Johnson’s experiments. “Johnson continuously built upon the body of knowledge that is still used today,” said Douglas Bowen, head of ORNL’s Nuclear Criticality, Radiation Transport and Safety section.” She was a pioneer among pioneers.”
Johnson also helped open doors for women in nuclear science – including her granddaughter, Autumn Higgins, who joined the Nuclear Navy in 1995, a year before Johnson’s death. In 2022, Higgins earned her senior reactor operator license for the Tennessee Valley Authority’s Sequoyah Nuclear Plant.
“If I could talk to her today, I would want her to know her struggle meant something for me and other women,” Higgins said. “Not only her scientific contributions, but also her personal contributions. They meant something.” — Sumner Gibbs