Lawrence Livermore National Laboratory (LLNL) recently led the successful execution of the first U.S. subcritical experiment (SCE) since 2021 at the Principal Underground Laboratory for Subcritical Experimentation (PULSE) facility, formerly known as the U1a Complex, at the Nevada National Security Site (NNSS). The last LLNL-led SCE was in February 2019.
The experiment - the first in a series of SCEs named "Nimble" - involved detonating high explosives nearly 1,000 feet below the desert floor allowing researchers to study special nuclear material properties. The Nimble series will provide key data to support certification efforts within the stockpile modernization program.
SCEs do not form a self-sustaining, supercritical chain reaction and remain below the threshold of criticality. The experiment was consistent with the zero-yield standard of the Comprehensive Nuclear-Test-Ban Treaty.
"The successful execution of the experiment represents an important advancement in research capabilities," said design physicist Fady Najjar, who helped lead the experiment design. "The high-fidelity diagnostic data we collected will answer key weapon physics questions and are essential in advancing modeling and simulation capabilities across the Nuclear Security Enterprise."
As the high explosive detonated, experimentalists utilized a complex suite of state-of-the-art diagnostics to capture comprehensive scientific data.
The SCE was conducted in a mined space at PULSE known as a "zero room" where SCEs are contained within a robust confinement vessel. This configuration prevents the release of radiological material and allows the room to be used again in future experiments.
"Completing the first SCE in the Nimble series marks the culmination of several years of dedicated effort," said Tim Beller, experiment director. Preparations for the SCE began long before fielding activities at PULSE, with precursor experiments and demonstration exercises conducted to verify the designs. The entire experimental area, including the zero room, was set up based on the experiment configuration. Throughout the process, the experiment team regularly conducted integrated dry runs of the diagnostics, timing and firing systems.
"I appreciate the tenacity of the Nimble team who overcame many challenges to field and execute this experiment," said weapon physicist Garry Maskaly, a principal investigator working on the project. "I am continually impressed with the professionalism and skills of our key partners at NNSS. These teams and the capabilities they provide are a key part of our national deterrent and fulfilling the U.S. commitment to not return to nuclear explosive testing."
The Nimble series is a multi-institution collaboration between LLNL, NNSS and Los Alamos National Laboratory. The campaign supports the National Nuclear Security Administration's (NNSA) Stockpile Stewardship Program, created to ensure the safety and reliability of the U.S. nuclear weapons stockpile now and in the future.
"The collaboration and technical expertise of our multidisciplinary team and our extraordinary partners have been crucial in achieving these results," said JB McLeod, integrated experiment team lead. "The combined efforts of engineers, scientists and technical experts across multiple organizations were instrumental in the experiment's success."
The experiment was the 34th SCE performed by NNSA since 1992, when the United States began its self-imposed moratorium on nuclear explosive testing. NNSA Administrator Jill Hruby announced in June 2023 plans to increase the frequency of SCEs to continue to gather important data on nuclear weapons materials. The United States is preparing to conduct approximately three SCEs per year by the end of the decade.
