It was a normal morning for design physicist Madison Martin at Lawrence Livermore National Laboratory (LLNL). At 7:45 a.m., she settled into her classified workstation with a cup of tea to check the results of a numerical calculation she ran overnight. If the calculations proved correct, the experiment she was designing on the National Ignition Facility (NIF) would deliver the data her colleagues needed to verify that a refurbished nuclear warhead would perform as expected.
Martin is one of many physicists at LLNL whose careers are defined by the stockpile stewardship era — using modern experiments, historic test data and high-performance computing to ensure the long-term viability of the U.S. deterrent without additional underground nuclear tests. Martin was in kindergarten when the last nuclear test took place, and the nuclear weapon her colleagues are refurbishing entered the stockpile in 1982, long before she was born.
The experiment was one of the high-energy density NIF experiments conducted in FY18 in support of the W80-4 life extension program (LEP). The data acquired in those experiments is helping weapon designers at LLNL assess replacement options for aged materials in the W80 warhead, marking the successful completion of one of the 12 key objectives on the National Nuclear Security Administration’s (NNSA) “getting the job done” list of top priorities for FY18. LLNL is the lead nuclear design agency for the W80-4 LEP. Some aged materials and components that need replacement cannot be produced exactly as they were originally manufactured. Researchers conduct extensive studies to evaluate whether various replacement options can be certified to be safe (won't go off by accident), secure (can't be set off without formal permissions) and effective (will work as designed) without having to conduct a full-scale explosive nuclear test of the system. That's where facilities like NIF come in.
NIF was funded, designed and developed to help replace the role underground nuclear tests played in maintaining the nuclear stockpile. The experiments conducted in support of the W80-4 LEP in FY18 were built on many years of development of experiment configurations and diagnostics. NIF provides access to the highest pressure and temperature regimes needed to assess weapon physics and material responses. In this case, physicists were able to use NIF to inform decisions about whether they could replace aged materials in the W80 without sacrificing the warhead's safety, security and effectiveness.
There are roughly four phases in a two-stage nuclear detonation. In the first “high explosives” phase, high explosives compress special nuclear material, creating a supercritical assembly. The “primary phase” comes next, when the supercritical assembly fissions and initiates fusion reactions, ultimately creating a burst of neutrons and X-ray energy. Those X-rays travel from the weapon primary to the secondary in the third “energy transfer” phase. Finally, the weapon secondary produces energy, explosion and radiation in the “secondary phase.”
While high explosives kick-start a nuclear detonation, the majority of a weapon's energy output is produced in the “high-energy density” (HED) state of the matter, where temperatures and pressures are equivalent to those found on the surface of the sun. ln the absence of underground nuclear testing, HED experimental facilities like NIF are the only way to answer questions about matter under these extreme conditions.
In addition to the FY18 experiments in support of the W80-4 LEP, NIF is making significant contributions in a number of areas relevant to stockpile stewardship:
Primary boost: Researchers are using NIF to better understand the physics that underlie “boosting” a nuclear weapon primary, a complex process where neutrons from the ignition and fusing of heavy hydrogen atoms are used to more efficiently fission the special nuclear material in the primary phase of the detonation.
Energy balance: Researchers used NIF to help solve a mystery about how energy flows in a nuclear detonation — euphemistically called "energy balance" — that eluded explanation for half a century. This breakthrough significantly advanced confidence in the accuracy of scientific assessments of the stockpile.
Material properties: Researchers are using NIF to measure the phase, strength and equation-of-state of plutonium — perhaps the strangest and most enigmatic material on the periodic table — under previously unmeasured pressures and temperatures. These measurements are relevant to weapon physics and are expected to continue to pay dividends not only for certification of life-extended weapon systems, but also for the annual assessments that NNSA laboratories conduct to evaluate if the U.S. stockpile is aging acceptably.
Survivability: Researchers are using NIF to produce radiation sources to evaluate how well U.S. weapons will be able to survive — and function as expected — after traversing evolving adversary missile defenses. Without the ability to survive, these weapons would be unable to hold targets at risk, rendering the deterrent ineffective.
In the coming year, researchers plan to conduct additional experimental campaigns in support of the W80-4 LEP to further inform decisions about life extension options. In addition, ongoing stockpile stewardship research on NIF that aims to answer questions about ignition, boost, survivability and material properties will be included in the Laboratory's annual assessments of the stockpile, which culminates in a letter from the Laboratory director informing the president and Congress of LLNL’s assessments of the warheads for which LLNL is responsible.