ASSURING the safety and reliability of the nation's nuclear weapons stockpile without underground nuclear testing is a formidable scientific and engineering challenge for the Department of Energy and its national security laboratories. For the past three years, a team of Lawrence Livermore men and women has been working in an underground facility at the DOE's Nevada Test Site to help meet that challenge.
In a complex of mined tunnels with instrumentation rooms and experimental alcoves, the team conducts experiments involving chemical high explosives and weapons-grade plutonium. Results from the experiments are playing an increasingly important role in DOE's Stockpile Stewardship Program.
We call the experiments subcritical because the amount of plutonium used in each experiment is so small it cannot reach critical conditions. No self-sustaining nuclear fission chain reaction, like that in the detonation of a nuclear warhead, can occur. We use the experiments to better understand the nature of plutonium and its alloys by strongly shocking them with explosives. The shock is intended to reproduce the tremendous pressures and temperatures that occur when a nuclear device is detonated.
The subcritical experiments are revealing important new information about the properties of shocked plutonium. The experimental data are used to refine the advanced computer simulation codes that run on our supercomputers to help predict any problems with the nation's aging nuclear stockpile. The codes reflect what we know about the materials properties of warhead constituents, especially nuclear materials. We can't make accurate predictions about how plutonium will perform in extreme conditions without a more complete understanding of its properties.
In particular, we need to know more about how plutonium changes behavior as it ages over several decades inside a nuclear warhead. Many nuclear weapons in the current stockpile must be maintained far beyond their intended lifetimes. To investigate the consequences of aging, we compare the shocked behavior of newly machined plutonium with that obtained from dismantled warheads that are decades old.
As described in the article entitled Underground Explosions Are Music to Their Ears, a team of experts from Livermore's Engineering and Defense and Nuclear Technologies directorates is responsible for designing and conducting the Livermore subcritical tests. The experiments are conducted underground at the remote Nevada facility to minimize any possible environmental impact. Underground tests also minimize costs to taxpayers, because they are far less expensive than those conducted in a comparable aboveground facility.
The subcritical experiments benefit from experience gained from a longstanding program of hydrodynamic experiments of mock warheads conducted at Livermore's remote Site 300 test facility. Indeed, most of the instruments we deploy to characterize subcritical experiments in Nevada are identical to those used at Site 300 for looking deep inside an imploding mock warhead. However, the hydrodynamic tests do not use plutonium and so cannot completely answer all of our questions about what takes place in a nuclear device in the microseconds following high-explosive detonation. Only subcritical tests can provide that information.
As we continue our program of subcritical experiments, we're learning how to perform them better, faster, and cheaper. For example, we're currently saving considerable money-and advancing our test schedule-by conducting experiments in steel cylindrical vessels that act as miniature laboratories to confine the explosive products from each experiment. That innovation, which significantly reduces the need to mine new experimental alcoves, is one of several we have made to the program during the past year.
The results from subcritical tests, combined with the steady progress from other elements of the nation's stockpile stewardship program, are strengthening our confidence that we can continue to keep the nation's stockpile safe and reliable.

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