There are approximately 20,500 nuclear warheads remaining in the world. The United States and the Russian Federation together have about 19,500 of them, according to the best estimates.
A new understanding
Jeffrey G. Lewis, a nuclear weapons expert and the director of the East Asia Nonproliferation Program at the Monterey Institute of International Studies, said that years of underground nuclear tests helped show weapons designers that the bombs worked under certain conditions, “but they could never fully explain how or why.”
“The best argument against the test ban was always that we didn’t understand how nuclear weapons really worked and couldn’t simulate them, so underground nuclear explosions were an important reality check,” he added. “But even then, there was never enough testing to establish the kind of confidence that comes from actually understanding the process of a thermonuclear explosion.”
As a result of the computer modeling, he added, “for the first time, nuclear weapons designers understand why and how thermonuclear weapons work.”
In recent years, physicists at Livermore surmounted one of the oldest and most difficult challenges they faced. In many nuclear weapons explosive tests, measurements suggested that the detonating bombs appeared to violate a law of physics, “conservation of energy,” which states that in a closed system, the total amount of energy remains constant, and thus energy cannot be either created or destroyed.
For decades, the nuclear weaponeers puzzled over why the test results appeared to break from this principle. Then, the “energy balance” problem, as it was known, was solved by a Livermore physicist, Omar Hurricane, who won the 2009 E.O. Lawrence Award from the Department of Energy for his work, which remains classified.
The supercomputers at Livermore and the other national laboratories do not do the job alone. Scientists use data from the 1,054 U.S. nuclear tests between 1945 and 1992, of which about 200 are relevant to today’s arsenal. They also cross-check the computer findings with laboratory experiments.
One of the most elaborate and ambitious is the National Ignition Facility at Livermore, housed in a stadium-size building where scientists hope to use 192 laser beams to achieve fusion ignition in a laboratory setting, a process that has never been witnessed. If it works, the lasers will heat a tiny fuel pellet of tritium and deuterium to 100 million degrees, causing some of the nuclei to fuse, generating energy and producing conditions close to those inside the core of stars — and inside a detonating nuclear weapon.
“No one has ever seen hydrogen fusion bare naked in a vacuum. It’s always been buried in the middle of an atomic weapon,” Goodwin said. “You can infer what happened.” If ignition is achieved, he added, scientists will be able to not only see it, but measure it.
The facility has suffered long delays and setbacks, but the target is to achieve ignition by next autumn.
Another, the Dual-Axis Radiographic Hydrodynamic Test Facility at Los Alamos, uses X-rays to follow the shape of sections of plutonium when they are compressed as they would be in a nuclear weapon explosion.
Teraflops and beyond
When the stockpile stewardship program was begun in the 1990s, the goal was to build a generation of supercomputers capable of 100 teraflops. A teraflop is a measure of a computer’s processing speed that is 1 trillion floating point operations per second; an operation could be a single mathematical calculation, such as addition or multiplication. This was accomplished, but new machines are now pushing far beyond.
Next May or June, Livermore plans to put into operation an IBM supercomputer, Sequoia, capable of 20 petaflops. A petaflop is a thousand trillion floating point operations per second. The machine, on 96 refrigerator-size racks, will contain 1.6 million processing cores and will be 10 times faster than what is now the fastest computer in the world. By comparison, all the computing power at Livermore today is about 2.5 petaflops.
With such vast computing capability, scientists can attempt to create a realistic model of what happens inside a nuclear explosion, when tremendous pressures and temperatures squeeze metals, including uranium and plutonium, to set off the nuclear blast. Fred Streitz, director of Livermore’s Institute for Scientific Computing Research, said the ultra-fast machines are “opening doors to new science,” such as models of how atoms behave, or how the crystal or grain structure of a metal changes under pressure.
Streitz said that over time, it became clear that smaller computer simulations were returning incorrect answers; only with finer resolution and more power could scientists grasp what was really happening.
In one example involving molten copper and aluminum, Streitz said, 9 billion atoms were modeled. It took more than 212,000 computer processors more than a week to carry out the simulation, he said, but the result was a near-perfect resolution of how the metals behaved.
“This is millions of times finer than you could ever do in a nuclear test,” Goodwin said. “You could never see this process go on inside a nuclear explosion.”
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