When people think about nuclear weapons, most probably picture warheads sitting silently on missiles or within secure hangers near airbases. The bombs themselves appear cold and unchanging.
Nothing could be further from the truth. Nuclear weapons are changing all the time, says Scott Doebling of Los Alamos National Laboratory, who recently took leave from managing the lab’s Verification and Validation (V&V) program to take an assignment with the National Nuclear Security Administration (NNSA).
“You’re dealing with systems that have radioactive isotopes in them. As the isotopes decay over time, they emit radiation that changes the properties of the materials surrounding them in the warhead.”
Just how does prolonged exposure to nuclear radiation change a material’s properties? How do those changes alter the way a weapon performs? Will an older bomb still perform as intended? Will it remain safe and intact if it crashes in an aircraft or burns in a fire?
At the height of the Cold War, the United States had easy answers to those questions. First, it replaced weapons every 10 to 15 years. Second, it tested weapons to ensure they performed as intended. Finally, America kept adding to its stockpile.
But in 1992, the United States halted underground nuclear testing, reduced the size of the existing nuclear stockpile and stopped making nuclear weapons.
Since the nation no longer built new warheads, it needed to extend the lifespan of the ones it had. It also needed to assess the performance of those aging and reconditioned weapons — and it had to do it without testing.
Today, the Department of Energy relies on computational simulations to predict weapons performance, employing several of the world’s largest supercomputers and some of the most sophisticated computer models ever created. Yet the models raise questions of their own. They are large, complex combinations of smaller models. Often, those models are based on different understandings of how physics works. They sometimes deal with temperatures and pressures where the physics are not well understood. Running such complex software also requires compromises that could change a model’s ability to predict future warhead behavior.
The task of the Los Alamos V&V team is to quantify those uncertainties and improve our confidence in the simulations.
Inherent Problems
Today’s computer models produce incredibly detailed results that appear to precisely simulate the behavior of weapons and materials as they age.
Yet do they? The uncertainties start with construction of the models themselves.
“Many of the mathematical models we use to understand physics are limited in what they explain,” Doebling says. One theory, such as electrodynamics, may elucidate the behavior of a few atoms individually. Yet it cannot portray the transfer of heat or force through a block of material made of those atoms. That would take a second theory, such as thermodynamics or mechanics.
Sometimes, just picking the right model is hard. Thermonuclear weapons operate under extreme conditions that aren’t easy to understand. As Doebling notes, weapons models must simulate a broad range of environments, from room temperatures and pressures to conditions in the center of the sun. A theory may work in one regime but prove less reliable in another.
Knobs
There are several ways to get a handle on how aging and reconditioned warheads will perform. One is to learn more about how they age. NNSA does that by dismantling weapons and examining the pieces as part of its stockpile stewardship program.
Deciphering the physical evidence is harder than it sounds. Radiation may have turned a polymer a different color, but its other properties may remain unchanged. A metal may look new but behave differently.
Those changes are critical only if they affect the bomb’s performance. To predict whether they will, model builders need to understand how those materials behave when exposed to the extreme heat, temperature and radiation of a nuclear explosion.
Researchers finessed the problem by including what they call “ad–hoc parameters” in the mathematical models. These parameters accommodated both the unknowns of aging materials and unknowns in the models themselves.
The ad–hoc parameters, however, were based on data accumulated during underground tests that ended more than 15 years ago. Researchers adjusted their materials models until the model outputs matched the test outputs.
Modelers call ad–hoc parameters “knobs.” “From the weapon design perspective, they are necessary to get the job done in design engineering,” Doebling says. “Now we’re going back and trying to understand the physics of those knobs.”
Verification
Verification and validation, Doebling says, is about ensuring the calculations yield the right answers for the right reasons, which means reducing and eventually eliminating dependence on knobs. “If we want our models to predict unknown behaviors, they cannot be right just because we tuned the model to our experimental data,” he adds. “They have to be right because the physics embedded in the models represents how the system truly behaves.”
The Los Alamos V&V team approaches that issue in two ways. First, it attempts to verify that its models solve equations properly. This takes both finely honed math and programming skills and judgment.
The team starts by looking for programming errors and re-checking the math.
Equally important, verification ensures that the model provides enough precision without sacrificing run speed.
Validation
The Los Alamos team’s second approach is to validate a model’s underlying physics. In other words, it ensures the model reflects reality.
New testing facilities make this possible. NNSA now operates several sites that can produce conditions of extreme temperature, pressure and radiation.
The experiments are costly and time-consuming to set up and run. So instead of testing everything, the V&V team identifies areas where precision counts most.
“These are incredibly complex projects, but verification and validation has to be tightly linked to testing,” Doebling says. “Decisions about the safety, security and reliability of nuclear weapons have pretty high consequences. For them to act as a deterrent, we have to know that after aging and modifications, they will still operate as designed.”