HEALTH

When President Obama and Russian President Medvedev signed an agreement in June 2009 to cut American and Russian strategic nuclear arsenals by at least 25 percent, it was an important step in the effort to reduce the global threat of nuclear war. As our leaders move forward with the political aspects of stockpile reduction, scientists are at work determining what to do with the highly radioactive uranium and plutonium in decommissioned nuclear weapons.

Fundamental questions about the nature of these elements are at the heart of research being carried out by Dr. Asok Ray, professor of physics at The University of Texas at Arlington and his group of postdoctoral fellows, graduate and undergraduate students.

“Let’s say the United States and the former Soviet Union come to agreement on how to destroy some of their nuclear weapons, which have plutonium and uranium in them,” Ray said. “When you destroy them, what happens to the uranium and plutonium? Or if you want to store them underground, how, over hundreds of years, will they react with atmospheric gases?”

To find answers to these questions, Prof. Ray and his group simulates the electronic structures of uranium, plutonium and other actinide materials on the Lonestar supercomputer at the Texas Advanced Computing Center (TACC). These simulations help Ray’s group understand the basic nature of radioactive elements and better predict their reaction processes when exposed to common elements like oxygen, hydrogen water, and carbon dioxide, among others.

Uranium and plutonium are part of the actinide family of fourteen elements — the last line of elements in the periodic table. Uranium is the final actinide found in nature; the others are produced in laboratories. The electrons in the actinides have unique properties that are strongly influenced by relativistic effects.

“Because of the high speeds of the electrons, you have to take into account Einstein’s theory of relativity, which says that as the speed increases, there is a connection to the mass formula,” Ray said. It is only with advances in computing technology that scientists are beginning to understand the properties of actinides and learn how elements like uranium and plutonium derive their power.

From a practical point of view, the most pressing questions address how to safely store or dispose of hazardous radioactive materials

“The storage of uranium and plutonium and the nuclear stockpile has been a concern to both the government and scientists,” said Ray. “They are stored in containers and over time these containers interact with all the atmospheric gases. The question is: how much energy is released in the process, and how can we minimize the release of that energy into the atmosphere?”

Using more than 300,000 computing hours on Lonestar in 2008, Ray examined the electronic and geometric structures of several actinide elements and simulated the dynamics of their surface interactions with the atmospheric gases. His simulations plotted the trajectories of the actinides’ large number of electrons as they reacted with each other and outside elements, reflecting the quantum mechanical and relativistic effects that influence the behavior of these electrons.

It is impossible to map the locations of each electron, so Ray uses density functional theory, which converts the many-body system into a one-body system to solve for the motion of one electron in the presence of the other electrons.

“We could not do this research without supercomputers. Even 10 years ago, we couldn’t study these actinides because of their complex behavior,” said Ray. “These are massive systems with relativistic effects and you simply cannot solve it on a PC or a small grid. You need parallel computing to do the work in a reasonable time.”

The group’s simulations, for example, showed that oxygen molecules break up spontaneously on a plutonium surface, but that water molecules and hydrogen only break up in the presence of extra energy. They also determined the amount of energy that will be released if a molecule brakes up in this process.

Oxygen adatom located at a bridge adsorption site on a the α-Pu(020) surface modeled by a four-layer slab.

The study of nuclear elements, for energy and for national security, is a hot topic. Most of the research in the field is performed at government high-performance computing centers in closed conditions. These studies largely focus on the bulk form of these materials, Ray said, whereas his group’s study of the atomic-level surface interactions is one of the first to show scientists and decision-makers how uranium and plutonium react at the smallest levels. This information will help the government dispose of dangerous unwanted materials with greater knowledge and safety.

Above and beyond the useful application of simulations for nuclear waste disposal, actinides serve as an effective lens through which researchers can understand the fundamental forces shaping the behavior of all elements.

“Actinides are very complex elements and studying them gives us significant insights about other elements, like transition metals, lanthanides, and others,” Ray said.

The only way to study elements like actinides is to set them in motion in the circuits of a supercomputer, as Ray’s research on Lonestar does. By applying the parallel-processing power of TACC’s supercomputers, researchers are able to investigate the most complex systems at the smallest level, fundamentally broadening our scientific knowledge.

“If we truly understand the actinides, we’ll have a very good feeling for how the rest of the periodic table works,” said Ray. “It helps us understand relativity in a general way.”