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Cleaning up the Cold War

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Thursday, August 6, 2009

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Media Contact: Sue Stevens

ARLINGTON - The United States and Russia agreed this summer to cut their strategic nuclear arsenals by at least 25 percent. Researchers led by University of Texas at Arlington physics professor Dr. Asok Ray now are helping determine how to safely dispose of the highly radioactive uranium and plutonium in the decommissioned nuclear weapons.

Professor Ray and his team of postdoctoral fellows, graduate and undergraduate students are using the Lonestar supercomputer at the Texas Advanced Computing Center (TACC) In Austin, the National Energy Research Scientific Computing Center (NERSC) operated by the Department of Energy and the UTA Supercomputing Center, to simulate the electronic structures of uranium, plutonium and other actinide materials. 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 14 elements - the last line of elements in the periodic table. Uranium is the last 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.

Difference charge density plot indicating significant charge transfer from the Pu atoms to O (ionic bonds). Red and blue denote regions of charge gain and charge loss respectively.

Difference charge density plot indicating significant charge transfer from the Pu atoms to O (ionic bonds). Red and blue denote regions of charge gain and charge loss respectively.

"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?"

"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.

"The storage of uranium and plutonium and the nuclear stockpile have been a concern to both the government and scientists," Ray said. "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 in Lonestar in 2008 and thousands of computing hours in NERSC and UTA, Professor Ray and his group 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.

"Even 10 years ago, we could not study these actinides because of their complex behavior," he said. "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 the a-Pu(020) surface modeled by a four-layer slab.

Oxygen adatom located at a bridge adsorption site on the a-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 in government high-performance computing centers in closed conditions. These studies largely focus on the bulk form of these materials, Ray said. 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. Beyond the issue of 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 those 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, NERSC, and UTA 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."

Ray's research is funded by grants from the Welch Foundation of Houston and the U.S. Department of Energy. His work is typical of the cutting edge research at UT Arlington, classified as a Carnegie Research University/High Activity, as the University progresses toward national research university status.

(This release contains previously published information from Aaron Dubrow, Texas Advanced Computing Center, University of Texas System.)


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