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Radiation studies key to nuclear reactor life, recycling spent fuel

 

Two UW-Madison projects to study advanced materials and fuel forms for both current and future nuclear reactors recently received funding of approximately $1 million under the Department of Energy Nuclear Energy Research Initiative (NERI).

The NERI program supports research and development under three Department of Energy nuclear initiatives: Generation IV nuclear energy systems, advanced fuel cycles, and nuclear hydrogen.

In one three-year project, UW-Madison nuclear engineers will study the resistance of oxide, carbide and nitride nuclear fuel “matrix” materials — the vessels that contain nuclear fuel — to radiation damage. A second project will exploit recent advances in computational power and technique to develop computer models of how a reactor's structural materials behave as a result of long-term radiation exposure.

The projects were among 24 selected for total funding of $12 million; UW-Madison is among five universities to receive funding for multiple projects.

Matrix materials are a key element of future fast-spectrum reactors, which are capable of safely and efficiently recycling spent nuclear fuel. The nuclear fission process produces high-energy radioactive neutrons, called “fast” because of their great energy. Current thermal reactors use a moderator to reduce the neutrons' velocity, making them capable of sustaining the nuclear fission reaction using simpler fuel.

James Blanchard

James Blanchard (large image)

Todd R. Allen

Todd R. Allen (large image)

But to recycle and minimize the waste impact of the spent fuel, you need to keep those neutrons fast, says Todd Allen, an assistant professor of engineering physics.

He and James Blanchard, a professor of engineering physics, are studying proposed matrix materials such as zirconium nitride or titanium carbide as a replacement for the current carbon matrix used in gas-cooled reactor fuel. “Replacing a lot of the carbon with zirconium atoms, for example, means you'll slow down neutrons less,” says Allen.

While they know that materials in the nitride or carbide families are more effective at keeping neutrons up to speed, what's not clear is how the materials hold up under constant radiation. Allen and Blanchard have constructed a radiation damage test facility on one beam line of the Department of Engineering Physics ion accelerator. In an experiment that simulates long-term radiation exposure, they will bombard their candidate materials with a high-energy ion beam and study how each one holds up.

“It's all in the context of devising new fuel forms that will allow you to efficiently recycle reactor fuel in a way that minimizes the net waste output from the entire fuel cycle,” says Allen. “And the reason for looking at recycle is to limit the number of underground repositories you have to build.”

Dane  Morgan

Dane Morgan (large image)

Another project involves applying complex materials modeling to nuclear reactors. In it, Allen and Dane Morgan, an assistant professor of materials science and engineering, will incorporate the properties of iron, chromium and nickel into more complete computer models of radiation damage in steel, a common reactor structural material.

Previously, a lack of computing power limited such models to single pure materials like copper or iron. “People have learned a lot about radiation damage,” says Allen. “But you never build anything out of just copper or just iron.”

The effort may lead to structural materials that are better able to withstand long-term exposure to radiation — in some cases, nearly 60 years.

In a reactor, high-energy neutrons can knock the individual atoms in steel out of their normal positions, bumping them elsewhere in the material or wedging them between their normal positions. As a result, vacant spots form in the steel and allow diffusion that can lead to unacceptable changes in shape or creation of brittle materials. “It all happens because atoms diffuse and form structures that either change the volume or make it brittle,” says Allen.

And that means costly reactor components may need to be replaced sooner than desired.

In addition, the researchers have seen evidence that when vacancies cluster together and form larger voids, the composition of the steel around those voids is different from the composition of the material as a whole. “We'd really like to know how to predict these local composition changes, but to do that requires you to understand how these diffusion events happen, and how they happen as a function of composition,” says Allen.

That's where Morgan's computer models will come in handy. Current diffusion data is measured at approximately 1,000 degrees Celsius, but reactors operate at much lower temperatures. Researchers can only speculate that the diffusion and composition-changes occur in the same manner at lower temperatures. Add radiation and the diffusion mechanisms become more complex. Morgan's models enable the researchers to calculate diffusion parameters in complex materials under radiation.

Coupled with accelerated ex situ reactor experiments, which are fast to perform but are not strictly representative of the multi-year damage that occurs in a nuclear system, the duo's more accurate models can help researchers predict how materials such as steel might behave over a reactor's lifetime. “And if you really start to understand the fundamental mechanisms of radiation damage, you gain the ability to predict how changes to the material could improve its properties,” says Allen. “If you understand things on a fundamental basis, it's easier to translate your accelerated experiment into the performance of the real system.”

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1/5/2006