Professor works toward safer nuclear options

August 11, 2009

Research that Arizona State University faculty member Pedro Peralta is pursuing with Los Alamos National Laboratory scientists to make nuclear power use safer and more effective will get support from a recent U.S. Department of Energy grant.

Peralta is an associate professor in the School of Mechanical, Aerospace, Chemical and Materials Engineering in ASU’s Ira A. Fulton Schools of Engineering.

The grant of more than $184,000 will pay to upgrade a scanning electron microscope and obtain a new “glove box” needed to advance research on nuclear fuels.

The equipment will enable researchers to more safely and effectively handle, prepare and analyze the microstructure of materials – called “surrogate” materials – that are used to simulate nuclear fuels.

The microscope will be used to map the microscopic structures and composition of fuel samples. A glove box is a box with attached rubber gloves that is sealed from the outside environment. It enables researchers to maintain an atmosphere of low oxygen and moisture inside the box.

The glove box preserves the chemical composition of depleted uranium oxide samples and keeps radioactive material that results from sample preparation from spreading to the environment outside the glove box.

With this equipment, Peralta can better analyze materials meant to simulate new nuclear fuels.

These new fuels are designed to close the nuclear “fuel cycle” – in other words, the point in the process in which used or “spent” fuel that powers nuclear reactors is reprocessed to make new fuels. At the same time, it reduces the amount of heavily toxic radioactive materials and the plutonium inventory, which are a typical byproduct of the use of uranium-based nuclear fuels.

The goal, Peralta explains, is to understand how the microstructure of these materials is affected by the processing that is used to make the surrogate samples, and in turn find relationships between that microstructure and the properties of these materials and their potential performance inside a nuclear reactor.

Three things that are important for nuclear fuels, he says. There’s porosity, which must be present in the right amount with pores of the right size. Secondly, composition needs to be as homogeneous as possible. Thirdly, grain size needs to be in the appropriate range (8 to 20 micrometers).

Grains are small crystalline regions that are present in most metallic and ceramic materials and are between 4 to 10 times smaller than a human hair.

The pores provide space for the gases that are generated inside the fuel as it is used in the reactor. The homogeneous composition is important because the presence of a higher concentration of, for example, plutonium, at one location, can lead to higher local temperatures and even melt part of the fuel.

If grains are too small, the radioactive gases produced during use can escape too easily. If they are too large, the fuel pellets can break. This can lead to melting of the fuel because heat cannot be transferred into the cooling fluid as efficiently as before.

“You have to know all these things to make sure they are in the right range and you must understand how to process and prepare the fuels to make sure that is the case,” Peralta says.

There are pools around the country, typically close to nuclear plants, where the spent fuel is kept underwater to reduce its temperature. That spent fuel contains plutonium and many other radioactive elements, many of which are as toxic and radioactive as plutonium.

A previous idea was to simply “store” the spent fuel in what is typically called a “geologic repository,” where spent fuel would be mixed with glass to stabilize them and then buried underground inside sealed containers.

Yucca Mountain in Nevada, was chosen for that purpose, but the toxicity and radioactivity of the materials in the spent fuel is so high and lasts so long that the repository must be guaranteed to remain geologically stable – no earthquakes, no landslides, etc – for several thousands of years.

“Imagine a strong earthquake exactly where you have some of most toxic and radioactive materials ever made by man,” Peralta says. “Furthermore, decomposition of the glasses used to stabilize the spent fuels and corrosion of the containers could also happen over the long times required, and the nuclear waste could then leach into the ground, contaminating the soil and eventually underground water sources. The technical challenges to ensure the geologic stability of Yucca Mountain and the safety of the nuclear waste itself for thousands of years are staggering.”

An alternative, he says, is to take the plutonium and other radio-toxic materials from the fuel and reprocess the materials using specially designed nuclear reactors.

This converts them into other elements – via a nuclear reaction called transmutation – that do not remain radioactive as long as the initial elements. This dramatically reduces the amount of time a spent-fuel repository must be geologically stable.

The processes Peralta and his colleagues are working on will also reduce the inventory of materials that could be processed to make weapons-grade plutonium or uranium, which will in turn reduce the risk of their accidental release into the environment or their becoming a target for criminal or terrorist activities.

For more information on this and related research, see the web site for the Advanced Fuel Cycle Initiative of the Office of Nuclear Energy, Science and Technology, at http://afci.sandia.gov/AFCI_index.htm