September 18, 2007

NC State and UNC Engineers Explore Ways to Mitigate Potential Hazards of Nuclear Waste

Dr. Yim. (Photo: Roger Winstead)

Ancient alchemists sought to transmute lead into gold. The conversion of one element into another proved to be a futile endeavor for these primitive chemists, but modern scientists and engineers are able to transmute chemical elements through nuclear reactions. Engineers at North Carolina State University and the University of North Carolina at Chapel Hill are conducting research to mitigate the potential hazards of nuclear waste through transmutation and other waste management schemes.

Dr. Man-Sung Yim, associate professor of nuclear engineering at North Carolina State University, and Dr. David M. McNelis, research professor of environmental sciences and engineering at the UNC Institute for the Environment are investigating ways to manage radioactive waste. The Russell Family Foundation is funding much of this research.

Certain radioactive isotopes — radioisotopes — in nuclear waste, such as Plutonium-239 and Iodine-129, have long half-lives of many thousands of years. Plutonium-239, a nuclear product, has a half-life of 24,110 years, and Iodine-129, a residue of atomic fission, has a half-life of 16.7 million years. These extremely toxic radioisotopes and others can be converted into short-lived or stable isotopes by bombarding them with a stream of neutrons in various kinds of reactors and accelerators.

“Hit an atom with a neutron, the atom absorbs the neutron,” Yim explained.

Isotopes are different forms of a chemical element with the same number of protons but a different number of neutrons. The process of converting one kind of isotope into another by changing the number of neutrons is called nuclear transmutation.

Although the short-lived isotopes may still be radioactive, their half-lives can be a matter of days or even hours as opposed to thousands of years.

“Adding a single neutron to Iodine-129 changes it into an isotope that decays in 12 hours,” explained McNelis. Nuclear transmutation, then, has the potential of reducing the storage demands of radiotoxic waste.

One of Yim's current projects examines the storage capacity of radioactive waste at the planned Yucca Mountain repository. Yucca Mountain is in a remote desert location in Nye County, Nevada. It is located within a former nuclear test site and has been designated as the first geologic repository for spent nuclear fuel and high-level radioactive waste. The U.S. Department of Energy (US DOE) has been in charge of studying the site for a number of years and has drawn on a large bank of experts from academic and research institutions to ensure the best possible science for this critical endeavor.

Graduate students, advised by Yim and McNelis, developed an analytical decay heat model for the site to represent spent nuclear fuel and performed a thermal loading analysis to show that the Yucca Mountain repository's planned capacity is not enough to accommodate the future generation of spent nuclear fuel without implementing nuclear transmutation or expanding the size of the repository. This kind of information is important for the US DOE because in 2010 they will recommend to Congress whether there is a need for a second repository.

Splitting uranium — fission — produces radioisotopes, such as Caesium-137 and Strontium-90, that give off a great deal of heat. The thermal design of the repository has limits. If the heat emitted exceeds the thermal limit, storage canisters can break down.

Nuclear transmutation is capable of converting these radioisotopes into short-lived isotopes that emit less heat and are less radiotoxic. With transmutation, only a small percentage of radioactive waste would have to be buried in the repository and perhaps for only a few thousand years as opposed to millions of years.

In addition to the analytical decay heat model and thermal loading analysis for the Yucca Mountain repository, Yim, McNelis and their students are involved in several other radioactive waste management projects. One student is studying the possibility of expanding the Yucca Mountain repository, using the thermal loading analysis scheme developed this year. Another student is investigating ways to improve the performance of an electron accelerator waste transmutation system. Others are collaborating with the Kurchatov Institute in Moscow on applying risk assessment and management techniques to the cleanup of radioactive waste at former weapons complex sites.

Yim and McNelis acknowledge that nuclear transmutation is not an easy solution for dealing with radioactive waste. Transmutation requires experts and sophisticated technologies, including separation technologies because long-lived radioisotopes must be separated from short-lived radioisotopes before transmutation in reactors can occur. There is a growing momentum in the industry for fast reactors and new technologies, such as proton accelerators, that provide the high energy needed to produce transmutation but currently are unavailable in the United States. According to Yim, any transmutation scheme must also consider time, proliferation prevention, economics, safety and impact on the repository.

Despite these challenges, Yim and McNelis are dedicated to finding optimal technological solutions for managing radioactive waste to meet current and future needs.

— mcblief —

/ News Index / News Archives Index /