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Department of Energy plans to locate a high level nuclear waste repository in
Nevada. One option that is being considered is transmuting the radioactive waste
by bombarding it with neutrons. This would be a very large-scale project since
77,000 tons of waste are planned for the repository. Is transmutation on such
a large scale practical? One important question is the materials question: can
materials be found that can withstand the high heat fluxes and high neutron fluxes
of transmutation? One important material is a mixture of lead and bismuth, lead-bismuth
eutectic (LBE), which has been proposed as both a spallation target(1) and as
a coolant(2). LBE has many advantages for this purpose: it has high thermal conductivity,
melts at a relatively low temperature (123 C), has a high efficiency as a spallation
target, and has a low neutron capture cross section. The Russian navy used LBE
in their nuclear submarines, so there is practical experience in the use of LBE
as a coolant. Unfortunately, LBE also is chemically corrosive: it corrodes the
stainless steel that makes up the piping and transmutation chamber.
The goal is to guide the selection of appropriate materials, or the treatment of existing materials, to minimize the corrosion.
One important result is the significance of surface preparation. Two steel samples of identical composition had different surface preparation: one sample was cold rolled and the other annealed. The two samples were then exposed to LBE. One might expect that the cold-rolled sample to be more susceptible to corrosion, because it has more grain boundaries, and grain boundaries are often initiation sites for corrosion. Instead, the cold-rolled sample was an order of magnitude less susceptible to corrosion.
See the Glossary for the Non-Scientist
Group publications, presentations, and graduate student theses and dissertations
(1) Spallation refers to the bombardment of LBE by high-energy protons, producing large numbers of neutrons, which transmute the waste.
(2) A coolant is needed because as many as 100 MW of power need to be dissipated to avoid melting the target.
For more information, see the AAA web site at UNLV devoted to the transmutation project.
Supported by the Advanced Fuel Cycle Initiative of DOE.