Wielding cotton swabs, inspectors with the International Atomic Energy Agency take swipes from grungy surfaces on counters and cabinets in a nuclear power plant. Some swabs may pick up only trace amounts of telltale plutonium, as little as several nanograms, or even femtograms, that apparently escaped cleanup. If the inspection reveals evidence of these tiny amounts, the agency is likely to begin an extensive investigation that could lead to accusing the country running the reactor with a violation of the Nuclear Nonproliferation Treaty. The charge: separating plutonium from the spent fuel for an illicit nuclear weapons program.
IAEA conducts inspections of nuclear sites to assure the international community that countries are complying with their own nonproliferation commitments. ORNL supports DOE's International Safeguards initiatives designed to support IAEA activities. For example, ORNL-developed mass spectrometry methods are helping IAEA measure uranium and plutonium concentrations in environmental samples to detect "undeclared activities."
Lee Riciputi and his team in ORNL's Chemical Sciences Division provide the IAEA with high-precision isotope measurements of actual uranium and plutonium samples from the surfaces of nuclear reactors and nuclear fuel processing facilities, such as gas centrifuge plants that could be used to separate fissionable and nonfissionable uranium isotopes.
"We are trying to improve the sensitivity of mass spectrometry so we can analyze smaller and smaller samples of uranium and plutonium while retaining the precision and accuracy required for the data," Riciputi says. "We work with samples as small as 1 to 1000 nanograms for uranium and down to a femtogram for plutonium. A femtogram is a millionth of a nanogram, which is a billionth of a gram. Natural uranium is found everywhere due to fallout from aboveground testing of atomic bombs decades ago. We are trying to detect uranium and plutonium traces that were not removed during cleanup at a nuclear facility inspected by IAEA. The minimal sample may be all we get." Scientists use a mass spectrometer to separate all the isotopes in the sample based on differences in mass. "We simultaneously detect many isotopes using our state-of-the-art equipment," Riciputi says.
With funding from DOE's National Nuclear Security Administration, the ORNL group has enhanced the mass spectrometer's thermal ionization efficiency. By successfully developing a cavity ion source and coupling it with the magnetic sector of the mass spectrometer, Riciputi was able to detect 1 in every 4 uranium atoms present, much better than 1 in every 10,000 atoms of uranium, when the instrument was operating at its lowest efficiency.
The cavity ion source consists of a metal rod that replaces the traditional filament. The sample is placed in a hole bored into one end of the rod.
"Using electron bombardment, we can heat our metal rod 1000°C hotter than the filament, vaporizing more atoms in the sample and improving the efficiency at which electrons are stripped from the atoms in the vapor," Riciputi says. "As a result, more ions are introduced into the mass spectrometer for analysis."
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