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Old plutonium, new tricks In December 2004, while excavating an old waste-burial ground at the Hanford site in Washington state, a cleanup team unearthed a curious safe. The safe held a 1-gallon glass jug, partially filled with a slurry of white precipitate in a clear liquid. During World War II, the Hanford facility produced plutonium for Trinity, the first tested nuclear weapon, as well as for the atomic bomb dropped on Nagasaki, Japan; ␥ spectrometry revealed that the jug contained several hundred milligrams of plutonium. In a new article in AC (DOI 10.1021/ ac802286a), Jon Schwantes and colleagues at Pacific Northwest National Laboratory assess the significance of the mysterious jug’s contents using several methods to determine age, origin, and history. Schwantes calls this process “nuclear archaeology.” Schwantes and his team used Gamma Energy Analysis (GEA) to analyze three samples of the slurry and then filtered them. The liquid fraction underwent a second round of GEA and then was split into four samples. The solid retentate was dissolved in a mixture of nitric and boric acids, analyzed by GEA, and then split into five samples. One of each type of sample was archived. The other samples underwent a succession of experiments: ion chromatography (IC), inductively coupled plasma optical emission spectrophotometry (ICP-OES), and inductively coupled plasma MS (ICPMS). The researchers used ICP-OES to identify transition metals and IC to detect major anions, including F⫺, SO42⫺, PO43⫺, NO3⫺, and Cl⫺, which revealed details about how the plutonium was originally purified. Processed plutonium needs to be separated from spent materials, and the separation leaves behind traces of reagents. The researchers determined that this sample had been separated using the bismuth phosphate process, the only industrial-scale reprocessing method used in the U.S. during the 1940s. ICPMS and nuclear-counting methods were used to determine the concentrations 1724
ANALYTICAL CHEMISTRY /
MARCH 1, 2009
of parent (plutonium-238, -239, and -240) and daughter (uranium-234, -235, and -236) actinide isotopes in the sample to calculate its age. During these analyses,
A safe found during excavation of the Hanford site contained processed plutonium.
the researchers had to worry the uneven distribution of uranium and plutonium in the inhomogeneous slurry. This fractionation means that when the slurry was transferred into the jug, the plutonium/ uranium ratio within the new container might not have reflected the ratio in the previous container. Because age calculations are based on parent/daughter ratios, this problem could bias the sample’s calculated age. But because isotopes of the same element are not expected to fractionate significantly, the researchers were able to cancel out the fractionation factor by deriving equations for each of the three parent⫺daughter pairs. Determining the amount of plutonium-238 was a particular challenge. An ␣ spectrum of a retentate sample fortunately revealed a small but distinct plutonium-238 peak. “It was very exciting that we could detect plutonium-238 in the presence of much more 239 and 240,” says Schwantes. Using three parent⫺daughter pair ratios, the researchers put the age of the sample at ⬃62 years. The amount of parent and daughter isotopes also was useful in assessing the reactor of origin. “We don’t just have neutrons of one energy in a reactor,” ex-
plains Schwantes. “The neutron environment within the reactor itself is highly dependent on the type of reactor and type of fuel used.” That’s when the researchers got a surprise. Using reactor model simulations, they determined that the plutonium in the jug could not have come from the Hanford reactor. With the help of documentation, the sample was traced back to the first batch of plutonium ever separated by the world’s first industrial-scale reprocessing facility in Oak Ridge, Tenn. The analysis showed that the sample was the oldest reactorproduced plutonium collection that has been located to date. In 2006, the contents of the jug were repacked into two bottles (one contained the samples analyzed here). This repackaging led researchers to develop a novel nuclear archaeology method using sodium-22 as a signature of sample splittings. Sodium-22 has a very short half-life and is produced when ␣ particles from plutonium-239 crash into fluoride particles. The rate of sodium-22 production is a function of the amount of plutonium in the sampleOa key point. By modeling sodium-22 concentration as a function of time, the researchers were able to determine not only how long it had been since repackaging but also how much plutonium existed in the original sample, even without information about the sample before repackaging. This technique, Schwantes says, could be used in the future to investigate radioactive samples of unknown origin. —Erika Gebel
10.1021/AC900093B 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 02/02/2009
