Determining the source of plutonium contamination
A paper published in February by scientists from the JRC Institute for Reference Materials and Measurements (IRMM) represents a significant advance in the 'fingerprinting' of plutonium contamination, giving nuclear safety and safeguards authorities more information on how nuclear material was produced and helping to identify sources of environmental contamination.
Using a technique known as Thermal Ionization Mass Spectrometry (TIMS), the researchers measured the ratio of plutonium isotopes with unprecedented accuracy in reference samples obtained from the International Atomic Energy Agency (IAEA) and the U.S. National Institute of Standards and Technology, and also on soil and moss samples collected from the site of the Chernobyl accident.
Plutonium (Pu) isotope ratios vary depending on the source of the contamination. For example, weapons-grade plutonium is characterised by a low content of the 240Pu isotope (<7%) with 240Pu/239Pu ratios between 0.03-0.07, whereas plutonium from nuclear reactors typically has a ratio of 0.4 or higher. This is because the nuclear fuel in a reactor is irradiated by neutrons for a longer time. The average 240Pu/239Pu ratio for global fallout, originating from nuclear weapons tests in the past, is 0.176 ± 0.014.
By measuring these isotope ratios of samples collected in the field, nuclear safeguards and safety authorities can determine whether any plutonium contamination originates from power plant accidents, nuclear weapons testing or discharges from reprocessing facilities.
The researchers published data with considerably low uncertainties not only for the 'major' isotope ratio of 240Pu/239Pu, but also – for the first time – for the minor ratios of 241Pu/239Pu and 242Pu/239Pu. The results on the reference materials and the samples from Chernobyl confirmed the suitability of the measurement technique, and the researchers could identify with confidence whether the environmental contamination originated from reactor-grade or weapons-grade plutonium.
The development of a technique for isotopic measurements of plutonium with Thermal Ionization Mass Spectrometry has lead to more precise isotope measurements of environment samples containing low amounts of plutonium. The published work represents a further advance in environmental sample analysis to strengthen the effectiveness and improve the efficiency of nuclear safeguards in line with global nuclear non-proliferation objectives.