Fission product activity ratios measured at trace level over France during the Fukushima accident

The nuclear accident of Fukushima Dai-ichi (Japan) which occurred after the tsunami that impacted the northeast coasts of Japan on March 11th, 2011 led to significant releases of radionuclides into the atmosphere and resulted in the detection of those radionuclides at a global scale. In order to track airborne radionuclides from the damaged reactors and to survey their potential impact on the French territory, the French Institute of Radiation Protection and Nuclear Safety (Institut de Radioprotection et de Sureté Nucléaire IRSN) set up an enhanced surveillance system to give quick results as needed and later give quality trace level measurements. Radionuclides usually measured at trace levels such as 137Cs and in a very sporadic way 131I were reported. Radionuclides that we had never measured in air since the Chernobyl accident: 134Cs, 136Cs, the mother/daughter pairs 129mTe-129Te and 132Te-132I, and 140La (from the mother-daughter pair 140Ba- 140La) were also reported. Except the 131I/137Cs ratio, activity concentration ratios were constant. These ratios could be used to help source term assessment, or as data for transfer studies realized after the passage of contaminated air masses, typically using the 134Cs/137Cs ratio. ► Measurements by low-level gamma-ray spectrometry of many rarely measured fission products, besides 134Cs, 137Cs and 131I: 136Cs, 129mTe-129Te, 132Te-132I and 140La. ► Activity concentrations determined for more than 2 months over the whole French territory. ► Constant radionuclide ratios characterized except for 131I and 140La.