Study on chemical separation method for the analysis of isotope ratio of 135Cs to 137Cs in soil
Asako Shimada, Taro Shimada, Seiji Takeda, Tetsuji Yamaguchi
Nuclear Safety Research Center, Japan Atomic Energy Agency
In order to confirm the completion of the decommissioning of a facility, the radioactivity concentration of soil at the site has to comply with criteria for site release. However, extensive area in the eastern Japan was contaminated by radiocesium as a result of the accident at the Fukushima Daiichi Nuclear Power Station. It is essential the contamination by radiocesium during operation and decommissioning of the facility is discriminated from that by the fallout due to the accident for the decision of site release.
The isotope ratio of 135Cs to 137Cs could be used to determine the origin of 137Cs. Zheng et al. reported an analytical method to estimate the isotope ratio of 135Cs/137Cs in soil contaminated by the accident, which involved the ammonium molybdophosphate (AMP)-selective adsorption of Cs and subsequent two-stage ion-exchange chromatographic separation, followed by the detection of 135Cs and 137Cs via triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS) . Nuclides that could possibly interfere during the measurement of 135Cs and 137Cs using ICP-MS/MS are 135Ba, 137Ba, 95Mo40Ar, 97Mo40Ar, 119Sn16O, and 121Sb16O. Zheng et al. minimized the influence of Mo from AMP by using a collision cell with N2O gas. However, N2O gas has been designated as a banned substance in Japan since February 28, 2016. Thus, we developed a scheme for the chemical separation of Cs that does not use AMP.
Environmental soil contains approximately 628 ppm of Ba, 1.1 ppm of Mo, 2.1 ppm of Sn, and 0.4 ppm of Sb ; our sample solution contained similar amounts of Ba, Mo, Sn, and Sb in a 1 M HCl solution. The major elements and Ba were crudely separated by co-precipitation with iron hydroxide and carbonate by adding 10 mg of Fe as FeCl3, 0.2 g of NaHCO3, and NH3 solution and subsequent boiling to grow the precipitation, followed by suction filtration. In this instance, anionic Mo, Sn, and Sb were also roughly removed by an Anion-SR disk set under a filter paper. The transfer rates of Ba, Sn, and Sb into the filtrate were less than 3%, 1%, and 18%, respectively. There was a considerable variation in that of Mo. The filtrate containing Cs was evaporated to dryness, the residue dissolved in a 3 M HNO3-H2O2 solution, and then, refluxed for 5 h to remove the ammonium salt. After evaporation and dissolution in 3 M HNO3, Cs was purified by solvent extraction using calixarene-bis(t-octylbenzo-crown-6) in 1-octanol.
This research project has been conducted as the regulatory supporting research funded by Secretariat of Nuclear Regulation Authority (S/NRA/R), Japan.
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