Presentation Determination of Pu isotopes in soil samples affected by FDNPP accident

KIEREPKO, Renata  ,  Hideki, Arae  ,  Sorimachi, A.  ,  Sahoo, S.K.

Introduction: Plutonium as the other actinides was distributed in the environment as a result of nuclear testing, nuclear fuel production and reprocessing, nuclear accidents (Windscale – 1957, Rocky Flats - 1969, Chernobyl - 1986, etc.) and due to a lesser extent accidental releases. On 11 March 2011, the gigantic tsunami in the northeast Japan region caused the serious damages of the Fukushima Dai-ichi nuclear power plant (FDNPP) resulted in large radionuclides releases to the atmosphere. The great public attention was and still is attracted by Pu which being recognized as a one of the most radiotoxic elements. The previous simulation (Povinec et. al., 2014) allowed to found total amount of Pu isotopes released into the atmosphere at the level 1012 Bq (including 238Pu, 239Pu, 240Pu, 241Pu). Pu aerosols due to wet and dry deposition contaminated environment. It has become necessary to investigate an impact of this actinide on the ecosystems. The first step in that case was to determine the activity level of Pu isotopes in samples material and second to find isotopic composition like: 238Pu/239+240Pu identifies as a fingerprint of nuclear accident. According to Yamamoto et al., 2012 this specific activity ratio for Japanese background (global fallout) was close to 0.03 before FDNPP disaster. Every higher result than those, can be considered as a signal of burned nuclear fuel and due to presence of the other artificial radionuclides (e.g. 137Cs, 134Cs, 90Sr, etc.) can be linked to the Fukushima accident. This study focused on the determination of Pu isotopes in soil samples collected during first few days after FDNPP accident. Material and Methods: The surface (0-10cm) layer soil samples from Fukushima Prefecture were collected during the first few days after FDNPP accident (March 2011). All samples were dried at the temperature 1050 C overnight and passed through 2 mm mesh sieve. Subsequently, samples were placed in the polypropylene cylindrical containers for gamma spectrometric measurements which were carried out at the National Institute of Radiological Sciences (NIRS) using p – type coaxial HPGe detector (ORTEC, GEM 100210) with 100% relative efficiency and resolution of 1.9 keV (FWHM) at 1332.5 keV. The measurement time was relatively short and equaled 1h. After that, samples were homogenized using a ball mill <150μm size and about 10 g of each powdered sample was ashed at 600 0C for organic matter decomposition. Samples were spiked with 242Pu tracer and digested using HF, HNO3 and HCl. Chemically Pu fraction was separated from the aliquot on anion-exchange column filled by DOWEX 1x8 mesh 200-400 (Kierepko et. al., 2009). The alpha spectrometry sources were prepared by NdF3 co-precipitation method (La Rossa et. al., 1992). Pu isotopes activity results were obtained by alpha spectrometry (ALPHA ENSEMBLE – 8 ORTEC) equipped with PIPS detectors with 450 mm2 active area.Results and Discussion: Despite of the main goal of our study that was detection of Pu isotopes in soil material, all samples were subjected to gamma spectrometry measurements for radiocaesium activity determination. We could measure 134Cs as well as elevated level of 137Cs, the obvious signature of the FDDNP accident. The activity concentration of 137Cs ranged between 0.460 ± 0.029 kBq kg-1 and 113.2 ± 7.4 kBq kg-1 for the reference date 11 March 2011 whereas the activity ratio 134Cs/137Cs was close to 1. Isotopic composition of Pu will be presented at the conference. References: 1.Povinec, P.P., et. al., (2014), Fukushima Accident, Radioactivity Impact on the Environment, Elsevier, Amsterdam.2.Yamamoto, M. et. al., (2012), An early survey of the radioactive contamination of soil due to the Fukushima Dai-ichi Nuclear Power Plant accident, with emphasis on plutonium analysis, Geochem. J. 46, 341-353.3.LaRosa, J.J, et. al., (1992), Radiochemical methods used by the IAEA’s Laboratories at Seibersdorf for the determination of 90Sr, 144Ce and Pu radionuclides in the environment samples collected for the International Chernobyl Project, J. Environ. Radioact., 17, 183-209.4.Kierepko, R., et. Al., (2009), Plutonium traces in atmospheric precipitation and in aerosols from Krakow and Bialystok, Radiochim. Acta, 97, 253-255.
Convener, Steering Committee IARPIC-2016 Radiation Safety Division

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