Kenichiro Yasuda

Radionuclide release to stagnant water in the Fukushima-1 nuclear power plant1

After the severe accident at the Fukushima-1 nuclear power plant, large amounts of contaminated stagnant water have accumulated in turbine buildings and their surroundings. This rapid communication reports calculation of the radionuclide inventory in the core, collection of measured inventory in the stagnant water, and estimation of radionuclide release ratios from the core to the stagnant water. This evaluation is based on data obtained before 3 June 2011. The release ratios of tritium, iodine, and cesium were several tens of percent, whereas those of strontium and barium were smaller by one or two orders of magnitude. The release ratios in the Fukushima accident were equivalent to those in the accident of the Three Mile Island, Unit 2 (TMI-2).

Size distribution of radioactive particles collected at Tokai, Japan 6 days after the nuclear accident.

Airborne radioactive particles released by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in 2011 were collected with a cascade low-pressure impactor at the Japan Atomic Energy Agency (JAEA) in Tokai, Japan, 114xa0km south of the FDNPP. Size-fractionated samples were collected twice, in the periods of March 17-April 1, 2011, and May 9-13, 2011. These size-fractionated samplings were carried out in the earliest days at a short distance from the FDNPP. Radioactivity of short-lived nuclides (several ten days of half-life) was determined as well as (134)Cs and (137)Cs. The elemental composition of size-fractionated samples was also measured. In the first collection, the activity median aerodynamic diameter (AMAD) of (129m)Te, (140)Ba, (134)Cs, (136)Cs and (137)Cs was 1.5-1.6xa0μm, while the diameter of (131)I was 0.45xa0μm. The diameters of (134)Cs and (137)Cs in the second collection were expressed as three peaks at <0.5xa0μm, 0.94xa0μm, and 7.8xa0μm. The (134)Cs/(137)Cs ratio of the first collection was 1.02 in total, but the ratio in the fine fractions was 0.91. A distribution map of (134)Cs/(137)Cs - (136)Cs/(137)Cs ratios was helpful in understanding the change of radioactive Cs composition. The Cs composition of size fractions <0.43xa0μm and the composition in the 1.1-2.1xa0μm range (including the AMAD of 1.5-1.6xa0μm) were similar to the calculated compositions of fuels in the reactors No. 1 and No. 3 at the FDNPP using the ORIGEN-II code. The Cs composition collected in May, 2011 was similar to the calculation results of reactor No. 2 fuel composition. The change of Cs composition implies that the radioactive Cs was released from the three reactors at the FDNPP via different processes.

Depositional records of plutonium and 137Cs released from Nagasaki atomic bomb in sediment of Nishiyama reservoir at Nagasaki

In a sediment core of Nishiyama reservoir at Nagasaki city, depth profiles of (240)Pu/(239)Pu isotopic ratio, (239+240)Pu and (137)Cs activities were determined. Sediments containing plutonium and (137)Cs, which were deposited immediately after a detonation of Nagasaki atomic bomb, were identified in the core. Observed below the sediments were macroscopic charcoals, providing evidence for initial deposit of the fallout of the Nagasaki atomic bomb. This is the first entire depositional records of plutonium and (137)Cs released from the Nagasaki atomic bomb together with those from atmospheric nuclear tests.

Analysis of plutonium isotope ratios including 238Pu/239Pu in individual U–Pu mixed oxide particles by means of a combination of alpha spectrometry and ICP-MS

Isotope ratio analysis of individual uranium-plutonium (U-Pu) mixed oxide particles contained within environmental samples taken from nuclear facilities is proving to be increasingly important in the field of nuclear safeguards. However, isobaric interferences, such as 238U with 238Pu and 241Am with 241Pu, make it difficult to determine plutonium isotope ratios in mass spectrometric measurements. In the present study, the isotope ratios of 238Pu/239Pu, 240Pu/239Pu, 241Pu/239Pu, and 242Pu/239Pu were measured for individual Pu and U-Pu mixed oxide particles by a combination of alpha spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). As a consequence, we were able to determine the 240Pu/239Pu, 241Pu/239Pu, and 242Pu/239Pu isotope ratios with ICP-MS after particle dissolution and chemical separation of plutonium with UTEVA resins. Furthermore, 238Pu/239Pu isotope ratios were able to be calculated by using both the 238Pu/(239Pu+240Pu) activity ratios that had been measured through alpha spectrometry and the 240Pu/239Pu isotope ratios determined through ICP-MS. Therefore, the combined use of alpha spectrometry and ICP-MS is useful in determining plutonium isotope ratios, including 238Pu/239Pu, in individual U-Pu mixed oxide particles.

Automatic sequential separation with an anion-exchange column for ultra-trace analysis of Pu, U, Th, Pb, and lanthanides in environmental samples

Sequential separation of 12xa0pg Pu and 50xa0ng of U, Pu, Th, Pb, and lanthanides in a calibration solution, withxa0>93xa0% yield for the objective elements, was achieved within 6.3xa0h by an automatic system using a small anion-exchange column. The elements of interest in tree ring samples extracted from an 88-year-old pine tree were separated using this system, and 0.09xa0ngxa0g−1 U was detected in the dry sample by inductively coupled plasma mass spectrometry. This separation technique is useful for ultra-trace quantitative and isotopic analyses of elements in small environmental samples.