N. Momoshima

Overview of tritium: Characteristics, sources, and problems

Tritium has certain characteristics that present unique challenges for dosimetry and health-risk assessment. For example, in the gas form, tritium can diffuse through almost any container, including those made of steel, aluminum, and plastics. In the oxide form, tritium can generally not be detected by commonly used survey instruments. In the environment, tritium can be taken up by all hydrogen-containing molecules, distributing widely on a global scale. Tritium can be incorporated into humans through respiration, ingestion, and diffusion through skin. Its harmful effects are observed only when it is incorporated into the body. Several sources contribute to the inventory of tritium in our environment. These are 1) cosmic ray interaction with atmospheric molecules; 2) nuclear reactions in the earths crust; 3) nuclear testing in the atmosphere during the 1950s and 1960s; 4) continuous release of tritium from nuclear power plants and tritium production facilities under normal operation; 5) incidental releases from these facilities; and 6) consumer products. An important future source will be nuclear fusion facilities expected to be developed for the purpose of electricity generation. The principal health physics problems associated with tritium are 1) the determination of the parameters for risk estimation with further reduction of their uncertainties (e.g., relative biological effectiveness and dose-rate dependency); 2) risk estimation from complex exposures to tritium in gas form, tritium in oxide form, tritium surface contamination, and other tritium-contaminated forms, with or without other ionizing radiations and/or nonionizing radiations; 3) the dose contributions of elemental tritium in the lung and from its oxidized tritium in the gastrointestinal tract; 4) prevention of tritium (in oxide form) intake and enhancement of tritium (oxide form) excretion from the human body; 5) precise health effects information for low-level tritium exposure; and 6) public acceptance of tritium leakage and waste disposal from reactors and fuel reprocessing plants.

Recent upward shift of the deep convection system in the Japan Sea, as inferred from the geochemical tracers tritium, oxygen, and nutrients

The thermohaline circulation system in the Japan Sea, an almost land-locked marginal sea in the northwest Pacific, is now in a transition state. Comparison of vertical profiles of tritium, oxygen and nutrients between 1984 and 1998 demonstrates the following two points: (1) the supply of dense surface seawater to the bottom layer (>2,500 m in depth) has almost ceased since 1984, and (2) the intrusion of surface seawater to an intermediate depth range (between ∼700 m and ∼2,000 m in depth) has been reinforced during this 14-year period. Such transitional upward shift of the thermohaline conveyor belt in the Japan Sea is thought to have separeted the bottom water from the conveyor belt and brought about a stagnant mode of the bottom water.

Atmospheric radionuclides transported to Fukuoka, Japan remote from the Fukushima Dai-ichi nuclear power complex following the nuclear accident

Radionuclides were detected from the Fukushima nuclear accident at Fukuoka, Japan, 1000 km west of the Fukushima Dai-ichi nuclear power complex. Iodine-131 was first detected 3 d after the accident, indicating that it was probably transported dispersively because of local meteorological conditions, and not global air circulation. The maximum concentrations, 5.07 mBq m(-3) for (131)I, 4.04 mBq m(-3) for (134)Cs, and 4.12 mBq m(-3) for (137)Cs, were recorded in particles collected on April 6, 2011. However, these concentration levels decreased below the detection limit by April 26, 2011. Gaseous (131)I accounted for 30%-67% of the total (131)I content. The increase in dose by inhalation was negligible at Fukuoka.

The radial distribution of 90Sr and 137Cs in trees

Abstract The distributions of 90 Sr and 137 Cs in tree trunks were measured for eight species. The concentration of 137 Cs was relatively uniform in the wood xylem and did not show any correlation with the temporal record of cumulative deposition of fallout in the northern hemisphere, indicating that 137 Cs was mobile in the wood xylem. The distributions of 90 Sr were classified into three groups. All of the gymnosperms—red spruce, eastern hemlock and white pine—showed a maximum 90 Sr specific activity in rings formed in the 1960s and the overall distribution was closely associated with the temporal record of the cumulative deposition of fallout. Three of five angiosperms—hickory, elm and American beech—showed similar distributions to the gymnosperms except that the maximum 90 Sr specific activity corresponded to the 1970s. The other two angiosperms—yellow poplar and sugar maple—did not show any correlation with the cumulative fallout deposition. The radial distribution of 90 Sr in gymnosperms was simulated considering a steady-state cycling of 90 Sr in the forest ecosystem. The model suggested that a few per cent of stable Sr are replaced annually in the nutrient pool of gymnosperms.

Identification of the contamination source of plutonium in environmental samples with isotopic ratios determined by inductively coupled plasma mass spectrometry and alpha-spectrometry

Concentrations of239+249Pu in environmental samples were detemined by ICP-MS and α spectrometry, showing consistent results, which suggests an applicability of ICP-MS to239Pu and240Pu measurement. The activity ratios of238Pu/239+240Pu and240Pu/239Pu were significantly different in samples from the general environment and near Sellafield reprocessing plants, indicating the usefulness of these ratios for identification of the Pu contamination source.

Tritium in pine needles and its significant sources in the environment

Abstract Tissue-free water tritium (TFWT) and tissue-bound tritium (TBT) were analyzed in pine needles collected at 21 locations of the general environment in Japan. The TFWT was recovered by freeze-drying and the TBT was obtained in the form of water by combustion of dried samples. Tritium was measured by liquid scintillation counting. The concentrations of TFWT were comparable with those in natural waters in Japan. The concentration of TBT, however, was higher than that of TFWT. This excess TBT may be attributed to direct assimilation of HT that is present in the atmosphere with extremely high specific activity.

An improved method for 85Kr analysis by liquid scintillation counting and its application to atmospheric 85Kr determination

Atmospheric (85)Kr concentration at Fukuoka, Japan was determined by an improved (85)Kr analytical method using liquid scintillation counting (LSC). An average value of 1.54 +/- 0.05 Bq m(-3) was observed in 2008, which is about two times that measured in 1981 at Fukuoka, indicating a 29 mBq y(-1) rate of increase as an average for these 27 years. The analytical method developed involves collecting Kr from air using activated charcoal at liquid N(2) temperature and purifying it using He at dry ice temperature, followed by Kr separation by gas chromatography. An overall Kr recovery of 76.4 +/- 8.1% was achieved when Kr was analyzed in 500-1000 l of air. The Kr isolated by gas chromatography was collected on silica gel in a quartz glass vial cooled to liquid N(2) temperature and the activity of (85)Kr was measured with a low-background LS counter. The detection limit of (85)Kr activity by the present analytical method is 0.0015 Bq at a 95% confidence level, including all propagation errors, which is equivalent with (85)Kr in 1.3 l of the present air under the analytical conditions of 72.1% counting efficiency, 0.1597 cps background count rate, and 76.4% Kr recovery.

Variation of atmospheric tritium concentrations in three different chemical forms in Fukuoka, Japan

Atmospheric tritium concentrations of tritiated water vapor (HTO), tritiated hydrogen (HT) and tritiated hydrocarbons (primarily tritiated methane, CH3T) have been measured in Fukuoka prefecture, Japan from 1984 to the present to establish a general database on the behavior of atmospheric tritium. HTO concentrations expressed in Bq/l-H2O vary within a range of 1.19 to 2.45, giving an overall average value of 1.86±0.077. HTO concentrations expressed in-mBq/m3-air vary within a range of 7.8 to 46.1 and have a strong correlation with the atmospheric humidity, being high in the summer and low in winter. In the case of HT and CH3T, no seasonal variations were observed with average monthly values of 23.1 to 61.0 mBq/m3-air and 8.3 to 23.9 mBq/m3-air, respectively. The present HTO concentrations are already close to the tritium level before nuclear testings. However, the present HT and CH3T concentrations are still higher by a factor of about 140 and 30, respectively, than those before the testings. Specific activities are estimated to be 14.6–16.7 TU for HTO, 5.5·105–1.0·106 TU for HT and 3.2·104–4·104 TU for CH3T. The apparent difference in the specific activities suggests a very slow transformation of these species in the atmosphere or a continuous supply of HT and CH3T with high specific activity. Residence time for atmospheric HT was found to be 6.5 years over the period 1988–92 and 10 years for 1988–95. These times are longer than 4.8 years given by Mason and Östlund in the 1970s, and thus indicate a supply to the atmosphere of HT from various tritium sources.

DETERMINATION OF TECHNETIUM-99 FROM THE ASPECT OF ENVIRONMENTAL RADIOACTIVITY

A radioanalytical procedure has been developed for the determination of99Tc in environmental samples. The procedure consists of precipitation, solvent extraction, ion exchange, electrodeposition and radiation measurement. Rhenium was used as a non-isotopic carrier of99Tc. On the basis of 3σ counting error, the detection limits were 4.9·10−5 Bq/l, 7.4·10−3 Bq/kg wet and 7.4·10−2 Bq/kg dry for water, biota and soil samples, respectively. Sea water, seaweeds (brown algae) and soils were collected to evaluate the present levels of99Tc in Japan. The level of99Tc in sea water was ca. 1·10−4 Bq/l around Japan. Among the seaweeds (brown algae), Ishige okamurai showed the highest concentration of 5.8·10−2 Bq/kg wet tissue and the highest concentration factor of 583. The level of99Tc in the organic rich surface soil was ca. 1 Bq/kg dry soil in Fukuoka.

Biologically induced Po emission from fresh water

Behavior of Po in fresh waters was examined in laboratory culture experiments using fresh water collected from a small pool, Xi river and Xiqing lake, showing formation of volatile Po compounds followed by emission to air. Addition of tryptone to the fresh water cultures increased the emission of Po considerably along with a growth of microorganisms, suggesting a connection of chemoheterotrophs to Po emission. Participation of photoautotrophs was also considered because Po emission was increased when NaHCO3 was added to the fresh water cultures. The emission behavior of Po and S in these experiments appeared in different ways. The quantity of Po emitted was comparable to the previous culture experiments (Momoshima, Song, Osaki & Maeda, Environ. Sci. Technol., 35, 2956-2960, 2001) in which artificial culture medium containing 3% NaCl was used and inoculated with sea sediment extract. The biological support for Po emission, thus, would be a general phenomenon in fresh water as well as a seawater environment and is possibly a source for atmospheric Po.