T. Okai

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.

TRITIUM CONCENTRATION IN OCEAN

Surface seawater and water vapor about 10 m above the sea level were collected in the Pacific and Indian Oceans during the expedition of KH-96-5 to examine tritium concentrations in open sea. The tritium concentration in the water vapor was one order of magnitude higher than that in the surface seawater, attributed to downward movement of naturally occurring tritium from stratosphere to troposphere.

Measurement of krypton-85 in the atmosphere with a portable apparatus

A portable apparatus was developed for the rapid collection, separation and counting of krypton-85 in the environmental air samples. The apparatus consists of a molecular sieve trap for the elimination of H2O and CO2, a charcoal trap for the collection of krypton, a chromatographic column for the isolation of krypton in the air sample and a vial for the dissolution of krypton in the scintillation solution. With this apparatus, krypton can be collected from half a cubic meter of air with 94% recovery in about 3 hrs. The measurements of krypton-85 have been done several times in Fukuoka area between the periods Jan. 30, 1981 and Dec. 9, 1981. And then, the krypton-85 concentration was found to be approximately 21 pCi/m3 air. The results were compared with those obtained by foregin investigators.

Tritium in a pine forest ecosystem: Relation between fresh pine needles, organic materials on a forest floor and atmosphere

Tritium concentrations were analyzed in organic and water fractions of fresh pine needles, dead and degraded pine needles accumulating on a pine forest floor to examine tritium cycle in a forest ecosystem. Tritium concentration was higher in the organic fraction of dead and degraded pine needles compared to others, suggesting two tritium sources. Rain is responsible for water fractions in the samples, while atmospheric hydrogen and methane are speculated to be responsible for high tritium level in the organic fraction of dead and degraded pine needles.

Analytical method for atmospheric tritium with a portable tritium sampling system

A portable tritium sampler was developed for the stepwise collections of water vapour (HTO), hydrogen (HT) and hydrocarbons (CH3T) in the atmosphere. First, water vapour was collected in an electronic cooler and an HTO collection column containing 400 g of molecular sieve. Next, dried air was introduced into an HT collection column containing 150 g of palladium catalyst. Hydrogen was then converted to water by catalytic oxidation at room temperature and the resultant water was immediately adsorbed on the molecular sieve bed supporting the catalyst. The remaining gas was finally introduced into a CH3T collection column containing 100 g of molecular sieve through a platinum catalyst column, in which hydrocarbons were burnt at 400°C. The resultant water was adsorbed in the CH3T collection column. The collection efficiencies of water in the HTO, HT and CH3T collection columns were all estimated to be nearly 100%. This newly developed method was found to be useful for the routine tritium monitoring by applying it to actual air samples.

An improved apparatus for the determination of krypton-85 and xenon-133 in an emergency situation

A simple and portable apparatus was developed for measurements of85Kr and133Xe that would be released into the atmosphere in an emergency situation of nuclear facilities. The method is based on cryogenic adsorption of these gases on charcoal followed by chromatographic separation from other gases. The85Kr and133Xe recovered from atmospheric air are determined separately by liquid scintillation counting. It takes about 1 hour for the stepwise determination of85Kr and133Xe. The atmospheric concentration of 3·10−3 μCi per m3 air (1.1·102 Bq/m3 air) is measurable for both nuclides with 20% counting error.

Variation of tritium concentration in the course of the degradation of fresh pine needles on a forest floor

Variation of tritium concentration was examined for 100 days in the course of degradation of fresh pine needles, which were left on a pine forest floor. No difference was observed on free water tritium (FWT) and organically bound tritium (OBT) concentrations of sterilized samples by gamma-ray irradiation or fumigation and control samples, attributable to incomplete sterilization. The OBT concentrations did not increase within the experimental period as the level of humus collected from the forest floor. The results suggest that a longer degradation time, more than 100 days, is necessary to elevate OBT up to the level, which is observed in the general environment.

Uptake kinetics of deuteriated water vapor by plants: Experiments of D2O release in a greenhouse as a substitute for tritiated water

Potted plants were exposed to D2O vapor in a greenhouse and uptake of D2O by leaf and deposition of D2O to pot soil were examined. Atmospheric D2O concentration in the greenhouse increased rapidly after starting the release and reached constant level in a few hours. Although the variation of D2O concentration in leaf followed that in air with showing a time delay, D2O concentration in leaf did not become the same level as that in air and vein showed lower concentration that lamina. D2O concentration in the pot soil increased slowly with diffusing in deeper layer.

Simple measurement of 14C in the environment using a gel suspension method

A simple analytical method for environmental14C with a low background liquid scintillation counter was developed. We used a new gelling agent, N-lauroyl-L-glutamic-α,γ-dibutylamide, for the liquid scintillation counting of14C as CaCO3 (gel suspension method). Our procedure for sample preparation was much simpler than that of conventional methods and required no special equipment. The samples prepared with the standard sample of CaCO3 were measured to evaluate the self absorption of the sample, the optimum condition of counting and the detection limit. Our results indicated that the newly developed technique could be efficiently applied for the monitoring of environmental14C.

Evaluation of the self-absorption of 14C beta-rays in gel-suspension samples by Monte Carlo simulation

In order to investigate the self-absorption of the β-rays from 14C in a gel-suspension sample, the Monte Carlo code, simulating the sequence of stages occurring in the sample, has been developed. The trajectory of the electron was calculated by the continuous slowing down approximation and the multiple Coulomb scattering theory. The effects of the self-absorption, strong quenching and particle size distribution of calcium carbonate on the output counting efficiency and the shape of the energy spectrum were evaluated.