H. Kakiuchi

N. Momoshima

T. "Okai

Y." Sakuma

National Institute for Fusion Science (NIFS-Repository)

§2. Study on Environmental Tritium: Development of Monitoring Method and Analysis of Factors Control I ing the Variation Okai, T. (Faculty of Eng., Kyushu Univ.), Momoshima, N., Kakiuchi, H. (Kyushu Univ.), Sakuma, Y. National Institute of Fusion Science (NIFS) is now constructing a new fusion device and related facilit.ies at Toki city in Gifu prefecture, Japan. Although no tritium burning experiments ,,,,,ill be carried ou t in the Toki site, it should be appropriate and useful to establish the general database on behavior of atmospheric tritium in and around an actual place of the experiments. Atmospheric tritium consists of three different chemical forms, tritiated water vapor (BTU), tritiated hydrogen (HT) and tritiated hvdrocarbons (primarily tritiated methane, CH3T). Th ~s, tritium concentrations of atmospheric HTO, HT and CH:lT have been measured in the Toki site a few times a veal' from 1991 to the present. .. Fig.] shows variation of average annual tritium concentrations for atmospheric HTO, BT and CH3T in the Toki site. The present HTO concentrations are 1.4 Bq/L-H 20 and close to the tritium concentration in Kobe In 1953. Since the Japanese islands are surrounded by oceans with low tritium levels atmospheric BTO would be easily diluted by wate; vapor from sea water and its concentration would become low quickly. Thus we conclude that the present HTO concentrations have already decreased to the levels before nuclear testings. HT concentrations gradually decreased from 1992 to 1997 and residence time for atmospheric BT are calculated to be about 9 years for 1992-98. Mason and Ostlund reported an average residence time for atmospheric H T of 4.8 years. This value would result from the physical decay of tritium and atmospheric HT removal processes. The dominant phenomenon among these removal processes is the conversion ofHT to HTO by 0 xidation in surface soils. HT oxidation in soil is usually parameterized by a deposition velocity and the deposition velocity is mainly dependent on the water content of the soil, while temperature is onlv of secondary importance . Assuming that the ave~rage waLeI' content of soil was almost constant for 1963-1998 on the whole earth, removal of atmospheric HT by the soil must have remained constant. Therefore. the residence time of 9 years longer than the 4.8 years sug.gest t hat atmospheric H T has been supplied from varIOUS sources. BT concentrations have decreased by a factor of about sevpn since the high tritium level in 1963. If, during t he period 1963-1998. BT concentrations had decreased at a rate corresponding to Mason and Ostlund's residence time estimation of 4.8 years, then currently we would expect a level of about 3 mBq/m~Lair. However, the present HT levels of about 24 mBq/m3-air are distinctly higher than those expected. Since atmospheric HT has the highest specific activity of the three tritium species, transformation 0 f H T from the other tritium species can not possibly explain the present H T concentrations in t he a tmosphere. These facts would indicate the supply of BT from tritium ijources to the atmosphere. The present H T concentration is still higher by a factor of about 120 than that before the nuclear testings. On the other hand, the present CH:/f concentrations are higher by a factor of about 30 compared to values observed In the southern hemisphere in 1953. Methane gas is chemically degraded in the atmosphere by the reaction with OB radicals and it has a residence time of 12 veal's. Therefore, the residence half] ife of atmospheric ·C H:IT is estimated to be about 6 years in addition to the physical decay of tritium. Decrease in atmospheric CB 3T concentrations was not apparent from our observations. The fact that, unlike HT concentrations CHaT concentrations have not been decreasing is 0 n~ of the most interesting problems in our study. The causes could be the transformation of HT into CH:JT with less specific activity or a continuous release of CH;:JT from t.ritium sources. The oxidation of atmospheric HT by mjcroorganisms in soil has continuously added HTO to the soil surface and the HTO h as high specific activity of tritium. When the HTO was transformed into CB3T by methane-producing bacteria in the soil, the resultant CRsT should have high specific activity and could be a source of atmospheric CHJT. This is reasonable , because the specific activity of BT is one order of magnitude higher than that of CH:iT. .cp 0 r.N ME 0-~ 3 30 al E C" al r:::.-c:- HT: .2 .2 ~ ~ 2 _. - - ~.- 20 c: CI) u E r:::. Q) 0 u 0 c: I-0 r.M 0 1 10 () 0 · CHT "0 ..... • 3 r:::. r. ctl I-o t r. 0 1990 1992 1994 1996 1998 2000 Year Fig.l Annual average tritium concentrations 393 

Study on Environmental Tritium: Development of Monitoring Method and Analysis of Factors Controlling the Variation

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