Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 117-120, 2007

Tritium in the Atmospheric Environment
N. Momoshima,*, a T. Yamaguchi,b T. Toyoshima,b Y. Nagao,b M. Takahashi,b M. Takamura,b andY. NakamurababRadioisotope Center, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Graduate School of Science and Technology, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan

Received: October 27, 2006; In Final Form: April 26, 2007
Three chemical forms of atmospheric tritium, i.e., tritiated water vapor (HTO), hydrogen gas (HT), andhydrocarbons (CH 3T), were measured at Kumamoto, Japan during the period between 2003 and 2005. The
average concentrations were in the order of HT, CH3T, and HTO. The yearly average HT concentrations were notso different in each year, while that of CH3T showed a large variation. The concentrations of all chemical species
have decreased compared to those measured during the period between 1984 and 1995 at Fukuoka, about 100 km
north of the present sampling location, suggesting a decrease in emissions of HT and CH3T from their sources forthese 8–10 years. The decrease in HTO concentration would be attributed to the decrease of the bomb-produced
tritium and to a regional characteristic of the sampling locations; Kumamoto showed lower tritium concentrations
in rain than Fukuoka.

1. Introduction
The production rate of natural tritium, produced constantlyin the upper atmosphere by nuclear reactions between secondary cosmic rays and nitrogen or oxygen atoms, is estimated tobe 0.2 to 0.25 tritium atom cm–2 s–1 which yields 1–1.3 EBq tritium inventory in the world.1 The naturally occurring tritiumhas been found in various environmental materials which contain hydrogen atom as a constituent element. The concentration of tritium in the environment had increased by nuclear
tests in 1950s–1960s that had released a large amount of artificial tritium to the environment. The amount of tritium emitted
from the nuclear tests would be 240 EBq1 and we can stilldetect the bomb tritium in environmental samples such as
underground water more than 40 years after the test ban treaty
in 1963. Peaceful use of nuclear energy has also increased
environmental tritium concentrations such as around nuclear
power stations and nuclear fuel reprocessing plants due to controlled release of tritium as airborne and liquid effluents.
Tritium is found in the atmosphere as 3 different chemicalforms2 that are water vapor, hydrogen, and hydrocarbon. We
report atmospheric tritium concentrations of 3 chemical species measured in the general environment of Japan.

measured by a mass flow meter. Water vapor is at first collected on molecular sieve 3A (MS-3A) in an HTO column
made of glass, 50 cm in length and 5 cm in diameter, and filledwith 500 g of MS-3A. The water vapor content in air varies
with season; in summer it sometimes exceeds the MS-3A
adsorption capacity packed in the HTO column. Then an HTO
cold trap, being made of glass and cooled at 2 °C, is placed justbefore the HTO column to avoid breakthrough of the HTO column.
Air inlet
Air filter 7 µm
Mass flow meter
Cold trap 2 °C
HTO column (MS-3A)
Generation of H2 by electrolytic
decomposition of tritium free waterDe-humidification column (MS-3A)

2. Experimental
2.1. Sampling apparatus. Although three chemical speciesof tritium are found in the atmosphere, it should be noted that
our measurement technique does not distinguish between theisotopic configurations of each chemical species.3 The chemical species, “HTO” includes DTO and T2O, “HT” includes DTand T 2 , and “CH3T” all volatile gaseous hydrocarbons. The
sampling apparatus of atmospheric tritium is designed to collect successively 3 different chemical forms: in the order ofHTO, HT, and CH3T. The block diagram of the atmospherictritium sampling apparatus is illustrated in Figure 1. Air is
introduced to the sampling system with a diaphragm pumplocated at the last position of the system. An air filter removes
dust in the air and then the volume of air and the flow rate is*Corresponding author. E-mail: momorad@mbox.nc.kyushu-u.
ac.jp. Fax: +81-92-642-2706.

Pt honeycomb catalysts heated at 100 °C
HT column (MS-3A)
Tritium free bomb CH4
Pd catalyst heated at 300 °C
CH3T column (MS-3A)
Diaphragm pump

Figure 1. A block diagram of atmospheric tritium sampling apparatus.

© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

118 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
Hydrogen gas is converted to water by catalytic oxidationand collected as water on MS-3A in an HT column. The designof the HT column is the same as that of the HTO column and500 g of MS-3A is packed in the column. The hydrogen concentration in air is about 0.5 ppm which would yield a quite
small amount of water and it makes difficult to quantitatively
recover water from the MS-3A and to measure tritium concentration by the liquid scintillation counting. To increase theamount of water available, tritium free H 2 that was generated
by electrolytic decomposition of deep well water is added tothe sampling air after the HTO column. Water vapor accompanied with the generated H2 is removed with a de-humidification column which is the same as the HTO column and packed
with 500 g of MS-3A. The H2 generation rate is controlled toyield about 15 mL of water with oxidation of H2. The original
sampling apparatus adopted Pd coated MS catalyst for H 2 oxidation which is prepared by vigorous shaking of PdCl3 solutionand MS-4A followed by baking in H 2 atmosphere.4 The Pd
coated MS catalyst undergoes H 2 oxidation and collection ofproduced water on the MS-4A simultaneously. To maintainthe oxidation capacity of the Pd coated MS catalyst, reactivation is carried out regularly by which H2 is flowed under heating the catalyst. To reduce the task of this reactivation process,we prepared Pt honeycomb catalysts for H2 oxidation, in which
0.12 g of Pt is dispersed on the surface of the honeycomb basemetal (25 mm in diameter, 40 mm in length). The apparatus
uses 3 pieces of the Pt honeycomb catalysts and is operated at100 °C. Hydrocarbons are oxidized to water with the Pd catalyst ( DASH-220D, 0.2 4 w t.% Pd, 3m m i n d ia meter,NECHEMCAT Co. Ltd.) at 300 °C. Tritium free bomb CH4 isadded to the sampling air through a mass flow controller before
the Pd catalyst column. The Pd catalyst column is made ofquartz and packed with the 150 cm3 Pd catalysts. The flow rateof CH4 is controlled to yield about 15 mL of water by the sampling. The water yielded is collected on 500 g of MS-3A in aCH 3T column as the case for the water vapor and hydrogen.
Teflon tubes (8 mm in outer diameter and 6 mm in inner diameter) and metal conjunctions (Swagelok Co. Ltd.) are used forconnecting the columns and other units. The performance ofthe catalyst has been reported elsewhere.3
2.2. Sampling and measurement. We collected air at aflow rate of 1.2 L min –1 which requires the 69.4 h samplingtime for the total of 5000 L air. The samplings were carriedout at Kumamoto, Japan (130°43’E, 32°48’N) at about 25 m
above the ground. Water adsorbed on the MS-3A is desorbed
by heating the MS-3A in the column at 400 °C with flowing N2gas. The desorbed water is recovered on a cold trap cooled at –9 °C with taking care to avoid contamination of the sample
water with ambient water vapor. About 15 mL of water is
expected to be recovered from each HT and CH 3T column.
The water recovered from the HTO column is combined withthat collected on the HTO cold trap. Tritium radioactivity ismeasured by the low background liquid scintillation counting
after mixing the water with the liquid scintillation cocktail(Ultima GOLDTM LLT, Packard). For the preparation of theHTO counting sample, the 50 mL HTO water and 50 mL scintillation cocktail is mixed in a polyethylene vial (AlokaPolyvial 145) of which internal wall is coated with Teflon. 10mL water recovered from the HT and CH3T columns are mixed
with the 10 mL scintillation cocktail in the Teflon vials to prepare counting samples. The background counting samples areprepared as the same way for either HTO or HT/CH3T using
tritium free deep ground water. No tritium in the ground water
was confirmed by an electronic enrichment technique. Eachsample is measured for 1300 min with a low background counter (Aloka, LB-III). The typical background count rate for theHTO samples was about 3.5 cpm and that for the HT/CH 3T
samples was about 2.8 cpm. Counting efficiencies were determined using standard tritium water (Nycomed Amersham),

Momoshita
showing 18.5% for HTO samples and 22.5 % for the HT/CH3T
samples.
3. Results and Discussion
The atmospheric concentrations of HTO, HT, and CH3T are
shown in Figures 2, 3, and 4, respectively. The samplings werecarried out 24 times in the period from June 2003 to June 2005.
The atmospheric concentrations of HTO, HT, and CH 3T are
distributed from a few to a few tens mBq m –3 and the differences in concentrations among species are not so large.
Average concentrations of HTO, HT, and CH3T in the experimental period are summarized in Table 1 and the order of theaverage concentration is HT > CH3T > HTO. The present tritium concentrations are lower than those of other gaseousradionuclides in the general environment such as 222Rn whichshows in the order of a few tens Bq m–3 (References 5–8), 85Kr
the order of a few Bq m–3 (Reference 9) and 14C the order of a
few tens mBq m–3 (Reference 10).
The distribution of HTO concentrations (Bq L –1) is shown inFigure 5. The change of atmospheric concentrations in Figure
2 will be related to the amount of water vapor in the atmosphere and the HTO concentration. In general the water vapor
content in the atmosphere is high in summer and low in winterbecause water vapor saturation is controlled by temperature.
The atmospheric concentration of HTO in Figure 5 showed aseasonal variation; the highest value was observed in August
2004 and the lowest one in December 2003 though the HTO
concentrations distributed widely from 0.1 to 1.3 Bq L –1. This
result suggests that the atmospheric concentration of HTO ismostly determined by water vapor content rather than its concentration (Bq L –1). The average HTO concentration was 0.69
± 0.35 Bq L –1 as shown in Table 1 that is significantly higher
than the average HTO concentration of rain, 0.30 ± 0.13 Bq L –1
observed at the same sampling location during the corresponding period of the present atmospheric samples.11 The higher
HTO concentration of water vapor than the rain would beattributed to the upward flux of HTO to the atmosphere fromthe soil surface where HTO concentration is elevated by microbial oxidation of the atmospheric HT and CH3T (Reference 12)
and to the downward flux of naturally occurring HTO from thestratosphere to the troposphere.13
The atmospheric concentrations of HT and CH3T in Figures
3 and 4 are lower than those measured in 1980s–1990s at
Fukuoka, Japan,2,14 which is located about 100 km north fromthe present sampling location. The present data revealed alarge variation in the concentrations, ranging from 3.6 to 33.5
mBq m –3 for HT and from 0.7 to 32.3 mBq m –3 for CH3T. A
degree of scatter of the data strongly depends on duration ofsampling; daily basis sampling gives larger scattering thansuch as weekly basis sampling. We collected the samples forabout 3 days basis sampling, the samples collected in 1980s–
1990s at Fukuoka also had showed large scatter for the HT andCH3T concentration, though they were taken by weekly basis
sampling. Neither seasonal variation nor change in yearly average concentration was observed in the present HT data: 12.6for 2003, 12.9 for 2004, and 11.4 mBq m –3 for 2005. On theother hand, yearly averages of CH3T concentrations were 14.1for 2003, 6.7 for 2004, and 3.0 mBq m–3 for 2005. The present
yearly average HT concentration, 11.4–12.9 mBq m –3 (2003–2005) was about half of that measured in 1995 at Fukuoka2 andalmost comparable to that measured in 2002 at Miami, USA(10.2 mBq m–3).15 The yearly average CH3T concentration measured in 1995 at Fukuoka2 was 12 mBq m–3 which was almost
equal to the present data of 2003.
The concentration order of the present data shown in Table 1
is HT > CH3T > HTO, which was different from that observed
at Fukuoka during the period between 1984 and 1995.2 In the1984–1995 period, the order was HT > HTO > CH3T; the CH3T

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 119

404035353030

CH3T / mBq m-3

HTO / mBq m-3

Tritium in the Atmospheric Environment

2520152520151010550

2003/1

2003/7

2004/1

2004/7

2005/102005/7

Figure 2. HTO concentrations in the atmosphere.

2003/1

2003/7

2004/1

2004/7

2005/1

2005/7

2005/1

2005/7

Figure 4. CH3T concentrations in the atmosphere.

40

2.535
225

HTO / Bq L-1

HT / mBq m-3

30

20
15

1.5

1

10
0.5
50

2003/1

2003/7

2004/1

2004/7

2005/1

2005/70

2003/1

2003/7

2004/1

2004/7

Figure 3. HT concentrations in the atmosphere.

Figure 5. HTO concentrations of atmospheric water vapor.

concentration was lowest among them. As mentioned above,the yearly average CH 3T concentrations of the present data
showed marked variation, probably suggesting a change of a
CH3T source intensity. Although the sources of CH3T are not
identified, emissions associated with activities of nuclear facilities would be involved. A regional characteristic, the present
data were taken at Kumamoto while the data between 1984 and1995 were taken at Fukuoka, would be one of the reasons forthe low HTO concentrations because tritium concentration inrain at Kumamoto were lower than that at Fukuoka.16 It has
been shown that very marked peaks in the HT concentrations
at Fairbanks, Alaska in 1973 are clearly correlated with large
underground events at the Novaya Zemlya test site, in the former USSR.17 One HT source from the late 1950s to the early
1990s was the testing of nuclear devices conducted underground.15 Tritium can escape from underground to the atmosphere as HT through natural and bomb-produced faults andfractures because low O2 environment would limit the formation of HTO. A coincident increase in yearly average HT concentration was observed at Fukuoka and Miami in 1988 andthe underground nuclear tests yield in 1988 was high; the fact
would be an indication that underground nuclear test is one ofthe major sources in atmospheric HT. The UNSCEAR18 estimated HT emission from the nuclear power industry (Fuel rodreprocessing, power plants) and the emission has been
increased. The HT observed in the present data would result
from the release from such nuclear industry.

TABLE 1: Concentrations of atmospheric tritium
Chemical species
HTCH3THTO
HTOConcentration
12.5 ± 6.9 mBq m–3
9.0 ± 8.2 mBq m–3
6.7 ± 5.4 mBq m–3
0.69 ± 0.35 Bq L –1

Acknowledgments. The work is performed with the support and under the auspices of the NIFS Collaborative Research
Program (NIFS04KOBS008).
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