analytical method for atmospheric tritium with a portable tritium sampling system

Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 130, No. 2 (1989) 399-407

ANALYTICAL METHOD FOR ATMOSPHERIC TRITIUM WITH A PORTABLE TRITIUM SAMPLING SYSTEM

T. OKAI,* Y. TAKASHIMA**

*Department of Nuclear Engineering, Faculty of Engineering, Kyushu University 36, ttakozaki, Higashiku, Fukuoka, 812 (Japan)

**Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka, 812 (Japan')

(Received August 26, 1988)

A portable tritium sampler was developed for the stepwise collections of water vapour (HTO), hydrogen (HT) and hydrocarbons (CH 3 T) 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 CH~T collection column cgntaining 100 g of molecular sieve through a platinum catalyst column, in which hydrocarbons were burnt at 400 ~ The resultant water was adsorbed in the CH~ T collection column. The collection efficiencies of water in the HTO, HT and CH~ T 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.

Introduction

Tritium originating from several different sources is present in the atmosphere in

various chemical forms, such as tritiated water vapour, tritiated hydrogen and tri-

tiated hydrocarbons. It is important to know the present background levels in each

chemical form of tritium in the atmosphere before large-scale tritium handling is

performed.

Monitoring systems for atmospheric tritium were reported by many investigators. 1 -4

Among these systems, OSTLUND's device is most popular in the community of tritium

investigators. The processes are based on the catalytic combustion of the atmospheric

hydrogen and immediate adsorption of resultant water on the molecular sieve bed

supporting the catalyst. Unfortunately, however, his system is too large for use in the

field work. Therefore, we intended to design a simple and handy sampler, which can

be applied ~o tritium monitoring in the environment. We first examined hydrogen

oxidation .and water vapour adsorption efficiencies on the palladium catalyst. Besides

Elsevier Sequoia S. A., Lausanne A kad~miai Kiad6, Budapest

T. OKAt, Y. TAKASHIMA: ANALYTICAL METHOD FOR ATMOSPHERIC

hydrogen, we also examined the oxidation efficiency of methane on the platinum

catalyst in relation with reaction temperature. On the basis of the experimental results, the sampler was designed and constructed

in order to collect the tritium in the form of water vapour (HTO), hydrogen (FIT) and hydrocarbons (CH3 T), successively. According to the" procedures mentioned above, we are not able to discriminate the isotopic configurations, hence HTO includes T~ O and DTO, HT includes DT and T2 iand CH3T includes all volatile

tritiated hydrocarbons, aldehydes and alcohols. Analytical results for actual air samples are also given.

Experiment~

Reagents and appara,'us

Molecular sieves (WAKO PURE CHEMICAL INDUSTRIES, LTD., one-sixteenth inch 4A pellet) were used as adsorbents for water vapour. Palladium catalyst pre-

pared by the same method as reported by OSTLUND and MASON a was used for

hydrogen oxidation and water vapour adsorption. The catalyst consists of molecular

sieves carrying finely dispersed palladium metal. Platinum catalyst (NIPPON ENGEL- HARD Ltd. 0.5% platinum-alumina pellet) was used for methane oxidation. Secondary standard tritium solution of 65 600 dpm/ml was prepared by diluting the standard

solution purchase d from the Radiochemical Center, Amersham, England. The solution was used to prepare the quenched standard samples for tritium counting efficiency. calibration. A commercially available scintillation cocktail (NEW ENGLAND NUCLEAR Ltd., Aquasol-II) was used as liquid scintillation solution. An ALOKA low background liquid scintillation counter, LB-1, specially developed for low level counting, was used. This equipment can be operated using 20 ml or 100 ml ~ teflon counting vials.

Measurement of apparent oxidation and adsorption efficiencies of hydrogen on the palladium catalyst surface

A test system for the performance of the palladium catalyst is shown in Fig. 1. Air was first passed through a filter and 400 g of molecular sieve, where all of

water vapour was removed. Dried H2(HT) from the electrolytic cell was then added

in the air stream. The mixture was passed through the palladium catalyst column 1 containing 80 g of catalyst, where H2 (HT) was converted to water under room temperature and the resultant water was immediately adsorbed on the molecular sieve. Hydrogen oxidation on palladium catalyst 1 was carried out within its capacity. Beyond the hydrogen oxidation capacity of the catalyst 1, excess H2 (HT) was

400

T. OKAI, Y. TAKASHIMA: ANALYTICAL METHOD FOR ATMOSPHERIC

introduced into the catalyst column 2 and adsorbed in the column after catalytic oxidation to water. On the other hand, beyond the water adsorption capacity of catalyst 1, excess water vapour was introduced into and adsorbed on the molecular sieve column.

Air catalyst 1 H mo a }"1 catalyst Z~.--.*ou t in 80g I | . . . . I |100 g l

~ ~ (HT) ['rrit ioted 6 ml/min woter

Electrolytic cell

Fig, 1. A test system for performance of palladium catalyst

Thus the hydrogen oxidation efficiency Eox and the water vapour adsorption efficiency E~t) were expressed as

Eox : C1/(Cx + C2) and

EA D = C l / (C l + CM),

respectively, where C~, C2 and CM are tritium ~ctivities in water adsorbed in the catalyst column 1, catalyst column 2 and molecular sieve column, respectively.

:Measurement of apparent oxidation efficiencies of methane on the platinum catalyst surface

A test system for the performance of the platinum catalyst is shown in Fig. 2. The mixture of 500 1 of air and 1 1 of methane in Balloon 1 was first dried by passing through a molecular sieve column at a flow rate of 2.0 ml/min by regulating a needle valve. The methane concentration in the mixture was 0.5% in volume. The

- - : - - ' 'l'-~'"iF~h%" H c~tQ~yst I . . . . I I l oo g ~ l v e

Air 500 I 2 I/rain / CH~ 11 1

Gas chromato- graphy

Fig. 2. A test system for performance of platinum catalyst

401


dry mixture was then passed through the platinum catalyst column which-was heated with an electric heater. Its temperature was changed from 325 to 400 ~ and regulated

within -+12 ~ by a temperature controller. Most methane was oxidized to water on

the catalyst and the resultant water was adsorbed in the following molecular sieve

column, while methane passed through without catalytic oxidation, was introduced

into Balloon 2. Consequently, the oxidation efficiency of methane was calculated

from the amount of methane in Balloon 2 and that in Balloon 1. The amount of

methane in Balloon 2 was determined by comparing the recorder response of 1 ml

methane monitored with a thermal conductivity detector. The experiments were

carried out until the combustion of 8 1 methane was completed.

Results and discussion

Apparent oxidation and adsorption efficiencies of hydrogen on the palladium catalyst surface

The results are shown in Fig. 3. The oxidation efficiency of hydrogen was estimated

to be 100% up to 13 I of hydrogen for 80 g of palladium catalyst. The adsorption

efficiency of the resultant water vapour was also estimated to be 100% up to 10.5 g of water for 80 g of palladium catalyst. The other experiments carried out at an in-

creased air flow rate of 4 l/rain showed the same results as those in Fig. 3. I;~ is

A

~1oo r "

.~_

"~ 9 9 - E 0 0

98~ I 1 I 1 .. 5 10 15 20

A

c~ 100[ -~ : - I ~ ._o >~ '-~ 991-

97 I I _ I 0 4 8 12 16

H209g Fig. 3. Oxidation efficiency of hydrogen and adsorption efficiency of water vapour on. palladium

catalyst; palladium catalyst: 80 g; air flow rate: 2 l/rain; hydrogen concentration: 0.2%

402


reported that the adsorption saturation of water vapour onto molecular sieve is 0.22

g/g-sieve at 25 ~ Therefore, 17.6 g of water should be adsorbed on 80 g of the

palladium catalyst at an efficiency of nearly 100%. However, the results show that

the estimated value is smaller than the expecte4 one. The difference between these values probably results from a decrease in adsorption capacity of water due to the palladium coating over a molecular sieve.

Apparent oxidation efficiency of methane on the platinum catalyst surface

The results obtained are shown in Figs 4 and 5. These results indicate that the

oxidation temperature should exceed 375 ~ for the quantitative oxidation of memthane

on the platinum catalyst. An electric heater was thus set to 400 ~ for the analysis and

A

100-

98 .~_ .u_

c 94 0

~- 92

I I I 350 375 400

Temperature ,~

Fig, 4. Dependency of methane oxidation on reaction temperature; platinum catalyst: 100 g; air flow rate: 21/min; methane concentration: 0.2%

A

~ I00-

~ L �9 ~- !

c 0

-g_

990 I I I I I t I l - v

1 2 3 4 5 6 7 8

CH4,i

Fig. 5. Dependency of methane oxidation on the amount of methane; platinum catalyst: 100 g; reaction temperature: 400 ~ air flow rate: 2 l/min; methane concentration: 0.2%

403


the temperature was kept constant within +12 ~ by a temperature controller. On monitoring, approximately 5.6 1 of methane was burnt in order to Collect about 9 g

of water. Consequently, Oxidation efficiency of methane on the platinum catalyst was

estimated to be above 99.5% in this case.

Portable tritium sampling system

A portable tritium sampler (Fig. 6) was constructed On the basis of the experimental data. The sampler was originally constructed at the Power Reactor and Nuclear FuelDevelopment Corporation, Japan. s However, since it was designed for

• Gas' - '~ EI~ t ronic ~i Filter l-- 1 meter [--[cooler

H2 Micro valve

Solenoid valve

Tritium- F 1 free | water L

Electrolytic cell

CH~

Needle vol ve ~J

Mett~an e gas cylinder

Air out

Fig. 6J Flow diagram for stepwise collections of atmospheric HTO, HT and CHa T

collection of only water vapour and hydrogen, we modified it by adding a methane

collection unit: Each unit was built in a rack whose dimension is 80 cmjn length,

65 cm in width and 40 cm in depth. In addition to HTO collection column, a

electronic cooler (NETSU DENSHI INDUSTRIES, LTD., GC-11) was used to remove a considerable amount of water vapour during humid seasons, i.e., in the period be-

tween May and October. HTO and CH3 T collection columns were fried with 400 g

and 100 g of molecular sieve, respectively, Hydrogen was supplied into the system from a cell by electrolysis of tritium-free water. A drying column containing 400 g of molecular sieve was used in order to remove the mist from the electrolytic cell. An HT collection column was filled with 150 g palladium catalyst, on which up to 24 1 of hydrogen can be oxidized and up to 19.5 g of the resultant water adsorbed.

A platinum catalyst column containing 150 g of catalyst was enclosed with a 30 cm long electric heater. Methane was supplied into the system from a methane gas cylinder. The flow rate of methane was controlled with a micro valve. A solenoid valve was used to cut off the methane supply so that the platinum catalyst column should not

404


be full of methane. This prevents explosion of the column in case of unexpbcted

power off. A small diaphragm-type air pump was used to introduce air into the

sampler and the flow rate of air, ranging from 1 to 2 1/min, was controlled with a needle valve.

Analytical method for atmospheric tritium

Air sample (5000 1 in summer to 12 000 1 in winter) was introduced into the sampler through a filter at a flow rate of 1-2 1/min. Water vapour was collected in an electronic cooler and an HTO collection column and dried air was passed through an

HT collection column. Since atmospheric hydrogen concentration is very low (about 0.6 ppm) 6,7 some hydrogen generated by the electrolysis of tritium-free water was

added as a carrier. Thus hydrogen was taken up in the HT collection column after catalytic oxidation to water. The remaining gas was finally introduced into a CH3 T collection column through a platinum catalyst column, in which methane was burnt.

The resultant water was adsorbed in the CH3T collection column. Tritium-free

methane gas was used as a carrier in this case, because the atmospheric methane con-

centration is very low (1A-1.8 ppm).S, 9 Flow rates of hydrogen and methane were

regulated so that about 9 g of water could be collected in the HT and CH3 T collec-

tion columns, because 8 g of water was needed for the measurement of tritium acti-

vity. The quantity of water collected in the HTO, HT and CH3 T collection columns

was determined by weighing each column before and after sampling. Water samples were recovered from the columns into the cold traps by flowing nitrogen at a rate of 1 l/rain through the columns and then heating them with an electric heater to 400 ~ 40 ml of recovered water from the HTO collection column and 60 ml of scintillation solution were mixed by shaking in a 100 ml teflon vial. 8 ml of water

samples recovered from the HT and CH3 T collection columns and 12 ml of scintillation solution were mixed by shaking in each of 20 ml teflon vials. The background samples

were prepared using tritium-free water in the same way as in the case of other en-

vironmental samples. The samples thus prepared were kept for 4 days in the counter

at 13 ~ and then the activity of each sample was counted for 1000 minutes. In this

experiment, the correction for counting efficiency was carried out by using an external standard ratio method. 1 ~ external standard ratio was measured before and

after the activity measurement. The counting efficiency correction curves were made

by measuring several standard samples with the same activity but different water

contents, i.e. from 37 to 43% for a 100 ml teflon vial and from 35 to 45% for a 20 ml teflon vial.

405

T. OKA[, Y. TAKASHIMA: ANALYTICAL METHOD FOR ATMOSPHERIC

Examples of measurements of atmospheric tritium

The aoove analytical method was applied to actual air samples collected weekly

in Kyushia University in the period between January 1984 and December 1984. Week-

ly and averaged tritium concentrations in the atmosphere are given in Fig. 7 and

g~ ~cc 50

E~ o 2.0 eHTO |

�9 - ~ �9 CH~T | ' ~ 1.5 a . i l I t , ,

_A .A 0.5 ...., x =A -"

0 1 J I L I ] L L ) I i I ~. 2 3 4 5 6 7 8 9 10 11 12

1984

Fig. 7. Atmospheric tritium concentrations at Kyushu University in 1984

Table 1 Annual averaged concentrations of atmospheric tritium in 1984

Chemical form Concentration

HTO(pCi/mLair) 0.59 + - 0.34 HTO(pCi/M-I 2 O) 49.8 +-13.1 HT(pCi/m3 -air) 1.27+- 0.19 CH3 T(pCi/m3 -air) 0.39+- 0.I4

The precision corresponds to +-1 m

Table 1, respectively. In Fig. 7, the solid circles, squares and triangles show the 019-

served tritium concentrations expressed in pCi/m 3 air in the atmospheric water vapour,

hydrogen and methane, respectively. The open circles show the specific activity ex-

presse d in pCi/1-H2 O in atmospheric moisture. Atmospheric HTO concentration varies within the range from 0.2 to 1.64 pCi/m 3 air or from 22 to 70 pCi/1 H2 O,

406


giving an annual averaged value of 0.59 pCi/m 3 air or 48 pCi/l H2 O~ Atmospheric HT

and CH3T concentrations vary from 0.952.o 1.90 pCi/m 3 air and from 0.22 to

0.77 pCi/m 3 air, giving annual averaged values of 1.27 and 0.39 pCi/m 3 air, respec-

tively. These data obtained appear to be very reasonable and thus we believe that

this newly developed method is useful for routine tritium monitoring.

References

1. R. C. MILHAM, A. L. BONI, Report DP-MS-76-38, 1976. 2. W. R. GRIFEIN et al., Tritium, A~ A. MOGHISSI, M. W. CARTER (Eds), Messenger Graphics,

Phoenix, 1973. p. 533. 3. H. G. OSTLUND, A. S. MASON, Tellus, 26 (1974)91. 4. W. E. SHEEHAM, K. M. MULDOON, Report MLM-2345, 1976. 5. K. SKINOHARA et al., Hokenbutsuri, 18 (1983) 231. 6. P. FABIAN et al., J. Geophys. Res., 84 (1979) 3149. 7. M. I. SCRANTON et al., J. Geophys. Res., 85 (1980)5575. 8. D. R. BLAKE et al., Geophys. Res. Lett., 9 (1982) 477. 9. M. A. KHALIL, R. A. RASMUSSEN, J. Geophys. Res., 88 (1983) 5139.

10. N. MOMOSHIMA et al., Int. J. Appl. Radiation Isotopes, 34 (1983) 1623.

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analytical method for atmospheric tritium with a portable tritium sampling system

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