M. Takamura

N. "Momoshima

T. Okai

T." Uda

National Institute for Fusion Science (NIFS-Repository)

Pt honeycomb catalyst§ 4. Study on Measurement Technique ofAtmospheric Tritium for DifferentChemical FormsMomoshima, N. (Fac. Sci., Kumamoto Univ.)Takamura, M. (Grad. Sch., Kumamoto Univ.)Okai, T. (Fac. Eng., Kyushu Univ.)Uda, T.There are several chemical forms of tritiumin the atmosphere of the general environment;those are water vapor (HTO), hydrogen (HT),and hydrocarbons (CH 3 T). Measurements oftritium in these different chemical forms in theatmosphere have conducted by Okai from 1984at Fukuoka, Japan. The observed annual averageconcentrations (Bq m-3 ) are in the order of HT >HTO > CH3 T. Specific activities on thesechemical species have showed quite differentlevels, which are highest on HT with 106 TU,then CH3 T with 104 TU and HTO with 101 TU.The TU is defined as atom ratio of one tritiumatom to 1018 hydrogen atoms. The quite differentspecific activities speculate us different sourcesof these tritium species in the atmosphere.In this study, we have designed andconstructed an atmospheric tritium samplingapparatus, which is usable for environmentalmonitoring around feature nuclear fusionfacility as well as general environment.The schematic diagram of the developedtritium sampling apparatus is shown in Fig. 1.Atmospheric tritium is collected successively inthe order of HTO, HT and finally CH3T. All ofthe tritium species are adsorbed on molecularsieve 3A(MS 3A, 500g) as water molecule afterconverting their chemical forms.The water content existing as water vapor inthe atmosphere varies with temperature andrelative humidity, in summer it sometimesexceeds the MS adsorption capacity packed inthe HTO column. Then a cold trap cooled at 2 °Cis located before the HTO column. Hydrogen isoxidized to water by metal catalyst. Weprepared Pt honeycomb catalyst, in which 0.12 gof Pt is dispersed on surface of honeycomb basemetal (25 mm in diameter, 40 mm in length).The Pt honeycomb catalysts can oxidize 4000ppm Hz to water completely at 100 °C, tritiumfree Hz generated by electrolytic decompositionof tritium free water is added to the sampling airto ensure recovery of HT. Oxidation of CH4 bythe Pt honeycomb catalyst was examined,showing no conversion of CH4 to water at thisoperation condition.Hydrocarbons are oxidized to water by Pdcatalyst (DASH-220D, 0.24wt%Pd, 3mm indiameter, 150 cm3 , NECHEMCAT) at 300°C.Tritium free bomb CH4 is added to the samplingair as to become 2000 ppm using a mass flowcontroller before the Pd catalyst column.Teflon tube and metal conjunction(Swagelock) is used for connecting each unit.Total of 5000 liter air is sampled at flow rate of1.2 L/min (2.89 days) or 4.8 L/min (17.3 h). Theperformance of the catalysts is confirmed to besatisfactory for the both flow rates and noleakage is observed on the apparatus duringabove sampling periods, suggesting goodtightness of the lines. Water adsorbed on MS isrecovered by introduction of N z gas heated at400 °C to the HTO, HT and CH3T columns.About 15 ml of water is expected to recoverfrom the HT and CH3 T column, respectively.Tritium activity is measured by low backgroundliquid scintillation counting after mixing therecovered water with liquid scintillationcocktail (Pico Flow LLT, PerkinElmer) inTeflon vial.HTO column (MS 3A)ITritium free H,Fig. 1 Schematic diagram of atmospherictritium sampling apparatus411

Study on Measurement Technique of Atmospheric Tritium for Different Chemical Forms

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