Plasma and Fusion Research: Regular Articles

Volume 14, 3405099 (2019)

Development of Field Estimation Technique and Improvement ofEnvironmental Tritium Behavior Model∗)Sumi YOKOYAMA, Tomoyuki TAKAHASHI1) , Masakazu OTA2) , Hideki KAKIUCHI3) ,Shinji SUGIHARA4) , Shigekazu HIRAO5) , Noriyuki MOMOSHIMA6) , Toshiya TAMARI6) ,Nagayoshi SHIMA6) , Mariko ATARASHI-ANDOH2) , Satoshi FUKUTANI1) , Shunya NAKASONE7) ,Masahide FURUKAWA7) , Masahiro TANAKA8,9) and Naofumi AKATA8)
Fujita Health University, 1-98 Dengakugakubo, Kutsukake-cho, Toyoake, Aichi 470-1192, Japan1)
Kyoto University, 2 Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan
2)
Japan Atomic Energy Agency, 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
3)
Institute for Environmental Sciences, 1-7 Ienomae, Obuchi, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
4)
Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
5)
Fukushima University, 1 kanayagawa, Fukushima, Fukushima 960-1296, Japan
6)
Kyushu Environmental Evaluation Association, 1-10-1 Matsukadai, Higashi-ku, Fukuoka, Fukuoka 813-0004, Japan7)
University of the Ryukyus, 1 Senbaru, Nishihara-cho, Nakagami-gun, Okinawa 903-0213, Japan8)
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan9)
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan(Received 10 January 2019 / Accepted 3 April 2019)To estimate the environmental transfer of tritium into the atmosphere and to establish tritium analysis techniques, this study improves our previous compartment model and proposes new analytical technique based on themicrowave heating method and a water-sample purification technique using ion resin. This article introduces anew conceptual approach for estimating environmental tritium behavior.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: environmental tritium transfer, compartment model, environmental tritium analysis, soil-water transfer, microwave heating method, simple water-purification, small amount water sample
DOI: 10.1585/pfr.14.3405099

1. Introduction
The Large Helical Device of the National Institutefor Fusion Science (NIFS) started deuterium plasma experiments in 2017 [1]. The deuterium plasma experiments will assist our physical understanding of hightemperature plasma. However, to realize the fusion reactor,
deuterium-tritium plasma experiments are necessary fromboth physics and fusion engineering perspectives. Such experiments were completed by the Joint European Torus andthe Tokamak Fusion Test Rector in the 1990 s, and will befinished by the International Thermonuclear Experimental
Reactor (ITER) [2–4] in the near future. Tritium is a radioactive material that emits beta-rays with a maximum
energy of 18.6 keV. To ensure the safety of fusion facilities, methods that evaluate tritium transfer in the environment are vital. When gaseous tritium (HT) and/or tritiatedwater (HTO) are/is released from a facility into the atmosphere, HT and HTO disperse and are deposited on soil,plants, terrestrial water or seawater surfaces, and are eventually transferred to the body interior [5]. In particular, re-leased HT is oxidized to HTO by bacteria in the soil. The
dose coeﬃcient of HTO is 10,000 times higher than thatof HT [6]. HTO in the soil, water, and air is transferred toplants, where it becomes free water tritium (FWT); moreover, organically bound tritium (OBT) formed by photosynthesis accumulates in the plant and is slowly discharged
from it (see Fig. 1).

author’s e-mail: sumi0704@fujita-hu.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

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Fig. 1 Tritium transfer in terrestrial environments.
c 2019 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 14, 3405099 (2019)

Fig. 3 Developed compartment model for tritium transfer in theterrestrial environment.
Fig. 2 Outline of this research project.

The present study aims to develop a tritium analysis
technique and to improve the tritium transfer model basedon the simple compartment model, establish practical parameters, and hence estimate tritium transfer in the environment. The models are developed under normal operation conditions of the facilities in previous studies. Here
we describe the improvements to the model, and develop
an analytical technique for obtaining the parameters. To
obtain several environmental sample data, we also establish methods for rapidly analyzing liquid scintillation measurements of plant and water samples. An outline of thisresearch project is shown in Fig. 2.

2. Tritium Behavior Model
The tructure of the developed model is shown inFig. 3. This model is composed of an air-soil-plant system.
Tritium in plants is divided into FWT and OBT. The OBT
component is subdivided into fast and slow components,
each containing exchangeable OBT (Ex-OBT) and nonexchangeable OBT (Nx-OBT) according to recent knowledge [7]. The source terms of this model are the HT andHTO concentrations in air. Although tritiated methane(CH3 T) can also be released from nuclear facilities suchas fuel reprocessing plants, we focused on HT and HTO
because the solubility (and hence the plant availability) ofmethane in water is low. The transfer coeﬃcient of eachpathway would be estimated by using previous studies andfrom experimental or field data obtained in this study. To
validate the developed model, the results would be compared with those of SOLVEG-II [7].

3. Methods for Estimating Soil-Water
Transfer
Soil water was collected by a lysimeter system withglass fiber wicks (Fig. 4). This system is inserted perpendicularly to the soil-water flow in the soil profile ata specific depth beneath undisturbed soil. As no vacuum
is applied, saturated flow conditions are established at theupper boundary of the sampler, enabling water to perco-Fig. 4 Schematic of the system for sampling soil water.

late into the system. After removing the percolate with apump, the volume of soil water and tritium concentrationin the soil water can be measured. This sampling system issimple, non-destructive, and convenient for assessing soilwater transfer. The system can acquire continuous monitoring data, and provides information on travel time. Thesystem, constructed from a 0.5 cm-thick polyvinylchloride(PVC) plate, provides a sampling area of 0.25 m2 whichis covered with glass fiber. In the present study, the sampler plate was set at 20 cm below the soil surface. The
sampler was connected to a 4-L high-density polyethylene
container by flexible PVC tubing. From the data collectedby the lysimeter system, we attempted to estimate the soil–water transfer.

4. Environmental Tritium Analysis by
the Microwave Heating MethodConventionally, FWT and OBT are analyzed by theliquid scintillation counting (LSC) method after freezedrying and combusting the sample. However, the pretreatments for FWT and OBT analysis are complicated andtime-consuming, requiring several weeks. To reduce thetime and eﬀort of pretreating the plant samples, we propose a microwave heating technique prior to the FWT andOBT analysis. We first tried recovering the water from3405099-2

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Fig. 6 Simple water-purification method.

Fig. 5 Photograph of the hexagonal microwave heating system.

fresh pine needle samples by a multimode-type large- volume microwave cavity with 2.45-GHz magnetron-type microwave generators, whose basic parameters had been determined in advance. The cavity collects the complete
moisture content of the sample. After the microwave heating experiments, the dried pine needle samples were redried in an oven dryer for calculating the FWT recoveryyield. The recovery yield exceeded 97% under the imposed experimental conditions (microwave output: 1.0 kW,
time: 30 min, N2 flow: 2 L min−1 ). This result indicatesthat sample drying and water recovery by microwave irradiation is more time-eﬃcient than the conventional method[8]. We then designed and constructed a new pretreatment
microwave heating system composed of the multimodetype hexagonal microwave cavity, 2.45-GHz magnetrontype microwave generators, an oxidation reactor packed
with a palladium catalyst, and a combusted water recovery cold-trap (see Fig. 5).
We preliminarily tested the combustion of plant powder. The dried plant sample was completely combusted
after approximately 2 hr. In future, we will optimize thehexagonal microwave heating system for OBT measurement using a semiconductor microwave generator.

5. New Analytical Methods for
Pretreatment and Tritium
Enrichment
5.1

Pretreatment of water samples for liquid
scintillation measurements

To suppress chemical quenching in the LSC method,
we must remove the impurities from the sample water.
The conventional pretreatment method is distillation of
the sample water, which is another time-intensive process.
Sample water for tritium LSC measurements has recently
been purified through ion-exchange resins, as shown inFig. 6 [9]. Toward an easy and eﬀective LSC pretreatment, we thus investigated the performance of anion- andcation-exchange resins and tritium columns produced by
Eichrom. We checked the absorption spectrum in the UV
range and the electrical conductivity of the water sam-ples to confirm the water quality (organic and ionic-species
contents in the recovered water).
The results confirmed that all impurities were removed from the simulated sample. As the next step, wewill attempt to purify more contaminated simulated samples.

5.2 Development of a compact electrolytic
tritium-enrichment cell for small watersamples
The tritium concentration in environmental water
samples such as rivers, rain, and seawater is too low tomeasure directly by the LSC method. Although an electrolytic tritium-enrichment system for environmental samples is commercially available [10], it requires a largevolume water sample and limits the electrolysis current.
For this reason, we developed a compact electrolytic
tritium- enrichment cell for small-volume water samples.
The glass cell holds approximately 300 mL of water sample, and the designed volume after enrichment is 12 mL.
The electrolysis employs three electrode plates (Ni-Fe-Ni)
and proceeds at 2.4 g/hour. In this system, the detection
limit of the electrolytic enrichment of a 60-mL water sample equaled that of direct measurement of a 50-mL water
sample in a 100 mL vial. Enrichment by this system will
be eﬀective if sample volumes of 100 mL or higher can beobtained. In a 100-mL water sample, the expected detection limit is 0.15 Bq/L for 1,000 min monitoring by LSC
measurement.

6. Summary
In future, the environmental behavior of tritium will
be eﬀectively evaluated by the simple compartment model,
and the model parameters will be obtainable by the rapid
and simple techniques developed in our research project.

Acknowledgment
This work is supported by NIFS collaboration research program (NIFS18KOCA003).
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[2] https://www.iter.org/[3] M. Keilhacker et al., Nucl. Fusion 39, 209 (1999).

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(1997).
[5] P. Calmon et al., “Tritium and the environment,” radionuclide fact sheet in IRSN, (2010), https://www.irsn.fr/EN/Research/publications-documentation/radionuclidessheets/environment/Documents/Tritium_UK.pdf[6] ICRP, Ann. ICRP, 45, 3/4 (2016).

[7] M. Ota et al., J. Environ. Radioact. 178-179, 212 (2017).
[8] N. Akata et al., Plasma Fusion Res. 11, 2405017 (2016).
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[10] http://www.permelec.co.jp/en/products/equipment_tritium.
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