Tritium Transfer from the Atmosphere to Crops
Takashi TANI, Ryuji ARAI, Yasuhiro TAKO, Shun’ichi HISAMATSU
Department of Radioecology
Abstract3

Trace amounts of tritium ( H) are expected to be released into the atmosphere from operation of thespent nuclear fuel reprocessing plant in Rokkasho, Japan. Since tritiated water (HTO) is transferred to crop
plants and can be incorporated into organic compounds, estimation of the 3H concentration in the edible partsof crop plants is necessary for the assessment of the local radiological impact of 3H from the reprocessing
plant. The purpose of this study is to establish a dynamic compartment model describing tritium
accumulation in crop plants.
Tritiated water is transferred from the atmosphere to free water of crop plants through the stomata andcuticle covering the epidermis. Because stomata are only open during the light period, the transfer rate of
HTO through the plant surface differs between light and dark periods. Moreover, the production rate oforganically bound tritium (OBT) in plants differs between light and dark periods. Therefore, experimental
data on tritium transfer from the atmosphere to organic matter of crop plants during both light and dark
periods are necessary for the establishment of the model describing tritium accumulation in crop plants.
In FY 2013, rice plants were exposed to deuterium-enriched water (HDO) vapor during a dark period
for 10 h at 83 d after seeding, and free water deuterium (FWD) concentrations in free water in the grain,shoot (leaf and stem) and root during the exposure were determined. Rice plants were also exposed to HDO
vapor at 74, 80, 87, 93, 100, 106 or 116 d after seeding, and organically bound deuterium (OBD)
concentrations in the grain, shoot and root harvested at 126 d after seeding were determined. During theexposure at 83 d after seeding, FWD concentrations in the grain and shoot increased but not in the root,indicating that the transfer of FWD from the aboveground parts to the root in rice plants was negligibly small.
Non-exchangeable OBD (NxOBD) concentration in the grain at harvest was the highest in the exposure at
the early stage of rapid grain growth.
By using our data on metabolism of deuterium in a leaf vegetable plant (Brassica campestris) reported
previously, we constructed a three-compartment model of deuterium metabolism in the shoot. In this model,
the shoot consisted of two compartments of free HDO (FWD1 and FWD2) and an NxOBD compartment.
Heavy water in the FWD1 compartment that entered from both the leaf surface and root was exchanged with
the FWD2 compartment and also removed by transpiration. Deuterium in the FWD1 compartment wastransferred to the NxOBD compartment mainly by photosynthesis. The FWD resulting from degradation ofNxOBD was returned to the FWD1 compartment. The values of transfer parameters in the model were
estimated with the results of exposure experiments by a least square method. The estimated concentrations ofFWD and OBD in the shoot obtained by the model agreed relatively well with the observed values.

Table 1 Deuterium concentrations in free water of grain, shoot (leaf and stem), and root of rice plants exposed todeuterium-enriched water vapor during dark period at 83 d after seeding (mean ± SD, n = 3)
Grain FWD concentration

Shoot FWD concentration

Root FWD concentration

exposure (h)（mmol D mol H）（mmol D mol H）（mmol D mol-1 H）

14710

1.59 ± 0.14
3.02 ± 0.36
4.27 ± 0.92
5.08 ± 0.22

0.64 ± 0.04
2.12 ± 0.60
1.76 ± 0.19
2.13 ± 0.06

0.147 ± 0.003
0.165 ± 0.007
0.167 ± 0.010
0.210 ± 0.014-1Grain NxOBD concentration-1Transpiration

Leaf uptake

NxOBD

FWD1
Root uptake
FWD20.25(mmol D mol-1 H)

Time after the start of0.20
0.15
0.10
Days after seeding (d)Fig. 1 Scheme of three-compartment modelFig. 2 NxOBD concentrations in grain of rice plants exposed toof deuterium metabolism in shoot ofdeuterium-enriched water vapor during dark period at 74, 80,Brassica campestris.

87, 93, 100, 106 or 116 d after seeding and at harvest 126 dafter seeding. The dashed line indicates the background-1

NxOBD concentration（mmol D mol H）

30
Leaf uptake(mmol D mol-1 H)

Estimated FWD concentration

concentration of deuterium (0.15 mmol D mol-1 H).

Root uptake
2010

y = 0.96x2

R = 0.90
0-1Observed FWD concentration (mmol D mol H)
Fig. 3 Observed and estimated deuterium

0.4

0.4
Light period

0.3

Dark period
0.3

Observed
Estimate

0.2

0.2

0.1

0.1

0.0

0.0
10152025

10152025

Days after seeding (d)Fig. 4 Observed and estimated NxOBD concentrations inconcentrations in free water of shoot ofshoot of Brassica campestris exposed toBrassica campestris.

deuterium-enriched water vapor during light or dark
periods at 10, 15, 20, or 25 d after seeding and at harvest
28 d after seeding. Dashed lines indicate the backgroundconcentration of deuterium (0.15 mmol D mol-1 H).