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 andcuticles covering the epidermis. The transfer rate of HTO through the plant surface differs between light anddark periods due to stomata activity, while the production rate of organic matter also increases in the lightperiod. Therefore, experimental data on tritium transfer from the atmosphere to the organic matter of cropplants during both light and dark periods are necessary for the establishment of the model describing tritium
accumulation in the plants.
In FY 2013, we obtained data for transfer of deuterium, which was used instead of tritium, from theatmosphere to rice plants by exposing them to deuterium-enriched water (HDO) vapor during the dark period.
In FY2014, rice plants at 87 d after seeding were exposed to HDO vapor during the light period for 10 h, andfree water deuterium (FWD) concentrations in the grain, shoot (leaf and stem) and root during the exposure
were determined. Rice plants were also exposed to HDO vapor at 67, 73, 82, 90, 100, 104 or 112 d after
seeding, followed by the measurement of non-exchangeable organically bound deuterium (NxOBD)
concentrations in the grain, shoot and root harvested at 126 d after seeding.
By using both data in FY2013 and FY2014, we constructed an eight-compartment model of deuterium
metabolism in rice plant. The model consisted of a FWD compartment of each grain, leaf, stem and root; anNxOBD compartment of each grain and root; and two NxOBD compartments of shoot. It is notable that thetwo NxOBT compartments of shoot included the leaf and stem, and one of the compartments was for only
storage of NxOBT. Direct synthesis of shoot NxOBD from HDO via root uptake was included in the model
for better describing NxOBD concentration in the compartment. Heavy water entered the plant from thesurface of the grain, leaf and stem as well as via the root, and transpired from surfaces of the grain, leaf andstem. The values of transfer parameters in the model were estimated with the results of exposure experiments
by a least square fitting method, while the transpiration rate constant through stomata was estimated from thephotosynthesis rate based on growth data of the plant. The estimated concentrations of FWD and NxOBD
compartments in the rice plant obtained by the model agreed relatively well with the observed values.

Transpiration
Degradation
Grain FWD

Grain uptake

Grain NxOBD

Translocation
Transpiration

Translocation

Degradation

Shoot

Leaf FWD

Leaf uptake

StorageShoot

NxOBD1Synthesis

NxOBD2

Transpiration
Stem FWD

Stem uptake

Synthesis

Translocation

Root uptake
Degradation

Root FWD

Root NxOBD

ReleaseFig. 1 Scheme of the eight-compartment model of deuterium metabolism in rice plant. FWD indicates free water

（a）
4020

y = 0.87x
R2 = 0.9600204060

Observed FWD concentration-1(mmol D mol H）

0.8
mmol D mol-1 H)

Estimated NxOBD concentration

60(mmol D mol-1 H)

Estimated FWD concentration

deuterium and NxOBD, non-exchangeable organically bound deuterium.

（b）Grains (light)

0.6

× Grains (dark)
0.4

Shoots (light)

0.2

y = 0.98x

Shoots (dark)

R2 = 0.85

Roots (light)
Roots (dark)

0.0
0.0

0.2

0.4

0.60.8Observed NxOBD concentration(mmol D mol-1 H）

Fig. 2 Observed and estimated concentrations of (a) free water deuterium and (b) non-exchangeable organicallybound deuterium in the grains, shoots and roots of rice plants; replicated data are separately presented.