Tritium Transfer from the Atmosphere to Crops
Takashi TANI, Ryuji ARAI, Yasuhiro TAKO, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Although its amount is normally limited, tritium (3H) is released into the atmosphere from operation ofthe spent nuclear fuel reprocessing plant in Rokkasho, Japan. Tritiated water (HTO) enters crop plants byfoliage uptake from the atmosphere and by root uptake after deposition to the ground, which may cause apossible increase in the internal radiation dose of residents in the vicinity of the reprocessing plant whoconsume these crop plants. Therefore, estimation of the 3H concentration in the edible parts of crops isnecessary for the assessment of the local radiological impact of 3H from the reprocessing plant. In this study,a dynamic compartment model was established to describe tritium accumulation in a root vegetable plant.
We investigated the deuterium enriched water (HDO) uptake of radish plant (Raphanus sativus L.) via
the leaf and the root. Radish plants were exposed to HDO vapor or HDO-enriched nutrient solution during alight or dark period at 14 d after seeding, and free water deuterium (FWD)

concentrations in free water inthe leaf and root during and after the exposure were determined. Radish plants were also exposed to HDO
vapor at 10, 12, 14, 15, 17 or 19 d after seeding, and organically bound deuterium (OBD) concentrations in
the leaf and the root harvested at 20 d after seeding were determined.
From the data obtained, we constructed a five-compartment model of deuterium metabolism in theplant. In this model, the leaf consisted of a FWD compartment and two OBD compartments (OBD1 andOBD2), while the root consisted of a FWD compartment and an OBD compartment. In the leaf, the FWD,OBD1 and OBD2 compartments were sequentially connected to each other, and the FWD and OBD
compartments in the root were connected similarly. The OBD1 had output to the root OBD compartment
through translocation. Part of the HDO uptake via root directly entered the leaf FWD compartment, and theroot FWD compartment had output to soil water.
The values of transfer parameters in the model were estimated from the results of the experiments.
Although the model well estimated the OBD concentrations with good agreement to the observed values,
estimated FWD concentrations in leaf and the root did not agree well with the observed values. Furtherstudy is required to improve the accuracy of the five-compartment model of deuterium metabolism.

k1(t)MLF(t)Leaf uptake

IL(t)

ALF(t)

IRL(t)

Root uptake

k6MRF(t)IR(t)

ALO1(t)

ARF(t)

Light period

Root OBDARO(t)

k11MRO(t)Dark period

20(A) Leaf

y = 0.85x

y = 1.03x
01020(B) Root00.8010(D) Root

0.40.4y = 1.46x
0.0
0.0

200.40.80.00.0y = 0.83x
0.4

0.80.010(A) Leaf
Observed
0.02
Estimate(C) Leaf

0.005

0.01
0.00
0.03

51015(B) Root

0.02

200.000
50.010101520(D) Root

0.005

0.01
0.00

103

Observed and estimated OBD
concentrations in leaf (A, C) and root
(B, D) of radish plants exposed todeuterium-enrichedwatervapor
during light or dark periods at 10, 12,14, 15, 17 or 19 d after seeding andharvested at 20 d after seeding. Valuesof the observed and estimated OBD
concentrations are normalized bydeuterium concentrations in air
moisture during each exposure.

Observed FWD concentration (mmol D mol H)
Fig.

0.000
15 20
5 10 15
Days after seeding (d)5-1

Fig. 2 Observed and estimated deuterium
concentrations in free water of leaf
(A, C) and root (B, D) of radish
plants exposed to deuterium-enrichedwater vapor during light or dark
periods at 14 d after seeding.

Dark period

Light period0.03(C) Leaf

1010

0.8

ALO2(t)k5MLO2(t)Scheme of five-compartment model of deuterium metabolism in radish plant.

20

0

Leaf OBD2

k8(t)MLO1(t)k7(t)MLF(t)k10(t)MRF(t)k4(t)MLO1(t)Normalized OBD concentration

Estimated FWD concentration (mmol D mol-1 H)

Leaf OBD1

k3MLO1(t)Root FWD

k9MRF(t)Fig. 1k2(t)MLF(t)Leaf FWD20