Tritium Transfer from the Atmosphere to Crops
Takashi TANI, Ryuji ARAI, Yasuhiro TAKO, Yuji NAKAMURA
Department of Environmental Simulation
Abstract3

Trace amounts of tritium ( H) are expected to be discharged into the environment from operation of
the spent nuclear fuel reprocessing plant in Rokkasho, Japan. Since tritiated water (HTO) is transferred toplants through the leaf surface and root, it is necessary to assess the local radiological impact of 3H from thereprocessing plant to predict the 3H concentration in the edible parts of crops. In this study, a dynamiccompartment model was established to describe tritium accumulation in a type of leafy vegetable plant.
In FY 2010, we investigated the deuterium enriched water (HDO) uptake of Brassica campestris(Komatsuna) via the root, and constructed a three-compartment model of hydrogen metabolism in the shoot,which has two compartments of free water with (FW1) and without (FW2) transpiration and a
non-exchangeable organically-bound hydrogen (NxOBH) compartment in the shoot. Water from both theleaf surface and root enters into the FW1 compartment and is exchanged with the FW2 compartment. Free
water in the FW1 compartment is transpired, and H in the free water is transferred to the NxOBH
compartment mainly by photosynthesis. The free water resulting from degradation of NxOBH is returned tothe FW1 compartment. Transfer parameters in the model, such as transpiration rate and rate constants of H
transfer between compartments etc., were cited from the literature. Although the model fairly well described
D concentration in free water in Komatsuna shoot, D concentrations in NxOBH were not examined.
In FY 2011, the HDO uptake via the leaf surface was studied using Komatsuna to obtain actual valuesof parameters in the model. Komatsuna plants were exposed to HDO vapor during a light or dark period at
15 d after seeding, and D concentrations in free water in the shoot during the exposure were determined.
Komatsuna plants were also exposed to HDO vapor at 10, 15, 20 or 25 d after seeding, and D concentrationsof NxOBH in the shoot harvested at 28 d after seeding were determined. The parameter values in the model
were estimated from the results of the experiments, and some parameter values differed by a factor of 2-7
from those in the literature. The model using the obtained parameter values mostly well described D
concentrations measured in free water and NxOBH of the shoot.

kFA(t)
FW2
MF2(t)

k12(t)

MF1(t)k21
IAF(t)

Fig. 1

FW1

PL(t)

kOF

NxOBH
MO(t)

IRF(t)

Scheme of the dynamic
three-compartment model.

0.40.04(A) Light-period exposure

0.03

0.02
Observed
0.01
Estimated
0.00
04

8120.012(B) Dark-period exposure
0.008

0.004

0.000

04812

Deuterium concentration in leaf NxOBH (mol D mol-1 H)Deuterium concentration in leaf free water (mol D mol-1 H)(A) Light-period exposure
0.3

Observed
Estimated

0.2
0.1

0.00

Observed and estimated deuterium
concentrations in free water ofBrassica campestris shoot exposed
to deuterium-enriched water vapor
during light (A) or dark (B) periods
at 15 d after seeding.

20300.4(B) Dark-period exposure
0.3

0.2

0.1

0.00102030

Days after seeding (d)Time after the start of exposure (h)Fig. 2

10Fig. 3

Observed and estimated deuterium
concentrations in NxOBH in Brassica
campestris
shoot
exposedto
deuterium-enrichedwatervapor
during light (A) or dark (B) periods at
10, 15, 20 or 25 d after seeding andharvested at 28 d after seeding.