Weathering of Iodine Deposited on Grass Leaf Surfaces
Hitoshi KAWABATA, Hirofumi TSUKADA, Masumi YANAI,
Yuichi TAKAKU, Shun'ichi HISAMATSU
Department of Radioecology
Abstract
Radionuclides released into the atmosphere are deposited on the leaf surfaces of crop plants, taken up
by the plants, and translocated from the leaves to other parts. Some amount of the radionuclides deposited
onto the leaves is removed from the surface by the environmental process called weathering, i.e. removal byrain, wind, etc. Although weathering, foliar uptake and translocation are important processes involved in theradiation dose assessment of radionuclides from crops, parameters to describe those processes for 129I, which
is an important radionuclide for the safety assessment of the nuclear fuel reprocessing plant in Rokkasho,
have not yet been elucidated. Since grasses are cultivated for livestock farming which is one of the important
industries in Rokkasho, a research project to determine the behavior of iodine on the surface of grasses waslaunched in FY 2011. In FY 2013, we studied the following subjects: 1) the effect of the growth stage ofgrasses on the foliar uptake and volatilization of iodide applied as liquid droplets onto leaf surfaces, 2) theeffect of light intensity on the volatilization of iodide applied as liquid droplets onto leaf surfaces, and 3) theeffect of rainfall on the weathering of iodine deposited on leaf surfaces as dry aerosol, solution or inorganic I
vapor (I2).
After applying droplets of iodide solution onto the leaf surfaces of Orchard grass (Dactylis glomerata L.
var. Akimidori II) at different growth stages, we placed each plant within an acrylic chamber and cultivatedthem for 7 d in the artificial climate chamber. The plant leaves were periodically collected, and the leaf
surfaces were washed with solution containing detergent. The foliar uptake of I was measured by analyzing
the plant leaf and the solution samples obtained by washing the surface. The I volatilized from the leaf
surface was calculated by subtracting the sum of the I amounts in the plants and the washing solution fromthe amount of I initially applied on the leaf surface. At all growth stages, 32 d, 39 d, and 49 d after sowing,
the amounts of I on the leaf surface of plants removable by washing decreased with time after the loading,while the amount of the volatilized fraction increased. The I content in the plant reached a maximum around3 d after applying and then decreased. The behavior of I applied on the leaf surface was almost the same at
all growth stages of the plant.
After applying droplets of iodide solution onto the leaf surfaces of Orchard grass at 32 d after sowing,each plant was placed within an acrylic chamber and cultivated for 3 d at different light intensities. The
volatilized fraction of applied I was almost constant and had no dependence on the light intensities of 5, 10and 30 klx.
After applying 1) gaseous I2, 2) liquid droplets containing NaI or NaIO3, or 3) dry aerosol containing
NaI or NaIO3, onto the leaf surfaces of Orchard grass, the plants were exposed at different rainfall intensities
and rainfall durations using a rain simulator. These plant leaves were collected and treated after the rain
exposure in the same manner as mentioned above. More I was removed from the leaf surfaces as the rainfall
intensity increased. However, the increase rate of the I proportion removed from the leaf surfaces decreased

as the rainfall intensity increased. In an experiment with the rainfall intensity of 1.2 mm h-1, the decrease inthe proportion of I remaining was approximated by a function with two exponential terms for rainfall
duration.

Remaining proportion

10△：Particle◆：Liquid
10.1

0.01
0Fig. 1

12
3Rainfall intensity

45(mm h-1)

6Remaining proportion of I and rainfall intensity.
Remaining proportion of I was defined as theratio of the sum of amounts on leaf surfaces andin plants to that loaded initially on the leaf
surfaces. Vertical bars indicate a standard
deviation of 3 samples. The lines show leastsquare approximation by a function with two
exponential terms of rainfall intensity.