Weathering of Iodine Deposited on Grass Leaf Surfaces
Hitoshi KAWABATA, 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. Part of the radionuclides deposited onto the leaves
is removed from the surface by the environmental process called weathering, i.e. removal by rain, wind, etc.
Although weathering, foliar uptake and translocation are important processes involved in the radiation dose
assessment of radionuclides in crops, parameters to describe those processes for 129I, which is an importantradionuclide 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 (I) on the surface of grasses was launched in
FY 2011. In FY 2015, we studied the following subjects: 1) the effect of the growth stage of grasses on thefoliar uptake and volatilization of I applied as inorganic vapor (I2) onto leaf surfaces, and 2) the weathering ofI deposited on leaf surfaces as I2, dry aerosol or solution by simultaneous exposure to fog and wind.
After exposing Orchard grass (Dactylis glomerata L. var. Akimidori II) to I2 vapor at 39 d or 49 d aftersowing, the plants were cultivated for 7 d in an artificial climate chamber. The plant leaves were periodicallycollected, and the leaf surfaces were washed with solution containing detergent. The foliar uptake of I wasmeasured by analyzing the leaves and the solution samples obtained by washing the surfaces. The I
volatilized from the plant was collected on activated carbon paper filters. Immediately after exposing to I2,
more than 98% of the I in the plants was found in the leaves, then I in the leaves gradually decreased withincreasing volatilized I. The behavior of I was almost the same at the two growth stages of the plant.
After applying 1) I2 vapor, 2) liquid droplets containing NaI or NaIO3, or 3) dry aerosol containing NaIor NaIO3, onto the leaf surfaces of Orchard grass, the plants were simultaneously exposed to fog and wind.
The exposure was carried out in a combination of fog with several atmospheric liquid water contents and windof different wind speeds. The exposed plant leaves were collected and treated in the same manner as mentioned
for the first study. For I applied as liquid droplets or dry aerosol, I retention in and on leaves in a fog with 47
mg m-3 liquid water content for 2 h exponentially decreased with wind speed. The I applied as I2 was removed
from the plants by the exposure to the fog and wind of 2 m s-1 for 24 h in contrast to the previous result showing
no removal under the same fog condition without wind.
To summarize the weathering of I applied as iodate in liquid droplets by rainfall in the previous studies,compartment models of the I were constructed for describing foliar uptake, volatilization and weathering, andtypical weathering half-life was obtained. A unit amount of radionuclide was assumed to deposit onto the leaf
surface at noon of a given date, and then the behavior of the radionuclide was simulated with the model andactual weather condition during 60 d after the date. A half-life was calculated from the remaining proportionin the plant and on the leaf surface on the assumption of exponential decrease. The half-lives were obtainedfor each day from May to October in 2014 and from May to October in 2015. The median values of 250 halflives obtained for iodate of liquid droplets was 38 d. The half-life in this study was longer than the weathering
half-life (14 d) used in the safety review of the reprocessing plant.

Table 1

Statistics of half-lives calculated for
IO3- using the compartment model
Statistic

Half life (d)Arithmetic mean

36.9Standard deviation

3.2

Geometric mean

36.7

Median38.0

Minimum

17.3

Maximum

38.1

n=250