Development of Dose Assessment Method for a Conifer
Yoshihito OHTSUKA, Masanori FUJII, Shinji UEDA, Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Protection of an ecosystem itself from radiation has become an important theme in the research field ofradiation safety. Since conifers are known to be more sensitive than other organisms in the generalenvironment, we planned to establish the radiation dose assessment method for one type of conifer and get its
natural background radiation dose rate. After considering that the first commercial spent nuclear fuelreprocessing plant is now under construction in Rokkasho, Japan, we selected a forest of Japanese black pine
(Pinus thunbergii), about 4 km away from the main stack of the plant, as our target field. In addition, we
studied the iodine dynamics in the forest for possible radioiodine release from the plant.
Four black pine trees in midrange size were felled as pilot samples in the target forest in 2016 for
preparing a phantom of the tree type. Samples of different-age leaves, branches and trunks were collected,
followed by measuring their dimensions, weights, and water contents. From the relation of the trunk diameters
at several heights above the ground, heights and lengths of every branch, the shape of the trunk, the outershape of the crown and the space inside the crown without leaves were found to be represented by cone shapes.
Mean wet-weight and the standard deviation were 399 ± 176 kg of the above-ground parts of those samples.
The wet-weight of their trunks, branches and leaves contributed about 89%, 8% and 3%, respectively, to thetotal weight on average.
For deciding the phantom composition, we measured stable elements in the plant parts, while wedetermined natural radioactive nuclide concentrations for calculating internal dose. For getting backgroundexternal dose rate, the horizontal distribution of environmental -ray dose rates in the forest were measured
with glass dosimeters. The dose rates at five or ten points in the forest were at a similar level with the meandose rate of 22±0.4 nGy h-1.
To study iodine dynamics in the forest, we made preliminary measurements of iodine concentrations in
the tree samples, dry deposition fluxes onto the tree and removal rates by rainfall and litter fall. Meaninventories of stable iodine in the crown and trunk were evaluated to be 3.8E-3 and 1.7E-3 g individual-1,respectively. The iodine dry deposition rate of 1.9E-4 to 9.6E-4 m s-1 to the forest crown was evaluated fromthe difference of dry depositions inside and outside the forest and gaseous iodine concentration outside theforest. The estimated deposition rate was comparable to that for methyl iodine reported in the literature. A
mean removal rate of iodine from the crown by 1 mm rainfall was estimated to be 3.3E-6 g m-2 by the differenceof wet iodine deposition inside and outside the forest. Litter fall including deciduous tree leaves contributed
to the iodine removal of 3.5E-6 g m-2 d-1 in the forest during April 2017 – January 2018.

Fig. 1Fig. 2

The external form of Japanese black pineconstructed based on the test trees in aRokkasho forest.

The average percentage of wet-weight of each part in the above-ground part of four pine trees.