Concentration Coefficients of Radioiodine in Different Chemical Forms
from Seawater to Fishery Products
Toshihiro SHIBATA, Yoshio ISHIKAWA, Yuichi TAKAKU, and Shun’ichi HISAMATSU
Department of Radioecology
Abstract
Radioiodine takes various chemical forms in the environment. Ion forms of both I- and IO3- were foundin the ocean for 129I discharged from the first Japanese commercial nuclear fuel reprocessing plant located inRokkasho. Since the concentration factor of iodine from seawater to marine products strongly depends on thechemical form of iodine, it is necessary for realistic assessment of radiation dose from the discharged
radioiodine via marine products to use the concentration factor of each chemical form of iodine. This study
aims to establish the concentration factor of radioiodine in I- and IO3- for marine products (seaweed, shellfishand benthos). In FY 2015, 1) the concentration factors of I- and IO3- for sea cucumber (Apostichopus japonicus)
were measured using an iodine radiotracer and 2) the chemical form of stable iodine in sea cucumber wasanalyzed by using X-ray absorption fine structure (XAFS) analysis.
We exposed sea cucumber samples obtained from local fishermen to 125I- or 125IO3- in seawater using airtight chambers, followed by collecting the samples at pre-determined time until 72 h after the start of exposure.
No feed was administered during the exposure. Three samples were collected at each time, followed by drying
in vacuo after removing seawater in their body cavity. The dried samples were measured for 125I concentrationby using a NaI auto-counter. The seawater in the chamber was also collected at the time of sea cucumber
sampling, and 125I- and 125IO3- in it were separately measured by their radiochemical separation using an anion
exchange column. The exposure experiment was repeated twice excluding exposure time of 48 h in the second
experiment, and each results are separately presented in Fig. 1 and 2. As shown in the figures, the intake rateof sea cucumbers in the first experiment was obviously higher than that in the second one. The 125Iconcentration in seawater in the first experiment also rapidly decreased in comparison to that in the second
one. The 125IO3- concentration in seawater did not significantly decreased in both experiments.
A single compartment metabolic model of iodine in sea cucumber with separate inputs of 125I- and 125IO3was constructed (Fig. 3), and transfer-rate coefficients in it were estimated by fitting the experimental data
(Table 1). The transfer-rate coefficients of input for 125I- (k1) and 125IO3- (k2) and of output (k3) in the firstexperiment were estimated as 6.E1, 6. and 1.E-1, respectively. The values of k1, k2 and k3 in the second
experiment were obtained as 5., 1. and 3E-2, respectively, showing that metabolism of 125I was less active inthe second experiment than in the first one by unknown reasons. Concentration factors of 125I- and 125IO3- inthe first experiment was 6.E2 and 6E1, respectively, while they were 2.E2 and 5.E1, respectively, in the second
one. Both results showed that the concentration factor of 125I- for sea cucumber was larger than that of 125IO3-.
It should be noted that ingestion intake of iodine into sea cucumber was not considered here.
The chemical forms of stable iodine in sea cucumber tissue samples was analyzed by XAFS. Although
no clear spectrum was obtained because of low iodine concentration in the samples, it suggested that most ofthe iodine in sea cucumber tissue was in organic form.

b)a)Exposed time (h)Exposed time (h)Measured 125I concentration in sea cucumber and seawater during the first experiment. Open square

Fig. 1shows 125I concentration in sea cucumber (Bq g-dry-1). Open circles and triangles show 125I- and 125IO3concentrations in seawater (Bq g-1). Solid lines show estimated value by the model.

b)a)Fig. 2

Measured 125I concentration in sea cucumber and seawater during the second experiment. Opensquares show 125I concentration in sea cucumber (Bq g-dry-1). Open circles and triangle shows 125Iand 125IO3- concentrations in seawater (Bq g-1). Solid lines show estimated value by the model.

Table 1 Obtained transfer-rate coefficients.

Fig. 3

Metabolic model of 125I in sea cucumber.

k1(h-1)k2(h-1)k3(h-1)

1st experiment

6.E1

6.

1.E-1

2nd experiment5.

1.

3.E-2