1.2.2 水産生物におけるヨウ素の形態別濃縮係数
Concentration Coefficients of Radioiodine in Different Chemical Forms
from Seawater to Fishery Products
柴田 敏宏, 石川 義朗, 高久 雄一, 久松 俊一
環境影響研究部

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 secondexperiment 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 of
the iodine in sea cucumber tissue was in organic form.

1. 目的

更に、同時に漁獲されたマナマコを、SPring-8 産

大型再処理施設の海洋放出管から排出される放業利用 II ビームライン BL14B2 を用いて、ヨウ素 K

射性ヨウ素は、海洋中で I 及び IO3 の化学形態を取吸収端（33.164 keV）の X 線吸収端近傍構造（XANES）るため、水産物を介した現実的な被ばく線量を評価測定を行い、マナマコ中の安定ヨウ素の存在形態をする際には、化学形態別の濃縮係数を用いる必要が調べた。

--ある。そこで本調査では、海水から六ヶ所村周辺の海産物（緑藻、褐藻、貝類及び底生生物等）への放3. 成果の概要

射性ヨウ素の濃縮係数を化学形態別に（I 又は IO3 ）3.1 水産物における放射性ヨウ素の形態別濃縮係求める。これまでに緑藻及び褐藻の形態別濃縮係数数--を求めており、平成 27 年度は、マナマコ(Apostichopus

1 度目及び 2 度目の実験結果をそれぞれ図 1 及びjaponicus) を対象に、化学形態の異なる放射性ヨウ

図 2 に示す。125I-添加海水中の 125I-濃度減少速度は 1素(125I- 又は 125IO3-)を添加した海水を用いて濃縮係度目の実験では 2 度目より速く、一方、マナマコ中数を求めた。また、大型放射光施設 SPring-8 を用い125 -てマナマコ中のヨウ素の化学形態を検討した。

ついては不明である。

I 濃度上昇速度も速くなっているが、その原因に
図 3 に示した代謝モデルにより、海水中 125I とマ

2. 方法

ナマコ中 125I の間の移行速度は移行元の濃度に比例

平内町漁業協同組合が漁獲したマナマコを入手するものとし、移行パラメータ求めた。得られた値

し、実験に用いるまで飼育用水槽で 1 か月間飼育しを表 1 に示す。125I-と 125IO3-について行った 1 度目のた。実験開始時時のマナマコの平均体重は 3.31.8 g

実験結果をまとめて解析した結果、k1=6.E1、k2=6.及であり、 I 又はIO3 を添加した海水を入れたばび k3=1.E-1 となり、125I と 125IO3-の濃縮係数はそれぞく露用水槽中に、マナマコ 12 個体を入れ、無投餌でれ 6.E2 及び 6.E1 であった。2 度目の実験結果から飼育を行った。継時的にマナマコ 3 個体と海水試料は、k1=5.、k2=1.及び k3=3.E-2 となり、125I と 125IO3-のを採取し、マナマコ体内 125I 濃度を測定するととも濃縮係数はそれぞれ 2.E2 及び 5.E1 であった。得らに、海水中れた濃縮係数には幅があるが、I-が IO3-に比べて吸収

125 -

125-

I を陰イオン交換樹脂カラムにより125 -

125 -Iと 125IO3-に分離して測定した。
1 度目の実験では、マナマコをばく露開始後 24、48 及び 72 時間に採取・分析を行ったところ、24 時間でマナマコ中I 濃度は概ね飽和していた。この125ため、2 度目の実験ではばく露開始後 6、12、24 及
び 48 時間に採取・分析を行った。

されやすいことが明らかとなった。
3.2 マナマコ中の安定ヨウ素の化学形態分析
XANES スペクトル測定結果より、マナマコに含まれるヨウ素の大部分が有機態であることが示唆された。

a)

b)

Fig. 1

Measured 125I concentration in sea cucumber and seawater during the first experiment. Open squareshows 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.

a)Fig. 2b)

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 1Fig. 3

Metabolic model of 125I in sea cucumber.

Obtained transfer-rate coefficients.

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

1st experiment

6.E1

6.

1.E-1

2nd experiment5.

1.

3.E-2