[M-AG38_2AM2] 福島原発事故により放出された放射性核種の環境動態

[MAG38-12] 福島第一原子力発電所事故前後における土壌中の¹²⁹Iと¹³⁷Csの深度分布

*松中 哲也¹、笹 公和¹、末木 啓介¹、高橋 努¹、松村 万寿美¹、佐藤 志彦¹、柴山 尚大¹、北川 潤一²、木下 哲一³、松崎 浩之⁴ (1.筑波大学、2.高エネルギー加速器研究機構、3.清水建設、4.東京大学)

キーワード：福島第一原子力発電所事故, 放射性ヨウ素, 放射性セシウム, 加速器質量分析, ガンマ線測定, 土壌分布

Massive nuclear fission products such as radioiodine and radiocesium were deposited on the land surface of Fukushima via radioactive pollution plumes derived from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. In order to evaluate inventory and penetration of accident-derived ¹²⁹I and ¹³⁷Cs in the land surface, depth profiles of ¹²⁹I, ¹²⁹I / ¹²⁷I atomic ratio and ¹³⁷Cs in 30-cm-long soil cores before (May 2008) and after (November 2012) the accident were compared at two sites (Iw-2 and Iw-8) on the western area within 10 km from the FDNPP. Total ¹²⁹I inventories in soil core at two sites after the accident were estimated to be 0.74 - 1.96 Bq m^-2, 14 - 34 times higher than those before the accident (53.6 - 57.0 mBq m^-2). Average ¹²⁹I / ¹²⁷I ratios ((1.4 - 6.2) × 10^-7) in soil core after the accident were consistent with the ¹²⁹I / ¹²⁷I ratio of the radioactively-contaminated surface soils in Fukushima (1.5 × 10^-8 - 7.2 × 10^-6, Miyake et al., 2012). We also estimated that total ¹³⁷Cs inventories after the accident were 0.60 - 3.15 MBq m^-2, 280 - 470 times higher than those before the accident (2.1 - 6.7 kBq m^-2). Average ¹³⁴Cs / ¹³⁷Cs activity ratios (1.07 - 1.08) in soil core fell within the activity ratio in Unit 1 - 3 (0.94 - 1.08) of the FDNPP calculated by ORIGEN2 code (Nishihara et al., 2012). These results suggested that accurate total inventories of accident-derived ¹²⁹I and ¹³⁷Cs in soil could be determined by deduction of those backgrounds at almost same site, thus, the FDNPP accident caused ¹²⁹I deposition of 0.69 - 1.90 Bq m^-2 and ¹³⁷Cs deposition of 0.59 - 3.14 MBq m^-2 on the western area within 10 km from the FDNPP. Moreover, deposited ¹²⁹I and ¹³⁷Cs at Iw-2 (4.2 km west from the FDNPP) were respectively, 2.9 and 5.3 times higher than those at Iw-8 (8.4 km west from the FDNPP).Depth profiles of ¹²⁹I concentration, ¹²⁹I / ¹²⁷I atomic ratio and ¹³⁷Cs concentration before the accident were essentially declined from upper layer with depth at two sites. On the basis of the highest values in these profiles, background levels were determined to be 420 ± 11 Bq kg^-1 for ¹²⁹I, 1.6 ± 0.1 × 10^-8 for ¹²⁹I / ¹²⁷I and 48 ± 2.5 Bq kg^-1 for ¹³⁷Cs. After the accident, significant elevated values of ¹²⁹I (40.2 - 130 mBq kg^-1), ¹²⁹I / ¹²⁷I ((0.9 - 9.3) × 10^-6) and ¹³⁷Cs (44.6 - 255 kBq kg^-1) were found in the uppermost layer at the two sites, then these profiles exponentially declined with depth. Approximately 90% of deposited ¹²⁹I and ¹³⁷Cs at two sites were absorbed upper 37.4 - 50.5 kg m^-2 (4.1 - 4.3 cm) and upper 13.3 - 21.3 kg m^-2 (1.0 - 3.1 cm) in depth, respectively. In addition, since the relaxation mass depths (h₀) of ¹²⁹I were 9.2 - 12.8 kg m^-2 greater than those of ¹³⁷Cs (6.8 - 11.7 kg m^-2) at two site, radioiodine was considered to penetrate slightly deeper than radiocesium in upper layer of both sites as Kato et al. (2012) found at 40 km northwestern site from the FDNPP. This is not contradicting to increasing tendency of ¹²⁹I / ¹³⁷Cs activity ratio with depth at both sites. Based on the fact that both ¹²⁹I and ¹²⁹I / ¹²⁷I in soil after the accident declined to a background level under 84.8 kg m^-2 in depth at Iw-2 and under 133 kg m^-2 in depth at Iw-8, about 8 - 9% of accident-derived ¹²⁹I were likely to penetrated 37.4 - 84.8 kg m^-2 (4.3 - 8.6 cm) in depth at Iw-2 and 50.5 - 133 kg m^-2 (4.1 - 10.2 cm) in depth at Iw-8.