The Great East Japan Earthquake and Tsunami happened on 11 March 2011, which caused the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The accident resulted in a substantial release of radionuclides, including 131I, 134Cs, and 137Cs, into the atmosphere, causing significant environmental contamination. This was a particular issue in many parts of eastern Japan, especially in the Fukushima Prefecture (Yoshida and Takahashi 2012). Among the above-mentioned radioactive isotopes, 131I is one of the most critical radionuclides to be monitored after an accidental reactor release due to its tendency to accumulate in the human thyroid gland. Because of the short half-life of 131I (8 days), it is difficult to determine its radioactivity within a few months after such an accident. Estimating the effective dose of released 131I is important, but the lack of data on the deposition of 131I immediately following such accidents makes retrospective dosimetry a challenge (Michel et al. 2005). This article summarizes what is known about the amount of 131I released into the environment due to the Fukushima Daiichi Nuclear Power Plant accident and also introduces the Fukushima Health Management Survey Project for the long-term health care of the residents of Fukushima Prefecture.

Knowledge of the total 131I released from the FDNPP accident is critical when estimating the exposure doses for Fukushima residents. However, it is difficult to obtain direct and accurate source information on the amount of 131I released by the FDNPP accident. This is because the radiation monitors were affected by earthquakes and tsunamis, and because the core meltdown of several reactors caused various sources of release from the plant. For this reason, atmospheric dispersion simulations and environmental monitoring were adopted to estimate the 131I release from the Fukushima accident. From this methodology, the total release amount was ~1.5 × 1017 Bq (Fig. 1) (Katata et al. 2015). Using the UNSCEAR dosimetry protocol, the total release of 131I was ~1–5 × 1017 Bq (UNSCEAR 2014).

The behavior of the contamination plume that contained 131I was investigated using an atmospheric dispersion model based on the aforementioned information on the release and on measurements made at various locations (Katata et al. 2015). Tsuruta et al. (2019) summarized the transit periods of major plumes at the FDNPP and reported that the ratios of 131I to 137Cs in the radioactive plumes were divided into three groups: A, having a ratio of 10; B, having a ratio of 75; C, having a ratio of 360. The ratios in group C were much higher than those of groups A and B and were observed from the afternoon of 21 March 2011 to 25 March 2011 in Tokyo, 200 km from the plant. Tsuruta et al. (2019) pointed out that these high 131I concentrations could have been released after emergency cooling water had been supplied to the FDNPP.

After the accident, 131I levels of 210 Bq/kg on 22 March and 190 Bq/kg on 23 March were detected in tap water derived from the Kanamachi Water Treatment Plant in Tokyo, exceeding the provisional standard value of 131I at 100 Bq/kg set by Japan’s Ministry of Health, Labor and Welfare to restrict the intake of tap water by infants. This was probably due to the fact that the timing of the radioiodine plume reaching Tokyo coincided with a period of rainfall, and the rainwater containing radioiodine made its way into rivers which flowed into the water purification plant. In general, iodine exists in rainwater in the form of iodide ions (I) and iodate ions (IO3), which are difficult to remove with activated carbon. This suggests that radioiodine in rainwater flowed into the river and was detected in the tap water after some degree of dilution. The Bureau of Waterworks of the Tokyo Metropolitan Government conducted an experiment to reduce the concentration of radioactive iodine by oxidizing iodide ions through chlorination and then adsorbing them on activated carbon (Ministry of Health, Labor and Welfare 2011).

In order to understand the dispersion of radionuclides, a large-scale soil sampling campaign was organized in June 2011 by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) with the cooperation of many researchers from a variety of universities and institutes. Radiocaesium was detected at all locations, and deposition maps for these nuclides were constructed (Saito et al. 2014). In the case of 131I, however, and because of its short half-life (8 days), there was not enough data to provide regional information on the deposition of 131I.

Muramatsu et al. (2015) focused on the long-lived iodine isotope 129I (half-life of 1.57 × 107 y), analyzed by accelerator mass spectrometry (AMS) from surface soil samples collected by the MEXT team in Fukushima Prefecture. In order to obtain information on the 129I/131I ratio released from the accident, the group determined 129I concentrations in soil samples in which131I concentrations had been previously determined. Analytical results yielded a positive correlation (R2 = 0.82) between the two radioiodine isotopes, and the atomic number ratio of 129I/131I was determined to be 21, after a decay correction from 11 March 2011. Miyake et al. (2015) reported that the 129I/131I in soil samples collected in Fukushima was 26.1 ± 5.8, which is in good agreement with the reported values. Based on these results, 129I concentrations (Bq/kg) in approximately 400 soil samples were determined and the radionuclide deposition densities (Bq/m2) of 131I were then estimated so one could reconstruct a deposition map of 131I (Fig. 2) (Muramatsu et al. 2015). It was found that the areas with high 131I values (>5000 Bq/m2) extended to the northwest and south. Compared to the contamination map of 137Cs, the distribution of 131I was different, and it was reported that131I/137Cs tended to be higher in the south of the plant (Saito et al. 2014; Muramatsu et al. 2015).

Fukushima Prefecture conducted the Fukushima Health Management Survey Project for the purpose of long-term healthcare administration and medical early diagnosis and treatment for the prefecture residents. The large-scale ultrasound screening in Fukushima Prefecture demonstrated a high detection rate of thyroid cancer in young individuals, revealing 187 cases after testing ~300,000 subjects (Yamashita et al. 2018).

The exposure doses for the Fukushima residents were much lower than radiation doses from the accident at Chernobyl (Tokonami et al. 2012). There is no strong evidence that supports a causal relation between thyroid cancer and radiation exposure in Fukushima. However, recent studies report a positive correlation between the incidence of thyroid cancer, the air-dose rate, and 131I in soils (Fig. 3) (Toki et al. 2020). The correlation between 131I in soil and the incidence of thyroid cancer is very weak; therefore, the causal relation of thyroid cancer with radioiodine exposure in Fukushima is still controversial. It is very important when interpretating thyroid cancers in Fukushima to understand the causal relation between thyroid cancer and radiation exposure. Thus, future studies will be needed to avoid any misinterpretation of the high detection rate of childhood thyroid cancer.

I started writing this article after returning from my field research in Fukushima. With the fifth wave of COVID-19 now under control in Japan, I was able to resume my fieldwork after almost two years. I am very grateful to the late Professor Yasuyuki Muramatsu. He was a leading researcher of radioiodine, and, after the FDNPP accident, he was appointed as an advisor to the Fukushima Prefecture. In April 2011, I joined Professor Muramatsu’s laboratory as an assistant professor, and I learned from him not only about radioiodine research but also about behavior that is appropriate for a researcher. The researchers who engage in curiosity-driven research can be of use to society as experts in geochemistry when catastrophes occur. I feel that there is still much left for me to contribute. In particular, the behavior of 129I in the ecosystem needs to be documented. The full-scale operation of the reprocessing facility in Japan is scheduled to start in 2022. Once the facility is operational, a large amount of 129I will be released into the environment but in a safe and controlled manner. As a generation that experienced these events firsthand, it is important to pass on the knowledge we have gained to the next generation.

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