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Radionuclides Released from the Fukushima Daiichi Nuclear Power Plant

Radionuclides released into the environment as a result of the nuclear accident were: iodine-131 (131I), iodine-133 (133I), cesium-134 (134Cs), cesium-137 (137Cs), and tellurium-132 (132Te) [43]. Other radionuclides of concern included strontium (90Sr), yttrium (90Y), lanthanide fission products, and actinides, but none of these have been measured in any detectable quantities within or beyond the established evacuation zone [10]. Most releases of noble gases (i.e. 133Xe) would have occurred in the early days after the accident [3]. It is estimated that 160, 88, 18, and 15 PBq of 131I, 132Te, 134Cs, and 137Cs, respectively, were discharged from the Fukushima Daiichi NPP into the environment [43].

A primary health concern for internal exposure to 131I is the potential development of thyroid cancer, since the thyroid gland is most sensitive to 131I [44]. Examples of deterministic health effects induced by inhalation of β-emitting 131I include bone marrow depression (1–10 Gy), hypothyroidism (10–100 Gy), and ablation of the thyroid gland (100–100 Gy). Increased stochastic effects induced by the inhalation of 131I are estimated to be observed at an exposure 10–100 Sv [4]. Children are more susceptible than adults to risks of cancer from radiation [11]. For example, children receiving a 100 mSv thyroid dose have a 0.3 % increased risk of developing thyroid cancer [45]. 131I has a half-life of only 8 days, meaning human exposure to an external source of this radionuclide is relatively short [41]. It is volatile and can be inhaled. It can also be ingested because it readily enters the food chain (131I deposits on the ground). Similar to stable iodine, 131I is actively taken up by the thyroid gland. Once 131I is taken up by the thyroid gland, a constant bombardment of surrounding tissue can overwhelm the repair mechanisms of cells and trigger cancer [3]. Tokonamii et al. calculated the median thyroid equivalent dose to 4.2 and 3.5 mSv for children and adults, respectively [44].

Stable iodine tablets were distributed to Fukushima accident evacuees within a week after the accident. An oral dose of stable iodine blocks the uptake of 131I by the thyroid, although the timing of the intake of stable iodine relative to exposure is important to optimize the effect of this protective measure [38]. Nagataki reviewed the results of thyroid equivalent doses in the initial phase of the accident in the most affected areas of Fukushima prefecture and concluded that 96 % of the children received <10 mSv, with a maximum of 35 mSv, which is lower than the IAEA intervention level (50 mSv) [46, 47]. It should be noted, however, that any increase in thyroid cancer cases may not be evident until several years following the incident (as was the case in the children and adolescent age groups in the Chernobyl region) [41]. 134Cs and 137Cs, with a half-life of 2.1 and 30.2 years, respectively, pose a longterm threat since they remain on the ground [48]. Examples of deterministic health effects induced by inhalation of β-γ emitting 137Cs include mild bone marrow depression and erythema (1–10 Gy), bone marrow failure, pneumonitis, and GI failure (10–1,000 Gy), with a very high risk of death above 100 Gy. Increased stochastic effects induced by inhalation of 137Cs is estimated to occur at a dose of 1 Sv [4]. Current recommended decorporation therapy in the event of cesium intake is oral administration of Prussian Blue. Overall, solubility of particles affects the biokinetics in the body. Soluble forms would be better absorbed into the blood and result in higher content in tissues. The system biokinetics of Cs is similar to that of K, although Cs does not cross cell membranes as readily as K does. Inhaled or ingested, Cs is readily absorbed either from the GI tract or the lungs and is subsequently taken up by most tissues [10]. Upon reaching the systemic circulation, Cs distributes uniformly in the body, with a higher concentration in skeletal muscle than in most other tissues [4]. According to the Japanese Ministry of Health, Labor and Welfare, radioactive cesium in foods is less than 1 % of 1 mSv/year as of April 2014, and that

radiation levels in public water supplies are below allowable limits [49].

The third largest source of radioactivity released from the Fukushima Daiichi NPP is 132Te. This radionuclide has a half-life of 3.2 days and decays to 132I, which has a half-life of 2.3 h, and then becomes 132Xe, which is a stable isotope. Hence, 132Te is biologically relevant during the first few days after a nuclear accident [43].

Health Effects and Consequences

Taking into account uncertainties associated with the LNT model of human exposure at low doses, Ten Hoeve and Jacobson used the model to quantify long term health effects. They factored in ingestion exposure, inhalation exposure, and external exposure pathways of radioactive 131I, 137Cs, and 134Cs released from Fukushima. They estimated 130 mortalities and 180 morbidities related to cancer, chiefly in the most affected areas of Fukushima. These estimates do not account for the increased radiation risk for roughly 20,000 workers at the plant in the months following the accident [39].

Because most people were exposed to radiation doses that were just slightly above background, attributing carcinogenic effects to radiation exposure from the Fukushima accident is difficult [32, 50]. This challenge is mainly due to the multitude of variables that should be taken into consideration, such as smoking, diet, geographical location, etc. Furthermore, cellular damage incurred by irradiation may not manifest until many years after exposure. Some researchers assert that even a well-implemented study will not yield statistically significant data on stochastic effects, such as cancer. It should also be noted that 40 % of all Japanese develop cancer [32].

It is also important to consider the shortand long-term psychological effects following a devastating accident. The intangible nature of radiation exposure heightens the public's feelings of fear and vulnerability [51]. The Chernobyl disaster has illustrated that long-term psychological effects, including post-traumatic stress disorder, depression, anxiety, fear, and unexplained physical symptoms, may increase following a nuclear accident [12, 39, 51].

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