Introduction: Basic concepts regarding the Fukushima accident and radiation and radioactivity

The accident of the Fukushima Daiichi (First) Nuclear Power Station (FDNPS) of the Tokyo Electric Power Company (hereafter, Fukushima accident) transpired after the Tohoku Region Pacific Coast Earthquake occurred in March 2011. Table 1.1 summarises the main events of the accident. After the earthquake occurred at 14:46 on 11 March 2011, tsunami waves of 13 m in height arrived at the FDNPS (TEPCO, 2011); the diesel power engine stopped at 15:41. Due to this electricity loss, the nuclear reaction became uncontrollable. The Fukushima Daini (Second) Power Station was able to make a controlled stop for cooling even after the intrusion of seawater from a tsunami wave with a height of 9 m. The estimated maximum height in the design of the Daiichi and Daini Power Stations was 5.1 m. In contrast, the estimated maximum tsunami height in the design of the Onagawa Nuclear Power Station of the Tohoku Electric Power Company, which avoided serious damage, was 14.8 m (Matsumoto, 2007).