An accident occurs even if you do everything you can to prevent it-this does not represent fatalism, but rather it means to make assurance double sure, which is often used as a caution for personnel who work on taking safety measures. In most cases, however, actual accidents occurred as human intellect was not exploited.

In the 1970s-about 40 years ago-nuclear power plants were booming around the world and were actively constructed in many countries. Of the six reactor units at the Fukushima Daiichi Nuclear Power Plant, Unit 1 started operating in 1971. Since then, the other units commenced operation in 1974, 76, 78 (Units 4 and 5), and 78, in sequence. Let us look back over the arguments regarding safety in the 1970s.

In 1975, the United States Nuclear Regulatory Commission published a report entitled Reactor Safety Study, which is a well-known report evaluating the risks of nuclear accidents. Following this report, people who promote nuclear power generation have argued that an authoritative agency assured the safety of nuclear power plants in this report. Those who have doubts about this report have soberly examined the data supporting the report and pointed out problems (some members in the Nuclear Regulatory Commission also criticized the report as being overly optimistic).

The report estimated the probability of core melt to be once in 20,000 reactor years. This means that if a reactor is operated for 20,000 years, core melt occurs at least once during the same period.

Based on estimate, some maintained that the probability of a nuclear plant accident is approximately the same as that of two jumbo jet airplanes colliding with each other. Of course, this was an argument implying that such an accident was impossible to occur. Just such an accident actually occurred all too soon, however. How scary reality is! This was the Tenerife airport disaster in 1977, in which KLM and Pan Am jumbo jets collided. This accident is regarded as the worst airplane accident in history.

Now, let us turn to nuclear power plants. According to scientists who examined the report mentioned above, the process of calculating the probability of core melt contained some errors, and correcting them would generate the result that core melt occurs once in 8,000 reactor years, instead of 20,000 reactor years (In fact, some scientists challenge the assumptions of the calculation itself in the report, but now we will move the discussion forward irrespective of this point).

More than thirty years have passed since that argument. There are more than 500 nuclear power plants in the world. The average age of those existing plants (the number of years after commencement of operation) is about 24 years. Many plants have already been decommissioned. Now, let us count the reactor years of all the nuclear power plants in the world as a whole. It would be reasonable to use an approximation of 500 plants operating for 30 years. This gives us 500 plants × 30 years = 15,000 reactor years. Even based on the estimates in the report mentioned above-regarded as overly optimistic-it can be concluded that core melt could have occurred about twice.

In reality, Unit 2 of The Three Mile Island Nuclear Station (1979) and Unit 4 of the Chernobyl Nuclear Power Plant (1986) caused a serious accident, though the latter had a different type of reactor. Currently, it is said that Units 1 to 3 caused core melt at Fukushima. The report, which argued 36 years ago that an accident was unlikely to occur, can now be interpreted to reflect that accidents may often occur given today's proliferation of nuclear power plants.

In countries that are advanced in terms of nuclear power generation, the total number of nuclear plants will continue decreasing gradually, since decrepit plants will be decommissioned one after another even though new nuclear plants are steadily built as scheduled. Among emerging countries that pin their hopes on nuclear power generation, about 60 nations reportedly desire deployment of new nuclear plants. Jukka Laaksonen, the vice chair of the International Nuclear Safety Group (INSAG), a group of experts at the IAEA, said that only five or six nations among them have the skills to introduce atomic energy. The reality seems harsh, though we might also be able to expect that the passage of time might enhance the reliability of nuclear power plants.

In the past, it was estimated that the probability of a nuclear power plant accident was about the same as that of a plane falling on a nuclear plant. A battle plane that was believed to be from Israel, however, did in fact destroy a reactor in Iraq that was about to start operation (Operation Babylon in 1981). This military operation of bombing a nuclear reactor was a tremendous shock on the people involved, because it makes us anticipate a nuclear attack without using nuclear weapons. Furthermore, the radiation released may exceed even that released from an atomic bomb. While earthquakes and tsunamis are of course major concerns, terrorism is also likely to cause such a serious event from the international point of view. In the current accident in Japan, the total loss of all the power sources-a weakness of a nuclear power plant with defense in depth-brought the lethal outcome. Fukushima finally revealed this awful fact. Some have pointed out that national governments might be involved in activities to destroy nuclear facilities, such as the cyber attack using a computer virus that targeted a uranium concentration facility in Iran in 2010.

Atomic energy is surrounded by momentous situations. Arguments regarding the safety of nuclear power plants definitely require an accurately understanding of the diversified risks lurking therein.