Thank you very much for introducing me.I was asked from Prof. Okamoto to give a speech at NUTHOS-11, but he said my speech time was less than 25 min. including Q and A time.I would like to have questions and answers time enough, so try my speech about 20 or 25 minutes.So we will have enough Q and A time.Japan experienced the severe accidents at Fukushima-Daiichi nuclear power station.At that time, I chaired Nuclear Safety Commission, and watched inadequate countermeasures.Today, I will talk what I wrote in the summary with adding my personal impressions.I am afraid that my presentation is a lack of order and tidiness, but I hope it contains a lot of lessons learned from my experience.

Why we experienced severe accidents?I think there were three main reasons: insufficient tsunami countermeasures, insufficient countermeasures against severe accidents, and insufficient emergency countermeasures.Perhaps there were other reasons, but these three were the main, I think.In the following, I will explain why we could not have solved the issues.

Why tsunami countermeasures were insufficient?One reason is both TMI and Chernobyl accidents were caused by internal factors. But there are many other reasons.I know large typhoon attacked Busan area last week, and you experienced large earthquakes of M5.8 this year here in Gyeongju. Japan experienced earthquake of M9.0, perhaps you cannot imagine. In Japan, we have several types of natural disasters, typhoon, earthquake, volcano, so on.Some seismologists warned that huge earthquakes occurred soon in Japan. Seismic engineers made efforts to construct earthquake-resistant nuclear stations. I think they succeeded.Of course we have tsunami often. However, our tsunami experiences are less compared with earthquakes.One more reason! Estimation of natural disasters contains large uncertainties.I knew well that IAEA INSAG-10 demanded preparations not only for internal events but also external causes. I chaired NSC for the regulation system innovation. But I considered the innovation should have been done step by step. It was my mistake. I regret it.

This slide shows trial results of seismic PRA.There are large uncertainties.I frankly confess that before the accident I considered preparations for internal causes were the first. Because, we did not satisfy the international obligation of preparations for even internal causes.The preparations for external causes were the next, I considered.That was also my mistake.

In the safety analysis, either deterministic or probabilistic, we must take every possible event in consideration.The large uncertainties cannot be the reason to avoid the events from the consideration.I regret strongly that I considered the preparation for external causes was the next.Let me explain one more reason.We, Nuclear Safety Commission members, were not informed that NISA, Nuclear and Industrial Safety Agency, was worrying about tsunami.NISA was the regulatory agency, and should be supervised by NSC.Of course, they did not like it.It was quite natural that they did not tell us frankly.With these things Japan wasted time for finding a reason why measures against tsunami were difficult.I doubt some people thought countermeasures were unnecessary for a while, the while means "not in their term of office."

Then let us move to the next issue.Why severe accident measures were insufficient?Severe accidents were studied widely in Japan, but there were no regulatory requirements.The countermeasures were left to voluntary efforts of power companies.They did take measures if they believed the accidents happened actually.But very few people believed so.Besides, some people believed that recognition of the severe accident possibility would be misinterpreted as non-satisfaction of DBA(design basis accident)-based regulation.The law required “adequate prevention of disasters.”I heard with my ears them saying that litigation risk was higher than actual accident risk.

But adequate prevention of disasters does not necessarily mean adequate safety.If requirements for adequate safety were presented, then this problem could have been easily solved.I want to know the reason why the requirements were not presented.

This slide shows the severe accident research items.We experienced hydrogen explosions in the reactor buildings at Fukushima-Daiichi.But you can see hydrogen combustion here.But the possibility of hydrogen explosion was already warned by some articles for example by Manninen et al. in Nuclear Engineering and Design.I honestly confess that I read the articles after the accident.After watching the explosion on TV, I understood the reason why the explosion of reactor building occurred.So I explained the mechanism; water and zircaloy interacted, the containment vessel was kept high pressure and temperature long time, hydrogen leaked into the reactor building …But I was strongly criticized that I did not explain the possibility of explosion before watching TV.How they behaved, if I have found the possibility.Anyway, I was busy answering the politicians’ questions.I did not have a time to think about the possibility of explosion.

Here is a list of severe accident programs done in Japan.ALPHA program, VEGA program, verification tests, and so on.But I have not finished it yet.There may have been some more important projects.Please think these are merely examples.

We should have improved the regulation system with making use of the research results.Research for research was meaningless.The disaster occurs any time, so I had to make regulation changes as soon as possible.I dare to say that I tried my best to change Japanese safety standard so as to satisfy the international obligations.But I had only 10.5 months for regulatory system innovation.I am still thinking what I could have done before the accident.

Before worrying about the litigation risk, NSC, my predecessors, should have presented the requirements for adequate safety.I want to ask them why they did not do so, but they never answer my question.Let me add one more important problem.The severe accident researchers’ knowledge was not utilized unless the decision makers’ comprehension.It is important to build up a system for advising their knowledge to the decision makers.In case of emergency, misunderstanding produces other misunderstandings.They thought re-criticality meant nuclear explosion.Perhaps you cannot imagine.I knew their misunderstandings after I read their books more than 1 year after the accident.In a certain conference, I talked this episode, but a non-expert audience said it was a very natural reaction.Communication between experts and non-experts is very difficult.

This is the last issue.Why emergency measures were insufficient in Japan?The measures against simultaneous natural disaster and nuclear accident were ignored in Japan.I heard that nuclear budget could not be used for other purpose.The evacuation in the case of tsunami is unrelated to nuclear development.The bureaucrat cares about such a trifle.Here is one episode.The former head of secretariat of NSC took office as the director of NISA, and pressured the predefinition of Precautionary Action Zones.He was definitely a nuclear promoting person.It was very difficult even for NS commissioners to ignore his pressure because it was impossible to meet their obligations without secretariats’ collaboration.Therefor Precautionary Action Zones were not predefined in Japan.

Why the former head of secretariat of NSC took office as the director of NISA?I have to say that independence of NSC from NISA was not enough.We learned the next important lesson.The countermeasures against simultaneous natural disaster and nuclear accident should have been considered so as to be included in emergency drills.I have to say that our disaster drills were meaningless.We should have performed emergency drills for simultaneous natural disaster and nuclear accident.We could have solved the budgetary problem easily.

Here I summarized my presentation.Appropriate regulatory system and the enforcement are the key factors for preventing severe accidents.Severe accidents were studied widely in Japan, but we did not make use of the results in regulation.Perhaps Japan wasted time for finding a reason why it was not necessary to measure against severe accidents.The next item is very important.In order to avoid confusion, I want nuclear regulatory office members to have a minimum knowledge about severe accidents.In the case of Fukushima accident, they did not have a minimum knowledge, which caused many problems.

It is impossible to prevent severe accidents without prior preparation.Without prior preparation, we could not prevent severe accident.That is all I learned from my experience.This is only my personal impression, but the root cause investigation why Japan ignored the possibility of severe accident is still insufficient.Those who should take responsibility have not talked yet about the reason why they ignored the possibility of severe accidents.I think that Japanese culture in which apologizing people are exempted from cross examinations is in the background.There were two or more investigation committees, but they excluded the persons concerned.People who do not have enough knowledge can never reveal the root cause.The investigation of the root cause is the first step of the severe accident prevention.I am afraid that Japanese culture is not suited for nuclear energy development.But I still believe that nuclear power is necessary.Thank you very much.