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Report on the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission (NAIIC)
~What kind of panel was the NAIIC, and what was clarified by its investigation?~

The report can be downloaded from NAIIC’s Website:
Most of English language version is still being prepared.
by Mitsuhiko Tanaka*

Mission Impossible

    The National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) was established on December 8, 2011. It was probably a mere coincidence that this commission, the first Diet investigation panel comprised of private-sector experts in the history of Japan’s constitutional government, was set up on the anniversary of the outbreak of the Pacific War.

    The commission was set up under the Law on the Fukushima Nuclear Accident Independent Investigation Commission of the National Diet of Japan. The commission is required to submit to the presidents of both houses of the Diet the report on the results of its investigation and recommendations within around six months from the appointment of the commission chairman and its members.

    In the press conference held immediately after the swearing-in ceremony for the commission members, Chairman Kiyoshi Kurokawa expressed his bewilderment at the extremely short period of merely “six months” provided in the law for investigating the nuclear accident and compiling the report on the results. He described this difficult task as “a virtual mission impossible.”

    The amazingly short period was not the only concern for me. Another source of concern which seemed much greater and more serious was the fact that there were no experts on nuclear power plants among the commission members. Although the commission was tasked with the unprecedented job of investigating the disastrous accident at the Fukushima nuclear power plant, could its members manage to complete the job as scheduled?   
    The mass media described me as the only expert on nuclear power plants among the commission members. But this is not correct. Admittedly, I was engaged in the design of pressure vessels for boiling-water type nuclear reactors for several years up until the mid-1970s. During that time, I was charged with the detailed design of the No. 4 reactor at the Fukushima Daiichi Nuclear Power Station, and the basic design of the reactor pressure vessel at Tokai Daini Nuclear Power Station. But I resigned from the company shortly after that, and 35 years have already passed since that time.

    In addition, there was another problem. I felt I was under great pressure concerning the issue of the causal relationship between the massive earthquake and the Fukushima nuclear accident. At the time when I became a member of the commission, it was already evident that I was regarded as having a conviction that the earthquake had triggered the disaster.

    This, however, was more than natural because I began pointing out, two weeks after the hydrogen explosion occurred in the Unit 1 reactor at the Fukushima nuclear power plant, the possibility that the earthquake had damaged the reactor system piping and caused a loss of coolant.(1) Since then, I have written about my conviction in many books and magazines and talked about it in meetings and lectures on numerous occasions.(2)

    But I thought that, depending on the circumstances, while investigating as a member of the commission, I might be forced to deny my own belief. This would mean that I might have to hand down the final judgment on my own belief in the full view of the public, and this would not be a pleasant thing to do.


    The commission chairman and its members are bound to maintain the confidentiality of any facts they have learned in the course of performing their duties. It also states that the condition of confidentiality continues even after the members resign from the commission.

    This means that the members are not allowed to leak any information to third parties, including both facts they have learned through the hearings, or through reading the documents submitted to the commission. A member who is well versed in legal affairs said it is not a violation of confidentiality to write down the results of the investigation in the commission’s report because the members are required to compile the report. However, the members are not allowed to disclose facts not included in the report, and must maintain the confidentiality of such facts for good.

    In connection with confidentiality, I have received admonitions and warnings from the commission’s secretariat several times. For example, when I made a two-day trip to Shimane to give a lecture on nuclear power plants in a study meeting, after I flew back to Tokyo a secretariat official showed me a local newspaper article of the study meeting and asked me if I had maintained confidentiality during the lecture.

    One of the staff members supporting the commission’s investigation was accused of violating confidentiality, and his contract with the commission was suddenly cancelled in mid-February. He had talked about the investigation method to be adopted by the commission in a lecture. He never thought that the investigation method corresponded to a secret he had learned in the course of performing his duties. Unfortunately for him, a video of the lecture meeting was posted on the Internet. It would seem that, the secretariat attached greater importance to the maintenance of order than to the possible damage the commission might suffer from the loss of the staff member.

TEPCO lied in official announcements about the time when the tsunami hit the Fukushima NPP

    In conducting the investigation, the commission members were divided into four groups. Working Group 1 was in charge of investigating the nuclear accident, Working Group 2 was in charge of investigating the damage, Working Group 3 was in charge of reviewing related policies, and Working Group 4 was in charge of proposing new policies. The first three groups were composed of two members each, both members serving as co-chairman.(3) The horizontal relationship among the working groups was rather weak, and the result was that they took the form of typical vertically divided domains.  

    With regard to the relations between the huge earthquake and the Fukushima nuclear disaster, the commission came to the conclusion that it is difficult to rule out the possibility of the earthquake causing damage to major safety-related equipment in the nuclear power station. In particular, the commission referred to the possibility of a small-scale loss-of-coolant accident (LOCA) in Unit 1, saying that it is difficult to deny that this accident could have occurred. I am one of the commission members who were responsible for this text, and I would like to summarize the main points of this conclusion.
Yoshinori Ito, a lawyer supporting the commission’s investigation and who is well-informed about nuclear power plants, probed very carefully and closely into the allegation that the failure of the Fukushima Daiichi plant’s backup diesel generators (D/G), and ensuing station blackout (SBO), were caused by the tsunami tidal waves alone. These two incidents are believed to be the root causes of the nuclear disaster at the plant.

    His probe disclosed the fact that the times of arrival of the first tsunami wave, at 15:27 on March 11, and the second wave, at 15:35, as announced by TEPCO, were not the times when the waves reached the Fukushima Daiichi Nuclear Power Station. These were the times when the waves hit the wave-height meter installed at a point 1.5 kilometers offshore from the nuclear power plant. Mr. Ito pointed out that the second tsunami wave reached the coast where the nuclear power plant is located, at least about two minutes later, at around 15:37.

    In the commission’s hearing conducted earlier with a TEPCO reactor operator, it was confirmed that the emergency D/G 1A, one of the two backup generators for the Unit 1 reactor, had tripped at 15:36 or earlier. If the actual arrival time of the second, and larger, tsunami wave was two minutes later than the time released by TEPCO, at 15:37, the tripping of the stand-by generator could not have been caused by the second tsunami wave but must be due to some other reason. Moreover, there is a possibility that the Unit 1 reactor emergency D/G 1B and the Unit 2 reactor emergency D/G 2A also tripped before the tsunami waves hit the plant.
    The commission asked TEPCO about this time difference in writing. The utility company replied that the arrival time of the second tsunami wave released by the company, at around 15:35, was the time when the waves hit the offshore wave-height meter, and that the actual time when the wave arrived at the premises of the nuclear power plant is presumed to be about two minutes later, at 15:37 or 15:38.

    Despite this reply to the commission, TEPCO’s final report on the nuclear accident released on June 20 said the arrival time of the first tsunami wave was 15:27, and that of the second wave was 15:35.

Fault Tree Analysis

    In the commission’s report, there is a description of the fault tree analysis (FTA) on page 217. FTA is one of the methods used for identifying the root cause of an accident (e.g. an explosion), or some special phenomenon (e.g. a sudden fall in pressure).

    For example, when some incident such as an explosion (the top event) has occurred, you enumerate the potential causes of the accident, and then list the thinkable causes of those causes. While continuing this process, you add branches to a tree diagram. This is called the “Fault Tree”. Whenever you add a branch that indicates another possible cause, you analyze the probability of a causal relationship between the added cause with the top event and evaluate each cause until you find the most likely cause of the accident.

    The biggest obstacle hampering the investigation into the disastrous accident at Fukushima Daiichi Nuclear Power Station was the fact that no one can go into the reactor containment vessels to carry out a detailed inspection of the situation inside. However, this provides us with a good opportunity to use FTA effectively. In the commission’s report, on pages 220 – 223, there is a description of the FTA analysis carried out to determine whether the seismic ground motions had caused damage to the piping of the Unit 1 reactor. This analysis was made by the Incorporated Administrative Agency Japan Nuclear Energy Safety Organization (JNES). As a result of a series of analyses, they arrived at the perception that they could not deny small-scale damage to the piping (i.e. small-scale loss of coolant). This is a notable result.

No operating noise heard from the main steam radiator safety valve of the Unit 1 reactor!

    In the course of the investigation, Working Group 1 was confronted with another problem which was as serious as the abovementioned arrival times of the tsunami waves. This was the problem involving the testimonies by some TEPCO operators in Unit 1. They testified that they did not hear any operating noise from the main safety relief (SR) valve, not only in the central operation room but also in the nuclear reactor building as well (cf. page 239-243 of the report).

    In the case of Units 2 and 3, the SR valves operated repeatedly from a time immediately after the massive earthquake hit the nuclear power plant. The reactor operators in the central control room told Working Group 1 that they clearly heard the noise from the SR valves whenever the valves resumed operation.

    As for Unit 1, the manual operation of the isolation condenser (IC), which requires electricity for operation, became impossible during the SBO. (The IC is a device used when pressure within the reactor rises, and changes steam inside the reactor into water to reduce the pressure). Because it was not possible to operate the IC manually, the SR valves should have started to work frequently, just as in Units 2 and 3. The operators of Unit 1, however, testified that none of them heard any operating noise, despite the fact that it was extraordinarily quiet all around at that time. What made this difference? Does it happen sometimes that the SR valves resume operation without making any noise?

    In an attempt to get answers to these questions, we sent questionnaires to the officials in charge of reactor operations at the Fukushima Daini Nuclear Power Station (Units 1 to 4), the Onagawa Nuclear Power Station (Units 1 to 3), and the Tokai Daini Nuclear Power Station, and asked if they had heard any noise when the SR valves started operation on March 11, 2011. The operation records of the Onagawa Nuclear Power Station and the Fukushima Daini Nuclear Power Station show that the SR valves worked very frequently on that day. Despite these data, the reactor operators at the Fukushima Daini Nuclear Power Station said they did not hear operating noises from the valves. On the other hand, the reactor operators at the Onagawa Nuclear Power Station said no valve-operation noises were heard in the central control room but such noises were heard in the reactor buildings.

    How was this operating noise generated? It is believed that a hydrodynamical load was generated when a massive amount of steam flowed into the donut-shaped pressure suppression chamber, which is peculiar to the Mark-1 type containment vessel, and shook the huge pressure suppression chamber. Although the Onagawa Nuclear Power Station has the improved model of the Mark-1 type containment vessels, the basic structure of the pressure suppression chamber is the same. Meanwhile the four reactors at the Fukushima Daini Nuclear Power Station have the Mark-2 type containment vessels. Because the Mark-2 type has a different structure, it seems that vibration of the pressure suppression chamber does not occur so easily.

    It is seldom that the SR valves become operational and very few reactor operators are familiar with the operational noise from the valves. An operator at the Fukushima Daiichi Nuclear Power Station Unit 5 told us that he had previously opened the SR valves at Unit 4 on an experimental basis, and that he remembered a loud vibration was felt the moment the steam flowed into the pressure suppression chamber.

* Science writer and former nuclear engineer for Babcock Hitachi.

(1) Mitsuhiko Tanaka “The Fukushima Daiichi Nuclear Power Plant Accident was Never Beyond Assumption,” Sekai, May 2011.
(2) Katsuhiko Ishibashi (ed.), “Abolishing Nuclear Power Plants,” Iwanami Paperback, 2011
(3) In WG1, Mitsuhiko Tanaka and Katsuhiko Ishibashi jointly served as co-chairmen.

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