Current Situation of the Verification of the Fukushima Nuclear Accident by Niigata Prefecture Technical Committee

English summary of an article by Tanaka Mitsuhiko (Member of the Niigata Prefecture Technical Committee and former Member of the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission (NAIIC))


The Niigata Prefecture Technical Committee (TC) began the work of verification of the Tokyo Electric Power Company (TEPCO) Fukushima Daiichi Nuclear Power Station (FDNPS) accident in earnest in the autumn of 2013, about two-and-a-half years after the accident occurred. Almost seven years has passed since that time. Verification has been carried out on six important problems which have not yet been fully understood. For the verification , six task groups were organized inside the TC in September , 2013, with each group consisting of two to four “core members” The verification work in each task group has been basically performed through discussions with TEPCO about the group’s assigned task. Below are the six assigned tasks.[1]

Task 1. Impacts on important equipment due to seismic motion[2]

Task 2. Decision-making at important junctures, such as the injection of seawater

Task 3. TEPCO’s accident response management

Task 4. The nature of information dissemination, e.g. regarding the meltdowns

Task 5. Work under the high dose rate regime

Task 6. The response to severe accidents

Below, I would like to report on the current state of the verification work in Task 1 Group 1 (Task 1G), of which I am a core member.

From Open-session Task 1G to Closed-session “tripartite meetings”

In contrast to moves in society as a whole, which attempt to explain everything that occurred in the Fukushima accident as due to the “tsunami” only, I believe Task 1 stems from naturally raised questions such as ‘whether, for example, important equipment and piping were damaged by intense shaking due to the earthquake, and whether or not this had a great impact on the accident as a whole.’  However, the construct of tsunami vs. earthquake itself is a problem that constricts the scope of thought. Even if the trigger (direct cause) of the accident was the onslaught and drawback of the tsunami, or the intense shaking of equipment and piping arising from the huge earthquake, there is still the possibility that other factors unrelated to the tsunami and earthquake were strongly implicated in the progression of the accident after that. These might be, for instance, the problem of the material strength of piping and bolts, etc. in the extreme high-temperature environment under a severe accident, or whether or not what is written in the operational safety program and operation procedures were correctly observed, but as one of the core members I believe that these kinds of problems should be actively discussed in Task1D, and have been so. I am also aware that there was criticism that this kind of discussion would only draw out the time taken for the verification work. The verification of the Fukushima accident conducted by the above mentioned task-based discussions has as its sole purpose the discovery of “what Niigata Prefecture, which hosts Kashiwazaki-Kariwa Nuclear Power Station (KKNPS), now aiming for a restart, should learn” from the accident and not a verification to investigate the causes of the Fukushima accident per se. However, it is impossible to avoid discussion of the latent risk factors.

Now, I believe that, depending on the outcomes, in order to avoid misunderstanding by the general public due to holding discussions on important problems in closed session, the discussions should, as far as possible, be held with the public in attendance. I do not know if it was this thinking that was accepted, but, in contrast to the other task-based discussions being held in principle in closed session, Task 1G applied the principle of open-session discussions. If my memory is correct, Task 1G held 13 meetings, but was only once held in closed session.[3] However, as the number of meetings increased, I began to feel keenly that key discussions were not going deep enough due to being held in public. The first reason is that, in the open sessions, we just cannot persuade TEPCO to disclose their confidential materials that are really necessary to deepen discussions. One further reason is that it is not possible to have calm one-on-one discussions with TEPCO.

When I told the TC secretariat about this at the beginning of last year, with the approval of Chairperson Nakajima of the TC, from April last year at a pace of about one meeting a month, and with the TC secretariat in attendance, I began to hold one-on-one discussions with TEPCO while being allowed to see TEPCO confidential materials related to the discussion. What is being discussed in these “tripartite meetings” are matters that have mostly been already discussed in the Task 1G open-session, but at these tripartite meetings, while having the confidential materials presented, detailed discussions – within the limits allowed today by the “3Cs” – were held on whether there might be problems or contradictions with the explanations that TEPCO had given thus far.[4] The purpose of this, naturally, was to increase the degree of certainty of the verification outcomes.

Below are the “items for discussion” that I presented for the first tripartite meeting. However, it is necessary to understand that these items are not necessarily independent issues. Especially, it is possible that 1) to 4), depending on the case, have mutual causalities.

1) Reliability of the measured data for Unit 1 reactor pressure

2) Possibility of Unit 1 reactor steam release (SR) valve failing to operate

3) Seal function of Unit 1 reactor pressure vessel main flanges under the condition of a severe accident

4) Water release incident on 4th floor of Unit 1 reactor building immediately after the occurrence of the earthquake

5) Relation between the station blackout (SBO) and the onslaught of the tsunami

6) Possibility of damage to Unit 2 pressure containment vessel (PCV) due to the earthquake

7) Confirmation of several items related to TEPCO’s safety program and operating procedures

That this kind of closed-session discussion had been ongoing since April of last year as a part of the Task 1G verification work was made public at the 13th Task 1G open session held in Niigata on March 18, 2020 (only media-related observers were permitted due to the COVID-19 problem) and is now so-called common knowledge. On that day I gave an outline explanation of the discussions that had been held thus far entitled “Current Report on the Tripartite Meetings (March 18, 2020).” However, I am unable to state here the details of these discussions. The reason is that many of the discussions were held on the basis of confidential TEPCO materials which I am not permitted to divulge to third persons and that I simply cannot make a unilateral full disclosure of what were originally closed-session discussions.

Below, therefore, from the “Current Report on the Tripartite Meetings (March 18, 2020)” mentioned above, especially regarding the “seal function of (FDNPS) Unit 1 reactor pressure vessel main flanges under the condition of a severe accident” (see 3) above) that I believe to be a serious issue relating to a restart of KKNPS, I would like to state here in some detail based only on the knowledge that I have personally held since before the tripartite talks with TEPCO.

Was the seal function of (FDNPS) Unit 1 reactor pressure vessel main flanges maintained?

This issue has not been discussed in past Task 1G sessions, but, despite this, the reason why I made a point of including it in the tripartite meeting items to be discussed was that it was an issue that I had found very troubling personally since immediately after the accident. While it troubled me, it had not been discussed in either the NAIIC or other Fukushima accident investigative reports by experts. To my knowledge, whether inside Japan or abroad, this issue has never been addressed. Thus, I decided to include it in the tripartite meeting items to be discussed.

As stated in the “Current Report on the Tripartite Meetings (March 18, 2020),” according to TEPCO, “We believe there is a possibility that steam and radioactive gasses leaked from the main flanges, but have not especially investigated this.” There is a possibility, but it has not been investigated – it can only be said that this is a truly stunning statement.

  According to nuclear power proponents, the reactor pressure vessel (RPV) is one of the ‘five walls of protection’ that prevents radioactive materials from escaping into the environment. The top of the RPV can be opened for refueling purposes, etc. but the lid section is sealed in place by a specially designed flange. The flange must be able to withstand the design temperature and pressure of the nuclear reactor, but in the case of a severe accident, the reactor core temperature may reach around 3000℃.

  As someone who worked extensively on the design of the main flanges for Fukushima Daiichi Unit 4 back in the 1970s, I have serious concerns that the sealing function of the RPV main flanges was not maintained in the severe accident at Fukushima Daiichi. This is a crucial issue for all boiling-water reactors (BWR), including even improved models such as KKNPS Units 6 and 7. This underlying risk factor must be thoroughly investigated and resolved before the restart of any BWR.

(Tanaka Mitsuhiko is now writing a detailed technical analysis of the design problems which he believes could have led to the failure of the RPV main flanges’ sealing function during the severe accident at Fukushima Daiichi. This analysis will be available online in Japanese. We will post the link as soon as it is available) .

[1] Of the six task groups, with the exception of Task 1G, Task Gs 2 to 5 essentially completed their verifications by February 2016. Only the verification in Task 1G is currently ongoing.


[2] The term “seismic motion” refers to a rocking motion of the ground surface. Personally, I feel that there is a slight problem with the use of this term here.


[3] My recollection is that the third Task 1G, held on April 28, 2014, was held in closed session. It seems this was because the meeting had the special purpose of hearing individual explanations related to the time of arrival of the tsunami from TEPCO and Ito Yoshinori, at the time an NAIIC cooperating investigative member and who was the first to point out that there was a problem in the relation between the tsunami and station blackout (SBO).


[4] Meetings were frequently held in a conference room at the Tokyo office of Niigata Prefecture, but after the outbreak of COVID-19, the location was changed and discussions were held so as to avoid “3Cs” issues (closed spaces, crowded places, close-contact settings).



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