"New facts" in the 12th year of the Fukushima Daiichi Nuclear Power Plant accident March 3 17:22
"The head office told me to stop (Drywell) spray, so I think we ended up giving up on it and telling them to stop."
This is the testimony recorded in the so-called "Yoshida Report" of an interview with the late Masaro Yoshida, who was the director of TEPCO's Fukushima Daiichi Nuclear Power Plant at the time of the accident, at the government's accident investigation and investigation committee.
Several incidents surrounding the accident at the Fukushima Daiichi Nuclear Power Plant, such as seawater injection and withdrawal issues, have been intensively investigated by the public accident investigation committee and communicated to society. However, what we at NHK Meltdown Reporting Team focused on this time was the response to the accident surrounding "Splay". This is a problem that the public accident investigation committee has hardly examined, so to speak, a blind spot in the investigation.
Twelve years after the accident, the investigation of the Fukushima Daiichi Nuclear Power Plant accident has become almost untalked about.
However, by carefully connecting the "dots" such as data from the time of the accident, video conference conversation records, and the situation inside the containment vessel that had been revealed through investigations over the past few years, "new facts" about the 12th year of the accident have emerged.
(NHK Special Meltdown Coverage Team)
* This content will be explained in detail in NHK Special Meltdown File.12 "Second Part: "New Facts" of the 19th Year of the Accident" on Sunday, September 9 at 8 p.m. ~ General TV.
Unit 3 The only "dry wealth play" performed
What caught our attention was the response to Unit 3 on March 13.
After being hit by the tsunami on March 3, the first reactor to melt down nuclear fuel and explode hydrogen was the first reactor. The next crisis hit Unit 3. In the early morning of March 11, the cooling system stopped. After this, the site was concerned about a meltdown, the destruction of the containment vessel, and the release of a large amount of radioactive material. At that time, the pressure in the Unit 1 reactor was so high that it was impossible for fire engines to inject water. What the operators did was a "dry well play" to cool the containment vessel.
The containment vessel is divided into a flask-shaped "dry well" and a donut-shaped "suppression chamber" (commonly known as a sapchan).
According to TEPCO's report, "Spray is absolutely necessary to reduce the increase in drywell pressure! Somebody has to do it." The earnest expectations placed on Splay are recorded as the voices of the field. A diesel-powered fire pump is used to cool the dry well to reduce pressure build-up and prevent destruction. This was the aim of the drivers.
In order to do the spray, you had to be prepared.
Unit 3 had lost most of its power due to the tsunami. The valves required to implement the spray cannot be operated remotely from the central control room. Therefore, it was necessary to directly operate the valve in the vicinity of the containment vessel. According to the testimony of the person in charge who went to the scene, "hot and high-pressure steam flowed from the reactor to Sapchan, and the huge containment vessel was shaking under the pressure." "If steam blows out, you may not be safe." The person in charge was determined to work in the reactor building, where radiation levels were rising. In order to open the valve, I put my foot on the sapchan, which was hot. The valve is hot and cannot be held for long. And while working, the soles of the boots melted from the heat.
At 3:13 a.m. on March 7, a dry well-play was performed by operating several valves. The pressure in the containment vessel, which had been on an upward trend until then, leveled off as intended. However, just 39 minutes after the start, the Fukushima Daiichi Nuclear Power Plant was contacted by TEPCO head office via video conference.
It signaled the cessation of dry wealth play. The conversation was left on the video conference.
March 3,
13:7 a.m. TEPCO Head Office
: "I'm from the Ministry (country) right now, but I want to start venting as soon as possible and avoid accumulating hydrogen and things like that, so I want to blow the rupture as soon as possible, but Dry Wellsplay I've been told that it doesn't make sense to think about it too much. 57F?"
This conversation contains complex information. First of all, in the part that says "It's from the Ministry" at the beginning, at that time, the head office of TEPCO was always in contact with the Ministry of Economy, Trade and Industry, which was a regulatory agency. The person in charge of the head office informed that the government intends to vent and emit hydrogen. The day before this conversation, the hydrogen explosion at Unit 3 occurred on March 12. As with Unit 1, there was a common understanding that there was a risk of hydrogen explosion at Unit 3 and wanted to avoid it. Considering the safety of the many people who were working outside the building to inject water and restore power, it is understandable that the Fukushima Daiichi Nuclear Power Plant will prioritize responding to hydrogen explosions.
Like the spray, the vent is an operation to protect the containment vessel. However, this required the destruction of an unfamiliar device called a "rapture (disk)". The "rupture disk" is attached to the vent piping and plays the role of covering the vent to prevent radioactive materials from leaking from the containment vessel under normal conditions. If the pressure of the containment vessel does not exceed about 5 atm, it cannot be torn and venting is not possible.
However, at that time, the pressure of Unit 3 was below 5 atmospheres because it was cooled by spray. In other words, in order to vent, the only way to vent was to stop the "dry well play", which has the effect of cooling the containment vessel and lowering the pressure.
Although the purpose of protecting the containment vessel is the same, "dry well spray" and "vent" have the contradiction of "incompatible at the same time". The field was faced with a choice.
During a video conference, the Fukushima Daiichi Nuclear Power Plant announces to TEPCO's head office that the drywell pressure has calmed down by spraying. In response, the head office asks, "Can't you stop it?" When the person in charge of the Fukushima Daiichi Nuclear Power Plant was unable to answer immediately, the director Yoshida repeated the head office's question, saying, "The question at the head office now is whether we can stop dry wealth play." And officials at the Fukushima Daiichi nuclear power plant also agreed to stop spraying. Yoshida did not express any opposition to this in the video conference.
The operators stopped the spray only an hour after the start.
In the "Yoshida Report" introduced at the beginning of the article, "The head office told me to stop (drywell) spraying. This testimony was based on this situation.
Another role that Splay has
Together with experts, we decided to decipher TEPCO's accident operation procedure. TEPCO initially refused to publish these procedures, citing the protection of intellectual property and other reasons, but the Nuclear and Industrial Safety Agency of Japan (NRA) has released some of the procedures submitted based on the law since October 2011.
In the "Unit 3 Accident Operation Operation Procedure (Severe Accident)", in addition to cooling the containment vessel and lowering the pressure, another role of dry well play was described. It is to pour water under the reactor in a place called the pedestal, which is the floor part of the containment vessel. It stipulates that "if damage to the RPV (reactor pressure vessel) is confirmed, this operation will be performed." "If the flow rate is insufficient due to the number of pumps and alternative water injection cannot be performed in equilibrium operation, the following priority shall be given: 1, containment vessel 2, pedestal 3, nuclear reactor".
A meltdown progresses, and a melt-through in which nuclear fuel breaks through the bottom of the reactor. When that happens, the nuclear fuel melts down to the floor of the containment vessel. There is concern that the containment vessel will be destroyed by the pressure increase caused by the heat, and that a phenomenon called "shell attack" will occur, in which the containment vessel itself is destroyed by spreading high-temperature nuclear fuel on the floor and coming into direct contact with the containment vessel.
Injecting water into the containment vessel floor (pedestal) by dry well-play played an important role in cooling the melt-through "melt core (debris)".
Then, when faced with the choice between venting and spraying, why did the head office and Fukushima Daiichi not choose "spray"?
According to TEPCO's video conference, for example, the possibility of the reactor filling up was discussed in the conversation over the injection of water from the meltdown Unit 3.
March 3, 14:3 a.m. Vice President Muto: Well,
Vessel, you put in enough water to fill up.
Director Yoshida: That's right.
Vice President Muto: What is it? What's going on? I mean, it's overflowing, or something.
Director Yoshida: The furnace water level has not risen as in Unit 36, so even if it is injected, it will be injected. This means that there is a high possibility that there is a bypass flow somewhere.
Vice President Muto: Does the bypass flow mean that it is somehow crossing?
Director Yoshida: Yes, yes, yes, yes. Yeah.
According to an analysis by SAMPSON, which can simulate the development of the nuclear accident in detail, a melt-through occurred at Unit 6 at 3:13 p.m. on March 9, about six hours before this conversation. However, with most of the parameters lost, it was difficult for the parties concerned to have a common awareness of the problem of melt-through.
What is the impact of the suspension of dry wealth play?
So, did stopping the spray have any negative effects? This time, we asked Masanori Naito, who specializes in accident analysis, to perform various analyses. Naito is a world-renowned expert in the field of accident analysis, leading international accident analysis projects conducted by the Organization for Economic Co-operation and Development (OECD/NEA) after the accident at the Fukushima Daiichi Nuclear Power Plant. One of the analyses that Naito conducted was the difference between stopping dry wealth play as handled at the time of the accident and continuing thereafter. The amount of water injected in the case of continuation was set to "70 tons of water injection before melt-through" in TEPCO's procedure manual.
At first, Naito was also skeptical about the effects of continuing this spray. However, the results were surprising. It is said that the amount of hydrogen generated that causes a hydrogen explosion can be reduced.
The analysis results showed that the amount of hydrogen generated from the reactor when the spray was stopped was about 800 kg, while the amount of hydrogen generated was reduced to 25 kg, which is about 600% less when the spray was continued.
Naito analyzes, "By cooling the reactor from the outside by dry well-spray, it has the effect of suppressing the temperature rise of nuclear fuel, which suggests that the water zirconium reaction activated by the high temperature is suppressed, and the amount of hydrogen generated decreases." In addition, water can be filled about 1 meter from the floor inside the containment vessel, which is expected to have the effect of cooling the melt-through nuclear fuel and the containment vessel itself. Continuing to play could have prevented the accident from getting worse.
Verify a spray problem
The investigation team worked with experts to determine what was wrong with the response to the "spray stopping" accident.
The first thing that was pointed out was the design problem.
Hiroshi Miyano (former chief engineer at Toshiba), chairman of the Fukushima Daiichi Nuclear Power Plant Decommissioning Review Committee established within the Japan Atomic Energy Society of Japan, supported the continuation of the spray that had been made as a decision on the Fukushima Daiichi Nuclear Power Plant.
"I don't think it's basically 'no' to stop the spray and increase the internal pressure to break the rupture disc. So, by design, we will keep the spray."
Naito, who participated in the discussion, agreed with this opinion.
In fact, Naito said that experts had been discussing that dry wealth play should be used effectively from the day the accident occurred.
"We discussed that we should do dry wealth play properly in the hope that we could cool the reactor from the outside and make the core melt a little milder. With that in mind, I think TEPCO's procedure manual says the right thing. Dry Wellsplay causes water to accumulate on the floor of the containment vessel, and then cools and solidifies the melt-through core, thereby preventing it from spreading. With that in mind, it's hard to come up with the idea of stopping the splay and venting as fast as possible."
Miyano, who was an executive at a manufacturer that had been in charge of the design of nuclear power plants, took issue with the fact that the vents were not designed with the response in the event of an accident in mind.
"Before the accident at the Fukushima Daiichi Nuclear Power Plant, the basic idea in Japan was that radioactive materials should not be released, and the idea of an accident occurring and venting was not prioritized. There was a problem. We need to change the design from the idea of 'no accidents' to 'how can we respond when an accident occurs.'"
The rupture disc that was one of the causes of stopping Dry Wealth Play.
After the accident, nuclear power plants in various parts of the country removed the rupture disc itself to prevent venting in the event of an accident.
Decision-making issues
In the response to the spray, experts pointed out the most severe intervention in decision-making by the government and TEPCO's head office. In the event of an accident, the judgment of Director Yoshida, who was originally in charge of the Fukushima Daiichi Nuclear Power Plant, was respected, and the head office was divided in roles to provide support. TEPCO's accident investigation report states, "The implementation of operational measures necessary to prevent the spread of the accident belongs to the plant manager (Director Yoshida in this case), who is the nuclear disaster prevention manager, and the head of the head office emergency response headquarters (president) is responsible for providing personnel, materials, and equipment to the power plant emergency response headquarters."
But if you look at the exchanges surrounding Splay, that wasn't the case at all.
In the video conference exchange, Mr. Komori, managing director of TEPCO's head office, told the Fukushima Daiichi Nuclear Power Plant, "We should stop it." It stops the spray accordingly. However, as stated in the opinion from the Ministry (Government) at the beginning of the remarks by TEPCO's head office, TEPCO, the Ministry of Economy, Trade and Industry (METI) and the Prime Minister's Office have various communication routes, and sometimes Takeguro Fellow, who was at the Prime Minister's Office, and Mr. Tamame, who was the chairman of the Nuclear Safety Commission, made direct inquiries and instructions to Director Yoshida.
Naito points out that there was a problem with the system and authority in the nuclear accident, when the government could effectively "intervene" by conveying its opinions to the head office.
"The TEPCO head office said various things to Fukushima Daiichi because there are people who influenced the opinions of the head office, and they should not know the procedure manual at the time of the accident, and it is impossible to tell them that they should know the procedure manual.
At the Fukushima Daiichi Nuclear Power Plant, there was "government" intervention in various situations. In Unit 1, then-Prime Minister Suga visited the Fukushima Daiichi Nuclear Power Plant and directly urged Director Yoshida to "hurry up venting," and Chairman Mamame directly telephoned Director Yoshida to express his opinion on the issue of "SR valves should take precedence over vents" at Unit 2. In addition to the points we have examined in our interviews, a new issue of "intervention" has emerged.
How is the government going to respond to nuclear disasters in the future? The Nuclear Regulation Authority oversees power companies' response to accidents. We asked Counselor Toyoshi Sarada, who has served as a committee member and chairman for 10 years since its inauguration, about the role of the government in responding to the accident.
First, Sarada pointed out that the background to the government's intervention in the Fukushima Daiichi Nuclear Power Plant accident was the lack of preparation and capacity of both the national government and electric power companies.
"Before the accident at the Fukushima Daiichi Nuclear Power Plant, countermeasures against accidents under extremely severe conditions were left to the voluntary measures of the operators (electric power companies), and sufficient training was not conducted. In some cases, the government intervened because the operators could not explain the circumstances and response of the accident well."
He added that the government is continuing its efforts to make use of the lessons learned from the accident.
"Based on the lessons learned from the Fukushima Daiichi nuclear accident, we have been conducting training and preparing to deal with it again. At the time of the accident at the Fukushima Daiichi Nuclear Power Plant, the government intervened, resulting in confusion. The basic principle is that the business operator who knows the site best will take responsibility for the response, and this is still confirmed in training and other settings."
Dry Wellsplay Stops Even The Mystery of the Guarded Containment
Unit 1 stopped the dry wealth play in just one hour. After that, high-temperature nuclear fuel exceeding 3°C melted down to the floor of the containment vessel from the reactor, and there was a risk that the containment vessel would be destroyed due to increased pressure.
However, it is a surprising fact that has been revealed through surveys into the containment vessel of Unit 2000. Compared to Unit 3 and Unit 1, the water level accumulated inside was the highest. The water level was 2.1 meters in Unit 2 and about 8 centimeters in Unit 2, but 60.3 meters in Unit 6. This means that the containment vessel of Unit 3 "maintained its soundness" compared to the other units.
Is the soundness of Unit 3, which stopped spraying and could not sufficiently cool the containment vessel, maintained?
When I interviewed several experts, I found that a hypothesis was being examined among the people involved.
It is an impossible mechanism in which water flowed back from sapchan into the dry well, cooling the nuclear fuel.
What prompted experts to investigate this mechanism was the pressure difference between the dry well and the sapchan of Unit 3. Since the drywell and sapchan are connected, there is no large pressure difference by design. However, when looking at the fluctuations in the pressure of Unit 3, there was a phenomenon in which there was a difference in the pressure of both the dry well, the time when the pressure of the sapchan was high, and the time when the pressure of the dry well exceeded.
To explain this phenomenon, the reverse flow of water from Sapchang was being studied. The pressure decreases when water vapor in the dry well leaks from gaps. Then, the water accumulated in the sapchan flows back into the dry well and accumulates on the floor. There is melted nuclear fuel on the floor of this containment vessel, and when water comes into contact with it, water vapor is generated again at the same time as cooling. The mechanism is to increase the pressure on the drywell side again.
TEPCO was also verifying this mechanism. In November last year, the latest report investigating and examining "unclarified matters" in the Fukushima Daiichi Nuclear Power Plant accident was released. For the first time, the possibility of water flowing back from Sapchang into the dry well was acknowledged, and as a result, the nuclear fuel debris was cooled and the integrity of the containment vessel was maintained.
However, this reflux phenomenon was never intentional. It was, so to speak, a coincidence.
Cooling of containment vessels that worked unintentionally by humans. The difficulty of controlling water, which is essential for controlling nuclear power plants, was also exposed.
Naito: This phenomenon was not intentionally caused by people to regurgitate, so if we call it a natural phenomenon, can we always expect this natural phenomenon? I don't think so."
Miyano: "It just happened to happen, and if we want to aim for it, we have to create such a system."
Long-lasting validation
The Fukushima Daiichi Nuclear Power Plant accident still leaves deep scars, with more than 2,2011 people still forced to live as evacuees.
How can we extract the lessons of this accident, pass them on to future generations, and leave them behind?
One is to verify the response at the time based on the actual situation of the accident, such as the situation inside the containment vessel, which will become clear as the investigation for decommissioning progresses. In 2012 and 3, when accident investigations were intensively conducted, the situation was not clarified, and as a result, a "new perspective" of verification that had been overlooked emerged. This is exactly what we have to do with the Splay of Unit <>.
The situation inside the containment vessel and nuclear reactor gradually becomes clear over a long period of time. We look at each piece of data and continue to verify it against the vast amount of records that have been left so far. This is the mission not only of the government and electric power companies, but also of the media. The verification is not over yet.