Evan Osnos wrote in The New Yorker: Despite the venting and bleeding efforts, "at 3:36 P.M., Reactor Building No. 1 exploded, hurling chunks of concrete that injured five workers and destroyed cables that had been laid in the hope of restoring electricity. With that explosion, the crisis passed an invisible line, as each problem triggered another. High radiation levels hampered workers trying to vent Reactor No. 3, and on the third day after the tsunami it, too, exploded. The following day, another blast occurred, this time at No. 4. On television screens throughout the world, three exposed steel carcasses were seen smoldering. An American aircraft carrier and its fleet left waters downwind of the plant after seventeen helicopter crew members returned from missions with traces of radiation. [Source: Evan Osnos, The New Yorker, October 17, 2011]

When nuclear reactors are in operation, hydrogen tends to be generated by the radiolytic decomposition of water and other chemical reactions. But hydrogen generation was not the cause of most past explosions at nuclear plants, so close attention was not initially paid to the phenomenon during the Fukushima crisis.

Under ordinary circumstances, reactor containment vessels that cover pressure vessels contain nearly no oxygen, making it almost impossible for the substance to chemically react with hydrogen and set off a blast. Even if hydrogen is generated in a reactor, it should not leak from the containment vessel. But after the disaster damaged pipe joints in the reactor building, hydrogen from inside the reactor leaked into the outer structure.

"No expert had predicted that a hydrogen explosion would occur at the reactor building," Goshi Hosono, special adviser to Prime Minister Naoto Kan, told the Yomiuri Shimbun. Haruki Madarame, chairman of the Nuclear Safety Commission, had said, "The containment vessel has been refilled with nitrogen, so a hydrogen explosion won't happen" — an explanation Kan accepted. It was considered common sense among many nuclear experts that such an explosion would never occur. [Source: Yomiuri Shimbun, June 10 2011]

The 2002 report, compiled by TEPCO and five other power companies, on response measures to be taken in the event of core meltdowns and other severe nuclear accidents, stated, "There is no need to take a hydrogen explosion into consideration." This belief came from the common knowledge that such an explosion would not occur if the containment vessel was filled with nitrogen, which would keep the concentration level of hydrogen low.

Tomoho Yamada, director of the NISA's Nuclear Power Licensing Division, said, "[The reactor] was designed to keep hydrogen from leaking out of the containment vessel into the reactor building. In the safety screening, we assumed that a hydrogen explosion would not occur in the reactor building." Yamada admitted that due to this assumption, measures to prevent a hydrogen explosion in the reactor building were not included in the list of safety measure evaluations.

Why the Explosions Occurred and Defending TEPCO’s Response

The interim report also referred to a possible reason why hydrogen explosions occurred at the Nos. 1 and 3 reactors. The report said silicon rubber used to seal the spaces between doors and wall, and between the containment vessels and their lids, may have not functioned properly due to the high temperatures, opening gaps that allowed the release of hydrogen into the reactor building.

The panel defended the company's slow initial response to the nuclear disaster, such as preparations to vent steam from reactor, by saying, "It can't be helped that that [the initial response] took time, as workers had to prepare in the darkness."The interim report said the nuclear reactor's major equipment sustained no damage due to the earthquake itself, but instruments ensuring the safety of the nuclear reactors lost their functions simultaneously due to tsunami, worsening the situation exponentially.

Hydrogen Explosion at Reactor No. 1

The first blast occurred at the No. 1 reactor at 3:36 p.m. on March 12, about five hours after the radioactive steam was released. The event violently shook an earthquake-resistant building nearby that was housing the crisis headquarters in the compound. An employee of a TEPCO partner company told the Yomiuri Shimbun he was sitting on the floor when the explosion hit and it tossed him about 10 centimeters up in the air. "I thought an earthquake had hit nearby," he said. People around him started shouting, "Check the [radiation] dose!"

According to an analysis by TEPCO, after all of the nuclear fuel at the No. 1 reactor melted 16 hours after the massive earthquake and tsunami on March 11, the pressure and the containment vessels became damaged. Hydrogen, which was generated from the reaction between the zirconium cladding of the fuel rods and oxygen, leaked out and began accumulating in the reactor building. A former TEPCO executive said: "I'd never have thought such a large amount of hydrogen would be generated after the nuclear fuel was exposed. We must accept that we were overly confident." [Source: Yomiuri Shimbun, Yomiuri Shimbun , June 10, 2011]

The message an "An explosion was heard at the Fukushima No. 1 nuclear power plant” was relayed almost immediately from the Fukushima prefectural police via the National Police Agency to the government's crisis management center, located in the basement of the Prime Minister's Office. But officials with the government nuclear agency and TEPCO, who were at the Prime Minister's Office at the time refused to accept the information, some of them repeatedly saying, "That's not possible." It was not until five hours later that the government acknowledged that a hydrogen explosion had occurred at reactor No. 1. Such an explosion had not been factored into their scenarios, and it took some time for officials to analyze the situation.

At 8:40 p.m., Chief Cabinet Secretary Yukio Edano held a press conference. "The explosion destroyed the reactor building, but the containment vessel [housing the reactor] has not been damaged," he said, stressing that the reactor was safe. Meanwhile, TEPCO, the plant operator, had only made an announcement that it was analyzing the incident.

After the Hydrogen Explosion at Reactor No. 1

The hydrogen explosion at the No. 1 reactor was indeed a critical point leading to delays in responses to the nuclear accident. TEPCO attempted to ventilate other reactor buildings at the power plant, but workers struggled with high radiation levels, and failed to prevent a hydrogen explosion at the No. 3 reactor on March 14.

“On the night of March 12, workers from the headquarters tried to lay power cable from No. 4 reactor's turbine building to the No. 3 reactor's cooling motor, hoping No. 4's power supply was still functioning. But the door to the turbine building had been warped by the tsunami and was stuck shut. It took the workers about three hours to remove the door, but they soon saw their work had been for nothing. The No. 4 reactor's power supply was dead. Slowly, workers at the power station were realizing the tsunami had caused destruction beyond their imagination.”

“After the blast at the No. 1 reactor on March 12, the officials received a report from the nuclear safety agency saying, "The No. 2 reactor is unstable." But the only action TEPCO's crisis headquarters said it would take was to "respond to rising pressure [at the No. 3 reactor] by venting the reactor."

At 11 a.m. on March 14, the No. 3 reactor building exploded and again the headquarters in the quake-resistant structure was given a violent shake.Even after the blast at No. 3 reactor, the nuclear safety agency continued to say the reactor vessel was sound and the container was filled with water.

Hydrogen Explosion and Fire at No. 4 Reactor

At 6:10 a.m. on March 15, a hydrogen explosion occurred at the No. 4 reactor that was so strong it caused the outer walls of the reactor building to collapse. This was followed by a fire near a storage pool for spent nuclear fuel at the No. 4 reactor at 9:38 a.m. Radiation in excess of 400 millisieverts per hour was detected in the vicinity.

Until this point, the nuclear safety agency had been saying the No. 4 reactor was relatively stable. TEPCO had repeatedly said the No. 4 reactor had been stopped and as long as there was water in the spent fuel storage pool, things would be fine. But apparently what had been really happening was the fuel rods were overheating — the tsunami cut off electricity pumped coolant into the pool — and was close to melting down.

The Yomiuri Shimbun reportedly: “It was starting to dawn on government officials, including Defense Minister Toshimi Kitazawa, that the nuclear safety agency and TEPCO lacked the ability to make appropriate decisions. They were becoming frustrated by what they saw as constant changes to explanations about what was happening at the power station. Distrust in the agency and TEPCO was soaring....But TEPCO and the nuclear safety agency had their hands full responding to the volatile situation surrounding the No. 2 reactor after the blast at the No. 1 reactor. Dealing with one urgent crisis after another, they had no time to take actions that would have averted the more serious situation that was to develop.”[Source: Yomiuri Shimbun. April 14, 2011]

The hydrogen explosion appears to have helped cool the spent rods in Reactor No. 4 and prevented them from melting down, according to TEPCO by damaging a water gate and causing water to flow into the spent rod pool from a neighboring part of the facility.

When the earthquake occurred, the No. 4 reactor was under repair. Covers of the pressure vessel and containment vessel were open at the time and the whole of the well, including the pressure vessel, was filled with water. The water was injected to allow the removal and transfer of nuclear fuel rods from the pressure vessel to the pool without exposing them to air. [Source: Yomiuri Shimbun, April 29, 2011]

TEPCO assumes the following happened: 1) The water level inside the pool decreased and parts of the spent fuel rods became exposed. 2) Metal covering the overheated fuel rods reacted chemically with water and discharged a large quantity of hydrogen gas. 3) The gas was ignited and exploded, damaging the gate. 4) As a result, hundreds of tons of water entered the pool and the overheating of fuel rods ended.

No Explosion at No. 2 Reactor

An interim report released in October 2011 by a TEPCO panel investigating the nuclear crisis at Fukushima concluded that a hydrogen explosion did not occur at the plant's No. 2 reactor, overturning its previous conclusion that an explosion took place on March 15. According to TEPCO, the first hydrogen explosion took place in the No. 1 reactor building on the afternoon of March 12, followed by an explosion at the No. 3 reactor on the morning of March 14. Early on the morning of March 15, TEPCO confirmed the sound of an explosion, and then found damage in the No. 4 reactor building. [Source: Yomiuri Shimbun. October 3, 2011]

The power company also confirmed that pressure in the No. 2 reactor's pressure suppression pool, which is the lower portion of the reactor's containment vessel, dropped significantly immediately after the sound was heard. So the company concluded that explosions must have occurred almost simultaneously at the Nos. 2 and 4 reactors, and the government reported the same conclusion to the International Atomic Energy Agency in June.

However, the panel studied a seismometer at the plant and found only one explosion tremor recorded at 6:12 a.m. that day. Based on analysis of the tremor, the panel concluded that the explosion occurred at the No. 4 reactor. However, due to the fact that the pressure at the No. 2 reactor's pressure suppression pool dropped around that time, the panel said the reactor's containment vessel may have sustained other damage. The interim report did not further explain the damage or its cause. Indeed, the core of the No. 2 reactor is believed to have sustained the most serious damage among the power plant's six reactors, and the pace of cooling at the No. 2 reactor has been slow.

According to the draft of the interim report, a blowout panel installed at the upper part of the No. 2 reactor building opened accidentally after the No. 1 reactor's hydrogen explosion on March 12. The draft says a hydrogen explosion was avoided at the No. 2 reactor because a certain amount of hydrogen escaped through the blowout panel.

Design of the Fukushima Reactors Partly to Blame

The Yomiuri Shimbun reported: “The design of reactors at Fukushima made them prone to hydrogen blasts, a risk that was overlooked, according to some experts. At the plant, the Nos. 1-5 reactors are the Mark-1 model developed by General Electric Co. of the United States in the 1960s. The Mark-1 is one of the oldest light-water reactor models. "[The Mark-1] containment vessels are relatively small and pressure fluctuates greatly, so its operation is difficult," said a former senior TEPCO official who worked at the Fukushima plant. "As hydrogen accumulates easily, I felt a potential risk." In the United States, among 104 reactors in use at 65 nuclear power plants, the Mark-1 model accounts for 23 reactors at 16 plants, a major subject of criticism among antinuclear activists. [Source: Yomiuri Shimbun, June 10, 2011]

In the 1970s, U.S. nuclear regulators considered a ban on the use of the Mark-1 model out of concern that its containment vessels could be vulnerable to serious accidents. But as the model was already being widely used, authorities stopped short of forbidding its use. However after the 1970s, experts pointed to the need for measures to prevent rapid hydrogen buildup. As a result, plant operators have since taken steps such as installing equipment that regenerates water from hydrogen in turbine buildings and injects nitrogen into containment vessels. Such measures have also been taken at the Fukushima plant.

But in the 1980s, a senior official of the U.S. Nuclear Regulatory Commission named Harold Denton pressed the argument that Mark-1 reactors had a 90 percent probability of suffering an explosion if fuel rods overheated and melted in an accident. In the late 1980s, GE attempted to help the model survive such criticism by equipping it with improved devices such as a venting system to reduce pressure in the containment vessel.

Regarding the Mark-1 model and its 40-year history, a senior official of a nuclear reactor manufacturer said, "The biggest problem is we lacked knowledge of the workings of devices that were vulnerable to a loss of power." Even the device to regenerate water from hydrogen cannot work if power is lost.

TEPCO uses Mark-2 reactors at its Fukushima No. 2 nuclear power plant. Immediately after the March 11 disaster, the cooling system temporarily halted at the plant's three reactors, but a hydrogen blast did not occur there. In Japan, the Mark-1 model also has been used at the Nos. 1 and 2 reactors at the Hamaoka plant. Nuclear plants in Onagawacho, Miyagi Prefecture; Matsue, Shimane Prefecture; and Tsuruga, Fukui Prefecture, each have one Mark-1 reactor. In a report prepared for the International Atomic Energy Agency, the government has included the installation of equipment to eliminate hydrogen from reactor buildings as a measure to prevent hydrogen explosions.

Mistakes Made at Reactor No. 1 and No.3

According to the interim report released in December 2011 by a government panel TEPCO misjudged and poorly handled the situation at the Nos. 1 and 3 reactors, which eventually suffered meltdowns along with the No. 2 reactor. [Source: Kyodo, December 26, 2011]

As for the No. 1 unit, injecting water into the reactor by using fire trucks was delayed mainly because officials at the plant's emergency headquarters mistakenly thought that a cooling system called an isolation condenser was functioning when it was not. There was "a good chance" the actual situation could have been noticed, the report said, but reactor operators and headquarters staff apparently did not possess sufficient knowledge about the equipment itself or how to handle it, which was a "quite inappropriate" situation for a plant operator. [Source: Kyodo, December 26, 2011]

TEPCO "had not expected a situation in which all power sources would be lost at multiple reactors simultaneously due to an extremely severe natural disaster, and it had not provided enough training and education to respond to this situation," the report said.

In the case of the No. 3 reactor, meanwhile, operators on duty switched off an emergency cooling system called a high pressure coolant injection without securing alternative water injection methods, and did not seek instructions from their managers. If workers had been able to release the pressure in the Nos. 1 and 3 reactors and start injecting water earlier, the reactor cores might not have been damaged as much as currently believed and the amount of radioactive substances released might have been smaller, the report said.

Workers 'Didn't Check' Reactor Pressure Day Before Explosion at Reactor No. 3

The No. 3 reactor lost power due to the March 11 tsunami, and the operation of a reactor core isolation cooling system--another type of emergency cooling device--was suspended at 11:36 a.m. the following day. As a result, the water level in the reactor fell, which led to the automatic launch of the high-pressure core cooling system at 12:35 p.m. According to the sources, workers at the reactor stopped the high-pressure core cooling system at 2:42 a.m. on March 13 to switch to the emergency fire pump to inject water. [Source: Yomiuri Shimbun, December 17, 2011]

The Yomiuri Shimbun reported: A government panel has found that workers at the No. 3 reactor stopped operation of a high-pressure core cooling system without checking if a substitute pumping system would work on March 13, one day before a hydrogen explosion occurred there, sources said. By failing to check if the reactor's fire pump could inject water instead of the emergency cooling device, the workers may have failed to follow instructions in the plant's operating manual, according to the panel established to investigate and verify the facts of the nuclear crisis.

After stopping the operation of the high-pressure core cooling system in the early hours of March 13, the workers could not inject water through the fire pump because of the high pressure in the reactor. As a result, water injection into the reactor was suspended for about 6 ½ hours before a hydrogen explosion occurred just after 11 a.m. the following day.

According to the plant's manual on how to respond to severe accidents, workers must first confirm that there are about seven atmospheres of pressure or less in a reactor core before using a substitute pumping system. The fire pump uses relatively low pressure to inject water. However, the No. 3 reactor's pressure had jumped to about 40 atmospheres at that time, preventing the fire pump from injecting water to it.

Therefore, the workers tried to revert to the high-pressure core cooling system, only to find it could not be started due to a low battery charge. They then took steps to lower the pressure. Water was finally pumped into the reactor by fire-extinguishing vehicles at 9:25 a.m. that day. However, it did not prevent the hydrogen explosion from occurring.

Videos Records at the Time of the Hydrogen Explosions

Tokyo Electric Power Co. disclosed about 150 hours of video records of the company's teleconferences from the evening of March 11 to March 16 2011, at the outset of the nuclear crisis. The teleconferences linked TEPCO's Tokyo head office, the Fukushima No. 1 nuclear power plant and other locations. [Source: Yomiuri Shimbun, August 16, 2012]

Discussions were held on the afternoon of March 13 about measures to release the hydrogen from the building. "How about dropping something from a helicopter to destroy [the roof of the building]?" one says. Another says: "What about asking the Self-Defense Forces to blow off the panel [of the reactor building] with its firepower?" No sound was being recorded when the hydrogen explosion hit the No. 1 reactor building in the afternoon on March 12, although the video footage shakes violently.

According to video records of TEPCO’s teleconferences after the hydrogen explosion at the No. 3 reactor on March 14, Yoshida said: "Head office, head office. Big trouble, big trouble. An explosion just occurred at the No. 3 reactor, probably due to steam...It's 11:01 [a.m.]!" A senior TEPCO official at the head office said: "It's like the explosion at the No. 1 reactor [that occurred on March 12], isn't it?" Yoshida responded in a high voice: "We felt a horizontal shaking that apparently was different from that of an earthquake. As there was no aftershock like those linked to a quake, I think it was probably a [hydrogen] explosion, just like the one at the No. 1 reactor."

Shouts and a voice giving instructions can then be heard. "Look at the parameters, the parameters (data on the reactor), the parameters for the No. 3 reactor. Telephone the central control room." Executives at the Tokyo head office also issue many instructions. "Next, staff at the [accident] site evacuate, evacuate," Managing Director Akio Komori says. President Masataka Shimizu says, "Urgently notify related sections [about the matter] immediately.”

Then suddenly, probably because someone changed an audio switch, the slow voice of Chief Cabinet Secretary Yukio Edano interrupts. Edano was holding a press conference at the Prime Minister's Office. "...we are putting a burden on you. However, the reality of the power supply...” Edano had yet to be informed of the explosion and was calling on the general public to be understanding of rolling blackouts.

Yoshida then gives instructions in a firm voice, apparently having recovered his composure. "First, please confirm staffers' evacuation and safety. Measure radiation levels accurately and report them," he says. At the response headquarters at the nuclear plant, many staffers in work uniforms move around. At the Tokyo head office's disaster countermeasures office, four executives including Shimizu in business suits sit in front of a TV monitor, issuing instructions and making calls to the Prime Minister's Office.

Image Sources: Tepco and YouTube

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, Daily Yomiuri, Japan Times, Mainichi Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated August 2020

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