The March 11 tsunami that engulfed the Fukushima No. 1 nuclear power plant was horrific enough. But in the hours that followed, it became apparent that Japan had a major nuclear disaster on its hands.
The March 11 tsunami that engulfed the Fukushima No. 1 nuclear power plant was horrific enough. But in the hours that followed, it became apparent that Japan had a major nuclear disaster on its hands.
When precisely was that?
According to government and Tokyo Electric Power Co. sources, the night of March 14 through to the following morning proved to be the watershed.
Weather conditions on March 15, specifically wind direction and rain, are thought to have contributed to the formation of a high-radiation zone that stretched northwest from the crippled plant. In that sense, March 15 may be considered the "fateful day" that doomed the region to exposure to nuclear fallout.
The pressure inside the containment vessel of the No. 2 reactor at the Fukushima No. 1 plant rose above the maximum designed level, triggering fears of an explosion. At 11:46 p.m., the pressure had risen to nearly double the limit. A TEPCO interim report defined the situation as "critical."
Hydrogen explosions had torn through the buildings that housed the No. 1 and No. 3 reactors, the former in the afternoon of March 12 and the latter in the morning of March 14. Both their containment vessels remained intact, but with the No. 2 reactor, there was a possibility of a massive radiation leak.
Masao Yoshida, then director of the Fukushima No. 1 plant, recalled: "I just couldn't foresee what might happen from moment to moment. The worst-case scenario that crossed my mind was a meltdown continuing beyond control. We'd all be finished then."
Actually, a core meltdown had already begun by then, according to later calculations by TEPCO and the Nuclear and Industrial Safety Agency (NISA), an arm of the Ministry of Economy, Trade and Industry.
Water inside the reactor unit had evaporated, creating a situation akin to heating an empty kettle on the stove. Hydrogen and high-temperature steam, containing radioactive substances from the fuel rods, escaped from the pressure vessel's venting valves and filled the containment vessel, where the pressure kept rising.
TEPCO opened venting pipe valves to release the steam from the reactor and reduce the pressure, but was unable to determine if this operation was producing the desired results.
During a news conference on the night of March 14, Hidehiko Nishiyama, then NISA deputy director-general, pointed out that damage to the No. 2 reactor containment vessel "had to be prevented at all costs."
But even at this extremely critical juncture, NISA did not hold a news conference for the next eight hours. The next press briefing was held in the morning. This was the first time since the start of the nuclear crisis that the agency broke its routine of frequent press briefings.
A harried agency official noted at the time: "We want to hold a news conference, but the prime minister's office won't give us the go-ahead. The deputy director-general is really irritated by this."
But everyone was just as harried at the prime minister's office. Banri Kaieda, then minister of economy, trade and industry, had been informed that TEPCO was "planning to evacuate all its people from the Fukushima No. 1 plant."
According to Masaya Yasui, also a deputy director-general of NISA, he was asked by Kaieda, "What happens if TEPCO goes ahead with the evacuation?" Recalling the scene, Yasui noted, "At least a few Cabinet ministers seemed to believe that TEPCO was pulling out of the Fukushima No. 1 plant."
To a roomful of Cabinet ministers assembled in the parlor of the prime minister's office, Yasui and Haruki Madarame, chief of the Nuclear Safety Commission, explained what a full withdrawal would mean. They warned that the abandonment of all post-disaster recovery work at the plant could result in radiation spillage not only from the No. 2 reactor, but also from the No. 5 and No. 6 reactors that were still intact at the time as well as the spent nuclear fuel storage pools of all the reactors.
Naoto Kan, who was prime minister at the time, summoned TEPCO President Masataka Shimizu to the prime minister's office and asked him bluntly, "Wouldn't you yourself be in trouble if you let all your people evacuate the plant?"
Shimizu was seated kitty-corner from him, and Yasui, who was in the room, expected Shimizu to remonstrate. But to his surprise, the TEPCO president agreed with Kan immediately. Yasui recalls being taken aback.
Kan, however, interpreted Shimizu's response as noncommittal. Thus, when he visited TEPCO's disaster response center on the second floor of its Tokyo head office around 5:35 a.m., he told TEPCO Chairman Tsunehisa Katsumata and others in no unclear terms, "A withdrawal is not an option."
About half an hour later, at 6:10 a.m., "a big bang" was heard, suggesting that the containment vessel of the No. 2 reactor could have been damaged. Almost simultaneously, the pressure inside the suppression chamber, which was connected to the containment vessel of the No. 2 reactor, dipped to zero.
Destruction of the containment vessel would spell a massive radiation leak. Of 720 workers who were at the Fukushima No. 1 plant at the time, 650 were evacuated on buses. The remaining 70 were tasked with recovery work in the plant's earthquake-proof wing.
It has been suggested that leaving these 70 people behind was not in TEPCO's original plan. Those who subscribe to this "theory" point out that a faxed message, sent in the early morning of March 15 by the plant's chief, Yoshida, to the NISA, contains a crossed-out line that says, "Relocating the emergency response headquarters to the Fukushima No. 2 plant." Yoshida rewrote this to, "We are evacuating some of our people temporarily."
The night before, Yoshida had readied himself to sacrifice his life. When Shimizu discussed the situation on the phone with several people, including then NISA Director-General Nobuaki Terasaka, he never said anything about leaving some workers at the plant. The fact was the reason TEPCO was thought to have been planning for a "complete withdrawal." However, TEPCO's true intention remains unconfirmed to this day.
In its interim in-house investigation report published last December, TEPCO asserted, "We never considered a full withdrawal, nor did we ever mention it." Still, people at the prime minister's office clearly came to the conclusion at one point that TEPCO was definitely considering it.
Ultimately, the No. 2 reactor did not explode. It is believed that this was only due to pure chance. TEPCO theorized that when the No. 1 reactor exploded, panels of the adjoining No. 2 reactor building became dislodged, causing hydrogen to escape. The "big bang" that was heard is believed to have originated in the No. 4 reactor building, which blew at that time. The sudden drop in the pressure in the suppression chamber was ascribed to an instrument malfunction.
The pressure inside the containment vessel of the No. 2 reactor did not drop after the "big bang." It registered 7.3 atmospheres at 7:20 a.m., and finally dropped to around 1.5 atm at 11:25 a.m.
It is strongly suspected that the containment vessel's silicone rubber sealers gave in under high heat during that period, causing radioactive steam to escape.
After white steam was seen spewing out of the No. 2 reactor, radiation level readings taken at the front gates of the Fukushima No. 1 plant spiked to 11,930 microsieverts per hour. Three hours before, the level had been 73.2 microsieverts. At the off-site emergency response center about 5 kilometers away, outdoor radiation levels exceeded 2,000 microsieverts and indoor levels registered between 100 and 200 microsieverts around 9 a.m. NISA staffers and prefectural government workers evacuated to the Fukushima prefectural government office.
The Ministry of Education, Culture, Sports, Science and Technology did not disclose predictions on radiation dispersion via the System for Prediction of Environmental Emergency Dose Information (SPEEDI). In other words, this system did not serve its intended function.
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Three questions remain unanswered about the crisis that unfolded at the Fukushima No. 1 plant--the state of the melted nuclear fuel in the reactors, the mechanisms of the hydrogen explosions, and the amount of radioactive substances spewed.
(1) STATE OF NUCLEAR REACTOR INTERIORS
The No. 1, No. 2 and No. 3 reactors of the Fukushima No. 1 plant experienced meltdowns, in which nuclear fuel rods melted inside the reactors and seeped through to the base.
According to an analysis by TEPCO, the No. 1 reactor pressure vessel was the most severely damaged. It found that all nuclear fuel there fell through holes for control rods and other openings in the bottom of the pressure vessel to the base of the containment vessel.
TEPCO's analysis estimated that the melted fuel eroded the concrete floor to a depth of about 65 centimeters, leaving only 37 cm to reach the steel casing of the containment vessel.
Fuel debris fills concrete pits to a height of 81 cm, TEPCO said.
However, the utility maintains that water is cooling the nuclear fuel and is preventing further erosion. The melted fuel has not penetrated into the ground, TEPCO said.
At the No. 2 and No. 3 reactors, most of the melted nuclear fuel remains within the pressure vessels, although some has seeped through to the concrete floors.
At any rate, a precise determination of the state of the melted nuclear fuel requires the use of remote cameras and other equipment to verify the state of the interiors during the process of decommissioning the reactors.
(2) GENERATION OF HYDROGEN GAS
Explosions occurred at the No. 1, No. 3 and No. 4 reactor buildings of the Fukushima No. 1 plant.
A large volume of hydrogen gas was generated inside the nuclear reactors when they lost all means of cooling. The gas is thought to have leaked from the reactors, but details of precisely what happened, including the ignition mechanisms, have yet to be determined.
Computer simulations by the Japan Nuclear Energy Safety Organization suggested that 1,000 kilograms of hydrogen were generated in the No. 1 reactor, about 450 kilograms of which leaked into the reactor building during the roughly 10 hours leading up to the explosion. Hydrogen and radioactive substances leaked together, experts said.
At the No. 3 reactor, hydrogen is thought to have flowed back along a ventilation pipe, which is connected to but is separate from venting pipes, when gas was vented from the interior of the containment vessel to prevent damage from high internal pressure.
But even if that back flow took place, large amounts of hydrogen did leak directly out of the containment vessels in the No. 1 and No. 3 reactor buildings, according to the Nuclear and Industrial Safety Agency.
The No. 4 reactor was undergoing regular inspection when the explosion occurred.
Fuel in the No. 4 reactor's spent fuel storage pool was initially suspected as the culprit. However, it later emerged that hydrogen probably flowed back into the No. 4 reactor building through a pipe it shares with the neighboring No. 3 reactor when gas was being vented from the No. 3 reactor.
(3) LEAKAGE OF RADIOACTIVE SUBSTANCES
The amount of radioactive substances spewed by the Fukushima No. 1 plant and their leakage mechanisms have yet to be established.
NISA's simulation indicated that up to 770 quadrillion becquerels' equivalent of radioactive iodine (a quadrillion is one thousand trillion) may have spewed into the atmosphere. Most of that amount is thought to have leaked during the hydrogen explosions and the venting of gas that took place at the nuclear reactor buildings until March 15 last year.
The NISA estimate of 770 quadrillion becquerels is about 15 percent of the 5.2 quintillion becquerels spewed by the Chernobyl nuclear plant in 1986.
The hydrogen explosions at the No. 1, No. 3 and No. 4 reactor buildings blew out their ceilings, causing the leak of radioactive substances. The venting of gas from the No. 1, No. 2 and No. 3 reactors is also thought to have released radioactive substances, together with the gas, from their exhaust ducts.
The rate of leakage is thought to have peaked on March 15. That was the day a radiation level of 11,930 microsieverts per hour was detected at 9 a.m. near the main gate to the nuclear plant.
It is thought that a total of 10 million becquerels of radioactive substances continue to be released each hour from the No. 1, No. 2 and No. 3 reactor buildings.
(This article was written by Tatsuyuki Kobori, Jin Nishikawa and Naoya Kon.)