Jake Adelstein and David McNeill wrote on The Atlantic Online: Before the dawn on March 12, the water levels at the reactor began to plummet and the radiation began rising. Meltdown was taking place. The TEPCO Press release issued on March 12 just past 4am stated, “the pressure within the containment vessel is high but stable.” There was a note buried in the release that many people missed. “The emergency water circulation system was cooling the steam within the core; it has ceased to function.” [Source: Jake Adelstein and David McNeill, The Atlantic, July 2, 2011]

In May 2011 Tokyo Electric Power Co. (TEPCO) admitted that core meltdowns occurred at the Nos. 1, 2 and 3 reactors at the Fukushima No. 1 nuclear power plant. The melted fuel is now believed to be kept cool at the bottom of each reactor pressure vessel because of the emergency water injection measure. In a report was submitted to the Nuclear and Industrial Safety Agency of the Economy, Trade and Industry Ministry, said he meltdowns at the Nos. 1 to 3 reactors occurred in relatively short periods of time--about a half day to about four days--after the March 2011 earthquake and tsunami. As much as 800 kilograms of hydrogen was generated as a result of the overheating of nuclear fuel at the No. 1 reactor, according to TEPCO. The amount was enough to destroy the reactor building, according to nuclear experts. [Source: Yomiuri Shimbun, May 25, 2011]

The report was submitted two months after the disaster. Many people wondered why it took so long. The primary reason for the delay was that the reactors were without power, as their switchboards were submerged in water when the powerful tsunami struck. Most of the data TEPCO used to grasp what had happened in the reactors is normally recorded on computers at the reactors' central control rooms. Extremely high radiation levels near the control rooms in the early days of the crisis delayed the utility's ability to retrieve data that had been recorded.

It was not until early May when radiation levels declined and rubble was cleared from the area, that TEPCO workers were able to enter the control room to collect the data. Besides electronic data, paper records were also left inside the control rooms, which TEPCO scanned to add to the electronic records. To fill in the gaps when there was neither electronic nor paper records, the utility interviewed officials who were at the plant at the time, and looked at job sheets and other notes left on whiteboards in the control rooms. TEPCO then performed computer simulations of the meltdowns using an accident analysis program, based on various data including those already announced, such as water and pressure levels in reactors and operation records.

Early Hints of Meltdowns at Fukushima Nuclear Power Plant

According to The Chunichi Shinbun and other sources, a few hours after the earthquake extremely high levels of radiation were being measured within the reactor one building. The levels were so high that if you spent a full day exposed to them it would be fatal. The water levels of the reactor were already sinking. After the Japanese government forced TEPCO to release hundreds of pages of documents relating to the accident in May, Bloomberg reported on May 19 that a radiation alarm went off 1.5 kilometers from the number one reactor on March 11 at 3:29 p.m., minutes before the tsunami reached the plant. TEPCO would not deny the possibility that there was significant radiation leakage before the power went out. They did assert that the alarm might have simply malfunctioned. [Source: Jake Adelstein and David McNeill, The Atlantic, July 2, 2011]

On March 11, at 9:51 p.m., under the CEO's orders, the inside of the reactor building was declared a no-entry zone. Around 11 p.m., radiation levels for the inside of the turbine building, which was next door to the reactor, reached hourly levels of 0.5 to 1.2 mSv. The meltdown was already underway.

At 8:39 a.m. on March 12, about 18 hours after the earthquake, radioactive tellurium-132 was detected in Namiemachi, Fukushima Prefecture, six kilometers from Tokyo Electric Power Co.'s damaged plant, according to the report from the agency. The detection of Te-132 meant the temperature of nuclear fuel at the plant had shot up to more than 1,000 C. It also meant nuclear fuel pellets in the reactor cores had been damaged and nuclear material had leaked into the environment.

The Japanese government said the discovery of plutonium in the soil near the plant provided new evidence that the fuel in at least one of the plant’s reactors had experienced a partial meltdown. A full meltdown of the fuel rods could release huge amounts of radiation — including highly toxic plutonium — into the environment. Some scientists thought the plutonium came from the spent fuel rods.

In early April, based on levels of the radioactive xenon and krypton, TEPCO estimated that 70 percent of the fuel rods in reactor No. 1 had been damaged and 25 to 30 percent of those in reactors No. And No. 3 were damaged. The xenon and krypton — which are supposed to be contained in containment vessels — were found in gas outside the containment vessels, a sign there might be leaks in the vessel. The damage to the rods could involve in a hole in their zircalloy cladding — in other words a meltdown of the rods — however higher temperatures below the rods rather than next to the rods — a sign of meltdown — had not been detected. The No. 3 reactor uses a mixed fuel known as MOX — a combination of uranium and reclaimed plutonium that is more toxic than the fuel in the other reactors. The reactor vessel of the No. 3 unit may have been breached. That could release a very dangerous radioactive plume.

Radiation at reactor No. 1 peaked at 162 sieverts per hour on March 14. Radiation at reactor No. 2 peaked at 138 sieverts per hour on March 15. These readings were recorded after the reactors lost significant amounts of cooling water, leaving large parts of them exposed to the air. The same happened to reactor No. 3 The injection of water caused radiation levels to drop to between 20 and 31 sieverts, 11 to 22 percent of their peaks, by early April.

Meltdowns and Cesium

“Nuclear fuel rods normally are kept submerged in water inside reactor cores, thus preventing the rods' temperature from exceeding a certain limit. However, if the water level goes down and the fuel rods are exposed, the cooling system's efficiency rapidly deteriorates, causing the reactor core's temperature to rise. If the core's temperature exceeds a certain level, the fuel rods melt--in other words, a meltdown occurs.” [Source: Yomiuri Shimbun]

“The detection of traces of a nuclear substance called cesium outside the Fukushima nuclear power plant led the agency to infer that a possible meltdown occurred. Cesium is formed after uranium, which is used for nuclear fuel at the plant, undergoes fission. However, nuclear fuel is kept in a pellet form covered by a special metal. It is impossible for cesium to leak from the shell unless the metal melts--which occurs when the temperature reaches 2,700 C to 2,800 C, according to the agency, an affiliate of the Economy, Trade and Industry Ministry.”

“The detection of cesium indicates the temperature of the reactor core has reached a seriously high level. If the water level does not rise enough to stop the reactor core from overheating, the reactor core's stainless steel cover could melt. In the worst-case scenario, the reactor core itself might explode due to a build-up of excessive pressure.”


Meltdown at Reactor 1 at the Fukushima Nuclear Power Plant

Eric Talmadge and Mari Yamaguchi of AP wrote: By late afternoon on the day of the disaster, “Unit 1 was spiraling out of control, with its power and cooling systems down. The heat from decaying radioactive elements in the fuel rods was growing. As the core overheated, it burned off its coolant water, exposing the 13-foot (4-meter) rods. In turn, steam from the evaporated water was building up inside the containment chamber. As the heat and pressure rose, the uranium pellets inside the rods melted through their zirconium casings. When the zirconium reached 2,200 degrees Fahrenheit (1,200 Celsius), it reacted with the water, producing hydrogen.” [Source: Eric Talmadge and Mari Yamaguchi, Associated Press, July 2, 2011]

Most of the nuclear fuel inside the No. 1 reactor at the Fukushima No. 1 nuclear plant melted down within about 15 hours and 20 minutes of the tsunami striking the plant. TEPCO made this assessment in May after analyzing temperatures, pressure readings and other data taken from the reactor's central control room. Most of the fuel melted and fell to the bottom of the reactor pressure vessel. The government and TEPCO previously believed that about half of the fuel had melted down. [Source: Yomiuri Shimbun, Kyodo May 17 and 18, 2011]

The fuel is estimated to have started melting about four hours after the plant was hit by the massive tsunami triggered by the March 11 earthquake. The reactor automatically halted shortly after the 2:46 p.m. earthquake, but its water level dropped to the upper part of the fuel rods and the temperature began to rise around 6 p.m. The damage to the fuel had begun by 7:30 p.m. with most of it having melted by 6:50 a.m. the following day, the utility said.

A TEPCO report issued in late May said the meltdown started about four hours after the quake and most of the fuel rods melted and dropped to the bottom of the pressure vessel after about 15 hours. Welded parts of the bottom of the vessel, including a section for control rods to pass through, were damaged. The amount of hydrogen sharply increased immediately after the fuel rods started to melt, reaching nearly 800 kilograms, enough to cause a major hydrogen explosion, in about half a day. [Source: Yomiuri Shimbun, May 25, 2011]

Events During the Meltdown at Reactor No. 1 at Fukushima

Describing the events after the earthquake at Reactor No. , the Yomiuri Shimbun reported: “Immediately after the earthquake, the reactor was halted when control rods were automatically inserted to slow down power output. When outside power was cut off, two emergency diesel generators started up. Valves that send steam to the turbine closed and an isolation condenser to convert steam back into water started six minutes after the earthquake. When the tsunami struck, however, all electricity supplies at the plant were destroyed, rapidly aggravating the situation in the No. 1 reactor. [Source: Yomiuri Shimbun, May 25, 2011]

According to TEPCO's analysis, the water level in the pressure vessel declined in the two hours after power was lost, exposing fuel rods in the reactor's core. An hour later, the temperature in the reactor apparently shot up to as high as 2,800 C. The heat melted the casing for the fuel rods and nuclear fuel pellets inside began to melt and fall apart--a meltdown. It is believed all the fuel rods had melted and dropped to the bottom of the pressure vessel within 15 hours of the earthquake.

The fuel rods' casing is made of zirconium, which forms zirconia when it combines with oxygen in water. Hydrogen left over from this chemical reaction--water is composed of hydrogen and oxygen--quickly filled the reactor. TEPCO's report said about 800 kilograms of hydrogen was produced by this reaction, which leaked from the pressure and containment vessels to fill the building that houses the reactor. This massive amount of hydrogen is believed to be what caused the explosion at 3:36 p.m. on March 12. Freshwater was poured into the vessel to try to cool the reactor core shortly before 6 a.m. on March 12. According to the report, the fuel had completely melted by the time this began. Freshwater injections stopped at 2:30 p.m. and seawater injections started at about 8:20 p.m.

Meltdown at Reactor 2 and 3 at the Fukushima Nuclear Power Plant

Hidenori Tsuboya wrote in the Asahi Shimbun, “Inside the pressure vessel at the No. 2 reactor, pressure began dropping precipitously on the night of March 15, falling to almost the same level as the outside atmosphere. The following day, March 16, pressure in the surrounding containment vessel rose in the afternoon. The heat generated by a core meltdown likely burned holes through the bottom of the pressure vessel, accounting for the rise in pressure.” [Source: Hidenori Tsuboya, Asahi Shimbun, May 18, 2011]

“At the No. 3 reactor, pressure in the pressure vessel started to fall on the night of March 15.Similarly, as it fell to almost the same level as the outside atmosphere around midnight March 16, pressure in the containment vessel surged. Again, the bottom of the pressure vessel may have been damaged in a manner similar to the No. 2 reactor. Pressure in the containment vessel jumped March 20. It is believed this was caused by melted fuel rods dropping to the concrete bottom of the containment vessel through damaged piping, some of it used to gauge neutron levels. As a result, gray smoke rose from the No. 3 reactor building shortly before 4 p.m. on March 21.”

The Yomiuri Shimbun reported: “The progress of the meltdowns at the Nos. 2 and 3 reactors was slower than the pace at the No. 1 reactor because a "reactor isolation cooling system" was working at the two reactors for a certain period of time even after the tsunami that hit the plant after the quake. The No. 1 reactor's cooling system, which uses an isolation condenser, is different from that of the other two reactors and stopped working completely after the tsunami. As it was impossible to obtain accurate records of the reactors' water levels, TEPCO made calculations about the Nos. 2 and 3 reactors based on two possible scenarios: In one, the lower parts of the fuel rods were immersed in water that was injected into the reactor; in the other, the injection of water did not go well and the fuel rods were left completely exposed. [Source: Yomiuri Shimbun, May 25, 2011]

Under the partial-exposure scenario at the No. 2 reactor, the core was exposed about 75 hours after the earthquake and the fuel rods started to melt at 77 hours after the quake. At the same time, hydrogen started to be generated and a hydrogen explosion occurred in a pressure suppression chamber in the lower part of the reactor containment vessel at the 87th hour. However, half of the fuel rods remained at the bottom of the reactor core in a melted mass even after a week after the quake. As a result, the pressure vessel was not damaged. Under the full-exposure scenario at the No. 2 reactor, the meltdown rapidly progressed after the hydrogen explosion, and about 80 percent of the fuel rods dropped to the bottom within about 109 hours after the quake, damaging the pressure vessel.

Under both scenarios at the No. 3 reactor, the upper parts of the fuel rods started to become exposed above the water at about the 40th hour after the quake after a high pressure core cooling system, a type of emergency cooling device, stopped operating. At about the 42nd hour after the quake, the fuel rods started to melt. Under the full-exposure scenario at the No. 3 reactor, however, the pressure vessel became damaged about 66 hours after the quake. Despite repeated venting--attempts to lower the pressure in the containment vessel by releasing steam--the amount of hydrogen generated reached almost 600 kilograms, causing a hydrogen explosion.

Core temperatures at the Nos. 1 to 3 reactors once neared 3,000 C but currently are stable at around 100 C to 170 C.As the residual heat of the fuel rods has decreased, TEPCO thinks the risk that overheating will recur is low. It was probable that radioactive water escaped the reactor buildings through holes in the pressure vessels and in the outer containment vessels as well.

Events During the Meltdown at Reactor No. 2 at Fukushima

The Yomiuri Shimbun reported: “A reactor core isolation cooling system, which injects water into a reactor core in emergencies, began running at the No. 2 reactor in the time after the earthquake and before the tsunami. According to a TEPCO worker's log and other sources, staff switched the cooling system off and on trying to deal with fluctuations in the water level in the pressure vessel. The cooling system stopped working at about 1:25 p.m. on March 14, meaning it ran for about three days after the earthquake hit at 2:46 p.m. on March 11. [Source: Yomiuri Shimbun, May 25, 2011]

After the cooling system failed, seawater was sprayed on the reactor from fire trucks starting at 4:34 p.m. the same day, but the water level continued to decline. By 6 p.m. on March 14, the tips of the fuel rods were exposed, and just an hour later they were almost totally out of the water, according to the report. Since water gauges for the reactors may have been damaged and are no longer accurate, TEPCO speculated on the situation inside the reactor cores based on worst-case scenarios, in which the water levels in the Nos. 2 and 3 reactors sank despite the injection of seawater.

Based on these assumptions, TEPCO concluded damage to fuel rods in the No. 2 reactor began about 8 p.m. on March 14, an hour after the fuel rods were fully exposed. The rods melted and fell to the bottom of the pressure vessel by 8 p.m. on March 15. Eight hours later, or at about 4 a.m. on March 16, the bottom of the pressure vessel was damaged, according to TEPCO's analysis. Although "the worst" has likely happened, TEPCO said recent temperature measurements near the No. 2 reactor's pressure vessel showed the situation is relatively stable. If the water gauge is accurate, it would mean the seawater injection worked to a certain extent, meaning meltdown did not occur until at least a week after the earthquake, according to the report.

20110413-Jap Min of Land, Infr and TransFukushima_I_NPP_1975_medium_crop.jpg
Fukushima facilities in 1975

Events During the Meltdown at Reactor No. 3 at Fukushima

The Yomiuri Shimbun reported: “It was early on March 13, or 36 hours after the earthquake happened on March 11, that conditions at the No. 3 reactor began to clearly worsen. Though the No. 3 reactor lost its power source in the tsunami, TEPCO had managed to continue pouring water into the nuclear reactor, albeit with a gap of one hour, using two different types of emergency cooling systems--the high-pressure core flooding system and the reactor core isolation cooling system.” [Source: Yomiuri Shimbun, May 25, 2011]

However, the high-pressure core flooding system, which was considered the last resort, automatically stopped at 2:42 a.m. on March 13 because of falling pressure inside the reactor. The conditions inside the reactor drastically deteriorated after all batteries ran out and the reactor became uncontrollable.The reactor's pressure rapidly rose from 0.58 megapascals to more than 7 megapascals in only two hours. Operators tried to reactivate the reactor core isolation cooling system, but they could not. TEPCO then reported to the government at 5:10 a.m. that all functions to inject water into the reactor had been lost.

During that time, the water level at the No. 3 reactor kept going down. According to TEPCO's estimates, fuel rods became exposed above the water's surface at around 7 a.m. When operators released steam from inside the pressure vessel to the containment vessel to relieve pressure in the reactor shortly after 9 a.m., fuel rods were exposed to the air completely, and they began to be damaged. At 9:20 a.m., operators vented steam from the containment vessel to the outside. Soon after the operation, at 9:25 a.m., operators started to inject freshwater into the pressure vessel for the first time since the loss of cooling functions about seven hours earlier.On the afternoon of March 13, they changed the water injected into the vessel from freshwater to seawater. But, in the early hours of March 14, they stopped injecting water for about two hours to replenish their seawater supply.

According to TEPCO's worst-case estimates, fuel rods remained exposed and most of them melted and dropped down to lower parts of the reactor during that time. TEPCO believed the reactor's condition continued to get worse even after the company started to inject water and the pressure vessel was damaged at 9 a.m. About 59 percent of the metal covering the fuel rods apparently reacted chemically with water to produce a huge amount of hydrogen, which exploded around 11 a.m. on March 14.

On the other hand, according to TEPCO's estimates, if the water injection had been conducted successfully, the pressure vessel would not have been damaged, but about 70 percent of the metal covering the fuel rods would have reacted with water. This still could have generated enough hydrogen to cause an explosion.

Melt-Through at Fukushima?

Justin McCurry wrote in The Guardian: “Molten nuclear fuel in three reactors at the Fukushima Daiichi power plant is likely to have burned through pressure vessels, not just the cores, Japan has said in a report in which it also acknowledges it was unprepared for an accident of the severity of Fukushima. It is the first time Japanese authorities have admitted the possibility that the fuel suffered "melt-through" — a more serious scenario than a core meltdown. [Source: Justin McCurry, The Guardian, June 8 2011]

The report, which was to be submitted to the International Atomic Energy Agency (IAEA), said fuel rods in reactors No 1, 2 and 3 had probably not only melted, but also breached their inner containment vessels and accumulated in the outer steel containment vessels. The plant's operator, Tokyo Electric Power (Tepco), says it believes the molten fuel is being cooled by water that has built up in the bottom of the three reactor buildings.

the Yomiuri Shimbun reported: Sections of the bottom of the pressure vessels where control rods go through have been damaged. Highly radioactive water from inside the pressure vessels was confirmed to have leaked out of the containment vessels, even outside the buildings that house the reactors.”

The 750-page report, compiled by Japan's emergency nuclear task force, concedes that the country was wrongfooted by the severity of the accident, which occurred after the plant was struck by waves more than 14 metres high following the earthquake on 11 March. "We are taking very seriously the fact that consistent preparation for severe accidents was insufficient," the report said. "In light of the lessons learned from the accident, Japan has recognised that a fundamental revision of its nuclear safety preparedness and response is inevitable."

Minor Criticality at Fukushima Plant's No. 2 Reactor?

In November 2011, according to Yomiuri Shimbun, TEPCO reported, Small-scale criticality — a continuous nuclear reaction--may have taken place at the No. 2 reactor of the crippled Fukushima No. 1 nuclear power plant, as small amounts of radioactive xenon were detected. TEPCO injected boric acid into the reactor, as boric acid is effective in suspending nuclear fission reactions by absorbing neutrons. [Source: Yomiuri Shimbun, November 3, 2011]

Xenon 133 and xenon 135 are radioactive substances generated when nuclear fission reactions take place. The substances were found in gas from the reactor containment vessel, TEPCO said. Xenon 133 has a half-life of about five days, while xenon 135's is about nine hours. Because the two substances have very short half-lives, a small-scale fission reaction is likely to have taken place within the reactor, he said. "As the reactor's cooling is progressing, this finding will not have a major impact on the situation," Matsumoto said. "We don't believe criticality has been maintained.”

TEPCO suspected a nuclear fission chain reaction took place caused by existing neutrons within the reactor or that very local criticality took place as the water temperature in the reactor decreased and water density increased, due to the recent increase of water injection.At the No. 2 reactor, TEPCO activated in late October a gas management system to remove radioactive substances contained in the gas within the containment vessel with filters. The two types of xenon were detected in the gas. The amounts detected were about one-100,000th becquerel per cubic centimeter for both types.

A day later TEPCO retracted assessment that a continuous nuclear reaction, or a criticality, could have taken place in the damaged Number 2 reactor. The company said the small amount of xenon-135 it detected in gas taken from the reactor's containment vessel was the result of the spontaneous nuclear fission of radioactive curium-242 and -244, a form of radioactive decay, and not from sustained fission or criticality. The two substances are contained in nuclear fuel. The amount of xenon-135 detected almost matched the amount that would have been produced if the radioactive curium in the fuel had undergone spontaneous fission. TEPCO says a criticality event would have resulted in higher levels of xenon concentration. Spontaneous fission refers to the nuclear fission of radioactive materials other than uranium, and it does not lead to criticality. Such fission is said to occur constantly.

Fukushima facilities after the explosions in 2011

Acknowledgment of Meltdown at Fukushima Nuclear Power Plant

In mid April, the Nuclear and Industrial Safety Agency reported that nuclear fuel pellets in the Nos. 1 to 3 reactors at the Fukushima power station are believed to have partially melted. The report was the first time the agency acknowledged that nuclear fuel has melted at the Fukushima No. 1 nuclear plant. [Source: Yomiuri Shimbun, April 20, 2011]

Hidehiko Nishiyama, a spokesman for the agency, said damage to reactors can occur in three phases of increasing severity. In the first phase of initial damage to a reactor's core, the metallic casing surrounding the fuel pellets are damaged but the pellets remain intact. The second phase involves some melting of nuclear fuel. In the third phase, what is known as a meltdown, all the fuel pellets melt and accumulate at the bottom of the containment vessel.

The agency said it now believes the fuel pallets have melted because of the high levels of radiation detected at the Nos. 2 and 3 reactors. Melting fuel pellets also likely led to a hydrogen explosion at the No. 1 reactor, Nishiyama said but "We can't say for sure about how much has melted until the rods are actually taken out.”

Meltdown Without High Temperatures

In May TEPCO was able to better assess the condition of Reactor No. 1 because workers were able to get close enough to fix a water gauge. It showed that the water level in the reactor was much lower than expected despite the infusion of tons of water since a devastating earthquake and tsunami knocked out the plant’s crucial cooling systems.[Source: Hiroko Tabuchi and Matthew L. Wald, New York Times, May 12, 201]

Hiroko Tabuchi and Matthew L. Wald wrote in the New York Times, “One of the most startling findings was that water levels in the reactor vessel, which houses the fuel rods, appeared to be about three feet below where the bottom of the fuel rods would normally stand. Ever since the reactor shut down, workers’ primary task has been to keep pouring water into the reactors to ensure the nuclear fuel remained covered so that it would not melt. But the new information suggests the fuel was uncovered for at least some time, probably early in the crisis.”

“That indicated that the exposed fuel has probably melted and slumped to the bottom of the vessel in little pellets, Junichi Matsumoto, a Tepco spokesman...Still, the worst fears did not materialize. Experts have long worried that such melting would allow a nuclear chain reaction to restart, producing enough heat to burn through all barriers — resulting in a full meltdown and a catastrophic release of radioactive material.”

“Mr. Matsumoto said relatively low temperature readings on the surface of the reactor, between 100 and 120 degrees Celsius (or 212 to 248 degrees Fahrenheit), suggested that the slumped fuel was being kept cool to some extent by the water inside the reactor and therefore was not as dangerous as some expected.” “We are not seeing a China Syndrome,” Mr. Matsumoto said, using a term coined in the United States in the 1970s to describe a severe nuclear meltdown of the fuel, which could sink into the ground and cause an explosion. The term is a satiric reference to the idea that in such an uncontrolled reaction, the core could burn through the earth.

David Lochbaum of the Union of Concerned Scientists, a nonprofit group usually critical of the nuclear industry, agreed that the temperature readings were a good sign. He told the New York Times he believed that the damage to the fuel at Reactor No. 1 was already finished, and that even if some fuel rods were still standing — and therefore exposed — they were no longer hot enough to keep melting. “As bad as things are,” Mr. Lochbaum said, “they’re getting better.”He cautioned that dangers remain. Conditions could get worse, he said, if the continued addition of water creates conditions more conducive to a nuclear reaction.

Failure of the Emergency Cooling System at Reactor No.1 and Missed Opportunities to Cool the Reactor

The interim report released in December 2011 by a government panel questioned the operation of the plant's emergency reactor cooling systems, which are automatically activated when all power supplies are cut. The manuals stipulate there is "no necessity to consider" a situation in which all power sources are cut off. [Source: Yomiuri Shimbun, December 30, 2011]

According to a Yomiuri Shimbun article, “The March 11 tsunami, which hit the plant at a height of up to 15.5 meters, submerged even the emergency diesel power generators, leaving the reactors completely without power. In this kind of situation, an isolation condenser is supposed to be activated to provide emergency cooling to the reactors.

In fact, the Fukushima plant's emergency isolation condenser, which was equipped only for the No. 1 reactor, was operating before the tsunami arrived. But when power was lost, the isolation valves closed and the condenser stopped working. However, Yoshida and other staff at the plant's emergency response headquarters were not aware the isolation condenser had stopped, nor were people at the TEPCO head office.

According to the report, this meant people in charge misjudged the situation when an alternative water source was needed to cool the No. 1 unit's core, which delayed the injection of water. The report notes there were at least three opportunities for the emergency headquarters and the TEPCO head office to notice the isolation condenser had stopped functioning. Since they failed to do so, key opportunities to keep the reactor from overheating were missed.

The first missed opportunity was from 4:42 to 4:56 p.m. on March 11, when they confirmed the water level in the reactor vessel was declining. The next came an hour later at about 5:50 p.m. when radiation levels inside the reactor building were rising. Both of these were proof the condenser was not properly keeping the core cool. Shift operators on duty finally noticed the isolation condenser had stopped working, opened its valves at 6:18 p.m. that day and reported the situation to the plant's emergency headquarters. However, seven minutes later the operators closed the isolation valves again out of fear of damaging the piping. This was done without consulting the plant's emergency headquarters or the head office, and went unreported.

At about 9:19 p.m., the No. 1 reactor's water gauge began working again, and indicated the nuclear fuel inside was still safely submerged in water. According to a later analysis by TEPCO, the meltdown at the No. 1 unit was already under way at this time, and the water gauge was malfunctioning. However, the condition of the isolation condenser was misjudged and workers at the plant failed to prepare an alternative water source for cooling the reactor.

The emergency response at the time was focused on preparing to vent the No. 2 reactor to release steam that was causing pressure inside the reactor to reach dangerous levels, threatening an explosion. However, pressure later began to rise in the containment vessel of the No. 1 unit, and work shifted to preparing for a vent of this reactor early on the morning of March 12. Huge amounts of hydrogen, likely generated by the core meltdown then under way, accumulated inside the reactor building, eventually causing an explosion at 3:36 p.m. on March 12, just after the vent began.

20110413-Jap Min of Land, Infr and TransFukushima_I.jpg
closer look at the Fukushima facilities in 1975

All 68 Tons of Nuclear Fuel in Reactor No. 1 May Have Melted Down

In December 2011 TEPCO said almost all the nuclear fuel inside the No. 1 reactor has melted, damaging the pressure vessel and eroding the concrete bottom of the containment vessel by up to 65 centimeters. The determination was based on temperature, water levels and other data. TEPCO said the fuel inside other reactors has melted to various degrees. The No. 2 reactor's fuel is up to 57 percent melted, while that of the No. 3 reactor is up to 63 percent melted, TEPCO's analysis has shown. [Source: Yomiuri Shimbun, December 2, 2011]

According to a Yomiuri Shimbun article: Following the March 11 earthquake and tsunami, water injection at the No. 1 reactor was suspended for about 14 hours, resulting in damage more serious than in the Nos. 2 and 3 reactors, which had water injection suspended for six to seven hours, according to TEPCO.

The nuclear fuel at the No. 1 reactor melted as its temperature reached nearly 3,000 C at one time, TEPCO estimated. In the No. 1 reactor, TEPCO believes, almost all of the about 68 tons of fuel melted. This has not only seriously damaged the bottom of the steel pressure vessel enough to create holes, but the fuel has also fallen to the concrete bottom of the containment vessel, eroding it by up to 65 centimeters.

Only 37 centimeters of concrete remains between the fuel and the vessel's outermost steel wall in the most damaged area, TEPCO said. Without water, the No. 1 reactor's fuel temperature was more than high enough to have melted everything inside the pressure vessel, not only the fuel itself but also the fuel control rods, the utility said.

TEPCO currently maintains a steady supply of water to the three reactors, enabling the No. 1 reactor to always have about 40 centimeters of cool water at the bottom of the containment vessel, enough to cover the melted fuel, according to the utility.Both the government and the utility said the three reactors are experiencing no problems in maintaining cooling functions.

However, the melted fuel likely will be a major hurdle in removing fuel from the troubled reactors in the decommissioning process, which is expected to take more than 30 years.

Government Knew Meltdown Was Probable

The government's Nuclear Emergency Response Headquarters was aware that the Fukushima No. 1 nuclear power plant would likely suffer a meltdown just hours after the nuclear crisis began, according to internal meeting summaries. At the first meeting, which started after 7 p.m. on March 11, members of the headquarters were briefed that the plant had activated emergency cooling systems--which run mainly on batteries--after losing all of its power, the summaries said. [Source: Yomiuri Shimbun, March 10, 2012]

"After [the batteries run out in] eight hours, temperatures at the reactor cores will increase and the reactors will likely suffer a meltdown," an unknown speaker was quoted as saying in the summaries. At that time, the government had not officially referred to the possibility of a meltdown at the nuclear power plant.

At a meeting held after 10 p.m. the same day, Kan referred to the 1986 Chernobyl disaster and the 1979 Three Mile Island crisis in the United States. "Could a Chernobyl-type [meltdown] occur? Or a meltdown similar to the one at Three Mile Island?" the summaries quote him as saying. However, no responses were recorded in the summaries.

Fukushima No. 2 Plant Was 'Near Meltdown'

In February 2012, the Yomiuri Shimbun reported: Fukushima no. 2 nuclear power plant was "near meltdown" after being hit by tsunami following the Great East Japan Earthquake on March 11, according to the head of the plant. The No. 2 plant, on the border of Naraha and Tomioka towns in Fukushima Prefecture, is 12 kilometers from the Fukushima No. 1 nuclear power plant, which suffered a meltdown. Both facilities are operated by TEPCO. Plant chief Naohiro Masuda, in charge of plant operations since the crisis, said "The No. 2 plant almost suffered the same fate as No. 1 [which led to a severe crisis]." [Source: Yomiuri Shimbun, February 10, 2012]

On March 11, a 9-meter-high tsunami struck the No. 2 plant, while the No. 1 plant was hit by a 13-meter-high tsunami. The tsunami caused the No. 2 plant's seawater pumps, used to cool reactors, to fail. Of the plant's four reactors, three were in danger of meltdown. Luckily, one external high-voltage power line still functioned, allowing plant staff in the central control room to monitor data on internal reactor temperatures and water levels.

By March 15, the No. 2 plant's four reactors reached a state of cold shutdown without any leakage of radioactive materials. "[At that point, the situation at the No. 2 plant] was considerably different from the No. 1 plant where it was difficult to know what was going on," Masuda, 53, said. However, despite intense efforts by all employees, it took a long time to stabilize the reactors.

On March 11, about 2,000 employees of the No. 2 plant worked to stabilize the reactors. Some employees connected 200-meter sections of cable, each weighing more than a ton, over a distance of nine kilometers. Masuda noted the timing of the disaster was critical in saving the plant. "We were lucky it happened on a Friday afternoon [and not on a weekend]," he said. Masuda pointed out only 40 employees would have been at the plant if the earthquake had occurred in the evening or on a weekend. "[In that case] it would be have been difficult for us to deal with the disaster," he said.

The Fukushima prefectural government conducted an on-site inspection at the No. 2 plant in February 2012 and repeated a request to TEPCO to decommission the facility. The no. 2 plant's No. 1 reactor began operating in 1982.

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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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