MARCH 11, 2011: “A SITUATION THAT WE HAD NEVER IMAGINED” - Fukushima: The Story of a Nuclear Disaster (2015)

Fukushima: The Story of a Nuclear Disaster (2015)

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MARCH 11, 2011: “A SITUATION THAT WE HAD NEVER IMAGINED”

At 2:49 p.m., the chandeliers began to sway in an ornate hearing room of the Diet Building, home to Japan’s parliament. Prime Minister Naoto Kan glanced nervously toward the high ceiling. Legislators darted about, their voices rising as the shaking increased. One speaker advised everyone to duck under the desks. Aides hurried to the prime minister’s side, uncertain where safety actually lay.

NHK, Japan’s public broadcasting system, was televising the Kan hearing. As an unsteady camera captured the confusion in the hearing room, the network was receiving an earthquake alert from the Japan Meteorological Agency. Ground-motion sensors along the coast near Sendai, north of Tokyo, had picked up seismic activity offshore. Based on those readings, the agency estimated that a magnitude 7.9 quake was likely. In thirty seconds it dispatched a warning to residents along the northeastern coast. Personal cell phones lit up. Businesses, schools, hospitals, and the news media all received alerts; twenty-four bullet trains operating in the region glided to a halt.

Within ninety seconds, NHK had interrupted its coverage to provide early details about the quake. Almost instantly, TV screens across Japan began displaying emergency information.

In Sendai, an NHK cameraman hopped aboard the helicopter kept on permanent standby for the network and lifted off from the airport. It would be perhaps the last aircraft to depart the facility before the earthquake crisis took a dramatic turn for the worse. The footage shot from that helicopter would soon be replayed again and again around the world.

Japan sits atop one of the most earthquake-prone regions of the world, shaken by more than a thousand tremors each year. The ancient Japanese believed a giant catfish lay buried beneath the islands. When the creature thrashed about, the earth moved. Modern scientists have their own explanation: several massive plates of the earth’s crust abut each other around the islands of Japan, shifting continuously a few inches per year. If their movement is impeded, stress builds until it is released in a seismic event. That event may be barely detectable—or it can be catastrophic.

Along the northeastern coast, the westward-moving Pacific Plate is forced beneath the North American Plate that holds most of northern Japan. This downward movement along a seam in the earth’s surface is known as subduction. It is responsible for producing some of the world’s largest earthquakes.

A map of Japan showing the major cities …

A map of Japan showing the major cities. Perry-Castañeda Library Map Collection, University of Texas

Although northeastern Japan is no stranger to earthquakes, seismologists long believed that a subduction quake was unlikely to happen there because the 140-million-year-old Pacific Plate was sliding downward smoothly, never creating a huge buildup of stress. Unbeknownst to them, however, sections of the two plates had locked together, possibly for as long as a thousand years, while pressure continued to build.

At 2:46 p.m. on March 11, 2011, about eighty miles offshore, the strain along the tightly squeezed plate surfaces finally became too great. Like a stubborn load winched too tightly, the Pacific Plate broke free and lurched downward with a jerk. That freed the North American Plate to spring upward, the pressure relieved. Along a seam of more than 180 miles, the Pacific Plate dropped at an angle, moving 100 to 130 feet westward as the overlying North American Plate rose up and angled eastward the same distance.

Within moments, the northern half of the island of Honshu, Japan’s largest, was stretched more than three feet to the east, a movement of land mass so great the earth’s axis shifted by several inches.

Earthquakes send out packets of energy in a successive series of waves. First come the body waves, which travel through the entire body of the earth. The fastest body waves are primary (P) waves, which are nondestructive and travel three or four miles per second. This is the ground motion Japan’s earthquake early-warning sensors detect. P waves are followed by secondary (S) waves, which travel at about half the speed but have the potential to cause more damage. Following the P and S body waves are the even slower surface waves, which cause the most severe ground motion and are responsible for the most damage to surface structures. The lead time provided by P waves is crucial because even a few minutes’ advance notice allows people to seek safety and critical systems to shut down.

Japan’s earthquake warning network is regarded as the best in the world. It was created in the aftermath of the 1995 Kobe earthquake in western Japan, which killed more than five thousand people. The system relies on a thousand ground-motion sensors around the country that can pinpoint the location of an earthquake within a second or two and in most cases estimate its magnitude as fast.

On March 11, sensors near Sendai detected the first offshore tremors within eight seconds and transmitted the information to the Japan Meteorological Agency 190 miles away in Tokyo. Just two days before, four quakes had been measured in this same area of seabed. The largest had a magnitude of 7.3, which was nothing out of the ordinary by Japanese standards.1 Seismologists thought that the stress had been relieved and the earth had settled back to normal. Now, however, the sensors were recording an even larger quake, with an estimated magnitude of 7.9.

For all of its technological prowess, Japan’s early-warning system has a proven weak spot: because of its speed, the system broadcasts warnings before all seismic data have arrived, and thus it tends to significantly underestimate the size of an earthquake. Although a 7.9 magnitude quake could produce major damage, it was not beyond what Japan had often experienced before.

But nature was throwing technology a curveball.

The Fukushima Daiichi nuclear power station in 2010 …

The Fukushima Daiichi nuclear power station in 2010. The exhaust stacks between Units 1 and 2 and between Units 3 and 4 can be used to discharge radioactive gas vented from the containment structure during an emergency. Administrative buildings, including the emergency response center and Seismic Isolation Building, are located behind Unit 1. Tokyo Electric Power Company

The shaking continued for about three minutes, an unusually long time. About seventy-five seconds in, long after the system had sent out an alert, the massive undersea plates slid apart, releasing cascading amounts of energy. The U.S. Geological Survey would later estimate that enough surface energy was thrown off by the rupture to power a city the size of Los Angeles for a year. Over several days, the Japan Meteorological Agency upgraded the quake, ultimately to magnitude 9, forty-five times more energetic than the original 7.9 prediction. This was the largest earthquake ever recorded with instruments in Japan and one of the five most powerful in the world since modern record keeping began in 1900.

On a forested stretch of the coast south of Sendai, seismic sensors at Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Station also registered the P waves. At forty-six seconds past 2:46 p.m., the motion caused a sensor on the Unit 1 reactor to trip. Twelve seconds later, another sensor on Unit 1 tripped. The reactor began to shut down automatically, just as it was designed to do. At 2:47, an alarm alerted control room operators to Unit 1’s status: “ALL CR FULL IN.”2 The ninety-seven control rods—the brakes that halt a nuclear chain reaction—were fully inserted into the core. Units 2 and 3 soon followed suit. In about a minute, the three operating reactors at Fukushima Daiichi were in shutdown. (The other three reactors at Fukushima Daiichi, Units 4, 5, and 6, were out of service for routine maintenance.) Elsewhere along the northeastern coast, eight reactors at three other nuclear plants also automatically shut down. It was what U.S. reactor operators call a “vanilla scram”—a shutdown done by the book.

Fukushima Daiichi has six boiling water reactors (BWR), designed in the 1960s and early 1970s by the General Electric Corporation (GE) and marketed around the world. This type of reactor produces electricity by boiling water to make steam to turn a turbine generator. Three factors are at work: the amount of water, its temperature, and its pressure.

As the water circulates around the ferociously hot core, it turns into steam. The steam, under pressure, moves through pipes to the turbine generators. After forcing the turbines around, the steam flows down into a condenser, where it is cooled, converted back to water, and then recirculated through the core. Too little water in the core and the fuel rods can overheat and boil dry; too much steam, and pressure builds to unsafe levels.

The nuclear fuel used in the reactors consists of uranium oxide baked into ceramic pellets. These pellets are formed into fuel rods by inserting them into thirteen-foot-long tubes (or “cladding”) made of a metallic alloy containing the element zirconium, which contains radioactive gases produced in the fission process.

The fuel rods are assembled into rectangular boxes known as fuel assemblies, which are then arranged in an array approximately fourteen feet wide within a six-inch-thick steel reactor vessel. The reactor vessel itself is located within another structure called a drywell, which is part of the primary containment structure. In “Mark I” BWRs like Fukushima Daiichi Units 1 through 5, the drywell is made of a steel shell surrounded by steel-reinforced concrete. This shell is a nearly impermeable barrier designed to contain radioactivity in the event of an accident.

The Mark I boiling water reactor …

The Mark I boiling water reactor. U.S. Nuclear Regulatory Commission

Another part of the primary containment, the wetwell, sits below the drywell and is connected to it through a series of pipes. Half filled with water and often called a torus because of its doughnut shape, this system is designed to reduce pressure by drawing off excess steam and condensing it back to water. Without the “energy sponge” provided by the torus, the primary containment structure would have to be five times larger—and thus more expensive—to withstand the same amount of energy released during an accident.

Finally, the primary containment is surrounded by another structure, the reactor building, known as the “secondary containment.” Although the reactor building can help to contain radioactivity, that is not its main purpose.

The ceramic fuel pellets, the zirconium alloy cladding, the reactor vessel, the primary containment, and the reactor building constitute a series of layers that are intended to prevent the release of radioactivity to the environment. However, as the world would soon witness at Fukushima Daiichi, these barriers were no match for certain catastrophic events.

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After a scram, the reactor operators’ primary job in the first ten minutes of an abnormal event is to verify that appropriate automatic actions are taking place as the plant shuts itself down. However, with nuclear reactors, the safety challenges continue after the off switch is flipped. Although the chain reaction has stopped and uranium nuclei are no longer undergoing fission, the fuel in the reactor cores continues to generate a huge amount of heat from the decay of fission products, unstable isotopes produced when the reactor was operating. Therefore, pumps driven by electric motors are still needed to circulate cooling water around the nuclear fuel and transfer the heat energy to what engineers call the “ultimate heat sink”—in this case, the Pacific Ocean.

If cooling is lost, in as little as thirty minutes the water level within the reactor vessel drops about fifteen feet and falls below the tops of the nuclear fuel rods. Soon afterward, the exposed rods overheat, swell, and burst. The zirconium alloy cladding reacts with steam and generates potentially explosive hydrogen gas. The temperature of the fuel pellets continues to rise until they begin to melt, emitting more radioactive gases into the reactor vessel. After several hours, the melting core slumps and drops to the bottom of the vessel. The molten fuel is so corrosive that within a few hours it burns completely through the six-inch steel wall.

Once the reactor vessel is breached, the fuel flows through to the concrete floor of the primary containment, where it reacts violently with the concrete, churning out additional gases. At this point, any of several mechanisms can cause the primary containment to fail, either rapidly through violent explosions or slowly through gradual overpressure. The last barrier to the environment—the reactor building—may contain some of the radiation but cannot be counted on to do so. The end result: the release of plumes of radioactive material and contamination of the environment.

In addition to more than two hundred tons of fuel in the cores of Units 1, 2, and 3 combined, the Fukushima Daiichi site stored hundreds of tons of irradiated fuel that had been discharged from the reactor cores and was now being kept under water in swimming pool-like structures at each of the six reactors and in a common pool nearby. Most of this was “spent fuel” no longer useful for generating electricity. Some older spent fuel was stored in “dry” concrete and steel storage casks. Even though the spent fuel rods in the pools had been removed from the reactor cores months or even years earlier, they still generated enough decay heat to require active cooling systems.

Ordinarily, the electricity needed to power the cooling systems for both the scrammed reactors and the spent fuel pools would come from off-site through the power grid. However, conditions on March 11 were far from ordinary because of the earthquake, which had toppled electrical transmission towers and damaged power lines. Inside their control rooms, jolted by the tremors, operators at Fukushima Daiichi could only surmise what was happening in the world beyond. They now watched their monitors as temperature and pressure in the reactors decreased.

In the event that the primary cooling system fails, boiling water reactors have auxiliary systems. Fukushima Daiichi Unit 1 had isolation condensers: large tanks of water designed to provide an outlet for steam from the reactor vessel if it becomes blocked from its normal path to the turbine condenser. The other reactors were each equipped with a “reactor core isolation cooling” system, known as RCIC (pronounced rick-sea), which is powered by steam and can run reliably without AC power as long as batteries are available to provide DC power to the indicators and controls.3 In addition, BWRs are equipped with emergency core cooling systems should the primary and auxiliary systems fail.

Just before 2:48, as the shaking worsened, alarms in the Unit 1 control room signaled that power had been lost to the circuits that connected to the off-site power grid. Like homes and businesses all along the battered coast, the reactor was now without external power. The lights flickered as Unit 1 and the other reactors were automatically transferred from the external power supply to the on-site emergency system. Within seconds, Fukushima Daiichi’s thirteen emergency diesel generators (two per reactor, plus a third at Unit 6) automatically fired up. This restored the power, instrumentation, and cooling equipment needed to keep the nuclear fuel from overheating.

At the same time, the turbine generators at Units 1, 2, and 3 shut down, and the valves carrying steam to the turbines automatically closed, as they were supposed to do after a scram. At Unit 1, a rise in pressure was halted five minutes later, at about 2:52 p.m., when other valves automatically opened—also as expected—and allowed the steam to flow into the isolation condensers. About ten minutes later, operators managed to start the RCIC systems at Units 2 and 3.

Once again pressure levels headed downward, as did water temperatures. But just as heat and pressure spikes pose a threat to hardware in a reactor, so do rapidly falling temperatures; both can cause metal to expand or contract too quickly and ultimately break because of high stress. At 3:04 p.m., fearing that the Unit 1 reactor was cooling too fast, control room operators followed procedures and shut down the isolation condensers, figuring they could be turned back on when needed.

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When the tremors had subsided, Fukushima Daiichi workers assembled for a roll call in the parking lot in front of the main office building, which had been damaged by the shaking. Those assigned emergency management duties then moved to the plant’s emergency response center on the second floor of the earthquake-proof Seismic Isolation Building next door. From there, they could communicate with operators in the control rooms, who confirmed that Units 1, 2, and 3 had successfully shut down.

But the earthquake was just nature’s first assault. The next was about to strike. The quake’s hypocenter was located eighteen miles beneath the ocean floor, but the rupture angled upward through the crust, reaching the seabed and reshaping it. That displaced a mountain of water. Some was now heading east into the open ocean. (By the time the waves hit Antarctica, about eight thousand miles south of the epicenter, they still had enough power to break off more than fifty square miles of ice shelf, twice the area of Manhattan.) Waves were also racing westward—headed straight for northeastern Honshu at the speed of a jetliner.

Barely a swell on the ocean’s surface, the huge surge of water would slow when it hit shallower depths but then rear up and intensify, striking not once but multiple times. Just as the Japanese are no strangers to earthquakes, they know well the power of the massive wave that they—and the rest of the world—call a tsunami.

The northeastern shoulder of Honshu is known as the Tohoku region, a remote mountainous area that includes, from north to south, the prefectures of Aomori, Iwate, Miyagi, and Fukushima. While the region near Fukushima Daiichi is relatively flat, elsewhere steep hillsides and jagged inlets shelter small fishing or farming villages. The portion north of Fukushima is known as the Sanriku Coast and is home to some of Japan’s most spectacular scenery and renowned seafood.

For earth scientists, however, the name Sanriku is synonymous with the convulsive forces of nature, as evidenced by a June 1896 earthquake and tsunami, the deadliest in Japan’s modern history. A magnitude 7.2 earthquake struck offshore, rattling the coast but causing no alarm. Thirty-five minutes later, at about 8:00 p.m., the ocean suddenly receded hundreds of yards, then returned as a wall of water that destroyed everything in its path. The wave, estimated at 125 feet high in places, killed 22,000 people along the Sanriku Coast and swept away entire villages.4

Japan’s written records of earthquakes go back to 599 A.D. and document one that struck Sanriku in July 869 A.D., known as the Jogan earthquake. Believed to have had a magnitude of about 8.6, it generated a twenty-six-foot wave that swept inland at least two and a half miles, killing a thousand people, according to an official record.

That was the first of as many as seventy recorded tsunamis that struck the Tohoku region of Japan. The March 2011 tsunami that hit Sanriku and areas to the south, including Fukushima Prefecture, recorded forty-five-foot waves. The Japanese have officially designated this disaster the 2011 Tohokuoki Earthquake.5 Its toll: nearly nineteen thousand killed or officially declared missing. Of those deaths, more than 96 percent are attributed to the tsunami.

Fukushima Daiichi’s site superintendent, Masao Yoshida, was in the emergency response center when he learned from a television broadcast that the tsunami predictions had been revised upward. Three minutes after the early quake warnings, the coastal prefectures closest to the epicenter—Iwate, Miyagi, and Fukushima—had been alerted to prepare for a “three-meter or higher” wave (ten feet or so). The wave was now estimated at about twice that. Yoshida began to worry that the tsunami might damage emergency seawater pump facilities on the shore—the systems needed to carry residual heat away from the reactors and support equipment, such as the water-cooled emergency diesel generators. However, he expected that even in that case he would be able to compensate by using other available equipment. He could not anticipate the full extent of the disaster about to occur.

At 3:27 p.m., forty-one minutes after the earthquake began, the first tsunami wave hit the seawall extending outward from the Fukushima Daiichi plant. The wave, thirteen feet high, was easily deflected; the wall had been built to withstand water almost thirty-three feet (ten meters) high.

At 3:35 p.m., a second wave struck. This one towered about fifty feet, far higher than anyone had planned for. It destroyed the seawater pumps Yoshida had worried about and smashed through the large shuttered doors of the oceanfront turbine buildings, drowning power panels that distributed electricity to pumps, valves, and other equipment. It surged into the buildings’ basements, where most of the emergency backup generators were housed. (Two workers would later be discovered drowned in one of those basements.) Although some diesel generators stood on higher levels and were not flooded, the wave rendered them unusable by damaging electrical distribution systems. All AC power to Units 1 through 5 had been lost. In nuclear parlance, it was a station blackout.

The radioactive waste treatment facility at Fukushima Daiichi is engulfed in seawater at 3:35 p …

The radioactive waste treatment facility at Fukushima Daiichi is engulfed in seawater at 3:35 p …

The radioactive waste treatment facility at Fukushima Daiichi is engulfed in seawater at 3:35 p.m. on March 11, 2011, after a wave about fifty feet high slammed into the nuclear power plant. The image below shows the same scene one minute later. Tokyo Electric Power Company

Japanese regulators, like their counterparts around the world, had known for decades that a station blackout was one of the most serious events that could occur at a nuclear plant. If AC power were not restored, the plant’s backup batteries would eventually become exhausted. Without any power to run the pumps and valves needed to provide a steady flow of cooling water, the radioactive fuel would overheat, the remaining water would boil away, and the core would proceed inexorably toward a meltdown.

Because Japanese authorities, like those in other countries, believed the possibility of such a scenario was very remote, they dragged their heels in addressing this threat. They were confident that the electrical grid and the backup emergency diesel generators were highly reliable and could be fixed quickly if damaged. They refused to consider scenarios that challenged these assumptions.

But in the 1990s, after U.S. officials finally took action to address the risk of station blackout, Japanese regulators also recommended that plant operators develop coping procedures.6 These plans took advantage of backup cooling systems like the RCIC that operate by steam pressure alone, without the need for electric pumps. But the systems do rely on eight-hour batteries to power equipment that operators can use to monitor their performance and make adjustments to keep them stable. Coping with a station blackout is essentially a race against time to restore AC power before the batteries run down.

Across Japan, a network of remote cameras is mounted at ports, along highways and bridges, atop buildings, and in other critical areas. On the afternoon of March 11, these cameras provided the world with a real-time view of the disaster. Under its agreement with the government, NHK could activate the cameras from its newsroom, giving viewers stunning images: fishing boats tossed like bathtub toys and whole villages consumed by ravaging water, miles of mud, and debris. Video uploaded from personal cell phones and webcams appeared on the Internet within seconds. Billions of people became eyewitnesses to the natural catastrophe unfolding in Japan.

From a hillside behind the reactor complex to which they had evacuated, employees watched as the murky gray water roared up and around the buildings. Cars and machinery bobbed like corks, and debris borne on the surging water crashed into structures like battering rams. Then, with as much force as it had slammed into the plant, the water surged back into the roiling ocean, carrying away equipment and leaving enormous damage behind.

In the capital, officials at Tokyo Electric Power Company, known as TEPCO, assembled their own command center on the second floor of corporate headquarters. More than two hundred employees, hastily summoned, took their places at long rows of desks.

At 3:37 came the call from Fukushima Daiichi. Not only was Unit 1 without AC power, but it had also lost DC power: flooding had destroyed its backup batteries. The control room for Units 1 and 2 went dark; instrument panels faded to black. All power to these reactors had been lost or couldn’t be delivered where needed because of damage to power panels and cables. This was now the most severe type of station blackout. Without DC power to control cooling systems like the isolation condenser, only a very narrow time window was left before core damage began.

One by one, in the space of a few minutes, Units 1 through 5 lost AC power supplies, and high-voltage electrical panels were flooded. (Only one generator, located on higher ground near Unit 6 and cooled by air rather than seawater, continued to work.) For the first time in history, a nuclear accident was unfolding in multiple reactors at the same time.

This was a situation no one had prepared for—or even thought possible. TEPCO’s station blackout guidelines were incapable of addressing the challenge now playing out, because they assumed that only one unit would be affected and it could draw on the power supplies of adjacent units. Nor did the guidelines contemplate the simultaneous loss of both AC and DC power. “We encountered a situation that we had never imagined,” Yoshida would say later.

To compound the crisis, these reactors had just been subjected to a record-breaking earthquake and tsunami that may have caused structural damage. And now, with power gone, there was no way to know what was happening inside them.

If a natural disaster could trigger a crisis like the one unfolding at Fukushima Daiichi, then, one might wonder, why aren’t even more safety features required to prevent such a catastrophic thing from occurring?

The short answer is that developers of nuclear power historically have regarded such severe events as so unlikely that they needn’t be factored into a nuclear plant’s design. The experts could not imagine that such a cascading failure of safety systems would really occur. So the regulations required only that reactors be able to survive conditions occurring during far less severe accidents, known as “design-basis accidents.” As defined by the U.S. Nuclear Regulatory Commission (NRC), design-basis accidents were scenarios that were unlikely to occur during the lifetime of a nuclear reactor but were nonetheless conceivable enough to warrant measures to limit their severity. Reactor designs are equipped with emergency cooling systems and containment structures intended to function during design-basis accidents to limit core damage and radioactive release to a level regulators consider acceptable.

If these systems don’t work, then the plant enters the realm of “beyond design-basis” accidents, also called “severe” accidents. Severe accidents challenge plant operators in part because they involve complex and poorly understood phenomena. What is known comes primarily from tests and computer simulations that provide only limited insight. The postmortem review of the 1979 beyond-design-basis loss-of-coolant accident at the Three Mile Island reactor in Pennsylvania confirmed some predictions but raised questions about others. Fukushima Daiichi has raised even more questions, and one day should provide additional answers.

Well before Fukushima, critics argued that predicating reactor safety on the ability to handle design-basis accidents left nuclear plants vulnerable to far worse events that are more probable than the industry would like to believe and than the public would be willing to accept. Even so, design-basis accidents do create a stiff set of requirements. Reactor containments must be rugged enough structures to withstand the high pressures and temperatures that could occur in a design-basis accident without developing large leaks or rupturing. Containments are typically made of steel shells or steel-reinforced concrete with leak-tight steel liners. Because of their size and strength requirements, they don’t come cheap.

When GE designed its boiling water reactors, it equipped them with “pressure suppression” containments to reduce construction costs. These containments featured additional systems for converting excess steam to water and thus, the theory went, did not need to be built to withstand very high accident pressures.

After GE began selling the first boiling water reactors with this feature, safety critics called attention to what they believed was a dangerous vulnerability: the complex pressure suppression system was not well understood. If it failed to sufficiently reduce steam pressure during an accident, the too-small primary containment surrounding the reactor vessel might burst, releasing radioactive material into the environment.

And that was not the only threat. The Three Mile Island accident raised awareness of the danger of hydrogen. During that accident, hydrogen produced by the reaction of steam with the fuel rod cladding caused an explosion in the containment. Although the Three Mile Island containment was sufficiently large and robust to withstand the shock, engineers realized that the explosion would have been powerful enough to rupture the smaller, weaker pressure suppression containments. As a result, the NRC required that reactors with pressure suppression containments be retrofitted to control hydrogen accumulation in accidents either by filling the containments with nitrogen, an inert gas, or by activating spark plug-like devices called igniters to gradually burn off any hydrogen. The Japanese were monitoring U.S. developments closely and also required Mark I containments to be “inerted.” That addressed one accident contingency, but neither the NRC nor the Japanese worried about what would happen should hydrogen escape the containment and leak into other areas of the plant.

Fukushima Daiichi Unit 1 was a Mark I reactor that began service in March 1971 and ran for forty years. Units 2, 3, 4, and 5 were more advanced BWR models but also had Mark I containments. Unit 6 was a BWR with a Mark II containment, which also used pressure suppression. Across Japan, there were twenty-eight boiling water reactors. The United States has thirty-five, of which twenty-three have the GE Mark I containment. (See p. 279 for a list.)

In Tokyo, Prime Minister Kan was also struggling to grasp what was happening. The capital hadn’t been included in the initial earthquake alert because the Japan Meteorological Agency had underestimated the earthquake’s size and subsequent hazard zone. But when the shaking hit Tokyo, Kan had hustled out of the hearing room and headed across the street to his office. There, he gathered a small group of advisors in a basement situation room.

Kan had won election to the office of prime minister just ten months before. A member of the Democratic Party of Japan, he had served as finance minister and had campaigned on a promise to improve the nation’s weak economy. He also pledged to lessen the influence of the powerful but unaccountable bureaucracy that has long run the government.

When it came to nuclear energy in Japan, that bureaucracy was large. Responsibility was divided among multiple government agencies, whose missions sometimes overlapped—or conflicted. Japan’s fifty-four commercial nuclear power plants were regulated by the Nuclear and Industrial Safety Agency (NISA), which operated under the jurisdiction of the Ministry of Economy, Trade and Industry (METI). NISA shared some responsibilities with the Ministry of Education, Culture, Sports, Science and Technology (MEXT), which had a dual role: to promote nuclear energy and to ensure its safe operation. MEXT performed environmental radiation monitoring and assisted local governments with radiation testing in the event of an accident.

Also in the mix was the Nuclear Safety Commission (NSC), an independent agency that operated within the executive branch. The NSC supervised the work of MEXT and METI and provided policy guidance, but also worked to promote nuclear power. And finally, there was the Japan Nuclear Energy Safety Organization, which inspected nuclear facilities, conducted safety reviews, and, in the case of an emergency, made recommendations on evacuations.

Japan’s prefectures had a role, too. They were responsible for radiation monitoring and directing evacuations if needed. On paper, all these duties and responsibilities may have seemed clear. In practice, however, the system proved unworkable.

At 3:42 p.m., Tokyo Electric Power Company declared a “first level” emergency, a legal threshold meaning that an accident is predicted or has occurred. By law, TEPCO had to notify the head of the Ministry of Economy, Trade and Industry along with the governor of Fukushima Prefecture and the mayors of the towns of Okuma and Futaba, the communities in which the plant is located. The procedural requirements were clearly spelled out. The notification, according to the plant’s emergency plan, was to be done by sending a fax “all at once, within fifteen minutes.” (Plant managers were advised to follow up by phone.)

But the emergency plan didn’t fit this emergency. There was no power. Phone lines and cellular towers were damaged or destroyed. Faxes or phone calls would be difficult, if not impossible. No one apparently had thought that an event fierce enough to damage a reactor might also disrupt basic communications.

Inside Fukushima Daiichi’s emergency response center, a generator powered a video link to TEPCO headquarters. But communications within the plant itself were difficult. The paging system was disabled; TEPCO had provided only one-hour batteries for some of the mobile units and there was no way to recharge them. Crew members often had to return to the emergency center to report simple details—a time-consuming and risky procedure. In many respects, the emergency communication system at Fukushima Daiichi reflected the underlying premise of the plant’s comprehensive accident management plan, which read: “The possibility of a severe accident occurring is so small that from an engineering standpoint, it is practically unthinkable.” The follies resulting from this complacent attitude began to build catastrophically.

Under the provisions of Japan’s Act on Special Measures Concerning Nuclear Emergency Preparedness, regulators from NISA were to staff an off-site command post and help coordinate the emergency response. At Fukushima Daiichi, the designated center was located about three miles from the reactors. When three NISA workers arrived there, they discovered no power, phone service, food, water, or fuel; additional staff couldn’t reach the facility because of damage to roads and massive traffic jams. Equally problematic, the building was not equipped with air filters to protect those inside in case of a radiation release. (The lack of filters had been cited two years earlier by government inspectors, but NISA had failed to install them.) It seemed nobody in government imagined a nuclear accident could produce a cloud of radiation intense enough to pose a hazard a few miles away.

Back in Tokyo, things were not going much better. Kan was in the situation room with his close advisors, few of whom knew much about nuclear power plants. Cell phones didn’t work in the basement, making contact with the outside world difficult. Five floors above, the government’s nuclear experts had gathered in another emergency response center. Some senior managers from TEPCO joined them. But the two groups were not communicating with each other, despite being in the same building. Nor did anyone from the government head to TEPCO to ascertain what the utility was doing. In many respects, government officials were functioning much like the operators in the control rooms: without information to guide them.

Under normal circumstances, the reactor operators at Fukushima Daiichi had access to a wide range of information about the status of critical systems via the Safety Parameter Display System for each unit. But when the control rooms were disabled by the loss of electrical power, the steady flow of information had largely ceased.

At 3:50 p.m., someone in the shared control room for Units 1 and 2 wrote on a whiteboard about their reactor cores: “Water levels unknown.” Without DC power, operators could no longer monitor or manipulate the isolation condensers at Unit 1 or the RCIC at Unit 2 remotely from the control room. Even worse, if those systems were not working and water levels dropped significantly, operators could not start up the emergency core cooling systems at either unit to pump water into the reactors quickly. Things appeared to be a little better at Unit 3. There, control room operators still had some backup battery power that provided readings on pressure and water levels and enabled them to operate steam-driven cooling systems. At about 4:00 p.m., they were able to restart the RCIC system and add water to keep the fuel rods in the Unit 3 core covered.

Units 3 and 4 also shared a control room. With Unit 4 shut down for maintenance, that team focused primarily on Unit 3. The team’s colleagues in the Units 1 and 2 control room had their hands full with both reactors, although early on Unit 2 seemed to pose a greater threat because operators could not confirm whether the RCIC was operating or measure the water level in the core. In contrast, the operators believed the isolation condensers in Unit 1 were working, but they could not confirm this either.

To operate the instruments that could provide the information they needed most—the water, temperature, and pressure levels inside the reactors—the engineers at Fukushima Daiichi badly needed power. They thought they had one backup source left: emergency batteries. Soon workers would be roaming the muck- and debris-laden plant grounds, scavenging batteries from undamaged cars and buses in a desperate attempt to jury-rig some sort of power system.

But hooking up the batteries was a challenging task. With much of the plant’s electrical infrastructure damaged or destroyed, crews sometimes had to search for working connections behind control room panels or find circuitry elsewhere. Darkness and the presence of standing water made this a delicate and difficult task. Batteries were scarce and too small to provide adequate voltage. The few hours’ cushion the operators thought they might get was fast disappearing.

At 4:30 p.m., TEPCO issued a press release announcing that “a big earthquake” had occurred at 2:46 p.m. More than 4 million households were without power. “Due to the earthquake, our power facilities have huge damages, so we are afraid that power supply tonight would run short. We strongly ask our customers to conserve electricity.”

The apologetic press release included a reassuring status report on TEPCO’s various generating stations in the affected area, including the company’s seventeen nuclear reactors: six at Fukushima Daiichi, four at nearby Fukushima Daini,7 and seven at Kashiwazaki-Kariwa, located on the western coast of Japan. “At present, no radiation leaks have been confirmed,” the release noted.

But at Fukushima Daiichi and the utility’s command center in Tokyo, the tone was far less confident. At 4:46 p.m., exactly two hours after the first tremors had been detected, TEPCO officially notified the government that the emergency was worsening. Operators could not determine the water level in the reactor cores of Units 1 and 2 and had no assurance that the systems to supply additional water were working. Specifically, emergency core cooling had been lost at Units 1 and 2. This, under law, required the declaration of a “second level” emergency.

The highest priority for the harried team in the darkened Units 1 and 2 control room became restoring water-level indicators. At least then team members might have a better idea of the status of the two units. They salvaged two twelve-volt batteries from buses and additional batteries and electrical cables from a contractor’s on-site office.

For a few tantalizing moments, a water-level gauge returned to life, showing that the level was dropping inside the Unit 1 reactor vessel. Minutes later, the gauge died. But this fleeting indication pointed to the possibility that soon water would have to be injected from outside the reactor using portable pumps. A diesel-powered fire pump was started and allowed to idle, ready to inject water into the Unit 1 reactor through a portal normally intended for use in firefighting, not core cooling. In addition to a diesel-driven fire pump at each reactor, there were three fire engines at Fukushima Daiichi that potentially could be used. TEPCO had ordered fire engines deployed to all its reactor sites after a fire broke out at the Kashiwazaki-Kariwa plant following an earthquake in 2007. But the utility did not contemplate that the fire engines might have to be used for something other than firefighting.

Unfortunately, as the plant operators knew, trying to get water into the reactor using either the fire pump or the fire engines would not be easy. These sources could supply water only at a relatively low pressure compared to the pressure within the overheating reactor vessel. Unless operators could depressurize the vessel, they wouldn’t be able to force water into it.

As pressure increased inside the reactor vessel, steam flowed out through safety relief valves designed to keep the vessel from rupturing. Pipes leading into the torus carried the steam downward. If the pressure suppression system had been working properly, the steam would have been cooled and turned back into water in the torus, which would reduce pressure throughout the containment. To keep the torus itself from overheating, its water would be routed through tubes into heat exchangers, where seawater flowing around the tubes would absorb the heat and carry it away to the Pacific. But because the seawater pumps were destroyed, there was no effective way to remove heat from the torus.

With no access to the Pacific and no electrical power, there was only one way left to reduce the pressure within the containment: by venting some of the steam into the atmosphere. That would make it easier to inject water into the reactor and would lower the likelihood of a containment breach. Fortunately, the Mark I boiling water reactor was equipped with an emergency vent that, when opened from the control room, would release steam into the environment through a three-hundred-foot-tall stack. As part of the response to a safety review that took place after the 1986 Chernobyl nuclear plant accident, TEPCO had taken measures in the 1990s to improve the effectiveness of the vent system.

One scenario TEPCO had not anticipated, however, was that the vent might be needed during a station blackout, when the valves required to open it could not be operated remotely from the control room. Consequently, the emergency guide did not explain how to operate the valves manually. Nor was the vent equipped with filters to remove radiation from the steam if an emergency release was required. The designers of boiling water reactors believed that filters were unnecessary because radioactive steam would be naturally scrubbed by water in the torus before being vented. But this filtering mechanism had never been demonstrated under real-world conditions, and some experts doubted its effectiveness.

These were two more holes in an emergency plan that was turning out to be full of them. “When on-site workers referred to the severe accident manual, the answers they were looking for were simply not there,” investigators would later write. “[T]hey were thrown into the middle of a crisis without the benefit of training or instructions.”

Nobody was sure if the ability to relieve the rapid buildup of pressure inside a reactor was within reach. In a BWR accident, venting—a controlled release of radioactivity—is a last-ditch move to stave off a far worse disaster: core melting and failure of the containment, which could result in larger, uncontrolled releases of radioactivity. The amount of radiation released during venting depends on the extent to which the core has been damaged. If it is badly damaged, radiation levels in the steam could be deadly. At this point, no one knew the status of the Unit 1 core, so the relative risks and benefits of venting were not clear. But guided by scant data and instinct, the engineers knew that some sort of intervention to stabilize the reactor was needed—and needed quickly. They moved on two fronts: to bring in additional water supplies and to prepare, somehow, to vent.

Venting, in addition to being technically difficult, was fraught with political and public relations implications; as a result, both TEPCO management and the government in Tokyo would demand a say. Ultimately, however, the decision to vent rested with the most senior official on the scene. At Fukushima Daiichi that was Masao Yoshida, fifty-six years old, who had become the boss ten months earlier.

The superintendent of Fukushima Daiichi, Masao Yoshida …

The superintendent of Fukushima Daiichi, Masao Yoshida. Tokyo Electric Power Company

Like a ship’s captain, the site superintendent knows the equipment intimately, has firsthand knowledge of the unfolding crisis, and is best positioned to assess the options. Under TEPCO’s emergency plan, Yoshida was to make the calls with input from utility executives. At the moment, however, TEPCO’s top executives were missing in action.

Chairman Tsunehisa Katsumata was in Beijing on a business trip with Japanese media owners. President Masataka Shimizu had been sightseeing with his wife in western Japan. Shimizu had received an earthquake alert on his cell phone, but his efforts to return to Tokyo that afternoon were thwarted. He had traveled only partway, hoping to make the final leg of the trip home in a TEPCO helicopter. But Japan’s civil aviation law bars private helicopters from flying after 7:00 p.m. Late in the evening, he won approval from the government to fly aboard a Japan Air Self-Defense Force (SDF) airplane, but, twenty minutes after the 11:30 p.m. takeoff, the defense minister (unaware of the official authorization) ordered the plane to turn around and instead stand by for disaster relief duties. Shimizu was left on the tarmac. It would be 10:00 a.m. the next day before he and Katsumata made it back to TEPCO headquarters.

Even if TEPCO’s executives had been on hand, the decision to vent wasn’t solely the utility’s to make. Government approval is not required by law, but it was widely understood at TEPCO that government officials from several agencies needed to be brought into the loop. Although no one knew exactly what was taking place inside Fukushima Daiichi, everybody wanted a say. As a result, the decision-making process about venting Unit 1 came to a near standstill.

For the nuclear power establishment, the decision to vent radioactive steam holds serious implications. Releasing radiation into the environment demonstrates unequivocally to the public that this form of generating electricity is not as clean and safe as the industry’s public reassurances and promotional campaigns proclaim.

By late afternoon, conditions appeared serious enough that Japanese regulators decided to notify international authorities. Shortly after 4:45 p.m., NISA alerted the International Atomic Energy Agency (IAEA) in Vienna that Fukushima Daiichi had reached the state of “near accident at a nuclear power plant with no safety provisions remaining.” Under the IAEA’s seven-level scale of nuclear accidents, seven being the most serious, that constituted a level 3 event.

Prime Minister Kan put off declaring a nuclear emergency, despite a request that he do so by the heads of both NISA and METI. The events at Fukushima Daiichi weren’t waiting, however. Just before 6:00 p.m., a work crew was sent to the fourth floor of the Unit 1 reactor building, hoping to learn more about the status of the isolation condenser. They wore no protective clothing. As they arrived at the double doors of the reactor building, their dosimeters shot off the scale, and they hurried back to the control room. This strongly indicated that the Unit 1 core was now exposed and fuel rods had already ruptured. Full-scale melting of the core would soon begin.

But the official word from TEPCO was vague and outdated. At 5:50 p.m., the utility issued a press release announcing the “malfunction” of the diesel generators and the resulting loss of backup power more than two hours earlier. “There have been no confirmed radioactivity impact to [the] external environment,” the English-language version of the announcement said. “Further details are in the process of being confirmed.”

Across Japan, the scope of the natural disaster that had hit the Tohoku coast was still sinking in. More than one hundred thousand members of the Self-Defense Forces, the Japanese equivalent of the National Guard, had mobilized; local disaster agencies were struggling to grasp where to focus their attention; in some communities, whole neighborhoods had simply vanished, their residents washed out to sea. In places along the coast, tsunami survivors were still trapped atop buildings where they had fled the oncoming water. It was cold and darkness was falling. The human toll was staggering. As Kan would later say, “The focus was on saving lives.”

Japan’s prime minister Naoto Kan announces the emergency response efforts on March 11 …

Japan’s prime minister Naoto Kan announces the emergency response efforts on March 11. After declaring that the nuclear power plants in the region had automatically shut down, he said, “At present, we have no reports of any radioactive materials … affecting the surrounding areas.” Cabinet Secretariat, Government of Japan

That rescue effort took on an added dimension shortly after 7:00 p.m., when Kan declared the nuclear emergency requested two hours earlier. At 7:45, chief cabinet secretary Yukio Edano alerted the nation and the world that an emergency had been declared. “Let me repeat that there is no radiation leak, nor will there be a leak,” Edano said in a reassuring voice. The prime minister’s office apparently was unaware of the high readings taken earlier at the Unit 1 reactor building. That news certainly wasn’t coming from TEPCO. At 9:00 p.m., the company issued another press release warning of a possible power shortage.

Shortly afterward, Yoshida got what he thought was a reprieve: the water gauge on Unit 1 suddenly started working, indicating the water level was still almost eight inches above the top of the fuel. (In all likelihood, the gauge was providing an inaccurate reading because it was not calibrated for extreme conditions.) Not long after receiving that bit of apparent good news, however, he got the bad news. The radiation levels inside the Unit 1 reactor building had risen so high that entry was forbidden, seriously complicating any emergency repairs. The radiation readings were positive proof that the fuel core now was exposed and most likely melting. That finding was passed on to Tokyo along with Yoshida’s alarming prediction that the Unit 2 water level and RCIC status were unknown and that the fuel there could also be uncovered soon.

By then, authorities had ordered the emergency evacuation of those living within about a two-mile (three-kilometer) radius of the reactors. Officials of the towns of Okuma and Futaba dispatched sound trucks and local firefighters to go door-to-door with the announcement. Many of the residents were still reeling from the earthquake and tsunami, searching for missing loved ones, or scavenging for their possessions. They were told to leave immediately. Those living a little farther out, between two and six miles (three to ten kilometers) from the reactor, were directed to stay indoors. All they were told was that there were problems at Fukushima Daiichi.

As people fled, the first of about a dozen power supply trucks were rumbling toward Fukushima Daiichi, dispatched that afternoon from TEPCO headquarters and from other utilities. These mobile generating units might provide the power so desperately needed. But the challenge of even getting to the plant was enormous. The drivers were forced to navigate roadways battered by the natural disaster and clogged with traffic leaving the area. By 11:00 p.m. the first trucks had made it, and workers attempted to connect the generators but had difficulty locating functioning electrical power panels. Unfortunately, some of the cables were too short and the plugs incompatible.8 False alarms of another tsunami interrupted the task, forcing workers to flee to higher ground. After twenty-four hours, only one generator was operating.

During the first hours of the accident, as crews at Fukushima Daiichi scrambled, government and utility officials in Tokyo lacked a similar sense of urgency, a response some later attributed to a failure of leaders there to understand what was happening at the plant. They seemed to feel that there was adequate time to decide a course of action. Prime Minister Kan, however, became increasingly frustrated that the venting was not happening.

The delay in venting was the result of a number of factors. One was the difficulty of the emergency evacuation. There was an informal agreement with the government of Fukushima Prefecture that until nearby residents were safely relocated, venting would not occur. But even if workers had tried to start the venting immediately, they would have had problems. Because the vent valves could not be operated from the control room, they had to be opened manually. It took hours to figure out where the valves were physically located and which could be opened by hand.

By the time the evacuation was declared complete and the decision to vent was finally reached on the morning of March 12, conditions at the plant had worsened. Accessing and opening the vent valves located deep inside the dark, intensely hot, and now radioactive reactor building was a far more dangerous mission than it would have been the previous evening. This was a scenario no accident drill had covered.

In Tokyo, Kan’s irritation over the slow flow of information—and its accuracy—was mounting. In addition to the two hundred TEPCO technical advisors on duty at company headquarters, four hundred plant personnel under Yoshida’s direction manned Fukushima Daiichi’s emergency response center. Communication between headquarters and the response center was occurring via TEPCO’s in-house videoconferencing system.

The government, on the other hand, had no similar ability to communicate with the plant. In Tokyo, NISA obtained information from phone conversations with TEPCO. (A videoconferencing system was not set up until March 31, when the government and TEPCO created a joint response center.) Dissatisfied, Kan and his advisors asked TEPCO to assign staff members to the prime minister’s office for briefings, and Kan and his aides eventually even began calling Yoshida for answers, an action for which Kan would be later accused of micromanaging.

Just before midnight on March 12, the plant’s emergency team was able to get a pressure reading of the Unit 1 drywell using a portable generator in the control room. The team found that it exceeded the design maximum operating pressure. At 12:49 a.m. on March 12, Yoshida decided the pressure in Unit 1 was likely so high that venting now must take place. TEPCO president Shimizu, who still hadn’t returned from his vacation, agreed with the decision at about 1:30 a.m. But TEPCO also wanted the government’s blessing. At this point, an unanswered question was whether TEPCO might have to vent Unit 2 as well as Unit 1.

Unit 1 was the only reactor of the six that relied on isolation condensers for emergency cooling. Apparently, even the shift team assigned to Unit 1 was unfamiliar with that design. Had team members been trained in the system, they would have recognized that it was not operating and that Unit 1 had been deprived of water for hours.

Meanwhile, the situation at Unit 2 also remained a mystery. Although the RCIC system had started up at the time of the earthquake, once DC power to the control panels was lost no one could tell whether the RCIC had continued to successfully inject water into Unit 2. Yoshida feared it hadn’t and that the top of the fuel was about to be exposed.

A crew wearing breathing gear and protective clothing ventured into the RCIC room in the basement of the Unit 2 reactor building to determine its status. The first trip was inconclusive. A second team was dispatched. This crew said it believed the RCIC was functioning based on pressure measurements. With that news, Yoshida decided that Unit 1 warranted first priority; Unit 2 could wait. Word was sent to Tokyo. Kan and Banri Kaieda, head of the Ministry of Economy, Trade and Industry, agreed.

TEPCO managing director Akio Komori, who had once worked at Fukushima Daiichi, joined Kaieda and the head of NISA at a joint press conference shortly after 3:00 a.m. to announce the venting. If it was meant to provide a reassuring message, it fell far short. Just before the press conference began, the three men found they had differing information about the status of the reactors. Uncertain of what was actually occurring at Units 1 and 2, they decided that Komori would announce the venting but not identify the reactor involved. When questioned by the media, he became confused. A few minutes later, government spokesman Yukio Edano took the podium to say that radiation would be released but the public should remain calm.

Edano was getting his information from NISA, which was getting it from TEPCO. When later asked to assess the accuracy of information coming from his office to the Japanese public at this time, Kan would say that NISA officials were “choosing their words carefully,” and as a result Edano was being misled.

The Unit 1 drywell, at twice its design pressure, was likely approaching a failure point. Venting had to happen, and it had to happen now. Engineers at the plant hurriedly calculated possible radiation exposure from the release. As the preparations continued, a worker was sent to check radiation levels at the Unit 1 reactor building. When he opened the door, he saw “white smoke” inside and quickly left without taking a reading. The smoke, whatever it was, clearly showed that something was leaking somewhere. At about 4:00 a.m., radiation levels near the plant’s main gate were measured at 0.0069 millirem (0.069 microsieverts) per hour. Twenty minutes later, they had jumped nearly tenfold to 0.059 millirem (0.59 microsieverts) per hour. The Unit 1 drywell was now venting itself.

RADIATION AND THE BODY: DOSES, DAMAGE, AND DEBATE

Radioactive materials emit ionizing radiation—that is, radiation energetic enough to detach electrons from atoms, turning them into charged particles (called ions). Exposure to ionizing radiation can have different effects on the human body, depending on the extent and nature of the damage it causes on the cellular level.

The relative biological damage in the human body resulting from radiation exposure is measured in units called sieverts. The United States, unlike most of the rest of the world, uses a rem (“radiation exposure man”) as its standard measure. One sievert is equivalent to 100 rem.

One class of radiation effects is known as “deterministic,” meaning that a certain level of exposure will almost always cause a particular outcome. Deterministic effects generally result from levels of radiation high enough to kill cells, causing widespread damage to tissues or organs. Depending on the nature of the injury, such doses range from tens to hundreds of rem delivered over a short period. The resulting injuries include burns, cataracts, thyroid nodules, hair loss, gastrointestinal distress, low blood counts, and cardiovascular disease. Recent studies have also identified statistically significant excess risks of certain circulatory diseases at low doses. These studies suggest that the mortality from such diseases due to low-dose radiation exposure may be comparable to that from cancer.

As the dose increases or wider areas of the body are exposed, the victim may develop an illness known as “acute radiation syndrome.” Although this sometimes can be cured with treatment, high enough doses—above several hundred rem delivered in a brief period—will almost certainly result in death within days or weeks. Following the Chernobyl accident, twenty-eight people—plant workers and firemen in close proximity to the damaged reactor—are known to have died in this manner. Death from acute radiation syndrome would be classified as an “early fatality,” occurring within a few weeks or months after exposure to a nuclear plant release.

Deterministic effects feature a dose “threshold” below which a particular effect will not occur. This is because cells must sustain a certain amount of damage before the cell dies. In addition, a certain number of cells must be affected before enough tissue damage occurs to cause clinical symptoms.

The other major class of radiation effects is known as “stochastic,” or random. Ionizing radiation can cause DNA damage that might produce changes in cellular behavior leading to cancer, but does not definitely cause such changes. Cancer risk does rise with increasing doses, however, because the more DNA lesions there are, the higher the chance that one of them will lead to cancer.

It is currently estimated, based on studies of survivors of the atomic bombings at Hiroshima and Nagasaki, that a dose of ten rem delivered at once will raise an individual’s lifetime risk of fatal cancer by about 1 percent on average. This risk is higher for children and other groups of people (for instance, those with certain genetic variations) who are more sensitive to the effects of radiation than the average adult. Doses delivered over lengthy periods may be less effective at causing cancer, but there is much uncertainty about whether this is true. Because most cancers take many years, even decades, to appear after exposure to radiation, deaths due to stochastic effects are called “latent cancer fatalities.”

Increases in cancer risk associated with low doses of radiation are small compared to the background level of cancer in humans. Thus, in epidemiological studies it is hard to detect a direct cause-and-effect relationship between radiation in the low-dose range (generally below about five rem) and cancer. However, there is broad scientific consensus that all radiation exposure, no matter how small, produces some increased risk of cancer. It is biologically plausible that even a single particle of ionizing radiation could cause enough damage in a cell to induce cancer.

Nonetheless, a small and vocal group, including some scientists, believes that the absence of observable evidence for an increase in cancer risk at low doses implies that ionizing radiation is harmless below some threshold. The logic is that at these doses, the rate of damage is so low that DNA repair mechanisms can combat it successfully. Some even believe in the theory of “hormesis,” which holds that low doses of radiation are actually beneficial and can stimulate the immune system like a vaccine.

Although these arguments have been reviewed and largely rejected by authoritative scientific bodies, such as the National Academy of Sciences’ Biological Effects of Ionizing Radiation (BEIR) VII Committee, advocates of a threshold continue to cite them as justification for the belief that the harmful effects of radiation in general, and nuclear energy in particular, are exaggerated.

At the extremes of pressure and temperature that the drywell was now experiencing, the bolts and seals used to make it leak-tight were giving way, allowing radioactive steam and hydrogen gas to escape directly into the reactor building. This was the last barrier preventing the release of radioactivity into the atmosphere. At least the pressure inside the containment was dropping, although not enough to obviate the need for operator-controlled venting. The plant operators needed to reduce the containment pressure by venting through the torus instead of the drywell; then some of the radioactivity might be filtered as the gases passed through water. Otherwise, unfiltered releases directly from the drywell would continue.

To vent the Unit 1 containment, workers needed to manually open two valves in different locations. The first was on the second floor of the reactor building. Without electric power, workers would have to open it with a hand crank—provided they could reach it. The second valve was in the basement torus room. Ordinarily the basement valve required compressed air to operate. By poring over plant drawings, however, workers had located a wheel handle on the valve that they could use to open it manually.

They now mapped out the route they would take to get to it. Their path would be through the dark and hot reactor building. In the past few hours, the plant site had been rattled by twenty-one aftershocks, raising fears of another tsunami. What if another wall of water were to sweep in and surround them all?

But the more immediate threat was invisible: the rising radiation level. Crews now had to wear protective gear and breathing equipment if they ventured out of the emergency response center. Soon, radiation levels rose even inside the Units 1 and 2 control room, and operators there had to wear full face masks and protective clothing. Most of the control room team moved to the Unit 2 side of the room and crouched down on the floor, where the radiation was a little lower, to do their work.

Workers wearing protective clothing and respirators inside the Unit 2 control room on March 26, 2011, after power is restored at the plant …

Workers wearing protective clothing and respirators inside the Unit 2 control room on March 26, 2011, after power is restored at the plant. Tokyo Electric Power Company

Meanwhile, workers continued their efforts to force water into the Unit 1 core by any means available. The diesel-driven fire pump idled for hours, but it remained useless because operators could not depressurize the reactor vessel and the pump was not powerful enough to inject any water into it. Finally, the pump shut down and could not be restarted.

And then, an apparent miracle happened, but it was a mixed blessing. At around 2:00 a.m., operators were able to recover some instrumentation using batteries and discovered that the reactor vessel pressure had dropped significantly all by itself. On one hand, the pressure, although still high, was now low enough to give the fire engines on-site a chance of getting water to the core. On the other hand, the depressurization was an ominous sign that the reactor vessel had sprung a leak somewhere—although it is not clear that anyone appreciated that at the time.

At about 4:00 a.m., workers finally managed to connect a fire hose to a portal on the Unit 1 turbine building, providing a pathway for one of the fire engines to pump freshwater into the reactor core. It took almost two more hours to establish a consistent flow rate, but for the first time in nearly fifteen hours, water was reaching the core. However, it was too little, too late. The fire engine pump pressure was still too low to force much water into the vessel. And in any event, it is likely that by then the core had already melted through the bottom of the reactor vessel and dropped to the containment floor. If so, much of the water being injected into the core was probably flowing out into the containment.

The whole exercise had taken much longer than anyone had anticipated in part because no plant worker knew how to operate the fire engines maintained for emergencies at the reactor site. A contractor had to be convinced to help with the arduous task. With rising radiation levels, ongoing aftershocks, and the threat of another tsunami, the contractor was reluctant to agree.

In Tokyo, at both the utility and the prime minister’s office, everyone was asking the same questions: When was the venting going to begin? Why was it taking so long? When Prime Minister Kan asked the TEPCO official assigned to his office to explain the delay, the only answer he got was: “I don’t know the reason.”

Shortly after 6:00 a.m., Kan decided to find out for himself. In the midst of the growing crisis at the plant, Yoshida was informed that the prime minister was en route via helicopter. Before Kan left Tokyo, he ordered authorities to widen the evacuation zone around the plant from about two miles (three kilometers) to six miles (ten kilometers). Without careful coordination, however, that would put a lot of people on the roads when the venting finally did take place. Nonetheless, while Kan was airborne, METI minister Banri Kaieda ordered TEPCO to vent.

At 7:11 a.m., Kan, accompanied by Haruki Madarame, the chairman of the Nuclear Safety Commission, landed at Fukushima Daiichi. Yoshida explained the difficulties to Kan, who calmed a bit when he discovered that he and Yoshida had attended the same college, the Tokyo Institute of Technology. Kan repeated the order: vent. Yoshida promised that would happen by about 9:00 a.m. Fifty minutes after he arrived, Kan headed back to Tokyo and Yoshida back to managing the disaster. Although Kan’s dramatic fly-in garnered much publicity (and eventually was portrayed by his critics as dangerous meddling), Yoshida remained the man calling the shots.

Three two-person teams suited up with protective clothing. They were about to enter a dark, highly radioactive reactor building that they might have to flee at any moment because of the intermittent earthquake tremors. They knew they would have just a few moments to accomplish their task before they would reach their allowable radiation dose and have to retreat. The makeup of the teams reflected the radiation exposure risks everybody recognized: young employees were excluded from the mission.

At 9:02 a.m., the plant was notified that the evacuation of residents was complete and that the venting could now begin. (The information about the evacuation turned out to be incorrect.) The first team entered the second floor of the Unit 1 reactor building, flashlights providing the only illumination as team members searched for and located the hand crank. With tools about the size of those used to change a car tire, they cranked the vent valve open a quarter of the way before they hastily left. In the ten or so minutes they had been inside, each man had received a radiation dose of two and a half rem (twenty-five millisieverts), one-fourth of the total dose they were permitted to receive under emergency conditions for an entire year. When members of the second team entered the torus room in the basement of Unit 1 to open the second valve, they found dose rates so high that they were unable to reach it, and they fled. Even so, one of the operators exceeded the emergency dose limit of ten rem (one hundred millisieverts). Entry efforts were then abandoned.

A new plan was devised; it, too, proved problematic. There was another, larger valve that might be opened from the control room with battery power and a compressed air supply. But the available compressor didn’t work without electricity, and nobody had a portable unit. Once again, contractors’ offices on and off the plant site were searched. More time elapsed.

Workers then attempted to open the small air-operated valve in the torus room from the control room in the hope that there was still enough residual air in the valve to make it work. For venting to be successful, this valve would have to remain open long enough to create pressure in the vent sufficient to burst a rupture disk, which served as the last barrier between the radioactive gas and the environment. When radiation levels spiked at the main gate and at several monitoring posts at about 10:40 a.m., operators thought their venting attempt had worked and the disc had ruptured. However, soon afterward radiation levels began to fall and it was no longer clear that there had been significant venting.

Finally, shortly after noon, a portable compressor was located and jury-rigged so it could be connected to the system that normally supplied air to plant equipment. Together with temporary DC power, this allowed operators to open the large air-operated valve from the relative safety of the control room. At 2:00 p.m., the compressor was started and the large valve opened. Pressure dropped inside the containment, and presumably inside the reactor vessel as well, allowing operators to inject water at a higher rate.

Another sign that the operators might have succeeded came when NHK cameras trained on the plant from a distance captured white smoke emerging from the exhaust stack shared by Units 1 and 2 and rising high above the sky-blue reactor building. Yoshida and the exhausted crew in the emergency response center thought they had finally caught a break. At 3:18 p.m., he notified TEPCO in Tokyo of the venting.

Captured by an NHK camera positioned about twenty miles away, the explosion inside Unit 1 at 3:36 p.m. on March 12 destroys the roof of the reactor building and blows out a panel in the adjacent Unit 2 reactor building. NHK

Captured by an NHK camera positioned about twenty miles away, the explosion inside Unit 1 at 3:36 p.m. on March 12 destroys the roof of the reactor building and blows out a panel in the adjacent Unit 2 reactor building. NHK

Yoshida had other progress to report as well. Recognizing that freshwater supplies were limited, he had ordered workers to come up with a method for utilizing seawater to cool the reactors. After freshwater supplies were exhausted shortly before 3:00 p.m., he gave the order to prepare to use the Pacific. It was a complicated endeavor—workers had to position three fire engines with interconnecting hoses to pump seawater transferred into a pit—but by 3:30 p.m., the lines to inject seawater into the Unit 1 reactor were almost in place. A steady supply of vital cooling water for the core was now within reach.

And then, at 3:36 p.m.—almost exactly twenty-four hours since the tsunami had roared in and flooded the plant—a powerful explosion ripped through the Unit 1 reactor building, blasting off the roof and sending debris everywhere. The impact blew out a panel in the neighboring Unit 2 reactor building. Inside the emergency response center, stunned workers watched the explosion on television, not sure exactly what had blown up.