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 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. 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 Da
iichi 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.
Fukushima: The Story of a Nuclear Disaster Page 2
