Fukushima: The Story of a Nuclear Disaster

by David Lochbaum

  The Ronald Reagan and six other navy vessels from the Seventh Fleet had been en route to South Korea to participate in joint naval activities there when they were diverted to Japan after the earthquake to help in relief efforts. The ships had taken up position about one hundred nautical miles off Fukushima Daiichi, with the officers thinking they would be outside any radiation plume. Then radiation detectors in the Ronald Reagan’s engine room picked up readings two and a half times normal.

  Radiation was also detected on crew members and helicopters that had landed on a Japanese ship fifty miles offshore from the plant. Although the levels were low, the admiral was worried. The measurements were higher than the navy had expected, based on the scant information available. The prospect that U.S. personnel might be at risk became real. (There were more than 3,400 sailors and aviators on the Ronald Reagan alone.) The crew and equipment were decontaminated and the navy ships headed for safer water about 130 nautical miles offshore. En route, the Ronald Reagan continued to detect contamination.

  For the navy vessels, getting out of harm’s way was a matter of weighing anchor and moving farther out to sea. It would not be as simple for the tens of thousands of Americans living in Japan. They no doubt were beginning to wonder whether they were safe and where they could go if they weren’t.

  Approximately 160,000 U.S. citizens were living in Japan in March 2011. Responsibility for their well-being rested with the U.S. Embassy in Tokyo. U.S. ambassador John V. Roos had put out a call to Washington for guidance almost immediately after the earthquake and tsunami; soon the embassy was also caught up in responding to a nuclear accident. The Japanese were not providing information and relations were growing strained.

  The NRC scrambled to get one of its experts aboard a military flight about to depart for Japan late on the evening of March 11. It was such a hasty departure that Anthony Ulses, the chief of the NRC’s reactor systems branch, had no time to pack. By March 14, a total of eleven NRC experts had been dispatched to Japan.

  It was that same day, March 14, that the Japanese relented and accepted the U.S. offer of assistance, and as a result more systematic data collection began. The National Nuclear Security Administration sent thirty-three people and eight tons of equipment that are part of its Aerial Monitoring System (AMS). The AMS uses specially equipped aircraft to identify and measure radiation in an emergency. Eventually, a U.S. Global Hawk drone went aloft over the reactors to begin gathering data and images that were shared with the Japanese government.

  On the morning of March 14, radiation levels at monitoring posts around Fukushima Daiichi were high, and the drywell pressure in Unit 3 was again starting to rise. Workers had successfully vented the containment three times the day before—or at least thought they had—and now needed to do so again. Although they managed to get power and compressed air to the necessary valves by using a portable generator, the drywell pressure continued to increase. The crew was aware that there had been a gap of more than five hours between the time it shut down the HPCI and the time it managed to set up the fire engine to inject water into the core: more than enough time for significant core damage to occur.

  Even worse, although water was being pumped into the reactor, the level inside did not appear to be rising. No one had known for quite some time just how much water had been getting into the reactor vessel, because of the intermittent operating histories of RCIC, HPCI, and the fire engine. Workers feared that Unit 3 might now be following a trajectory similar to that of Unit 1. Meanwhile, another crisis was about to emerge in an unexpected place.

  While plant personnel focused on cooling the three reactors that had been operating when the earthquake struck, trouble was slowly brewing in one of those that had been shut down on March 11. In the spent fuel pool of Unit 4, the water temperature was steadily rising, inching toward the boiling point. At the time of the earthquake, Unit 4 had been out of service for maintenance, and the entire reactor core was sitting in the spent fuel pool, high above the reactor vessel. Consequently, the fuel pool was loaded with more than 1,500 fuel assemblies. More than one-third of the spent fuel assemblies had been removed from the core just three months earlier, meaning they were still relatively “hot” in terms of both temperature and radiation levels. Of the seven spent fuel pools at Fukushima Daiichi—one for each reactor plus a common pool for extra storage—the Unit 4 pool had the highest heat load by far.

  The spent fuel pools are big tanks of water, forty-five feet (fourteen meters) deep, with walls and floor made of reinforced concrete and lined with steel. Spent fuel assemblies are stored in racks in the lower third of the pools. The large volume of water above the racks provides radiation shielding. Without the water to provide shielding, the radiation level even at the rim of the pool would be quickly lethal to workers. Because spent fuel generates heat and high levels of radioactivity for many decades after discharge from a reactor, active cooling systems are critical. During normal operation, pumps circulate water to maintain a pool temperature of about 100°F (40°C). Once cooling is interrupted, the water in the pool will begin to heat up, and if it should reach the boiling point losses can mount rapidly.

  The spent fuel pools at Units 1 through 4 had been without power to circulate cooling water for almost three days. Yet until now operators had paid little attention to them. For decades, engineers believed that spent fuel pools posed much less safety risk than operating reactors and hence required far less protection. Compared to fuel assemblies in an operating reactor core, most assemblies stored in spent fuel pools are cooler and less radioactive, because they have been out of a reactor for years and their shorter-lived fission products have decayed away. If a spent fuel pool were to lose cooling, operators typically would have days to respond before the massive volume of water in the pool could heat up enough to boil and eventually uncover the fuel. More severe scenarios—such as a massive earthquake that might damage the integrity of the pool structure itself—were regarded as too improbable to worry about.

  This lackadaisical attitude informed the way spent fuel pools were designed and built. Regulators in the United States and elsewhere who reviewed boiling water reactor designs did not require that the spent fuel cooling systems be as robust as those for the reactor core. Nor did they require that the pools be surrounded by leak-tight, pressure-resistant containment structures such as those mandated for the cores. And, in approving GE’s Mark I and II reactor designs, the regulators saw no problem in having the spent fuel pool perched on the fifth floor of the reactor building and topped only by a steel frame structure, despite the obvious dangers such an arrangement might pose in a serious accident.6

  The safety of these pool designs was predicated on the ability of operators to restore cooling within a few days. Apparently little thought was given to the question of what would happen if they couldn’t respond promptly—say, if they were occupied with a prolonged loss of power and three reactor meltdowns simultaneously. Neither did the designers or regulators worry much about what would happen if the operators didn’t have as much time to act as everyone assumed—for instance, if a large earthquake caused the pool to lose water by sloshing it over the sides or cracking the liner and causing it to leak.

  What could happen is this: if the water level did decrease and uncover the fuel rods, they could overheat and melt, much like those inside the reactor—but with one critical difference. The spent fuel is not enclosed by the robust primary containment.

  Even though the amount of heat in a spent fuel pool is far lower than that in a reactor, the zirconium alloy cladding encasing the hottest rods can catch fire if it reaches a temperature of 800°–900°C, generating its own heat source. This in turn could rapidly damage the fuel pellets themselves and allow the release of gaseous isotopes, like the long-lived cesium-137. Under certain circumstances, such a fire could spread to other assemblies in the pool. And because typical spent fuel pools hold more fuel than a single reactor core, a fire that involved the entire pool could release more ces
ium-137 than a reactor meltdown. (Iodine-131, another major isotope of concern in a reactor accident, poses much less risk in a spent fuel pool. It would not be present in significant amounts unless the fuel in the pool was less than three months old; after that time, the iodine-131 would largely have decayed.) And finally, reaction of the zirconium with steam would produce explosive hydrogen gas, just as it did in the damaged reactor cores at Fukushima Daiichi.

  At that point, the only barrier between these isotopes and the environment would be the leaky, flimsy top of the reactor building. Such a structure had already proven to be no match for the hydrogen explosion that occurred at Unit 1. When that happened, radiation levels did not increase very much because the primary containment inside the building remained intact. But a hydrogen explosion resulting from a spent fuel fire at Unit 4’s spent fuel pool, occurring in a largely uncontained environment, would release a catastrophic amount of radiation.

  By the time operators began to focus on ways to cope with the rising temperature in the Unit 4 pool, the radiation level there was already so high that it precluded any human intervention to get water into the pool using temporary pumps. As they began to explore other approaches, the operators knew time was against them.

  But on this Monday morning, March 14, the next explosion to hit Fukushima Daiichi came not from Unit 4, but from Unit 3. At 11:00 a.m., a huge blast blew out not just the upper sections of the reactor building, as had happened at Unit 1, but also large sections of the walls, injuring a number of people nearby. Debris fell into the Unit 3 spent fuel pool and onto the ground, where it damaged the fire engine and hoses positioned to inject seawater into the reactor. Once again, hours of painstaking work were undone in an instant.

  Smoke pours from Unit 3 following a blast at 11 a.m. on March 14. Falling debris injured workers on the ground and damaged firefighting equipment that was being readied to inject vital cooling water into the reactor. Tokyo Electric Power Company

  By early afternoon, radiation levels above thirty rem (three hundred millisieverts) per hour were detected just north of Unit 3, raising concerns that the core had already undergone significant damage and that the explosion had dispersed radioactive material around the site. The rising radiation also rendered part of the Units 3 and 4 control room unsafe, further hampering operators’ ability to gauge the condition of the reactor.

  Compared to the seriously damaged Units 1 and 3, Unit 2 had seemed like a success story. In contrast to Unit 3, the RCIC at Unit 2 had apparently continued to operate on its own for nearly three days, keeping the water level from dropping and exposing the core. This was remarkable, because ordinarily the RCIC should not have been able to operate for that long without battery power. But operators knew it couldn’t keep running indefinitely.

  The RCIC transferred heat from the reactor vessel to the torus, but that heat remained within the containment, which itself was already overheated. Without any means of external cooling, pressure and temperature in the containment continued to increase. The RCIC had not been designed to function under such punishing conditions. And at around 1:30 p.m. on March 14, it looked like it had finally quit. The Unit 2 water level started to drop. Now it was clear that, without prompt action, Unit 2 would suffer the same fate as Units 1 and 3.

  For exhausted shift operators, some of whom had been on duty for three days without sleep, the crises seemed to have no end. The operators needed to inject water into the Unit 2 reactor vessel as soon as possible to keep the core from being exposed. But just as at Units 1 and 3, they first needed to vent the containment. Otherwise, they would not be able to depressurize the reactor vessel adequately to force water into it with the fire engines. Yoshida had anticipated this eventuality right after the Unit 1 explosion and had ordered workers to prepare the means both to vent the Unit 2 containment and to inject water into it when needed.

  But Yoshida had not anticipated the Unit 3 explosion, which disrupted both the Unit 2 containment vent line and the alternate water injection line that workers had established. This setback marked the third strike against the doomed plant. Working amid rubble, aftershocks from the earthquake, and high radiation levels, a crew raced against time to repair the damage to the Unit 2 vent line and to construct a new water injection line. Workers tried to vent the containment at 4:00 p.m. but failed. Meanwhile, the water level inside the reactor vessel kept dropping. By 4:30, the top of the core was exposed.

  TEPCO president Shimizu ordered Yoshida to try to inject water into the reactor vessel without waiting for venting to succeed. But to make the attempt, workers needed to open a safety relief valve that would move steam from the reactor vessel into the torus. Efforts to open the valve were complicated by a lack of battery power. (This relief valve is different from the containment vent valves that workers struggled with on Units 1 and 3.)7

  Once again, a search was mounted for batteries to energize the safety relief valve. These were carried to the control room and connected, but they lacked enough voltage. The water level continued to drop. Finally, at about 6:00 p.m., additional batteries allowed the valve to be opened. But the water was now so low it didn’t even register on the gauge, meaning that the core might be completely uncovered. Also worrisome was the fact that, although hydrogen and radioactive gases were flowing into the containment, pressure there was not rising, as it had in the other units. The containment possibly had sprung a leak, allowing gases to escape into the reactor building and out through the hole blown in the side of the building when Unit 1 had exploded.

  By 7:00 p.m., pressure inside the Unit 2 reactor had dropped far enough for the fire engine pump to force water into the core. But a passing worker discovered that the fire engine, which had been idling while awaiting the injection operation, had run out of fuel. The engine was restarted shortly before 8:00 p.m., but water levels in the core remained too low to measure. Soon pressure inside the reactor rose again, thwarting the fire engine pump. The intense heat from the exposed fuel was quickly vaporizing the water being injected. As steam refilled the reactor vessel, the pressure increased. To enable the pump to inject water again, the operators managed to open a second relief valve. That appeared to do the trick; water inched up to cover part of the fuel. But the progress came at a cost: steam pressure inside the reactor was on the rise too. For the next several hours, the operators played cat and mouse with water level and pressure. The steam pressure periodically spiked above the water pressure of the fire engine hoses, and the operators could not be certain how much water was getting into the reactor vessel.

  At the same time, operators were trying without success to vent the Unit 2 containment. According to the gauge readings, by now the containment pressure should have forced the vent open by blowing through a rupture disk, but it had not. The pressure in the drywell suddenly began to rise rapidly, and Yoshida was afraid it would burst. He decided it was necessary to try to vent the drywell directly—an action that was considered a last resort because it would allow radioactive gases to escape into the environment without first being filtered through the water in the torus. However, for better or worse, operators were not able to vent the drywell either.

  The operators continued to struggle through the early hours of March 15 to find a way to vent the containment. At 6:00 a.m., just as a shift change in the Units 3 and 4 control room was taking place, a noise that sounded like an explosion was heard in the area around the torus beneath the Unit 2 reactor vessel. Pressure in the torus dropped, suggesting that the explosion had damaged the torus and allowed its contents to escape into the reactor building. If the containment had actually been breached, vast amounts of radiation could soon spill out into the environment.

  Then, around the same time, a hydrogen explosion ripped through the Unit 4 reactor building, collapsing the top two stories of the five-story structure. The incoming and outgoing control room crews fled to the safety of the Seismic Isolation Building, retracing their route on foot this time because the concrete and rubble covering the ground ne
ar the reactor made car travel impossible. In their cumbersome radiation suits and breathing equipment, it took them nearly two hours to make their way to the building and alert Yoshida of the damage to Unit 4.

  At about 6:20 a.m. on March 15, a huge explosion rips apart the top two stories of Unit 4, exposing the spent fuel pool. Until then, Unit 4 had seemed to pose the least threat in the first days of the accident. Tokyo Electric Power Company

  The workers at Fukushima had experienced many surprises in the past few days, but the explosion at Unit 4 must have come as a particular shock. Although they were aware that the temperature was rising in the spent fuel pool, they believed that Unit 4 was the least of their problems. Now it appeared that the pool had heated up much faster than they had expected, so rapidly in fact that the spent fuel might have become uncovered, sustained damage, and generated explosive hydrogen. But even more worrisome was the realization that the other spent fuel pools at the site might also be in jeopardy—a particular concern given that the pools at Units 1 and 3 were now directly exposed to the environment. In an instant, the spent fuel pools went from being a low priority to an immediate threat.

  In response to the deteriorating situation, at 11:00 a.m. Prime Minister Kan ordered residents between about twelve and eighteen miles (twenty to thirty kilometers) of the reactor to remain indoors. Those who hadn’t already fled now became housebound, relying on their small frame homes to protect them from whatever might happen next, and not knowing when they might be allowed out again. No one had ever warned them to prepare for a situation like this.

  With the catastrophic events at Fukushima Daiichi showing no sign of slowing, the radiation exposure levels of the workers grappling with the disaster had begun to worry government and utility officials in Tokyo. The government came up with a solution: increase the allowable limits of exposure.