On March 11, 2011, a powerful earthquake triggered a series of events that heavily damaged the Fukushima Daiichi (Fukushima “number one”) nuclear power plant on the Pacific coast about 60 miles (97 kilometers) south of Sendai, Japan. The earthquake, known as the Tohoku earthquake, was the most severe in Japan's recorded history. The seismic shock generated a huge ocean wave (tsunami) that inundated the facility and led to a complete power outage and breakdown of the plant's safety mechanisms. In the days that followed, the failure of the cooling systems of three of the six nuclear reactors at Fukushima Daiichi caused the nuclear fuel in each to severely overheat and melt. Several nuclear reactor buildings were blown apart by explosions of hydrogen gas, releasing radioactive materials that contaminated the environment.

The emergency at Fukushima led to the evacuation of nearly 300,000 persons. Their displacement compounded the devastation throughout the region and the heavy loss of life caused by the earthquake and the subsequent tsunami. The accident contaminated an area greater than 10,000 square miles (26,000 square kilometers), with major medical and socioeconomic consequences. The magnitude of the Fukushima accident is considered comparable to that of the 1986 Chernobyl, Ukraine (then part of the Soviet Union), nuclear power accident. Both were rated as level-7 accidents, the highest level of severity on the International Nuclear Event Scale.

Owing to the radiation at the site and the enormous logistical difficulties of recovery procedures, the Tokyo Electric Power Company (TEPCO), which operates the complex, had still not managed to stabilize the reactors and caches of used nuclear fuel at Fukushima Daiichi four years after the event. As of 2015, leaks and periodic releases of radioactive materials continued. The Japanese government acknowledged that the process of safely decommissioning Fukushima Daiichi could take as long as 40 years.

Many of Japan's nuclear reactors are situated on or near the ocean coast to take advantage of the ocean as a heat sink (absorbing the heat and allowing it to dissipate) and as a source of water for emergency cooling. The Japanese coastline is also a zone of significant seismic activity. Engineers and nuclear regulators, taking into account the risk of earthquakes, incorporated multiple safety features and emergency backup systems into the design of each power plant to minimize the risk of any prolonged interruption of electric power or flow of cooling water. The technical specifications required that the nuclear facilities be constructed so that their components could withstand what the industry terms a “design-basis accident” (a theoretical natural disaster or set of emergency conditions) without jeopardizing public safety.

The boiling water reactors at TEPCO's Fukushima Daiichi facility employed a primary containment system known as Mark I, which was designed by the General Electric Company. In Mark I reactors, the reactor core is surrounded by an airtight steel structure known as a “drywell.” In the case of an overheated or leaking reactor, radioactive steam entering the drywell would be expelled downward into a wet well, a vessel half filled with water. The Mark I design was controversial because of concerns that the containment was inadequate for dynamic loads that could develop under high-stress conditions, such as a major earthquake. The company subsequently mandated modifications for all existing reactors with Mark I containments.

The Fukushima Daiichi reactors included other backup systems. Unit 1, the least powerful of the six, featured an auxiliary container called an isolation condenser to receive excess steam whenever a shutdown would separate the unit from its ordinary flow of cooling water. For the other units, a reactor isolation cooling system provided a passive, secondary means of circulating coolant water. Nearly all of the reactors’ safety features, Page 599  |  Top of Articlehowever, had one common feature: they required electrical power. Under normal conditions, the plant obtained this power from the electrical grid. In the case of an accident to the grid or to a single reactor, the plant could use power generated by the adjacent units. A pair of diesel generators was located in the basement of each reactor building, along with auxiliary batteries, which were always kept fully charged. Nonetheless, the conditions brought about by the Tohoku earthquake vastly exceeded the parameters of a design-basis accident.

On March 11, 2011, at 2:46 p.m., the two tectonic plates that meet beneath the ocean floor off the eastern coast of Japan underwent a major shift. The resulting magnitude-9.0 earthquake was one of the largest ever recorded. It caused the coastline to drop by more than 2.5 feet (0.8 meters) and slide 8 feet (2.4 meters) eastward. Hundreds of thousands of buildings in Japan were damaged or destroyed, but damage to the Fukushima Daiichi nuclear plant was minimal. Units 1, 2, and 3 shut down from full power automatically. Within seconds, the backup generators fired up to power the cooling systems, and everything appeared to be working normally.

The sudden undersea seismic shift displaced a colossal amount of water, triggering the tsunami that sped westward toward the Japanese coast. It was the tsunami that took most of the 18,000 lives lost that day, devastating coastal settlements and scattering cars, trucks, buildings, and entire villages in its wake. The first massive wave, 13 feet (4 meters) high, reached Fukushima Daiichi at 3:27 p.m. but was blocked by the 18.5-foot (5.6-meter) seawall constructed for this very purpose. Eight minutes later a second wave arrived that was between 45 and 50 feet (14 and 15 meters) high. It swamped the plant and immediately cut it off from the electrical grid. It also flooded the basements and lower levels of the reactor buildings, wrecking all but one of the diesel generators and most of the stored batteries.

With the instrument panels no longer working, operators in the control rooms had no way of reading the temperature, water, or radiation levels inside the reactor cores. Furthermore, all means of supplying water to the overheated nuclear fuel rods had been disabled. One hour after the tsunami struck the plant, its emergency core cooling system failed. Shortly after this news reached the capital, the Japanese government ordered the evacuation of everyone within about 2 miles (3.2 kilometers) of the power plant. Plant workers scavenged for car batteries they could use to light up their control panels, if only briefly. A momentary display confirmed their fears: the temperature and pressure were way up inside Unit 1. After going without a supply of cooling water for a mere half hour, the reactor fuel was boiling off the water in which it was immersed. The evacuation zone was soon widened to a 6-mile (9.7-kilometer) radius around the plant.

Without intervention, the intensely hot fuel would inevitably be exposed to air, which would create three dire hazards. The steam, mixing with the radioactive materials generated by nuclear reactions, would generate a large amount of hydrogen gas that could easily explode and scatter radioactive debris into the environment. A more grave threat was the possibility of a meltdown. First, the exposed fuel rods would lose their zirconium coating, and then the uranium dioxide and other fuel components would melt and fall to the bottom of the reactor vessel, perhaps even burning through the floor. This toxic mix would make it impossible to prevent radiation from escaping. Lastly, the steeply rising pressure could surpass the capacity of the Mark I containment system. All three hazards eventually developed at Fukushima Daiichi.

An immediate priority was to relieve the excess pressure in Unit 1, and the plant manager was convinced that opening the unit's vent stack (an exhaust pipe) to release Page 600  |  Top of Articlehigh-pressure gases was the only possible step that might avoid a nuclear disaster on the scale of the Chernobyl accident. Risking their lives, workers finally succeeded in opening the vent stack by 2 p.m. on March 12, and by 3:30 p.m., managed to connect auxiliary water pumps to a power source and bring fire hoses into position to refill the unit's condensing tank. At 3:36 p.m., however, a large explosion took place. The hydrogen gas that had accumulated under the ceiling of Unit 1 had ignited, blowing off the roof and exposing the reactor core, still intact, to the outside air. The outside of Unit 2 was also damaged in the blast, and radioactive debris was strewn throughout the area.

A fire truck sprays water at the No. 3 reactor of the Fukushima Daiichi nuclear power plant in Tomioka, Fukushima prefecture in this still image taken from a video by the Self Defence Force Nuclear Biological Chemical Weapon Unit on March 18, 2011. Engineers had successfully attached a power cable to the outside of the damaged nuclear plant in a first step to help cool reactors and stop the spread of radiation. © Reuters TV/Reuters/Corbis. © Reuters TV/Reuters/Corbis.

At 6:30 p.m. on March 12, the evacuation order for the area around the plant was extended for the second time, to 12 miles (20 kilometers). Units 1 and 2 were both melting down, although government spokespeople avoided use of the word meltdown in addressing the press. By midday on March 13, the core of Unit 3 was also melting after its passive system of circulating coolant water failed. With freshwater supplies running low, a unit of Japan's Self-Defense Forces attempted to pump seawater into the core of Unit 1 to bring the meltdown under control. Another hydrogen explosion blew the top off Unit 3 at 11 a.m. on March 14, once again sending concrete and a radioactive mix skyward and once again setting back efforts to get water into the reactors. Much of the area, and even parts of the control rooms, were now so contaminated with radioactive materials as to be off-limits.

On March 15 a third hydrogen explosion blew out the top two floors of Unit 4, which had been off-line at the time of the earthquake. Managers had believed that the unit was relatively stable, but the unit's 1,500 spent fuel rods were still extremely hot. Spent fuel rods were stored on a high floor in each unit in pools that had been Page 601  |  Top of Articlestagnant since the tsunami swept through and were now completely open to the environment in damaged structures. Plant managers belatedly recognized that the spent fuel pools represented a radiation danger potentially even more severe than the meltdown of the reactor cores.

In their desperation to get water into the exposed fuel pools, authorities ordered the Self-Defense Forces to make water drops by helicopter. From a height of 300 feet (91 meters), pilots released 30 tons (27 metric tons) of water over the roofless reactors. The wind blew most of the liquid off target. On March 22 the first of several trucks arrived that had a remotely operated boom 190 feet (58 meters) tall, which was capable of shooting about 30,000 gallons (114,000 liters) of water per hour in a precise stream. The cooling water supplied by these vehicles enabled plant operators to bring an end to the most acute phase of the crisis.

TEPCO and the Japanese prime minister, Naoto Kan (1946–) received considerable criticism over their handling of the Fukushima Daiichi nuclear accident. Critics launched accusations of collusion between the government and the nuclear industry, which had often worked closely together, in controlling the disclosure of information during the crisis and the pace of remediation in the aftermath. Even members of Kan's administration voiced frustration with TEPCO for disclosing information slowly during the first few days of the disaster. Government spokespeople said that they wanted to avoid inducing a panic, but the withholding of information had unfortunate consequences. In some cases, local officials led evacuees into or through heavily irradiated areas because the national government neglected to furnish them with information it possessed, such as maps and forecasts of where the radiation was most likely to spread.

While authorities delayed acknowledging the extent of the Fukushima accident, mistrust of official information became widespread. Citizens and civil organizations offered their own, sometimes conflicting assessments of radiation and health dangers. The people with the least information, in many cases, were the evacuees themselves. Some were forced to move multiple times as the government successively widened the evacuation zone. The government also came under fire for raising the level of allowable radiation exposure for plant workers. Defenders of the decision said officials were worried that the power plant could end up short staffed if too many workers had to be sidelined after experiencing the maximum exposure.

Days and weeks after the initial accident, the crisis at Fukushima Daiichi and radiation releases from the plant remained ongoing concerns. The large amount of water used to try to cool the reactors and stabilize the spent fuel pools itself became a significant radiation hazard. Much of this water sat in storage tanks on the power plant site, and some of it leaked into groundwater sources and into the Pacific Ocean. Molten fuel continued to sit at the bottom of vessels housing the ruined reactors, still cooled by makeshift pumps and vulnerable power systems.

In 2015 roughly 6,000 workers were employed at the hazardous facility. Shoring up the safe storage of contaminated water and other radioactive waste, safely transferring spent fuel to new locations, completing and implementing a plan for protecting the defunct power station from future seismic activity, and on-site and offsite decontamination activities were some of TEPCO's priorities for mid-term risk reduction at the nuclear plant. The company envisioned that it would take 30 to 40 years to decommission the facility.

Although the Fukushima Daiichi accident appears to have been responsible for little loss of life, the contamination of the environment has been extensive. Iodine-131, a radioactive but short-lived isotope of iodine, was detected in raw milk in the Fukushima area as soon as one week after the earthquake. Levels of strontium-90, another radioactive isotope, in groundwater near the plant increased 100-fold between late 2012 and mid-2013, indicating ongoing uncontrolled releases of radioactive materials. Cesium-137 and other radioactive isotopes were soon found in farmed vegetables nearly 100 miles (160 kilometers) from the site.

Japan's agricultural production shrank by nearly one-fourth in the two years following the disaster. A formerly thriving fishing industry in the area was completely shut down. Radioactive cesium was found in Pacific tuna at great distances from the site, and fish sold in Japan was subject to radiation testing. Monitors found increased levels of radiation in every one of Japan's prefectures (administrative districts), with dangerous “hot spots” downwind to the plant's northwest. An area of roughly 1,500 square miles (3,900 square kilometers) has been deemed unfit for human habitation.

The Fukushima accident brought about a thorough shift in Japanese attitudes regarding nuclear energy. Before the Tohoku earthquake, nuclear power accounted for nearly one-third of Japan's total electricity supply, and government plans called for the construction of new nuclear plants. After the accident, public opinion polls and increasingly vocal antinuclear protests revealed that the majority of Japanese citizens supported eliminating nuclear energy, despite the expected increase in cost from rising fossil fuel imports. In a dramatic reversal of his government's prior policy, Prime Minister Kan called for a complete phaseout of nuclear power, and by May 2012 all of Japan's nuclear reactors had been removed from commercial operation. In 2015, however, despite widespread public opposition, Japan's Kyushu Electric Power Company sought to restart one of its nuclear reactors.

The seismic epicenter, radioactive contamination zones, and the evacuation zones are shown for the Fukushima Daiichi nuclear event. © Gurgen Bakhshetsyan/Shutterstock.com. © Gurgen Bakhshetsyan/Shutterstock.com.

Two months after the Fukushima Daiichi accident, Germany's chancellor, Angela Merkel, announced that Germany, Europe's strongest economy at that time, would shut down its 17 nuclear reactors by 2022, which was 14 years earlier than had been previously planned. In Italy, which closed its nuclear power plants in 1990 following the Chernobyl accident, voters in a national referendum in June voted overwhelmingly against allowing the industry to start back up. Before the year was out, Switzerland and Belgium had also moved to phase out nuclear power.

Assessments of the health consequences of the Fukushima meltdowns vary widely and have led to controversy. One generally accepted result of radioactive contamination is an increased risk of thyroid cancer, especially in the young. In Fukushima Prefecture, more than 360,000 individuals who were 18 years of age or younger at the time of the accident had been screened for thyroid disorders by 2015, and just over 100 cases of thyroid cancer had been confirmed. A comprehensive health assessment published by the World Health Organization (WHO) in 2013 concluded that the overall risks were minimal for populations outside the immediate area surrounding the Fukushima plant. In the most heavily affected areas, the WHO reported a substantially increased risk of thyroid cancer among females exposed as infants and a slightly elevated risk—less than 10 percent above normal—for leukemia and other cancers.

The United Nations Scientific Committee on the Effects of Atomic Radiation echoed these findings, projecting no significant increase in cancer rates, birth defects, or other health effects among the general public attributable to radiation released from Fukushima. In contrast, however, the International Physicians for the Prevention of Nuclear War, a Nobel Peace Prize–winning advocacy group, characterized these findings as “systematic underestimations” based on overly optimistic presumptions and narrow interpretations of the data on radiation-related illness. It will likely be years before a consensus emerges about the health issues resulting from the large discharges of radioactive materials caused by the accident.

In view of the estimated exposure levels, an increased risk of cancer is the potential health effect of greatest relevance. The relationship between radiation exposure and lifetime risk of cancer is complex and varies depending on several factors, mainly radiation dose, age at time of exposure, sex and cancer site. These factors can influence the uncertainty in projecting radiation risks, in particular when assessing risks at low doses.

Outside the geographical areas most affected by radiation, even in locations within Fukushima prefecture, the predicted risks remain low and no observable increases in cancer above natural variation in baseline rates are anticipated.

Some health effects of radiation, termed deterministic effects, are known to occur only after certain radiation dose levels are exceeded. The radiation doses in Fukushima prefecture were well below such levels and therefore such effects are not expected to occur in the general population.

The estimated dose levels in Fukushima prefecture were also too low to affect fetal development or outcome of pregnancy and no increases, as a result of antenatal radiation exposure, in spontaneous abortion, miscarriage, perinatal mortality, congenital defects or cognitive impairment are anticipated.

In the two most affected locations of Fukushima prefecture, the preliminary estimated radiation effective doses for the first year ranged from 12 to 25 mSv. In the highest dose location, the estimated additional lifetime risks for the development of leukaemia, breast cancer, thyroid cancer and all solid cancers over baseline rates are likely to represent an upper bound of the risk as methodological options were consciously chosen to avoid underestimation of risks. For leukaemia, the lifetime risks are predicted to increase by up to around 7% over baseline cancer rates in males exposed as infants; for breast cancer, the estimated lifetime risks increase by up to around 6% over baseline rates in females exposed as infants; for all solid cancers, the estimated lifetime risks increase by up to around 4% over baseline rates in females exposed as infants; and for thyroid cancer, the estimated lifetime risk increases by up to around 70% over baseline rates in females exposed as infants. These percentages represent estimated relative increases over the baseline rates and are not estimated absolute risks for developing such cancers. It is important to note that due to the low baseline rates of thyroid cancer, even a large relative increase represents a small absolute increase in risks. For example, the baseline lifetime risk of thyroid cancer for females is just three-quarters of one percent and the additional lifetime risk estimated in this assessment for a female infant exposed in the most affected location is one-half of one percent. These estimated increases presented above apply only to the most affected location of Fukushima prefecture. For the people in the second most affected location, the estimated additional lifetime cancer risks over baseline rates are approximately one-half of those in the highest dose location. The estimated risks are lower for people exposed as children and adults compared to infants.

In the next most exposed group of locations in Fukushima prefecture, where preliminary estimated radiation effective doses were 3 to 5 mSv, the increased lifetime estimates for cancer risks over baseline rates were approximately one-quarter to one-third of those for the people in the most affected geographical location.

Among Fukushima Daiichi nuclear power plant emergency workers, the lifetime risks for leukaemia, thyroid cancer and all solid cancers are estimated to be increased over base- line rates, based upon plausible radiation exposure scenarios. These scenarios and their corresponding estimated risks are detailed in the body of this report. A few emergency workers who inhaled significant quantities of radioactive iodine may develop non-cancer thyroid disorders.