On March 11, 2011, Koide Hiroaki was in his laboratory in Kyoto, Japan. It was a gray, wet afternoon, and the 61-year-old nuclear engineer was hard at work when the earthquake hit. Fifteen miles beneath the surface of the sea, one tectonic plate rumbled beneath another. A slippery clay layer helped the great pieces of crust slide, releasing centuries of stress. The seabed rose up 16 stories, and slipped sideways 165 feet.

The shaking lasted for six minutes. In Tokyo, skyscrapers swayed. The very planet shifted on its axis. The 9.0 magnitude earthquake was powerful enough to shorten the Earth’s day, throwing an extra wobble into its rotation. When the ground stilled, people looked to the ocean.

That’s when the next disaster hit: In less than an hour, the first of many tsunamis overcame the country’s northern shores. The waves reached heights of 128 feet, topping protective seawalls, and traveling inland for six miles to the town of Sendai. Residents had only minutes of warning, and even evacuation locations were inundated.

“I saw the images of the Sendai airport being swept away by the tsunami,” Hiroaki later told historian Katsuya Hirano. “Then I wondered about the safety of the nuclear plants.”

Eleven nuclear reactors at four power plants automatically shut down when they detected the earthquake’s vibrations. Control rods dropped into the cores, stopping the nuclear fission of uranium. But nuclear fuel requires cooling even when a plant is stopped. The Fukushima Daiichi plant, run by the Tokyo Electric Power Company (TEPCO), had planned to use emergency diesel generators to continually pump cooling water to these reactors. But within an hour, the flooded generators failed.

As the pumps stopped, the water in the cores began to drop, and the reactors started to boil. The steam built up terrible pressure, generating hydrogen gas. TEPCO declared an emergency, and, fearing what was coming, the government evacuated thousands of people in an expanding radius around the power plant.

Desperate, TEPCO tried to vent the steam, releasing radiation. They also injected seawater, corroding the reactors’ pumps and pipes. A hydrogen explosion tore through reactor Number 3 and spent fuel storage burned, releasing further radiation. Ultimately, three reactors were severely damaged.

So far, there has been one death from radiation certified by the Japanese Health, Labor, and Welfare Ministry, but that fails to account for the accident’s total impact. Over 164,000 people were evacuated from 230 square miles, of whom some 43,000 are still refugees. 

Ten years later, we’re still learning exactly what went wrong—with big implications for both human health impacts and remediation efforts. But even as scientists continue to investigate, Fukushima has changed global energy policy and raised big questions about the future of nuclear energy. 

Fukushima’s fallout

Years before disaster struck Fukushima, a prominent seismologist warned that large earthquakes and tsunamis might hit the coast. But the probability was small, so the Japanese Nuclear and Industrial Safety Agency didn’t insist that the utility company make plans for the loss of both main and backup power.

An earthquake of that magnitude was what the nuclear industry calls a low-probability, high-consequence event. In fact, the company destroyed a natural sea wall to save money by building the plant closer to sea level. “In some cases, as we become more sophisticated, we’ve lost the ability to see what’s most obvious,” says Rod Ewing, Frank Stanton Professor in Nuclear Security at Stanford University. “You calculate the probability of an event against the expense—and often cost is the driver.”

After the disaster, an investigative panel found that instead of supervising the company, government agencies had colluded with it. Ignoring the risks, the panel wrote, made the 2011 nuclear disaster “profoundly man-made.”

But determining how much—and what type—of radiation was released as a result of the accident has been harder. The radioactive fallout included many volatile radioisotopes, including iodine-131, cesium-134, cesium-137, and xenon-133. Of these, cesium-137 comprises the majority of the long-term contamination, lingering in the environment for decades.

Experts have spent the last 10 years trying to identify which particles have been released and in what quantities. Initial radiation estimates by the Japanese government didn’t account for the large amounts of radioactivity that traveled to North America and Europe, and some of their monitoring equipment was too contaminated to provide reliable data.

Later, independent research found more xenon-133 was released than in Chernobyl, and about twice the amount of cesium-137 than the Japanese government had accounted for. (Overall, Chernobyl released far more total radiation, as well more cesium-137 and strontium-90 particles that pose human health risks.)

Ocean radiation

Research from the Woods Hole Oceanographic Institute registered radiation levels in the ocean near the Fukushima reactors “50 million times higher than before the accident,” what they called “a threat to marine life.” A study in 2019 found that radionuclide levels in fish off the coast of Fukushima are variable, but remain elevated. Tuna as far away as California have been found to have low levels of radioactive cesium from Fukushima.

But researchers suggest that the human health risks of eating fish caught in the Pacific Ocean are not significant. In 2020, the Japanese government lifted bans on Fukushima seafood, saying they met safety standards that are stricter than American guidelines for cesium in food.

The radiation levels offshore of Fukushima have dropped in the years since, but some of the reactors there are still leaking. And over the last decade, TEPCO has continued to cool the fuel cores with water, which is contaminated by the process. “Now, they are running out of room to store the water,” says Satoshi Utusnomiya, an associate professor at Kyushu University.

The Japanese government has proposed releasing the water into the Pacific Ocean, though fishing industry opposition has caused the plan to be postponed. Utusnomiya explains that the water contains the radioisotope tritium, as well as carbon-14.

Stalled land cleanup

On land, cleanup has not gone quickly. In 2013, TEPCO decided it would keep the three undamaged reactors closed, and started decommissioning the power plant. But the process has been delayed; the timeline for fuel removal from one of the ponds, for example, has now been postponed by 10 years. The three damaged reactors have proven particularly challenging to address: Radiation levels remain so high, much of the cleanup work must be done using robots.

Radiation hotspots continue to be found around the country, including in 2019, near an Olympic sports complex 12 miles away from the power plant. And researchers are still uncovering new information, like how rainfall concentrates cesium in forests, potentially posing health threats if land is cleared for agriculture or wood burned for fuel.

In the meantime, 122,000 people have returned to their homes in Fukushima after government decontamination efforts.The Japanese government has ended housing subsidies to the evacuees, forcing many to choose between financial hardship and returning.

“Some say the level of radioactivity is low, and that it’s safe,” Ewing says. “On the other hand, that begs the most important question: What do we mean by safe?”

It depends on who’s counting

Every source of radioactivity carries a potential health risk, but the danger varies based on a variety of factors, including how much someone is exposed, for how long, and by which isotopes. Who you are matters too: Genetics, lifestyle, and age all play a role. Around Fukushima, those living closest to the plant had the highest exposures, because the particles became less harmful as they traveled away from the reactor.

After a nuclear accident, explains Zhanat Carr, medical officer of the radiation and health program at the World Health Organization, in addition to initial exposures, experts assess ongoing exposure to residual radiation. To do so, they measure external radioactivity levels in the air and soil, as well as internal exposures from things like food and drinking water.

The exact doses estimated depend on who’s counting: A large survey of residents of the Fukushima prefecture estimated individual doses over the past decade for adult residents to be around 10 mSv. That’s about what you receive from a single CT scan; it’s a bit under half of what the average person in the world receives over 10 years from natural sources. Many others find slightly different doses, but all are generally in the same low range.

“These levels of exposure will not result in clinically detectable health effects at the individual level,” Carr wrote in an email.

Measuring health effects of low chronic radiation doses is difficult because increases in cancer are hard to attribute to any single cause. The risks accumulate over a lifetime of exposure, and it’s hard to detect small increases in cancer due to radiation exposure when cancer rates are generally rising.

In the first five years after the accident, researchers screened 571,000 children under 18, and found 187 cases of thyroid cancer. Some have speculated the cases could be linked to the sensitive equipment or the process of screening itself, as thyroid issues are often undiagnosed. But Japanese researchers have found a statistically significant relationship between radiation doses and cancer in this group.

Other health effects

Over 18,500 people died in the tsunamis, but the death count from the nuclear accident is still disputed. Two TEPCO employees died from “disaster conditions,” suffering external injuries. The Japanese Health, Labor and Welfare Ministry has so far ruled that radiation caused the illnesses of four more employees. The single death the ministry has attributed to radiation was in 2018, when one former employee who was exposed died from lung cancer. 

But the health effects of the accident were not limited to the radiation exposure itself. They were far greater in the population that was evacuated to avoid radiation.

The Japanese Reconstruction Agency found that subsequent stress, interrupted medical care, and suicide caused 2,202 deaths. In addition, there was a sharp increase in mortality—2.4 times normal rates—among elderly people in temporary housing, along with significant mental health impacts and high rates of post-traumatic stress disorder. In a study of the displaced, 59.4 percent were indicated to have PTSD.

“Death alone is an inadequate measure—it’s not the whole impact,” says Sheila Jasanoff, a professor of science and technology studies at the Harvard Kennedy School.

Non-communicable diseases like diabetes also rose in displaced populations because of the disruption of their healthcare, as did obesity, due to less time spent outside being active because of radiation fears. The mental health impacts have been so large the World Health Organization released a report in 2020 providing guidance for preparing and responding to a nuclear emergency.

A nuclear landscape

Jasanoff visited the TEPCO site in 2018, and her memories are of a landscape that still clearly showed the nuclear accident’s effects. “What one sees is complete devastation, with huge numbers of blue tanks of cooling water proliferating around the site,” she says.

Elsewhere in the prefecture, radioactive soil has been scraped up and left in tens of thousands of plastic bags of soil, buried in people’s yards or heaped on top of each other as temporary storage. “On a particularly drizzly day, the [bags] were sitting in fields with ponds of still water around them,” Jasanoff recalls with horror.

The government has proposed using soil with lower radiation levels as the foundation for roads and other infrastructure, an idea that has met with opposition.

When the landscape itself is dotted with warning signs, Jasanoff isn’t surprised that those who have been displaced don’t trust the government’s claims of safety. “There’s a contradiction between the experiential knowledge of people, and the gaps they see in the care taken by the government,” she says.

The future for nuclear

After the disaster, Japan temporarily shut down all of its nuclear reactors, and only nine have since restarted, while at least 24 are being decommissioned. In the wake of the disaster, Germany committed to closing all of its power plants. Belgium, Spain, and Switzerland also plan to phase out their nuclear programs by 2030. But the Council on Foreign Relations suggests that while the accident might have accelerated the timeline for countries already on the path to phase out nuclear power, many others have stayed the course.

In the meantime, Japan has embraced solar, hydrogen, and offshore wind, heavily investing in renewable energies—although in the short term, it’s also turned to coal and natural gas to supply additional power.

As climate change makes energy decisions urgent, nuclear advocates still argue that the technology can be a safe and effective way to meet energy goals. Jasanoff says she’s agnostic on whether nuclear belongs in a green energy portfolio, but points out that pro-nuclear arguments that accidents come from human error are misleading.

“Nuclear experts say that safety is in our DNA,” Jasanoff explains, “but things always go wrong. Personally, I think it would be better to say things did go massively wrong, and we have to listen to what the problems are from the people who, to this day, are suffering from the effects.

“Fukushima will never return to being totally habitable—pockets have been taken out forever, or at least for the imaginable future.”