Nuclear reactor accidents are so devastating and world-changing that you know them by one name: Three Mile Island (1979), Chernobyl (1986), and Fukushima.

March 11, 2011 was a day of unimaginable tragedy in northern Japan, a tragedy exacerbated by the reactor meltdowns and release of contamination. But the nuclear part of this horrible day was, if the longest-lasting, certainly the least lethal event. Yet it's the part that still engenders so much fear. With the fifth anniversary of the Fukushima accident upon us this month, let's take a look at where things stand today with recovering from this calamity, and what might be happening next.

What Happened

You know the outline of the disaster by now: A powerful earthquake caused a massive tsunami that crashed into Fukushima Daiichi Nuclear Power Plant and caused multiple nuclear reactor meltdowns. But to really understand what happened at the nuclear plant that day, you need to know a little more.

At the site of the earthquake, stress had been building up in the Earth's crust for decades. When it released, that stress caused one of the most damaging quakes on record. The earth moved more than 20 meters over a 500-mile zone and the resulting earthquake released as much energy as a 45-megaton hydrogen bomb (to put this in perspective, this is 30,000 times more powerful as the bomb that leveled Hiroshima). It was the fourth-strongest earthquake recorded since 1900 and the strongest earthquake to strike Japan in recorded history. The quake shifted the Earth's axis by somewhere between 4 and 10 inches, altering the length of a day by nearly 2 microseconds.

A lone tree sits on the tsunami scarred landscape, inside the exclusion zone, close to the devastated Fukushima Daiichi Nuclear Power Plant Christopher Furlong Getty Images

Then came the water. The moving rocks shoved a wall of water across the Pacific Ocean. The seafloor began rising towards the surface, and as the water ran into the shallower depths it piled up to a height of more than 40 meters (140 feet) before it swept over the land. The tsunami slammed into the coast of Japan, killing more than 15,000 people and destroying or damaging more than a million buildings. This was among the worst natural disasters to hit a nation known for natural disasters, and that was only the start.

Near the city of Fukushima was a complex of six nuclear reactors capable of producing more than 4500 MW of electrical energy. When the earthquake hit there were three operating reactors (units 1, 2, and 3). Units 4, 5, and 6 were shut down, albeit with spent reactor fuel sitting in pools that required cooling. The quake itself caused the operating reactors to scram (shut down) as they were designed to do. With the electrical grid busted by the earthquake, Fukushima's emergency diesel generators kicked on and powered the site including cooling water pumps—again, as they were designed to do. But then the tsunami hit. Seawater climbed over the seawall and inundated the diesel generators, shutting them down. Lacking cooling water, the fuel—including the radioactive fission products—heated up and began to melt.

As crews raced to contain the disaster, one of their biggest challenges was to add cooling water to the reactors and find a way to power pumps needed to circulate this water through the reactor cores and spent fuel pools. Ultimately, the answer was to bring in power barges to allow pumping seawater into the reactor plant to keep the core cooled. By the time this was accomplished, the core had already been damaged beyond repair. But it didn't matter. Once seawater has been introduced into a reactor plant, it will never operate again.

That wasn't the end of the immediate danger. High-temperature chemical reactions between the zirconium fuel cladding and water created hydrogen. Over the next several days, the hydrogen escaped the reactor plant and collected in the support buildings. Some of these pockets exploded in the days to come, damaging the support buildings.

As a result of these radioactivity releases to the environment, the Japanese government ordered the evacuation of everyone living within 20 km (12 miles) of the site; this included a number of hospitals. Japan also banned produce from the area around the reactor site to reduce radioactivity entering the food supply. When I was there, about a month after the accident, I heard daily radio announcements in Tokyo (in English!) informing people of the radioactivity concentrations in the drinking water. We were also told that people who remained in the "shelter-in-place" region close to Fukushima were having problems finding food at the stores because truck drivers were reluctant to enter the area. We ate a lot of meals at 7-11 stores, which somehow remained well stocked. Some things you can count on!

The quake shifted the Earth's axis by 4 to 10 inches, altering the length of a day

How Much Is Too Much?

In the five years since the Fukushima accident there's been a lot of information put out about Fukushima – some is accurate but much is uninformed, hyperbolic, or worse. Let's take a look at what actually happened and what the science tells us.

A fissioned uranium atom splits into two radioactive fission fragments. (Common fission products are Tc-99, Ru-106, I-131, Cs-137—isotopes of the elements technetium, ruthenium, iodine, and cesium respectively). These isotopes are contained within the fuel elements, but when those elements are compromised—by melting down, for example—they can be released. Heavier elements are also created in a reactor core when uranium that hasn't been fissioned captures neutrons (plutonium and americium are two of these). We call them neutron capture products.

The question to ask is not "Is there any radioactivity present?" but rather, "How much, and is it enough to be harmful?"

Although all these products are present in reactor fuel, not all are released equally when the fuel is compromised like it was at Fukushima. For example, cesium and iodine are volatile, and these are far more likely to be released into the atmosphere than elements like plutonium. The elements that are more soluble—cesium and iodine are among those—are more likely to dissolve into reactor cooling water and escape into groundwater or seawater if the reactor coolant leaks out. What this means is that the elements we're most likely to see in the air, on the ground (because they settled out from the air), or in the water are the elements that are volatile or soluble.

Given this, it's not surprising that scientists detected airborne iodine and cesium throughout the Northern Hemisphere in the aftermath of Fukushima. Radioactivity from the airborne plume over the nuclear plant settled onto the ground—aerial and ground surveys by the Japanese and American governments confirmed this in the weeks and months following the accident. I visited Japan shortly after the accident, measured elevated radiation levels, and identified some of these volatile nuclides on the ground in the few places we visited, including I-131, Cs-134, and Cs-137. The highest radiation dose rates I measured were clearly elevated, but were also too low to cause short-term or long-term health risks.

Thousands of bags of radiation contaminated soil and debris wait to be processed, inside the exclusion zone close to the devastated Fukushima Daiichi Nuclear Power Plant on February 26, 2016 Christopher Furlong Getty Images

"Radioactivity escaping into the environment" sounds scary no matter how small the levels, which probably explains why there was so much bad information put out about the environmental impact of Fukushima's radioactive releases. For example, it's true that radioactive cesium (Cs-134 and Cs-137) was measured in tuna caught in the Pacific Ocean. But it's not true that this cesium posed any risk to people eating this tuna. I interviewed the scientist who made these measurements and he pointed out that the radioactivity of the cesium was lower than the radioactivity content of the natural potassium in the fish.

Likewise, there were claims that dissolved radioactivity from Fukushima was entering the ecosystem and causing massive die-offs of marine life. This claim was refuted by oceanographers who studied the matter extensively. As for claims that the collapse of the Unit 4 spent fuel pool might render the American West Coast uninhabitable? I calculated that dissolving all of the fuel of all three operating reactors, plus the entire contents of all of the spent fuel pools at Fukushima into the waters of the northern Pacific would still give a person swimming in the ocean off Hawaii, Alaska, or California about one billionth the amount of radiation dose needed to cause any harm. In the words of oceanographer Miriam Goldstein, radioactivity from this accident in seawater is "detectable but not hazardous." I had no qualms about eating sushi when I was in Japan the month after the accident, as well as in later trips to the West Coast.

Here's the thing: There are demonstrably high levels of radioactivity in the seafloor sediments in the vicinity of the Fukushima site. But the question to ask is not "Is there any radioactivity present?" but rather, "How much radioactivity is present, and is it enough to be harmful?" Despite the very serious nature of the Fukushima calamity, the answer to the latter question isn't as worrying as you might have been led to believe.

In the case of the seafloor around the Fukushima site, radioactivity concentrations are elevated, but not dangerously so. Samples of seafloor sediments show that the highest Cs-137 concentrations in sediments near to the Fukushima site measured 73,000 Becquerels (Bq) per square meter, a unit of measuring concentrations of radioactivity. Now, this is a very high reading. Most such seafloor samples show Cs-137 present at less than 100 Bq. On the other hand, the EPA says that each Bq per square meter will give us a radiation dose of about 3x10-19 Sieverts per second (the Sievert is a measure of radiation dose). Do the math and you find that this one very contaminated location would expose a person (or aquatic organism) to a radiation dose of less than 1 mrem annually. To put this in perspective, we receive this amount of radiation every single day from natural sources; I received more than this on the 14-hour flight from New York City to Japan.

A great place to find information about radioactivity in seawater is the Woods Hole Oceanographic Institution. Ken Buesseler has been studying this throughout the Pacific since shortly after Fukushima and his data are available online.

We heard a lot in the last five years about the impact of this radioactivity on fish and other sea life in the Pacific, too: reports (and photos) of bloody tumors, starfish with abnormal numbers of limbs, even stories about vast swathes of seafloor covered with dead or dying sea creatures. Guess what? These stories have been pretty much debunked by oceanographers and marine biologists, as well as myth-busting sites and some well-informed bloggers. For one thing, the radioactivity concentrations in seawater are far too low to cause these sorts of problems. And for another, it turns out that many of the photos used to "document" these claims were old or showed fairly common afflictions that have nothing to do with radiation exposure. For example, a claim that sharks were found to have tumors despite the "fact" that sharks don't get cancer is misleading at best. The sort of tumor found in a great white shark turns out to have been reported in sharks for decades. We can certainly detect radioactivity from Fukushima in sea life, and that in itself can cause concern. But those levels are far too low to cause the sorts of problems that have been reported.

A stopped clock covered in five years of dust, sits in tsunami damaged home inside the exclusion zone close to the devastated Fukushima nuclear plant. Christopher Furlong Getty Images

Another big concern is the presence of radioactivity in the groundwater near the reactor site. There's no doubt that some of the contaminated groundwater contains levels of radioactivity that are far higher than regulatory limits permit, and possibly high enough to cause harm to an organism living in the water or drinking it on a regular basis. (We should note here and nobody is drinking this water and, because the groundwater in this area flows out to sea, nobody will be drinking it downstream.) As a result, TEPCO began pumping groundwater into holding tanks for later processing. It was also looking into ways to build a "freeze wall" by running refrigerant through pipes sunk into the soil. The idea here is to freeze the soil to isolate the site from incoming groundwater and to keep the contaminated groundwater on the site from flowing into the ocean. As of this writing the freeze wall is not yet in place, but the design has been reviewed and found to be a sound design by a number of expert agencies (including some of the American national laboratories). It is expected to be operational sometime during 2016.

The Fukushima catastrophe did release a large amount of radioactivity into the environment—into the air, into the sea, and the ground. We can (or could) measure this radioactivity, though just because we can measure it doesn't necessarily mean that it's harmful (with the proper equipment I can detect radioactivity in my own body, in a bunch of bananas, and in virtually any natural air, water, or soil sample on Earth). But the scientific consensus seems to be that this radioactivity has not (and likely will not) cause long-lasting devastation on land or in the sea.

This shouldn't surprise anyone who has studied the impact of the Chernobyl accident. While there was significant short-term impact in the areas close to the Chernobyl reactors—and the area right around the ruined reactor remains a forbidden zone where you just don't want to go—further afield the impact was fairly low. Numerous studies (summarized by the International Atomic Energy Agency in this 2006 report) concluded that the ecosystem in the exclusion zone around the Chernobyl site is among the richest ecosystems in Europe, teeming with large game as well as smaller animals, partly because there aren't any people there.

Human Health

Peter Blakely

The only topic more controversial than the environmental impact of Fukushima accident is its effect on human health. Since the accident there have been claims of cancers, birth defects, and more tied to the radiation, though these reports are contradicted by the conclusions of the World Health Organization and the United Nations Science Committee on the Effects of Atomic Radiation (UNSCEAR). Both organizations concluded that it is likely no member of the public will receive enough radiation to cause health problems.

Nevertheless, the Japanese government recently compensated a worker who developed leukemia after receiving just under 20 mSv (about 2 rem) of radiation exposure from the accident. This leaves me with mixed feelings. On the one hand, leukemia is one of the first cancers to appear after radiation exposure, so if any cancer is going to show up after only a few years this is it. On the other hand, the dose the worker received is quite low and the probability that this low dose of radiation caused this particular cancer after only a few years is also very low—less than 1 percent. In fact, the Health Physics Society (America's radiation science professional organization) recommends against even performing this sort of calculation for any radiation exposure of less than 5 rem in a short period of time (or 10 rem over a lifetime) because of the great uncertainties in the epidemiological data at these low doses. In other words, when the radiation dosage is so low, it's hard to sort the signal from the noise—the actual risk could well be even lower.

We've also heard a lot of stories about thyroid tumors in children—stories suggesting these are due to radiation exposure from the accident. Here's the hard truth: From a scientist's point of view, it's really, really tricky to know whether that's true. Because there was no comprehensive inventory of thyroid nodules before the accident, we simply don't know how many kids had growths on their thyroids before they were exposed to radiation, or how many of these nodules would have appeared even in the absence of radiation exposure.

It's entirely possible that the evacuations meant to get the public to safety might have been deadlier than the accident itself

The WHO realized the same thing about Ukraine and Belarus in the aftermath of Chernobyl in the 80s. They noted not only increased numbers of thyroid nodules, but also increased numbers of birth defects in a number of villages. That's very troubling. But if you use information collected by UNSCEAR to plot the number of "extra" birth defects against the radiation dose in each village, there seemed to be no correlation between radiation exposure and birth defects or thyroid nodules. (If radiation was causing these effects, then villages with higher levels of radiation exposure should have higher numbers of birth defects or thyroid problems.) Now, the fact that this correlation doesn't appear doesn't tell us Chernobyl's radiation is not responsible for at least some of these. It just fails to tell us that radiation is definitely the cause because if you don't know the baseline number then you can't tell whether or not you're above that baseline.

So let's get back to Fukushima. Some very reputable organizations – among them, the International Atomic Energy Agency, the World Health Organization, and the United Nations Science Committee on the Effects of Atomic Radiation – have stated publically that they think Fukushima will not cause any radiation-related deaths in Japan, and almost certainly not in the rest of the world. My measurements during my own admittedly limited visit (although we did make measurements as close as 20 km from the site in the centerline of the plume) support this conclusion. Meanwhile, though, the death toll from the evacuations that took place was over 1,500. If nothing else, we should remember that evacuation is not a benign exercise. In the case of Fukushima, it's entirely possible that the evacuations meant to get the public to safety might have been deadlier than the accident itself.

Finally, a quick word about the health of the workers. While a great deal was written about the "Fukushima 50" (and I certainly do not question their courage and dedication), the fact is that there have been only two confirmed radiation injuries from this accident—the two workers whose boots leaked and who ended up with radiation burns on their lower legs. I spoke with the physician who treated them during my trip and he told us that these two workers had second-degree radiation burns, but that these burns were only about a half-inch deep (damage from beta radiation, like beauty, is apparently only skin deep). He also said that both were released after a short hospital stay and that he expected no future problems. This is not to minimize the injuries, merely to point out that they were short-lived and unlikely to cause lasting health problems.

The Long Recovery

Five years after the disaster, there's a lot we still don't know. How much land must be decontaminated, before it's done, and how far must it be cleaned up before people can (or want to) return? And, of course, how much will this all cost and how long will it take?

One lingering problem is the contaminated groundwater and the water already collected onsite. Regardless of the success (or not) of the freeze wall, there will be a lot of water that needs to be treated to remove radioactivity before it can be discharged to the ocean—or anywhere else. It's going to take time, money, and possibly new technology.

Of course, there's still a nuclear site with three damaged reactors. These reactors have to be cleaned up at some point, but that won't be today and it may not happen in the next few years. It took decades to clean up the reactor at Three Mile Island, and a quarter-century after Chernobyl there's still a lot of work to be done. It might be decades before Units 1, 2, and 3 are cleaned up, and it might be even longer before people return to the evacuated zone around the reactor site.

Some of this delay is absolutely necessary to allow radiation dose rates to decay to the point where work can be done or villages can be reoccupied. Some will be due to the amount of time required to do all of this work. And some will be due simply to the amount of time it takes for people to feel safe working in the reactors or moving back to their homes.

Peter Blakely

I get it. As a scientist, I have to admit that it's frustrating when people fear the idea of radioactivity without understanding that it's part of our world, and without understanding the real risks, however high or low. At the same time, as a father and husband I can sympathize. I'd be hard-pressed to bring my family back to a place that might pose a risk to them, especially if I weren't an expert in radiation science. I hope that I would try to learn enough to determine for myself what the numbers mean, and I hope those who were evacuated will be able to do the same.

The people from areas hit by the tsunami and affected by Fukushima fallout must be wondering how they could ever come back from this double blow. I have visited the Atomic Bomb museum in Hiroshima, and looking at the photos from that bombing, you see people wondering the same thing. But to visit Hiroshima today is to visit a place that, besides its history, is an ordinary Japanese city. I would wager that someday we'll be able to say the same about Fukushima. Not for the five-year anniversary, and maybe not for the 25-year, but someday.

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