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.
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!
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.
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.
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.
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.