Determining when a radioactive isotope will decay—shed those extra neutrons and protons—is similar. Rather than saying exactly how long it will be before a specific atom decays, scientists give the amount of time it will take for half of any group of atoms in a particular isotope to decay. If an isotope is said to have a half-life of five years, you can expect roughly half of the atoms to have decayed in that amount of time. Of the remaining atoms, half of those will decay in the next five years.
This chart illustrates the decay rate of iodine 131, a radioactive isotope that is produced by nuclear fission. It has a half-life of 8 days, which means that after 24 days, there would be about 14% of the original iodine left. It would take about 56 days for the levels of remaining radioactive iodine to reach less than 1%.
A nuclear power plant that is humming along properly contains its radiation in the reactor. In an accident, radiation—the protons and neutrons that continue to fly from the broken halves of uranium in the fuel—may travel a short distance from the reactor. But radioactive isotopes—the broken bits themselves—can travel far from their source. Carried in smoke and steam, they drift on the wind or in the water. Eventually, they decay, emitting radiation in their new location. The impact of that decay isn’t much of an issue unless the atom happens to be on or in a living body when it breaks down. There, the radiation emitted by a decaying isotope will strike one of the body’s cells.
Nuclear operators go through rigorous control measures to make sure they don’t carry radioactive dust out of the plant at the end of a workday. Anyone working in an area where radioactive materials may have been is required to wear a radiation suit—usually coveralls that are fastened tightly at the ankles and wrists, gloves, special socks, boots, goggles, and a breathing mask with a filter. Without the suit, bits of dust containing radioactive isotopes could cling to clothes and hair, and might be inhaled or swallowed.
Control room operators at Daiichi were wearing full gear, but the airtight radiation suits weren’t made for long-term use. They were hot and uncomfortable and simply impossible to keep intact over long periods of time. And the workers had to eat and drink. To do that, they had to pull off their face masks. It was a major breach of safety protocol that risked the workers swallowing radioactive dust with their food or inhaling it while they ate. But it was that or starve. In a situation where every moment counted, they could not take the time to leave the control room and eat in a safer environment.
Workers at the plant were being exposed to high levels of radiation, and it became clear they wouldn’t be able to stay within the legal exposure limit. By law, Japanese nuclear workers could not be exposed to more than 50 mSv of radiation per year. In emergency situations, the limit went up to 100 mSv per year. The law protected workers by making sure they didn’t receive doses of radiation that were unsafe. To stay within the legal limit, the workers needed to leave the plant if they reached that dose, and they were desperately needed where they were. So on March 14, Japan’s Ministry of Health, Labor, and Welfare raised the legal limit for annual exposure in an emergency to 250 mSv.
It’s incredibly difficult to say how much damage a radioactive isotope will do in the human body. Large doses can cause acute radiation sickness. But at smaller doses, the results are less certain. In the end, the damage done comes down to chance. While radiation has a very real power to damage cells, the most likely effect of that damage is cell death. In small numbers, the loss of cells is not a serious threat. Our body constantly generates new cells to replace old and damaged ones. But in some cases, radiation may strike a cell’s DNA. Remember how ionizing radiation breaks apart the bonds in molecules? Each spiraling ladder of your DNA is a single, amazingly complex molecule. If one of the bonds in its DNA is affected, the cell is likely to repair the damage. But if it cannot, the error will affect the way the cell reproduces. The result, in that case, can be cancer. No one knows whether any one exposure will be the unlucky one that starts disease. But one thing is certain: the more radiation a person is exposed to, the greater the odds that it will strike a cell’s DNA. At higher levels of exposure, the probability of a worker later developing cancer begins to climb.
In order to do harm, a radioactive isotope must be in or on the body when it decays. That means that breathing in or swallowing a radioactive isotope poses the greatest danger. But what if a radioactive isotope is swallowed? Our bodies are incredibly picky. During digestion, a body only retains the elements that are useful to it. And it only holds on to radioactive isotopes if they are similar to elements it needs. Nuclear fission creates several isotopes that are close enough to minerals the body needs to be dangerous. Strontium 90 is chemically similar to calcium, and so it is absorbed by the body and deposited in the bones. Once there, it can remain for decades, releasing radiation into the bones and bone marrow. Cesium 137 behaves like potassium and is used throughout the body, winding up mostly in muscle tissue. Iodine 131 resembles naturally occurring iodine 127, which concentrates in the thyroid gland.
But if the body doesn’t need a nutrient at the time that it is taken in, it won’t absorb it. Iodine 131 is a particularly problematic radioactive isotope because it tends to drift for long distances on the wind. But in the event of a nuclear accident, people who may be exposed to iodine 131 can take potassium iodide tablets, which provide enough non-radioactive iodine 127 to saturate the thyroid. The body, no longer needing iodine, will simply let any radioactive iodine it may encounter pass through harmlessly.
Children evacuated from the area around Fukushima Daiichi are given potassium iodide pills to protect against the absorption of radioactive iodine 131 on March 14.
* * *
As the reactors melted down at Fukushima Daiichi, workers in and around the plant took regular readings to determine how much radioactivity had escaped. They weren’t the only ones.
Twenty-five years earlier, another reactor had melted down near Chernobyl, Ukraine. Cows living near the Chernobyl plant had grazed on grass covered in iodine 131. Children, who are particularly vulnerable to the effects of iodine 131, had drunk the cows’ milk. Many later developed thyroid cancer.
The contamination had reached far beyond the area surrounding the Chernobyl reactor; radioactive isotopes had also spread across Europe. Cesium 137 rained down more than a thousand miles away in Scotland, where it covered fields and wound up being concentrated in the flesh of livestock.
Four years after the Chernobyl disaster, the International Atomic Energy Agency (IAEA) developed a scale called the International Nuclear and Radiological Event Scale (INES) to rate the impact of nuclear accidents. Much like the moment magnitude scale, each number on the INES scale, which runs from 1 to 7, indicates an impact that is ten times worse than the number before it. A nuclear accident that occurred at Three Mile Island in Pennsylvania in 1979 was classified after the fact as a level 5. At the time of the Fukushima events, Chernobyl was the only accident to ever rate a level 7.
Now scientists from the IAEA were carefully monitoring radiation readings in the areas around Fukushima. They feared that Fukushima might become a repeat of the accident at Chernobyl. Radiation readings from the USS Ronald Reagan confirmed that radioactive isotopes had already blown miles out to sea.
When large amounts of radioactive isotopes are released into the air, as they were during the explosion at reactor 1, they can form a radioactive cloud of isotopes mixed with air, smoke, water, and other particles from the explosion. Like rain clouds, radiation clouds are driven by the weather. The same air currents that blow storms across the ocean and onto land push radioactive gases, too. The Comprehensive Nuclear-Test-Ban Treaty Organization, which monitors releases of radioactive isotopes around the world, was also watching the unfolding disaster on the eastern coast of Japan. The group created a weather-based map that showed the likely path of the radioactive plume from Fukushima swirling across the North Pacific and sweeping onto the west coast of the United States.
* * *
Operato
rs at the Fukushima Daiichi plant didn’t have time to think about the radiation drifting over the Pacific. They were caught in a seemingly endless cycle of emergencies. It had been three days since the tsunami, and conditions at the plant continued to grow worse. Hydrogen gas had been leaking into the unit 3 building since the day before. They had managed to vent the reactor vessel four times after the fuel began melting. They knew that if the gas built up, the containment building was likely to explode, just as unit 1 had. But an explosion in unit 3 would be much worse. The building had reinforced concrete walls on the upper floor, making it a sturdier structure that would be able to hold more gas before the system failed. The stronger structure might buy them more time, but it also meant that an even larger cloud of flammable gas was building up.
Fears of another explosion made it difficult to work. Supervisors had to balance the need to save the reactor against the safety of their workers. “We alternated between deploying and pulling back workers, because we were afraid of another hydrogen explosion,” Takeyuki Inagaki remembered.
On the morning of the 14th, they brainstormed ways to open up the unit 3 building to let out hydrogen steam. But they were too late. At 11:01 A.M., a hydrogen explosion bigger and more violent than the one they had experienced two days earlier rocked unit 3. The blast sent a cloud of black smoke, thick with concrete dust, hundreds of feet into the air. Massive hunks of concrete rained down around the building.
The explosion in the unit 3 building completely destroyed its concrete walls.
About fifty workers who were near the building scrambled for cover. The members of a team that had been working inside the building at unit 2 were shocked, when they came out, to discover that the car they had used to reach the building had been blown away by the force of the blast. Chunks of the unit 3 building littered roads around the complex, making it even harder to get around.
When Inagaki remembered the moment years later for a television interview, tears filled his eyes. “Since there were so many people out there, I was really afraid for their safety. I thought to myself, It’s very possible someone was killed,” he said. “Then, one by one, people started to trickle back. They were all very pale in the face and some were bleeding.” Eleven workers were injured in the explosion, but all of them had survived.
The supervisors’ relief was short-lived. After accounting for the crew that had been outside during the explosion, they turned back to their instruments and discovered that the water level in unit 2 had started to go down. The disastrous meltdown cycle was about to begin again.
At this point, the workers had been struggling relentlessly for days, with very little sleep. One worker later remembered, “What was happening was beyond what we trained for on a daily basis. Using what little information we had, we had to decide immediately what we’d do … It was a race against time.”
Takeyuki Inagaki put it this way: “From the fourteenth to early in the morning on the fifteenth … it was like being in hell.”
Every task was critical, the stakes life-and-death. And despite their best efforts and the endless hours of work, they were losing the race. The reactors were melting down, one by one.
The explosion in unit 3 had badly shaken the workers, many of whom had barely escaped with their lives. Now Superintendent Yoshida had to beg them to go back into danger to try to save unit 2.
Once all of the heads been counted and they were sure no one had been killed in the explosion, Yoshida sent workers who had been injured in the blast to another site where they could be treated. Then he gathered the remaining crew together. “Everybody was in a daze and could hardly think,” he later remembered. He called the workers together and took responsibility for having sent them into the line of fire, saying his judgment was to blame. Then he made a difficult request. He asked workers to go back out onto the grounds and clear away the highly radioactive rubble that surrounded Reactor 2.
Of the three reactors that had been operating when the tsunami hit, unit 2 was the only one that was still intact. It had been functioning pretty well, but around noon on March 14, that had begun to change. Just as it had in reactors 1 and 3, the water in reactor 2 boiled off and pressure in the reactor vessel began to climb. But while they had eventually managed to vent some steam from the other two reactors, operators were unable to vent unit 2 at all. And that meant that they couldn’t get any water in.
“We had come to a situation where [nuclear] fuel was really exposed, but we could not lower pressure or pump in water,” Yoshida remembered. “I thought then, though not for the first time, that we were going to die. I thought we were really going to die. With no water coming in, the number 2 reactor was going to melt. All fuel was going to really override pressure in the containment vessel and escape outside. That would have been a worst-case accident.” Yoshida knew that a meltdown of that magnitude would be catastrophic. If the containment layers at unit 2 were destroyed, it would be too dangerous for workers to continue pumping water into units 1 and 3. All three of the reactors would be completely out of control.
The water in reactor 2 fell below the tops of the fuel rods by 5:00 P.M. By 7:20, the core began to melt.
The race was over. All three of the active reactors at Fukushima Daiichi had melted down.
DAY 5
fukushima 50
Tuesday, March 15, 2011
Reactor Status
Reactor 1: Melted down, building destroyed
Reactor 2: Melted down
Reactor 3: Melted down, building destroyed
Reactor 4: Building destroyed
Reactor 5: Shut down
Reactor 6: Shut down
The three active reactors at Fukushima had broken down, one after another, following the same pattern. Now that unit 2 was clearly in trouble, operators figured it was only a matter of time before the building surrounding it, like the ones that housed units 1 and 3, exploded. So they weren’t overly surprised when, at 6:12 on Tuesday morning, a loud rumble shook the control room.
Everyone in the emergency response center assumed there had been a hydrogen gas explosion in the unit 2 building. But the explosion was actually in an entirely unexpected place: the unit 4 building.
As it turned out, unit 2 had caught a lucky break when unit 1 had blown. The force of that blast had knocked a square panel from unit 2’s exterior, creating a vent for the hydrogen gas building up inside. A cloud of white steam had been pouring from the side of the building since then, preventing the buildup that had been so disastrous in the other two buildings.
The leak may have saved unit 2 from explosion, but it was also releasing radioactivity into the air. And in the early hours of Tuesday morning, the favorable winds that had been sweeping the radioactive gases out to sea had shifted, carrying radioactive cesium inland.
A man named Toru Anzai, who lived in the town of Iitate, a full 25 miles northwest of the Fukushima plant, later remembered the explosion at reactor 4. “I heard the sound of the explosion, and the air turned hazy and rust-red. There was also a metallic burning smell, and even indoors my face and exposed skin started to sting. The radiation was very high around that time. My legs felt as though they were sunburned.” That night, a radiation monitor at the village hall in Iitate registered 44.7 mSv per hour. Remember, the legal limit for radiation exposure for a worker in emergency situations had just been raised from 100 to 250 mSv per year. With radiation levels at 44.7 mSv per hour, people would reach the emergency exposure maximum in less than six hours.
A radiation-monitoring car parked at the plant’s gates, two-thirds of a mile from reactor 4, had been taking readings regularly since March 11. On the morning of March 15, radiation levels spiked to 12 mSv per hour. At that rate, workers would exceed 250 mSv of exposure in less than a day. It was no longer safe for workers to stay in the control rooms near the reactors—or anywhere outside the emergency response center.
The emergency response team debated whether to evacuate the plant. Yoshida told his crew to prepare vehic
les in case an evacuation was necessary, and to find temporary shelter on the grounds of Fukushima Daiichi. But in the confusion of the moment his command was garbled, and nearly 650 workers clambered onto buses and immediately evacuated to Fukushima Daini, about 6 miles away. Superintendent Yoshida and a team of about seventy supervisors stayed behind to keep fighting.
For Yoshida, abandoning the premises was not an option. Leaving the plant without any workers would cause a major catastrophe—one far worse than the three explosions that had already happened. But that left him with the grim task of determining which employees were essential and asking them to stay. “It was like deciding who would die with me,” he later recalled. “The faces of my team appeared before me one after another … I couldn’t bear the idea that these people I had known for years might die on my orders,” Yoshida recalled, “but I knew that our only hope was to keep injecting water. I had no choice. I had to ask them to prepare for the worst.”
The plant workers who stayed were driven by a sense of duty. Nuclear engineer Atsufumi Yoshizawa had been working from a disaster-response station about 3 miles from the plant. When he and some of his coworkers volunteered to return to Fukushima Daiichi, they filed past a line of firefighters, policemen, and other employees who saluted them. “We felt like members of the Tokkotai,” he later said, referring to Japanese kamikaze pilots during World War II. “The people lined up outside never said as much, but I could tell by their expressions that they didn’t think we would return.”
Meltdown: Earthquake, Tsunami, and Nuclear Disaster in Fukushima Page 7