Nuclear Meltdown Disaster

PBS Airdate: July 29, 2015

NARRATOR: Japan's most powerful earthquake ever triggers a monster tsunami.

CROWDS (Translation): Many huge waves are coming!

NARRATOR: Fear washes over the nation.

CROWDS (Translation): Hurry up, it's coming!

NARRATOR: But that's just the beginning. Ten nuclear reactors, at two power plants, are crippled, threatening the unimaginable.

NAOTO KAN (Former Prime Minister of Japan): If Tokyo needed to be evacuated, I feared the entire nation of Japan would be paralyzed by chaos.

TAKEYUKI INAGAKI (Tokyo Electric Power Company): It became darker and darker, a terrifying situation. We were fighting an invisible enemy: out of control reactors.

NARRATOR: What will it take to save the country from radiation?

FIREFIGHTER (Translation): Two-hundred-twenty millisieverts. I'm getting 100 millisieverts here, 100 millisieverts!

Everyone not working take cover inside the truck!

CHUCK CASTO (Former Nuclear Regulatory Commission Executive): Mankind has never faced the forces of physics and the forces of nature that those people faced.

NARRATOR: Nuclear Meltdown Disaster, right now, on NOVA.

This is the road to nowhere, a once-thriving place in one of the most prosperous countries on Earth, Japan—radioactive Japan.

Time stood still here, on March 11, 2011.

Houses that aren't homes; schools that are silent; stores shuttered; towns without people; past the checkpoints, the scan and the meticulous suit-up—layer upon layer upon layer of protection—is the place we simply know as Fukushima,…

DAIICHI OPERATOR (Translation): Please be safe.

NARRATOR: …site of three nuclear reactor meltdowns.

DAIICHI OPERATOR: This is called the Central Control Room. All of the equipment at the power plant is operated from here.

NARRATOR: He is a nuclear plant operator. This is where he has always worked. He used to live nearby, but now he, too, cannot go home.

He and his co-workers are ashamed, scorned by the neighbors they once had.

DAIICHI OPERATOR: We've been through so much in this control room. It's hard to put into words.

NARRATOR: He was here when it happened. Now, he's hoping to make amends, by helping to clean up the toxic mess.

DAIICHI OPERATOR: Four years ago, this room was completely dark. We had only small fluorescent lights and flashlights. We had given up on our own survival. Now, talking to you with the lights on, it seems like a lifetime ago.

NARRATOR: This is the story of the Fukushima meltdowns, the infamous events at Fukushima Daiichi, or Number One, told by those who were there and risked everything,…

FIREFIGHTER (Translation) : Eighteen millisieverts.

NARRATOR: …and the lesser known story of its sister plant, seven miles away: Fukushima Daini, or Number Two. It faced the same onslaught and challenges, but thanks to a little luck and a herculean effort, it was saved from ending up like this.

March 11: a bad day for Japan and the world could have been so much worse.

SELF-DEFENSE FORCE SOLDIER (Translation): I can see a huge tsunami heading this way!

NARRATOR: The inevitable came, without warning, at 2:46 p.m. Two giant pieces of the earth's crust, tectonic plates, move suddenly and violently. One pushes down, causing the plate above it to spring upward, like a catapult, along a 300-mile fault line. In a nation all too familiar with earthquakes, it is the largest ever recorded, magnitude 9.

Propagating outward, at 9,000 miles an hour, record-breaking seismic waves careen toward land and the 10 nuclear reactors at the two Fukushima plants, all of them designed by General Electric, owned and operated by the largest utility in Japan, the Tokyo Electric Power Company, TEPCO.

They are there to feed the insatiable energy needs of Tokyo.

TAKEYUKI INAGAKI: Suddenly, I heard the earth rumble, like a fierce growl. It was an extremely intense earthquake. But it wasn't only strong, it was also terribly long.

NARRATOR: An American nuclear reactor service technician, Carl Pillitteri is there doing some upgrades.

CARL PILLITTERI (Nuclear Reactor Technician): It was just one big hammer. The entire building was moving, everything was coming down. The lights were crashing everywhere, and it just got worse from there actually. In one nanosecond, just, the entire floor went black.

DAIICHI OPERATOR: The shaking was like nothing I had experienced. The operators either held onto that bar or crouched down. That's how we endured the earthquake.

TAKEYUKI INAGAKI: I just wondered, “How long is this going to continue?”

NARRATOR: It lasts six minutes.

Takeyuki Inagaki is General Manager of the maintenance department for Units 1 through 4 at Daiichi. He reports directly to the superintendent, Masao Yoshida. Both men have spent their entire careers at TEPCO.

Inagaki worries about his wife and two sons.

TAKEYUKI INAGAKI: When the earthquake hit, I think I sent one email to my wife explaining that I didn't think I'd be able to come home for a while. After I sent it, we lost contact with the outside. We couldn't even make calls.

NARRATOR: Seismometers at nuclear plants are designed to trigger an automatic emergency response after an earthquake. Power plant operators routinely drill for this, but it is still a risky, tense event. Called a “scram,” it is designed to put the brakes on the nuclear chain reaction of sustained fission.

A nuclear reactor is fueled by uranium, an element that naturally splits apart, releasing neutrons. But uranium fission can induce more fission. When a loose neutron fires into a nearby uranium nucleus, it becomes unstable and quickly splits.

Each time an atom splits, it generates heat. To make fission robust enough to generate power, uranium is enriched, shaped into pellets and then stacked in long tubes called fuel rods. This ensures lots of uranium atoms are close enough to each other to allow a healthy chain reaction.

To manage the rate of the reaction, control rods that absorb neutrons are moved in and out of spaces among the fuel. During a scram, the control rods are pushed all the way in, terminating the chain reaction.

By 3:02, operators at Daiichi confirm the three reactors that were online have successfully scrammed. And at Fukushima Daini, all four reactors, running full throttle, also automatically shut down safely.

Even though the nuclear chain reaction has now abruptly stopped, the uranium fuel rods remain very hot. It's called decay heat.

DAVID LOCHBAUM (Union of Concerned Scientists): The nuclear chain reaction was stopped within seconds, but the decay heat continued to be a problem. It takes approximately 20 to 24 hours for the systems to cool the reactor down to less than 212 degrees and achieve what is called “cold shutdown.”

NARRATOR: At its core, a nuclear power plant is not unlike a pressure cooker on a stove. Water is heated to the boiling point creating steam, and steam under pressure turns turbines, generating electricity.

The complexity and cost of nuclear energy is about keeping the radioactive genie in the bottle. So, the uranium fuel sits inside rods, under water, in a steel pressure vessel, surrounded by a concrete and steel containment structure, inside a reactor building.

All those layers of protection are there, in case the water stops flowing. If that happens, it quickly boils away exposing the fuel, and it melts, turning into radioactive magma.

Unchecked, it will melt right through the steel reactor vessel and onto the concrete and steel containment structure that surrounds it.

So at the Fukushima plants, now, it is critical that the electric water pumps and valves keep running for the reactor core to safely cool down. The power normally comes from the electric grid, and the earthquake has caused a blackout. But there is a plan B. The plant is equipped with diesel electric generators to provide power in an emergency. They start automatically, as they are designed to do.

DAVE LOCHBAUM: If it'd only been the earthquake, we wouldn't be here today. The safety systems were doing their thing, the reactor cores were being cooled and things were going pretty well.

NARRATOR: When the seafloor suddenly moves upward, during the earthquake, it causes the water near the epicenter to rise with it. The giant swell is the start of a tsunami. Forecasters issue a tsunami warning. They predict 10-foot-high waves in Fukushima Prefecture.

The main buildings at Daiichi and Daini are about 30 feet above sea level, so no one worries much about a tsunami.

SELF-DEFENSE FORCE PILOT (Translation): Three, four waves or more. Many huge waves are coming.

NARRATOR: And then, at 3:27 p.m., the first of seven giant waves crashes over the seawall.

CARL PILLITTERI: Then I saw the tsunami coming. I was on top of that hill, and I wondered if I was high enough.

CROWDS (Translation): Hurry up, it's coming!

The tsunami is going to catch you.

CARL PILLITTERI: This thing was, you know, it was huge.

NARRATOR: The tallest surge of water is nearly 50 feet high, more than twice the height of the seawall.

DAIICHI OPERATOR: I assumed there would be a tsunami, but not one 50 or 55 feet high. I couldn't even imagine a tsunami like that.

NARRATOR: Fukushima Daiichi is inundated. Two workers drown, trapped in the basement of the Number 4 turbine building. Six generators, along with the wiring, switches and breakers connecting them to the plant, are located in the basements of the turbine buildings for Units 1 through 4.

The floodwaters ruin them all.

Two additional generators behind Unit 4 are high and dry, but their switching gear is in the basement, ruined by seawater. The generators would be useless.

Meanwhile, seven miles to the south, at the other Fukushima plant, Daini or Number Two, the crisis is equally dire. The big waves roll in, swamping key motors, wiring, pumps and generators.

NAOHIRO MASUDA (Tokyo Electric Power Company): Immediately after the tsunami hit, this room lost power. That's when I realized something very serious had happened.

NARRATOR: Naohiro Masuda is the plant superintendent on that fateful day. He began his career here at Daini, reporting for duty in 1982, while it was still being built. He knows it as well as anyone.

NAOHIRO MASUDA: This building is 40 feet above sea level. And since there was a power outage here, I imagined most of the buildings near the sea were damaged by the tsunami. From that point on, we knew we were in trouble.

NARRATOR: Operators in the control rooms give Masuda very bad news: Units 1, 2 and 4 no longer have any operative cooling systems.

NAOHIRO MASUDA: In nuclear power, we say, “Stop, Cool and Contain.” These are the three most important functions. When I realized that we had lost cooling, I knew the situation was extremely serious.

NARRATOR: But what needs to be fixed? And how? He orders a team of about 40 workers to go out and inspect the damage.

NAOHIRO MASUDA: There were several hundred aftershocks that day. I really hesitated to give the order to send people out on site.

NARRATOR: They arrive in the rooms housing electric motors that run the pumps that draw seawater to cool the reactors.

TAKAKI MISHIMA (Tokyo Electric Power Company): Cars had been washed right up to the door, and there were huge piles of debris. They said, “How could you have sent us to such a place?”

NARRATOR: Today, they have preserved some of the evidence of the flood that wiped out the motors and their wiring.

NAOHIRO MASUDA: Electrical supply systems are located in this room. The tsunami came up to here.

NARRATOR: Back at Daiichi, Reactors 1 through 4 are now in total darkness, and things are quickly spiraling out of control.

DAIICHI OPERATOR: We were in the middle of dealing with the accident, then, all at once, the lamps went out.

All sorts of alarms were going off. All of those went out, basically, a “station blackout.”

NARRATOR: “Station blackout,” the two most dreaded words in the world of nuclear power generation. At Fukushima Daiichi, Reactors 1 and 2 not only lose the alternating current that powers the pumps and valves, but also the direct current, there to keep the instruments working.

TAKEYUKI INAGAKI: It became darker and darker, a terrifying situation, and the operators weren't sure what was happening. We couldn't even tell if there was water in the nuclear reactors.

MASATOSHI FUKURA (Tokyo Electric Power Company): We were at the starting line of a race, but we didn't know if we'd need to run 100 yards or if we'd need to run a marathon.

NARRATOR: The oldest reactor at the site is Unit 1. It began operation in 1971 and is equipped with outmoded, last-resort cooling systems, called isolation condensers.

DAVE LOCHBAUM: The steam being produced by the hot reactor core was routed into this large tank of water. The water would cool the steam, convert it back into water, and it would drain back into the reactor core, to be recycled over and over again.

NARRATOR: It is a passive system, designed to work on natural circulation, without any electrical power at all. So is it working? In the darkened control room, they don't have a clue.

What they did not know at the time is when the power fails, the isolation condenser valves are almost completely closed, curtailing the flow of steam and water.

DAVE LOCHBAUM: When the isolation condensers failed on Unit 1, the reactor water level just started boiling away, because it was no longer being cooled and maintained by the isolation condensers.

NARRATOR: If the isolation condensers are working properly, vents on the side of the reactor building would be releasing huge amounts of steam.

CHUCK CASTO: There was steam coming out of the discharge outlet, but they didn't realize it wasn't sufficient. They thought they had more time than they did.

NARRATOR: Now, there is nothing to stop the meltdown.

At 3:42 p.m., 15 minutes after the tsunami waves rolled in, TEPCO notifies the national and local governments that there is a “special event” at the plant. Actually, it is an emergency that has never been envisioned.

MASATOSHI FUKURA: It was a situation that hadn't been anticipated. We couldn't use the procedure manuals.

DAIICHI OPERATOR: 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 can we do? What should we do?”

It was a race against time.

NARRATOR: They desperately need power, but from where?

TEPCO dispatches generator trucks, but they are slowed by earthquake and tsunami wreckage. When they finally arrive, late on the 11th, damage to the plant's electrical system makes it all but impossible to connect them. So they must improvise, hoping to jury rig their dead instruments back to life by raiding the parking lots, grabbing as many batteries as they can from buses and cars.

TAKEYUKI INAGAKI: It took several hours to figure out how to connect the batteries. They had the schematic wiring diagram and they were furiously examining it for several hours. We ended up wasting a lot of time.

NARRATOR: Meanwhile, the water is boiling away fast in Unit 1. But how fast?

Six hours after the tsunami, at 9:19 p.m., they finally get the batteries and some portable generators rigged, so the instruments can flicker on. The gauges show the water level in Reactor 1, normally 20 feet above the top of the uranium fuel, is now only eight inches above it. They pencil in a running record of the water levels, right beside the gauge.

But the pressure in the vessel is so high, it causes inaccurate instrument readings. They learn later the uranium fuel had actually been exposed for three hours.

DAVE LOCHBAUM: It's becoming more like a lava flow from a volcano. So it overheated, melted, released radioactive contents and started falling into the bottom of the reactor vessel.

NARRATOR: In Tokyo, Japan's Prime Minister Naoto Kan worries that he might have to order a mass evacuation from Fukushima all the way to Tokyo, 50 million people.

NAOTO KAN: If Tokyo had to be evacuated, I feared the entire nation of Japan would be paralyzed by chaos for quite a long time.

NARRATOR: At 9:23 p.m., he orders everyone living within two miles of Daiichi to evacuate immediately, so TEPCO can prepare to vent some steam, radioactive steam. At this stage, radioactive Iodine 131 is the isotope of greatest concern. It is linked to thyroid cancer. Children are most vulnerable.

TAKEYUKI INAGAKI: We knew we had to vent, but the question was how. There was no electricity, and the valve was pneumatic. We needed an air compressor to open it, but we didn't even have that.

NARRATOR: Kan becomes increasingly frustrated and impatient.

NAOTO KAN: Even though we approved it, many hours went by, and they still had not vented. We asked them why. A specialist from TEPCO told me, “I don't know the reason.”

CHUCK CASTO: Much of the information he was getting from his government and the utility turned out not to be true, and he had no source of independent knowledge of what was going on in those reactors.

NARRATOR: Kan decides the only way to know for sure is to go there. And so, early on the morning of March 12, he flies to Fukushima Daiichi.

He passes over mile upon mile of utter devastation from the earthquake and tsunami. Nearly 16,000 are dead.

He lands at Daiichi just after 7, about 15 hours after the tsunami.

NAOTO KAN: I met with Superintendent Yoshida for 45 minutes. While he explained the situation on site, I could see that he was a person who could be trusted.

NARRATOR: The Prime Minister endorses the superintendent and his plan. Yoshida vows to begin venting at 9 a.m. But there is much to do and much to consider.

TAKEYUKI INAGAKI: To go into a pitch-black reactor building with the containment vessel pressure so high, I don't know if I should say it, but it felt like we were putting together a “suicide squad.”

NARRATOR: To open the vent, they need to manually turn two valves, one in the basement and another on the second floor.

At 9:04 a.m., a pair of workers makes their way through a dark labyrinth to the second floor of the Reactor 1 building.

It takes them 11 minutes to open the valve. Nine minutes later, a second team heads for the valve in the basement.

DAVE LOCHBAUM: They get to a point about midway, and the radiation is higher than they thought they'd get. They basically had drawn, “If radiation gets to this point, we'll go back.” Well, they got to that point before they got to the valves.

NARRATOR: They abort their mission.

In the Emergency Response Center, they realize they must find a way to open the vent remotely. They scramble to find an air compressor that can be attached to the pipe that blasts the valve open.

Finally, at 2:50 p.m., steam starts rising from the exhaust tower, and the pressure starts dropping. Could the worst be over?

Actually, it is just about to begin.

At Daini, where the power is still out and the reactors are getting hotter, there is a stroke of luck. There is power inside the radiation waste building behind Reactor 1, but they need it down by the water.

Naohiro Masuda decides to lay cables, hoping to restore cooling to the reactors.

Each of the four reactors needs three operative pump motors. They need to lay five-and-a-half miles of cable to connect them all.

He needs supplies urgently, at least 50 big spools of heavy duty cable. Masuda orders the cable, but the shipment is delayed.

YUTAKA MIAZAWA (Tokyo Electric Power Company): The police were busy redirecting traffic for evacuations, and when the truck hit those detours, it wound up going in the completely wrong direction.

NARRATOR: While they wait at Daini, the crisis is getting much worse at Daiichi.

Uranium fuel rods are encased in zirconium.

DAVE LOCHBAUM: If it gets too warm, there's a chemical reaction between the zirconium and the water or steam, to produce large amounts of hydrogen.

NARRATOR: The containment structure on a boiling water reactor is sealed with a dome shaped top that is removed for refueling. The pressure in Reactor 1 is now so high that it slightly lifts the top. The hydrogen escapes through the gap and into the reactor building where it mixes with air. It would be just a matter of time before the highly flammable gas would explode.

It is 3:36 p.m.

DAIICHI OPERATOR: Suddenly there was an upward thrust, an impact that seemed to push the whole building upward, and it became completely dark.

TAKEYUKI INAGAKI: I thought it was just another aftershock, but we all sensed something was very wrong.

DAIICHI OPERATOR: Then headquarters told me that the top of the reactor building was completely destroyed. When I heard that, I was shocked.

NARRATOR: It is a devastating setback.

TAKEYUKI INAGAKI: I think that most us who were working at the time didn't feel like we were going to make it out alive.

NARRATOR: At Daini, workers are preparing to lay the heavy cables, when they get word of the explosion at Daiichi.

NAOHIRO MASUDA: After the explosion, I had everyone outside come back into the Emergency Response Room.

There were 500 to 600 people in the room. I said, “Please, trust me. I definitely won't do anything to harm you, but Fukushima Daini is still in trouble, and I need you to do your best.”

NARRATOR: The cables arrive on the morning of the 13th. Finally, the heavy lifting to save Fukushima Daini begins.

YUTAKA MIYAZAWA: These were large capacity motors, several hundred amperes, so the cables were fairly thick. Each person had to walk while carrying around 35 pounds of cable.

NARRATOR: It is a race against time and physics.

TAKAKI MISHIMA: Normally, if you wanted to lay that much cable, it would take you about a month. I didn't actually think it was possible in the amount of time that we had.

YUTAKA MIYAZAWA: A total of around 200 workers were involved. We swapped people out when they got exhausted.

NARRATOR: In the midst of this, Masuda is running out of fresh water to cool his crippled reactors. He asks TEPCO headquarters for a shipment.

NAOHIRO MASUDA: I asked Tokyo for 4,000 tons of water, but instead they delivered 4,000 liters of bottled drinking water. That made me realize that we were on our own. We couldn't count on Tokyo. So we started looking for water.

NARRATOR: He remembers a creek used as a water supply during construction of the plant. Workers repair the leaky old pipe with a scavenged bicycle tube.

At Daiichi, events are still overtaking them. As the dust settles at Unit 1, they learn five workers are injured. With freshwater reservoirs exhausted, they scramble to inject seawater into the reactors. By 7:04 p.m., success.

But using corrosive seawater means the reactors will surely be destroyed. TEPCO headquarters tells Superintendent Yoshida to stop while they seek government approval.

DAVE LOCHBAUM: So, Yoshida does this very dramatic thing, where, on the video conference, he orders the people to stop the seawater injection. Before that, he called them over and said, “I'm going to order you to stop the seawater injection. Ignore that. That's just for Tokyo. You continue seawater injection.”

NAOTO KAN: Personally, I think the decision that Superintendent Yoshida made was the right one.

MASATOSHI FUKURA: If we had stopped the seawater injection at that point, I think things would have been much worse.

TAKEYUKI INAGAKI: In the meantime, Unit 3 was becoming unstable. Now our mission was, “Don't let Unit 3 turn into Unit 1.”

NARRATOR: With no battery power, they are unable to open the valves to begin venting Unit 3. And there are no handles on these valves. So, they grab car batteries hoping to open them from the control room.

By 9 a.m., Sunday, March 13, the Unit 3 reactor core is exposed and melting. The pressure keeps rising.

Fearing another hydrogen explosion, Yoshida orders workers to retreat to the Emergency Response Center, early on the morning of March 14. But the pressure plateaus and he lifts the order an hour later.

TAKEYUKI INAGAKI: We alternated between deploying and pulling back workers, because we were afraid of another hydrogen explosion. Unfortunately, we had close to 50 people positioned around Unit 3 when the explosion happened.

NARRATOR: It is 11:01, March 14.

TAKEYUKI INAGAKI: 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.” Then, one by one, people started to trickle back. They were all very pale in the face and some were bleeding.

NARRATOR: When Unit 3 explodes, 11 workers are injured. Amazingly, no one is killed.

TAKEYUKI INAGAKI: And when we finally accounted for everyone, that's when we noticed the water level in Unit 2 was dropping.

NARRATOR: It seems inevitable that Unit 2 will be the next to blow.

At noon, on March 14, an hour after the explosion at Unit 3, the water covering the hot radioactive fuel begins to drop precipitously. An hour-and-a-half later, its emergency backup cooling system fails.

Superintendent Yoshida quietly tells a few trusted workers to prepare buses for an evacuation. For now he has no choice but to order his men back into harm's way.

TAKEYUKI INAGAKI: After the explosion of Unit 3, he begged us to again, go to the field to save Unit 2. It was very impressive.

NARRATOR: But by 6:22 p.m., the water is gone. The uranium fuel is completely uncovered and melting. Again, they try to use car batteries to open the vents and relieve some pressure; no luck.

TAKEYUKI INAGAKI: From the 14th until early in the morning on the 15th, it was really—how can I describe it? —it was like being in hell.

NARRATOR: As day breaks, on the morning of March 15th, they hear a loud explosion.

But it is not what they dread. It is a complete surprise. There has been an explosion in Unit 4. But this reactor was shut down for maintenance when the tsunami hit. What could have happened there? And what saved Unit 2 from blowing up?

The investigation will have to wait. Superintendent Yoshida orders an immediate evacuation of 650 workers. He and nearly 70 supervisors would stay. In the confusion, they become known as the “Fukushima 50.”

TAKEYUKI INAGAKI: We were fighting an invisible enemy: out of control reactors. It was like fighting a war.

NARRATOR: It is midnight on the 13th. In the Daini control room, operators are girding for what seems inevitable: venting radioactive steam into the environment. Then, the cables and the motors finally start falling into place, and the mood shifts dramatically.

NAOHIRO MASUDA: When we got word that they were finished running the cable, there was applause. Then they tested the motors and reported “The motors are working!” And there was more applause. When the pumps were finally working for Units 1, 2 and 4, there were three bursts of applause, one for each.

TAKAKI MISHIMA: It was such a rush! I can't believe they did it. What a team they are, they pulled off something incredible. I thought “They did it!”

YUTAKA MIYAZAWA: The pressure in Unit 1 had risen to just under the containment vessel's pressure limit. We had about two hours to spare. I'd say we made it by the skin of our teeth.

NARRATOR: At Daiichi, Superintendent Yoshida has now lifted the evacuation order, and workers that had fled start to return.

TAKEYUKI INAGAKI: Things had bottomed out, and for the first time we were able to catch our breath. Then we started to worry about cooling the fuel pools.

NARRATOR: The pools are high up in the reactor buildings, there to store both spent and new uranium fuel. There are more than 3,100 fuel bundles, millions of uranium pellets in the pools for Units 1 through 4.

They are not inside the containment vessels, and there are no emergency pumps to keep them full. Should the pools go dry, the rods could overheat and catch fire in the open air, releasing a huge amount of radiation.

This is the worst-case scenario that has haunted Prime Minister Kan. It could force him to order a mandatory evacuation of everyone for 150 miles or more. Tokyo uninhabitable, maybe for decades? It is hard to fathom.

Nuclear plant operator Chuck Casto, then an N.R.C. executive, arrives at Fukushima Daiichi in the midst of this turmoil and uncertainty.

CHUCK CASTO: The biggest challenge, right away, as soon as I stepped foot on the ground, was should people take lethal doses to stop this accident? The situation was desperate.

(Translation): That's Unit 2.

Unit 1.

Concerns we had never faced before…

You can see the steam coming up in Unit 3.

That's the worst explosion that they had.

…and we were trying to work our way through them the best we could.

NARRATOR: It is time for desperate measures. On the morning of March 17th, Self Defense Forces helicopters fly four daring missions, hoping to dump seawater onto the Unit 3 spent fuel pool, which appears to be boiling away.

Radiation forces them to fly high above the plant, too high.

TAKEYUKI INAGAKI: In reality, barely any of the water got into the pool. And tensions were still running high.

NARRATOR: So they turn to an elite rescue squad from the Tokyo Fire Department.

FIREFIGHTER (Translation): The evacuation of the 3rd floor is complete.

NARRATOR: They plan to use equipment designed to fight high-rise fires.

They arrive at the plant late the next night.

FIREFIGHTER #1 (Translation): Should I get the lights ready?

NARRATOR: They are led by Deputy Chief Yukio Takayama, a 35-year veteran.

FIREFIGHTER #2 (Translation): I'll ride in this vehicle and lead the way. It's the way to the ocean.

YUKIO TAKAYAMA (Tokyo Fire Department): The plan was to get water into Unit 3 by any means necessary. It needed to be dead-on, not like when you put out a fire and spray the water anywhere you want.

FIREFIGHTER #4 (Translation): Current reading, point-4 millisieverts.

YUKIO TAKAYAMA: Two thoughts kept running through my mind: “Please be over soon,” and, “What will I do if this place explodes?”

FIREFIGHTER #5 (Translation): There's an open manhole to the left!

FIREFIGHTER #6 (Translation): That manhole, it's on the left?

FIREFIGHTER #5 (Translation): Yes, on the left!

FIREFIGHTER #6 (Translation): One hundred millisieverts here, 100 millisieverts!

Anyone not working take cover inside the truck!

YUKIO TAKAYAMA: I had never felt that kind of fear before. I thought, “This is what it feels like to really be in trouble.”

NARRATOR: They get the job done in 20 minutes.

Day breaks. The water is now flowing. The fuel in storage is never exposed to the air, and the feared radiation is not released. Tokyo is saved.

TAKEYUKI INAGAKI: Little by little, in a small way, we started to have some hope. Up until then, we were spiraling further down, and now we were dangling there. We weren't falling anymore.

NARRATOR: For the first time in days, Takeyuki Inagaki finds the time and a working phone to call home.

TAKEYUKI INAGAKI: My wife asked me, “Are you okay?” I could tell she was very emotional from her voice. I said, “I'm alive for now, and I have all of my limbs. Please take care.”

NARRATOR: In the days and weeks ahead, the nightmare does not end, but at least it gets no worse. Concrete pump trucks unleash steady torrents of seawater onto the fuel pools.

Power is fully restored to the plant, finally, on March 21st.

By June, they install a complex filtration system to remove cesium from the water washing through the radioactive debris and flowing into the Pacific.

And, slowly, the answers start trickling in. Why did Unit 4 explode? It used the same vent stack as Unit 3. When hydrogen built up there, it seeped into Unit 4 via a shared duct.

And why didn't Unit 2 blow up, as they feared? They realize the explosion at Unit 1 knocked out a door near the top of the Unit 2 reactor building. It allowed hydrogen and radiation to escape Unit 2. While it was venting, the wind shifted toward land, sending the highest concentrations of cesium to the northwest. The fallout will linger for decades.

Eight months after the earthquake and tsunami, reporters tour Fukushima Daiichi. Superintendent Masao Yoshida tries to downplay the worry.

MASAO YOSHIDA (Tokyo Electric Power Company): The plant is stable enough for local residents to have peace of mind. However, it's still very difficult to work under the current conditions.

NARRATOR: Yoshida had been reprimanded by his TEPCO superiors for disobeying orders and injecting seawater during the worst of the crisis.

What about his workers? Six that ventured into the reactor buildings, trying to open the vent valves, got the worst doses: as much as 678 millisieverts of radiation. Five-thousand millisieverts is considered lethal, and 250 is the maximum normally allowed for nuclear plant workers in an emergency.

The cancer risk for those six Daiichi workers is undoubtedly greater in the long run, but during that fateful week, they all believed there was no long run for them.

MASAO YOSHIDA: To be blunt, there were a number of times that I thought I would probably die. We couldn't predict anything. The worst-case scenario for the meltdown was that it would get out of control. I felt that was possible, and so, I thought, “Maybe this is the end.”

NARRATOR: One month later, the hot melted cores finally drop below the boiling point of water: cold shutdown.

By then, Yoshida is gone. He has cancer, unrelated to his job. He dies in July of 2013.

TAKEYUKI INAGAKI: Yeah, he was always keeping his head, always encouraging people.

MASATOSHI FUKURA: He was a good leader. You might even say he was super-human. Not just because of the quality of his decisions but how quickly he made them.

TAKEYUKI INAGAKI: Without him, I could not have, yeah, I could not be here.

NARRATOR: Fukushima, four years later: What was once one of the largest nuclear power plants in the world is now the center of the most complex, expensive, expansive cleanup ever attempted. It could take as long as 40 years. It will rely on technology not yet invented and the determination of people not yet born.

The man in charge of it all is the hero of Fukushima Daini. Naohiro Masuda is now TEPCO's chief decommissioning officer.

It is a very different challenge than what he faced in March, 2011.

NAOHIRO MASUDA: This is the first time anyone has attempted this kind of decommissioning. No one in the world has this experience. So, when we try to set a goal to work towards, I can't even give clear instructions, because we're still figuring out what it is we're trying to do.

NARRATOR: This is not Chernobyl, hastily abandoned, encased in a tomb and encircled by a fence. This is Japan, where land is precious, and they have a history of rising from ruin.

Here, they hope to erase the painful past and maybe one day, return to their homes.

DAIICHI OPERATOR: I was born and raised in this area, the same area where we are decommissioning. It's not possible to live here, now, but we all have a strong desire to make it habitable again. I think that's what keeps us working.

NARRATOR: It is a disaster with deep roots at high levels: bad design decisions, technological hubris, a broken safety culture. But in Japan, the sins of the company are the sins of its workers, so they are considered culprits as well as victims.

YUTAKA MIYAZAWA: We did what we had to do. I strongly regret the inexcusable situation that unfolded.

CHUCK CASTO: Mankind has never faced the forces of physics and the forces of nature that those people faced. The system may have failed, but those operators did the best they could with what they had. In my mind, they were absolutely heroes.

TAKEYUKI INAGAKI: There's nothing to be proud of. Most of the plant workers were born and raised here. They wanted to protect their hometown, protect their families. The reality is tens of thousands of people are still under evacuation and we're the ones that caused that. By no means are we heroes.