Original subtitles

- [Voiceover] More than three years have passed

since the Fukushima Daiichi Power Plant

became the site of one of the worst nuclear accidents

in history.

(beeping)

- [Voiceover] 8.6 microsieverts, 8.7.

It keeps going up.

- [Voiceover] Damaged reactors released huge amounts

of radioactive materials, forcing many people

to evacuate their homes.

Even now, no one fully understands how the leaks occurred.

We found some important information about five kilometers

from the plant.

Experts had assumed this monitoring post had stopped working

after the earthquake in March 2011.

No one knew it, but detailed data about radiation levels

right after the accident have been stored in this device

since that time.

An analysis led to a new discovery.

Enormous amounts of radioactive substances

began leaking the day after the earthquake.

The recently uncovered data

indicate that the substances began leaking

in high concentrations sooner than previously thought.

(dramatic music)

The nuclear reactors had multiple protective walls

but experts have determined there was a flaw

in their construction.

- [Voiceover] A new unexpected path has appeared.

- [Voiceover] The emergency venting operation

must have caused the release of radioactive materials.

(suspenseful music)

- [Voiceover] Experts are also starting to figure out why

a critical structure for containing radioactive substances

broke down.

The meltdown sent the temperature

of the nuclear fuel soaring.

Experts suspect that may have destroyed

the containment vessel, the final barrier

for keeping the substances inside the reactor.

(foreign language)

- [Voiceover] We reenacted what happened at the plant

during the crisis, based on accounts

from more than 400 people involved.

(rumbling noise)

(foreign language)

(chattering)

- [Voiceover] Throughout this series, Meltdown,

NHK has been conducting scientific investigations

into the causes of the nuclear disaster.

(beeping) (man shouting)

(foreign language)

On this program, we'll explore how huge amounts

of radioactive materials escaped from the nuclear plant.

(suspenseful music)

(foreign language)

- [Voiceover] I'm now in Kamihatori in Futaba town

Fukushima prefecture,

more than five kilometers from the nuclear plant.

Access to this area is strictly limited

due to its proximity to highly contaminated areas.

We have to wear protective clothing like this.

We found something here that has provided valuable clues

about why huge amounts of radioactive materials leaked.

It's this monitoring post for measuring radiation levels.

As it turns out, all the information recorded here

three years ago has remained intact all this time,

without anyone realizing it.

The data shows that before the hydrogen explosion

of the Reactor 1 building, there was a massive release

of radioactive substances.

This event coincided with the venting operation

at the reactor.

The discovery of this fact raises the alarm

about existing safety measures for nuclear power plants.

- [Voiceover] The monitoring post stored the data

for three years.

It shows radiation levels captured at 20-second intervals

over three days from the time the earthquake struck.

An analysis has revealed the seriousness

of the contamination for the first time.

About one full day after the earthquake

at 2:40 PM on March 12, the rapid rise in radiation

reached a peak.

It measured about 4.6 millisieverts per hour.

If it had stayed there, people would exceed

the annual limit for radiation exposure

in just 15 minutes.

When the radiation spiked,

what was happening at the Fukushima Daiichi plant?

(ominous music)

At 2:01 PM, white steam was coming out

of an exhaust stack.

Workers were venting Reactor 1 in an attempt to reduce

pressure in the containment vessel.

Data from this time showed that radiation levels

were rising on the plant's premises.

(foreign language)

- [Voiceover] I remember the readings were abnormally high.

In some places, the readings were off the scale.

We could guess how bad conditions must have been

outside the reactor buildings.

- [Voiceover] An expert has been studying how contamination

dispersed after the accident.

His name is Masamichi Chino.

He calculated how radioactive substances

released by the venting spread.

Here's the result of his simulation.

The substances drift northwest

and passed through kamihatori.

The estimated radiation level at 2:40 PM matched the data

recorded at the monitoring post there.

(foreign language)

- [Voiceover] It appears that particles were released

immediately after the venting began at 2 PM.

Substances such as cesium leaked.

That's clear, although we don't know in what ratio.

- [Voiceover] TEPCO officials had said

the amount of radioactive materials released

wouldn't reach dangerous levels.

(foreign language)

- [Voiceover] There is water below the reactor.

The steam will go through it first, and then be vented.

That step will keep radiation levels very low

even if the steam does contain radioactive substances.

(instrumental music)

- [Voiceover] The donut-shaped section toward the bottom

of the containment vessel is equipped with a mechanism

to limit the release of radioactive substances

during venting.

The suppression chamber is designed to reduce pressure.

The water inside serves as a filter.

When the containment vessel is depressurized,

the released steam goes through the water,

trapping most of the radioactive materials.

This should limit the amount of contaminants released

to just 1/10 of 1%.

(car engine noise)

So how did the venting procedure wind up

causing a massive radiation leak?

We went with the experts to a huge experimental facility

in Italy.

First, we conducted an experiment to understand

how the suppression chamber can limit the release

of radioactive materials.

A three-meter high water tank represents

the suppression chamber.

Steam is injected through a pipe.

This type of investigation could be very important

in order to enhance the safety

of the nuclear power plants.

(loud pops)

- [Voiceover] At first, the temperature of the water

in the suppression chamber is low.

High-temperature steam from the containment vessel

turns into bubbles that immediately disappear.

This footage was taken with a high-speed camera.

When steam is cooled, it instantly changes to water.

The moment steam from the containment vessel

turns into water, nuclear materials inside are captured.

(dramatic music)

But experts suspect something unusual was happening

in the suppression chamber at Reactor One.

(foreign language)

- [Voiceover] The water closer to the surface

rises in temperature, but water lower down doesn't,

resulting in thermal stratification.

The temperature of the suppression pool was probably

quite high.

- [Voiceover] It's believed that during the accident,

high temperature steam was already flowing

into the suppression chamber from somewhere

other than the containment vessel.

As a result, a phenomenon called thermal stratification

may have occurred.

Experts believe the upper layer of water was boiling.

(suspenseful music)

Would stratification make the suppression chamber

less effective at removing radioactive materials?

The researchers boil the water in the pool and inject

the same amount of steam as before.

Once they do this,

there is a dramatic change.

Now the steam turns into bubbles that rise all the way

to the surface.

This means that radioactive substances contained

in the steam don't get trapped like they're supposed to.

Instead, they're released into the atmosphere.

(suspenseful music)

This analysis indicates that when the temperature

of the water is low,

the amount of radioactive materials released

can be limited to 1/10 of 1%.

But when the water is boiling,

as much as half of the materials could be released

into the air.

The Fukushima accident highlighted the need

to re-examine the venting procedure.

(foreign language)

- [Voiceover] Three years after the nuclear accident,

some 135,000 people from Fukushima are still unable

to return to their hometowns.

Access to the area where I'm standing right now

is permitted during the day,

but no one is allowed to live here.

Rebuilding efforts are virtually on hold

in places with significant amounts

of radioactive contamination.

It's as if time has stopped.

Many people had to abandon their homes

after the massive radiation leaks.

Authorities and experts have conducted a variety

of investigations into the disaster.

But many issues remain unexplained.

More needs to be done.

Let's review the accident.

The day after the quake and tsunami,

Reactor 1 was the first to lose its cooling system

and suffer a core meltdown.

Then hydrogen, that had built up inside, exploded.

Two days later, the cooling system in Reactor 3 stop working

and the day after that, the one in Reactor 2 broke down.

The shutdowns triggered additional meltdowns.

The graph shows the amount of radioactive substances

that escaped.

There are brief spikes that coincide

with the hydrogen explosions at Reactors 1 and 3.

There was no such explosion in Reactor 2,

but this is the reactor that released the most radiation.

What exactly was going on inside Reactor number 2?

The truth about the massive leak of radioactive materials

is becoming clearer.

- [Voiceover] Meltdowns occurred one after the next

in Reactors 1 and 3, and hydrogen explosions

destroyed the buildings.

The building housing Reactor 2 didn't explode,

but the situation inside was actually more severe.

Reactor 2 released huge amounts of radioactive materials.

They're believed to have leaked from the containment vessel

through spaces around piping joints and the lid.

This is a simulation based on contamination data

from the time of the quake and tsunami.

Reactor 2 released large volumes of radioactive substances

to the northwest of the plant on March 14 and 15th.

The pollutants then spread further south

to the greater Tokyo area.

What caused the leaks in the first place?

In February 2014, our cameras were allowed inside

the main control room for Reactors 1 and 2.

At the time of the accident, the tsunami had knocked out

all power supplies, leaving this room in complete darkness.

Notes scribbled on a wall suggest how workers

struggled to deal with the unfolding emergency.

Experts suspect the failure of an emergency procedure

called venting led to the crisis in Reactor 2.

Venting is the last line of defense for protecting

the containment vessel and preventing a massive release

of radioactive materials.

Once a nuclear reactor's core begins to melt down,

the resulting heat causes pressure inside to surge.

This can destroy the outer container in no time.

Venting the internal steam lowers the pressure.

In this process, steam first passes through water

to remove radioactive substances,

then it's released into the atmosphere.

But for some reason, the vent at Reactor 2

didn't work at all.

What was going on?

Our investigation has uncovered some of the details.

We've reenacted the situation based on accounts

provided by people who were there.

(foreign language)

This is the main control room for Reactor 2.

The emergency cooling system continued running

after the earthquake, but on March 14,

three days after the quake, it stopped.

(foreign language)

By 1:25 PM, the reactor was rapidly losing water

needed to cool nuclear fuel.

A meltdown was looming.

At the time, workers in the emergency headquarters

some 300 meteres away were supervising the response.

The workers had been busy addressing the hydrogen explosions

in Reactors 1 and 3.

Now they issued an order for venting in Reactor 2,

to prevent a massive leak of radioactive materials.

(foreign language)

An operator at that time agreed to be interviewed.

He was involved in the crisis response.

(foreign language)

- [Voiceover] First and foremost, we had to think about

protecting the containment vessel.

We needed to vent it to maintain its integrity.

(chattering)

(foreign language)

(suspenseful music)

The main control room for Reactor 2 had lost all power.

(foreign language)

Operators attempted to vent the reactor

using backup generators.

(foreign language)

But

the pressure in the containment vessel didn't fall.

(foreign language)

(suspenseful music)

The workers suspected their efforts to open

the air-operated valve were failing.

This is an essential step in the venting process.

The valve opens when air is pumped in.

That releases pressure that is built up

in the containment vessel.

The workers figured there wasn't enough air

to open the valve.

(beeping)

(multiple footsteps)

Some members of the team raced to the reactor building

to check.

(foreign language)

The valve is inside the reactor building.

The power outage had knocked out all the monitoring gauges

so team members had to go inside and check the instruments

with their own eyes.

(beeping)

(multiple footsteps)

The entrance to the reactor building has double doors

to keep radioactive materials inside.

(beeping)

(foreign language)

(cranking noise)

(beeping)

The team members had no idea what they would find.

They proceeded slowly, checking radiation levels

as they went.

(beeping)

(foreign language)

(cranking noise)

(suspenseful music)

(beeping)

(multiple footsteps)

They were now barely 10 meters from the nuclear reactor,

which was on the verge of a core meltdown.

(suspenseful music)

They managed to locate the tanks that supply air

to the valve.

(multiple scurrying footsteps)

(foreign language)

(beeping)

(scurrying noise)

(creaking noise)

(suspenseful music)

(multiple footsteps)

(foreign language)

(foreign language)

(foreign language)

(suspenseful music)

If the valve itself were broken, there would be no way

to fix it right away.

(foreign language)

(suspenseful music)

The vent was not working.

Conditions in the reactor continued to deteriorate.

(suspenseful music)

Heat generated by the meltdown was starting to raise

pressure inside the containment vessel,

the structure that prevents radioactive materials

from escaping.

(foreign language)

The team did have a last-ditch measure.

They could use a backup valve,

but it was also inside the reactor building

protected by the double doors.

(beeping)

Team members went back to the reactor building.

They found conditions there had drastically changed.

(beeping)

(multiple footsteps)

(rapid beeping)

(man screaming)

(foreign language)

(rapid beeping)

The radiation reading was so high, a person's exposure

would reach tens of millisieverts in a matter of seconds.

The reactor building had become filled with steam.

(suspenseful music)

At this point, it was impossible to do any more work inside.

Masao Yoshida managed the plant

at the time of the accident.

Masatoshi Fukura was in charge of crisis management

at the reactors.

Fukura had received reports from workers at Reactor 2

about the vent failure.

(foreign language)

- [Voiceover] To vent the reactor,

we had to open a valve soon, a big one or a small one.

We knew what we needed to do.

In fact, the other workers on site were trying to do it.

But at that point, no one could get near Reactor 2 anymore,

let alone reach the valve inside.

Right around the time the team members found

they could no longer enter the reactor building,

massive amounts of radioactive substances

were beginning to be released.

At this stage, pressure inside the containment vessel

hadn't significantly exceeded the design limit

so experts assumed it wouldn't have been possible

for huge amounts of radioactive materials to leak.

(mysterious music)

Then why did the team members detect

high levels of radiation?

Panels, including those convened by Japan's

diadem government, have conducted various investigations

but they didn't identify the cause.

We enlisted the help of experts on reactor simulations

and nuclear power plant facilities in a bid to find out.

(mysterious music)

Rather than focusing on the containment vessel itself,

they zeroed in on something that most people had overlooked,

a device outside the containment vessel.

(foreign language)

- [Voiceover] Unless the shaft is sealed,

steam from the reactor's pressure vessel

could leak through the turbine.

- [Voiceover] They suspected a problem

with an emergency cooling system called the RCIC.

It uses steam to rotate a turbine that circulates water

and cools the reactor's core.

It doesn't need electricity to function.

Steam from the reactor flows directly into the turbine.

That's where the experts suspect the leakage occurred.

But the shaft of the turbine was tightly sealed,

with four layers of disc-shaped packing material.

Could a leak occur around the shaft?

We decided to try an experiment.

(suspenseful music)

We obtained a detailed diagram of the cooling system.

(foreign language)

- [Voiceover] The space here is 140th of a millimeter.

- [Voiceover] We worked with an expert

in fluid engineering.

(tapping metal noise)

We used the diagram to create a duplicate of the shaft

of the RCIC and the packing around it.

(mysterious music)

We injected steam into the device.

We applied the same amount of pressure as existed before

the nuclear accident.

No steam leaks from around the shaft.

Then we gradually raise the pressure,

just as happened during the crisis.

(hissing noise)

Large volumes of steam begin billowing out.

(foreign language)

- [Voiceover] It's much more than I'd expected.

It's possible that huge amounts

of radioactive substances leaked.

- [Voiceover] How did this happen?

It turns out, there was a flaw

in the cooling system's construction.

It's designed to have a little space

between the packing disks.

If the pressure rises and steam begins to escape,

there's a device to remove it.

But it needs electricity to function.

(suspenseful music)

When the device isn't working, the cooling system

starts spewing steam.

(foreign language)

- [Voiceover] At this rate, some 50 kilograms of steam

would flow out in an hour.

- [Voiceover] That much?

(suspenseful music)

The experts said that, judging from the amount of steam,

it wasn't surprising that radiation levels

rose so sharply.

(foreign language)

- [Voiceover] When steam comes out,

the inside of the building becomes completely white.

That's because steam takes up much more volume than water.

It spreads really fast too.

- [Voiceover] The former operator who took part

in the crisis management effort,

was surprised that the cooling system

could have been a source of the leak.

(foreign language)

- [Voiceover] It didn't occur to me

at the time of the accident.

But now that I think about it,

I realize the RCIC is one of the possible sources

of the leak.

(suspenseful music)

- [Voiceover] A safety device designed

to help cool the reactor failed.

As a result, workers were unable to vent the reactor

and contain the crisis.

(suspenseful music)

The workers couldn't do anything

inside the reactor building.

The situation was going from bad to worse.

The containment vessel would not be able to withstand

the increasing pressure.

TEPCO officials kept a record of discussions

at the plant's emergency headquarters during the crisis.

It recounts in detail how the core meltdown progressed

and tracks rising radiation levels

in the containment vessel.

The loss of the cooling system triggered a crisis

in Reactor 2, about 11 hours later.

(foreign language)

The containment vessel was on the verge of releasing

massive amounts of radioactive substances.

Then at 8:45 AM,

(foreign language)

(tense chattering)

- [Voiceover] this was the highest radiation recorded

on the plant's premises.

(suspenseful music)

(foreign language)

This photograph captures white steam

rising from Reactor 2.

It was taken at the moment that the containment vessel

broke and released huge amounts of radioactive materials.

Attempts to vent the containment vessel had failed.

As a result, the final barrier for keeping

radioactive substances sealed in broke down.

(dramatic music)

(foreign language)

- [Voiceover] we Wewere trying to do our best, of course,

but the situation just kept getting worse and worse.

That's how it became such a major accident.

There were so many things we wanted to do,

but we couldn't do any of them.

We felt powerless.

It's really regrettable.

High concentrations of radioactive contamination

prevent people from returning to their home towns

to this day.

The sequence of events inside Reactor 2,

illustrates how difficult it was to keep

radioactive substances inside the containment vessel.

(foreign language)

- [Voiceover] The massive release of radioactive materials

from Reactor 2 resulted in severe contamination.

The emergency cooling device has drawn little attention

in the past, but it may turn out that it played

a pivotal role in triggering the leak

by obstructing the venting procedure.

The containment vessel was considered the last means

of keeping radioactive materials from getting out.

Research is underway to find out where the vessel leaked,

but many questions remain about what exactly happened

inside Reactor 2.

Investigations into what happened in Reactor 1 offer a clue.

Venting did work there, but it was the first reactor

to lose its cooling system,

and it quickly went into a meltdown.

The containment vessel also broke,

allowing radioactive materials to get out.

We had experts analyze the results of the latest data.

They uncovered an unexpected weakness.

- [Voiceover] Where did the breakdown occur

in the containment vessels?

Research is underway to find exactly where

the failures occurred.

The initial focus is on Reactor 1,

it was the first to lose its cooling system

and go into a core meltdown.

Researchers are using a remote-controlled boat

to avoid exposure to strong radiation.

There's a pool of radioactive water

surrounding the containment vessel.

Experts insert the boat there and use an onboard camera

to look for cracks.

Here's what they found.

The camera captured images of contaminated water

pouring out of piping around the containment vessel.

That suggests there's a crack in the back somewhere.

Experts say the amount of radioactive water

indicates that the condition of the containment vessel

is more critical than previously thought.

There's a hole somewhere.

If there's no leak on this side, the hole is not here

but over here somewhere.

(dramatic music)

- [Voiceover] The location of the leak

indicates there's a crack in a lower part

of the containment vessel that's surrounded by concrete.

Specialists had assumed that cracks in this section

were unlikely.

(dramatic music)

We asked the experts to join us

in a more detailed analysis.

Right now, it's not possible to get a firsthand look

at the area thought to be damaged.

Our analysis is based on a diagram of the containment vessel

and a simulation of the meltdown.

(dramatic music)

Masanori Naitoh is an expert on accidents

at nuclear plants.

He's been using data to track

how the core meltdown progressed.

(dramatic music)

(foreign language)

- [Voiceover] There's actually an opening in one place.

From there, the melted nuclear fuel comes out.

Our calculations suggest the molten debris

didn't reach the wall.

- [Voiceover] We also consulted with experts on structures

made of metal.

In this simulation, melted nuclear fuel

breaches the reactor and pours out

onto the underlying concrete.

The spilled fuel flows toward the wall

of the containment vessel,

but it stops about a meter away.

If the fuel didn't touch the wall,

could it still have broken the vessel?

(foreign language)

- [Voiceover] The nuclear fuel is extremely hot,

with a temperature of more than 2000 degrees Celsius.

So the radiant heat raises the temperature

of the drywell wall.

The question is, would the structure of the drywell hold

in soaring temperatures?

(dramatic music)

- [Voiceover] The containment vessel is made of steel

about 24 millimeters thick.

The experts use this information and other data,

including the shape and material of the surrounding layer

of concrete to explore the effects of the heat

from the nuclear fuel.

They discovered an unexpected flaw

in the containment vessel.

The nuclear fuel, now at a temperature exceeding

2000 degrees Celsius,

heats the wall of the containment vessel

to nearly 600 degrees.

The metal part of the vessel, shown in red,

expands toward the outside.

A portion of the surrounding concrete is made

of a different material.

This section appears to be vulnerable to the extreme force

being applied.

The researchers' calculations show that this section

would become unable to bear the stress and break.

(foreign language)

- [Voiceover] Metals like this kind of carbon steel

are not meant to withstand such high temperatures.

They can't prevent thermal expansion.

(foreign language)

- [Voiceover] This was unexpected.

If molten debris from the nuclear fuel

comes close to the drywell wall, as it does here,

the structure can't maintain its integrity.

That's our conclusion.

- [Voiceover] In the past, experts thought leaks

were more likely at the joints connecting pipes

in the lid to the vessel.

But our research revealed a new weakness

in the containment vessel.

The heat from the nuclear fuel could break it,

even if the fuel didn't touch the vessel's wall.

What did the people in charge of Fukushima Daiichi

think of this finding?

(foreign language)

- [Voiceover] If melted nuclear fuel spreads out

to the containment vessel, that could cause strain

in various places, leading to radiation leakage.

I believe that's true.

At the very least, we'll take measures at Fukushima Daiichi

where large-scale leaks occurred.

And we'll think about how to prevent degradation

that could occur if hot melted nuclear fuel

landed on the containment vessel.

We can also cover it with materials that can withstand

extremely high temperatures.

There are still many things we can do, and we will do them.

- [Voiceover] Are Japan's power companies acting

on the lessons learned?

NHK surveyed six utilities with nuclear reactors

of the same type as the ones at Fukushima Daiichi.

TEPCO and two other utilities said

they will install new venting devices with filters.

They said this would prevent leaks.

The other three firms also said they plan to install

new equipment.

They said they are still deciding what to use

or declined to comment.

But experts say rising temperatures

in the suppression chamber may not completely explain

the high levels of radiation indicated by the data.

- [Voiceover] I wonder if enough thought had gone

into figuring out how radioactive materials

would behave in water that's tainted

with all kinds of chemicals.

- [Voiceover] During a core meltdown,

a variety of chemicals are generated inside a reactor.

Experts say the types and quantities of these chemicals

may affect the amounts of radioactive materials

that are released.

Have investigations into the causes of the accident

been thorough enough?

We posed this question to the Nuclear Regulation Authority,

which screens safety measures at the nation's

nuclear power plants.

(foreign language)

- [Voiceover] When it comes to safety,

the thing we need to fear most and guard against

is missing important points,

That's not always possible.

I believe we've made as much effort as we can

to cover every possibility concerning accidents,

but we shouldn't be satisfied with that.

We must constantly keep striving to make improvements

and identify anything we've missed.

- [Voiceover] Various flaws resulted in the release

of massive amounts of radioactive substances.

Actual events surpassed people's assumptions

and the measures that were in place.

The Fukushima accident has shown just how difficult it is

to contain radioactive materials and proven that

many things that happened during an accident

can't be foreseen.

Many issues still haven't been fully explained.

After the disaster, authorities drew up

new regulatory standards for nuclear power facilities.

The Nuclear Regulation Authority

is conducting safety reviews on 17 of Japan's 48 reactors.

Accidents like the one at Fukushima Daiichi

must never happen again.

(dramatic music)

Investigations must continue, leaving no stone unturned.

(dramatic music)

Have people truly learned the lessons of the accident?

Are they applying them?

We need to keep asking questions

and remember that the full truth about the catastrophe

has yet to be revealed.

(dramatic music)