Meltdown: Analyzing the Radiation Leaks

- More than three years have passed

since the Fukushima Daiichi Power Plant

became the site of one of the worst nuclear accidents

in history.

8.6 microsieverts, 8.7.

It keeps going up.

- 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.

The nuclear reactors had multiple protective walls

but experts have determined there was a flaw

in their construction.

- A new unexpected path has appeared.

- The emergency venting operation

must have caused the release of radioactive materials.

- 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.

- We reenacted what happened at the plant

during the crisis, based on accounts

from more than 400 people involved.

- Throughout this series, Meltdown,

NHK has been conducting scientific investigations

into the causes of the nuclear disaster.

On this program, we'll explore how huge amounts

of radioactive materials escaped from the nuclear plant.

- 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.

- 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?

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.

- 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.

- 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.

- 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.

TEPCO officials had said

the amount of radioactive materials released

wouldn't reach dangerous levels.

- 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.

- 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%.

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.

- 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.

But experts suspect something unusual was happening

in the suppression chamber at Reactor One.

- 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.

- 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.

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.

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.

- 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.