Original subtitles

[narrator] In 1938, in Berlin, Germany,

nuclear fission was born.

The nuclear age had arrived, promising both

greatness and peril.

In 1957, an uncontrollable fire

at Windscale, the UK's largest nuclear power station.

We saw, to our horror, four channels of fuel glowing

bright cherry red.

Nearly 30 years later, Three Mile Island Nuclear

Power Plant.

Mechanical failures and human errors trigger a partial

meltdown at the practically new plant,

built for 700 million dollars.

The Three Mile Island accident

is marking a real turning

point in the perception and the development of this technology.

But in 1986, the world is stunned by the most terrifying

of all explosions, the meltdown at the Chernobyl

power plant in what was then the Soviet Union.

[Emmanuelle, in French]

Have these tragedies taught us to approach

this powerful energy source with greater caution?

Or will humanity's relentless pursuit of nuclear supremacy

lead once again to a chilling example of fatal engineering?

[theme song playing]

50,000 residents are enjoying the beginning of their weekend

in Pripyat,

a city located in what was then northern Soviet Ukraine.

With spacious apartments, schools, hospitals,

cultural centers, sports facilities,

and an amusement park, it's considered a modern

and pleasant place to live.

[Rob] Life in Pripyat is pretty good, until one night when

an accident at the nearby power plant,

would prove to be fatal.

One that happened at a place

that everybody's heard of -- Chernobyl.

Saturday, April 26th, 1986.

It's 1:23 a.m.

Two massive explosions tear open reactor 4.

The first blast rips off the reactor's roof,

a structure weighing 1,000 tons.

Two or three seconds later, a second larger blast

sends huge lumps of toxic debris flying in all directions

and a cloud of radioactive material

high into the atmosphere.

[explosion roaring]

[Emmanuelle, in French]

[explosion roaring]

While the explosion itself leaves two dead,

the firefighters who immediately respond

to the site fall victim to acute radiation syndrome.

caused by high dose full body exposure.

[Shini] Vomiting, weakness and fever would quickly take hold

as the radiation enters the body, attacking the DNA,

turning healthy cells into cancerous ones.

28 firefighters and plant workers die of acute radiation

syndrome in the weeks following the accident.

For some, it's a brutal, fast-acting poison.

For others, a creeping, silent killer.

A number of people will develop cancer,

especially thyroid cancer.

Scientific organizations put the death toll at 4,000.

Chernobyl's exclusion zone now spans 4,760 square kilometers,

four times the size of New York City.

It may take thousands of years for it to be safely habitable.

[Emmanuelle, in French]

Was this a preventable disaster?

Or was reactor 4 doomed to be a case of fatal engineering?

to understand, we must return to the day

before the explosion.

Constructed between 1970 and 1977, the Chernobyl nuclear

power plant was the largest and most powerful in the USSR.

A symbol of Soviet nuclear ambition, the plant's

four reactor cores converted the thermal energy generated

from uranium fission into electricity.

On the eve of the disaster, all four reactors had been

running at full capacity for years, supplying electricity

to roughly 5 million people, which was 10%

of Ukraine's population.

At the heart of each reactor sits a giant graphite core.

[Emmanuelle, in French]

Every hour, more than 45 million liters of water,

the equivalent of 18 Olympic sized swimming pools,

were pumped through the system to cool

the 1661 uranium fuel rods.

These water pumps were essential.

Without them, the reactor core would overheat with potentially

devastating consequences.

It is precisely these pumps that will play a critical role

in the unfolding catastrophe.

[Emmanuelle, in French]

1:00 p.m.

The shutdown process for reactor four begins.

[Rob] To carry out the test, the operators begin to drop

the control rods into the reactor to slow down

the nuclear reaction.

Control rods.

These massive tubes are made to absorb neutrons,

the particles that split uranium atoms.

They release energy and create electricity.

By manipulating the position of the rods in the core,

operators could increase

or decrease the reactor's power output.

Control rods are inserted into the core to reduce its power

by half, down to 1,600 megawatts thermal.

All is going according to plan, until one unexpected delay.

[Shini] Elsewhere in Ukraine, another regional power plant

has unexplainably gone offline, and Kiev's electrical grid

controller requests Chernobyl postpone its power reduction

as power is needed to satisfy the peak evening demand.

This continues throughout the day until it becomes

nighttime, when the electricity demand isn't as high.

It's now deemed a safe time to continue the test of the backup

generators until a turning point in history where

everything would change for the families who live there.

Chernobyl Nuclear Power Plant

was the largest and most powerful in the USSR.

On the eve of the disaster, all four reactors

had been running at full capacity for years

\{{}Emmanuelle in French]

It's now deemed a safe time to continue the test

of the backup generators until a turning point in history

where everything would change for the families who live there.

A shift change.

While the day crew has experience with these tests,

the night crew does not.

At around 12:00 a.m.,

the team receives permission to continue the test

in reactor core four.

Then something goes wrong.

Without fully understanding why, the operators see

the reactor's power drop rapidly.

[in French]

To bring the power back up,

they begin pulling the control rods out of the core.

They have no way of knowing it, but this action further

destabilizes the reactor.

But this is not the only problem.

[Rob] Too many control rods have been removed, and now there's

a risk the chain reaction becomes uncontrollable.

As the rods are withdrawn, the power begins to rise again,

eventually stabilizing after around 20 minutes.

Although this is far below the 700 megawatts thermal

required for the test, they decide to proceed anyway.

[Shini] Part of the pre-planned test is to increase

the water flow rate around the core,

and two additional pumps are activated.

But this has a risk of lowering the temperature

and the power again.

And with it already low, the operators decide to remove

even more of the control rods.

1:23 a.m.

Despite the reactor's low power, the operators

continued the test.

[in French]

As the water flow slows down, it heats up more than usual.

Steam bubbles rapidly form inside the core.

For the Chernobyl reactor design,

this is a dangerous effect due to them

being positive void coefficients.

A positive void coefficient means the reactor becomes

more reactive as water turns to steam.

As the operators realize that the heat is becoming

out of control, they decide to drop the control rods back

into the core, shutting down the chain reaction and bringing

the temperatures back under their control.

40 seconds past 1:23 a.m.

Unlike the control rods in any other reactor, the control rods

at Chernobyl have graphite tips.

The rods themselves are made of boron carbide, which absorbs

neutrons and slows the reaction.

But their graphite tips do the opposite.

They speed it up.

[in French]

When the control rods travel the full height

of the seven-meter-tall reactor core,

equivalent to a three-story building, the graphite tips

come into contact with the steam, and the core itself.

Had the control rods not been fitted with those graphite

tips, it is likely that the Chernobyl disaster would

never have occurred.

The accumulative effect causes an unexpected and alarming

spike in reactivity.

Instead of a gradual controlled decrease, the fission reaction

causes a surge in power.

Within seconds, power output skyrockets, reaching 30,000

megawatts thermal, ten times the reactor's normal capacity.

This is the last measurement ever recorded.

44 seconds past 1:23 a.m.

Just four seconds after the rods are inserted,

the core becomes an unstoppable pressure cooker.

Superheated steam blows apart the reactor,

sending the roof flying.

A second explosion follows,

scattering chunks of radioactive graphite across

the site and igniting dozens of fires, including a towering

blaze directly above the gaping hole in the reactor.

[Emmanuelle, in French]

The reactor core begins to melt, forming corium,

an extremely radioactive lava-like substance.

It burns through 1.8 meters of reinforced concrete,

inevitably moving into the ground.

Anyone within several kilometers is in mortal danger.

100,000 people are forced to abandon their homes

and their lives.

Animals who have become highly radioactive are killed

to prevent them from spreading the radiation.

[Shini] Back at the ruined reactor, efforts are made to clear up

and contain the exposed radioactive material

from leaking further.

During the operation, a helicopter struck a crane

cable and crashed, killing all four crew.

The moment, caught on camera, became an iconic image

of the Chernobyl disaster response.

In total, 5,000 tons of sand, clay, boron, and lead are

dropped onto the reactor.

It will take 20 days to extinguish the fire and stop

the corium from reaching underground water sources.

[explosion blasts]

Two months after the disaster, a concrete and steel structure

known as the Sarcophagus is built to encase the reactor

and contain its lethal radioactive particles.

Thirty years later, in 2016,

a new, more durable structure replaces it.

It's a shield designed to one day allow for the safe

dismantling of the reactor.

[Shini] Even with the remains of Chernobyl reactor number

four gone, the legacy of the world's worst example

of fatal engineering will tragically remain

for thousands of years to come.

[Rob] This disaster remains one of the darkest moments

in the history of nuclear energy,

marking a real shift in the perception

and the development of this technology.

But just seven years earlier, another incident,

a nuclear plant on the east coast of the USA, where,

once again, nuclear fission is in question, would become

the focus of the world's attention.

Three Mile Island power plant.

It's March 28, 1979,

on the banks of the Susquehanna River,

in the quiet borough of Middletown, Pennsylvania.

Its 9,000 residents are fast asleep.

A crisis has started at the Three Mile Island

nuclear power plant.

It's 4:00 a.m.

A potentially deadly accident has occurred overnight

in the reactor TMI 2, and radioactive material has

been released into the atmosphere above them.

[reporter] Three Mile Island plant, an accident did occur.

It is my feeling that the people within the radius

of a few miles, especially pregnant women,

should be told that there is a significantly increased risk

to their babies.

To understand what happened, we have to go back 10 years

to a time when electricity demand in the United States

was surging.

By the late 1960s,

domestic energy consumption was booming.

Less polluting than coal and capable of generating

massive amounts of electricity at low cost, nuclear energy

seemed full of promise.

On May 18th, 1968,

construction began on a next generation

nuclear power plant built on an island

just five kilometers or three miles

from Middletown, Pennsylvania --

Three Mile Island.

[Emmanuelle, in French]

Together, its two reactors produce 1,700 megawatts

of electricity,

enough energy to power a city of 300,000 people.

Like all pressurized water reactors, the reactors use

steam generated by the heat produced in their core through

uranium nuclear fission to spin a turbine

and generate electricity.

This takes place in the two main circuits of each reactor,

the primary and secondary circuits.

[Shini] The primary circuit transports liquid water through

the reactor core, acting as both a moderator and coolant.

It absorbs the heat from fission and transfers it

to the steam generators.

The steam then powers a turbine, which in turn

generates electricity.

In the event of a worst case scenario, engineers have

designed three protective shields to contain

the radioactive materials inside each reactor core.

An ultra-resistant zircaloy cladding that encases

the uranium fuel rods,

a fully sealed water based primary cooling system,

and a reinforced concrete containment structure built

to withstand extreme pressure and temperature.

But for the system to function safely, one critical condition

must be met.

The heat produced must be effectively removed.

If the reactor becomes too hot, the uranium inside can

cause overheating, potentially causing a nuclear

meltdown and releasing lethal radioactive materials capable

of endangering thousands.

A key component at the heart of the disaster will prove

fatal for Three Mile Island -- the pressurizer.

Inside the reactor core, water heats up to nearly

300 degrees Celsius, a temperature at which water

naturally changes its state.

[Shini] At such high temperatures, as we all know,

water would normally boil.

But in the design of a reactor like TMI, it's crucial

that the water remains in a liquid state.

The pressurizer keeps the pressure in the primary

loop at 155 bars, preventing it from boiling,

a proven system that is used in around 60%

of nuclear reactors worldwide.

[in French]

A pressurizer valve opens to lower the pressure by releasing

water into a relief tank located inside the reactor's

containment structure.

Eight seconds later, the reactor core shuts down

automatically, while at the same time,

the emergency feed water system for the steam generator

is activated along with the turbine shutdown,

stabilizing the secondary side of the plant.

Nuclear fission comes to a stop.

[Emmanuelle, in French]

So far, for this type of situation, everything is

proceeding as expected.

The operators follow protocol

and engage the control to close the valve.

[Rob] The operators believe that having pressed the button,

the valve has indeed closed. But that's not the case.

But they've no way of knowing that.

It's 4:01 a.m.

With the reactor shut down, heat is no longer being

produced by fission reactions, but residual heat is still

being generated by the radioactivity

of the fission products.

Water from the primary circuit escapes through an open valve,

and as a result, the pressure begins to drop.

[Emmanuelle, in French]

At all costs, this heat must be removed to prevent

the temperature from rising inside the core.

4:02 a.m.

The water leak through the open valve causes the pressure

in the primary circuit to drop to 110 bars,

automatically triggering the high pressure safety

injection system to cool the core.

But because the have an inaccurate view

of the system's condition,

they misinterpret what they're seeing.

They're monitoring the water level in the pressurizer,

rather than in the reactor core.

[Shini] When they saw it rise, they assumed they had injected

too much water and manually closed the safety injection

after five minutes.

This was the point of no return.

4:05 a.m.

For over two hours, about 60% of the water in the primary

circuit is discharged into the relief tank inside

the containment building, and the operators have no idea.

With the valve still open, the second of three protective

shields designed to prevent radiation from escaping

is not working.

[Emmanuelle, in French]

At 6:00 a.m., the day shift takes over.

One operator fully realizes the seriousness

of the situation and closes the valve.

The second containment barrier

is now able to function properly again.

[Emmanuelle, in French]

6:14 a.m.

The radiation alarm goes off in the reactor 2 building

at Three Mile Island.

As a result, the temperature in the core

continues to rise,

and the dreaded scenario is unfolding.

[Emmanuelle, in French]

A core meltdown inevitably means the destruction

of a reactor worth several hundred million US dollars.

Worse still, if the final containment barrier fails,

radioactive materials will spread through underground

structures and escape through the exhaust stacks

of the Three Mile Island plant.

[in French]

Corium is highly radioactive.

If it isn't contained in time, this man-made,

ruthless and destructive magma can burn through even

the toughest structures.

[Shini] Meanwhile, in the control room, the operators are trying

to regain control of the cooling

of the damaged core.

As the pressure continues to rise, the operators decide

to reopen the valve, where the entire issue began.

It's 6:54 a.m.

This opening allows the now radioactive water

from the primary circuit...

to flow into the pressurizer storage tank,

located within the containment structure.

[in French]

[Shini] But the rupture disk on this tank has already broken,

normally due to the overflow of water.

As a result, the now radioactive water is

spilling into the building.

This containment structure is made of one-meter-thick

pre-stressed concrete.

That concrete is now the final barrier and the last chance

to prevent a major disaster from devastating

the town of Middletown

and possibly the whole state of Pennsylvania.

[Emmanuelle, in French]

[Rob] At this point, the alarm is raised

more generally,

alerting those in the area what is happening.

It's 7:20 a.m.

Within hours, fear grips the small town.

Schools close.

Many people take shelter in their basements.

200,000 people take to the roads, fleeing something

they can't see, but fear more than anything else.

Panic spreads everywhere.

Meanwhile, plant operators begin to regain control.

They restart the high-pressure safety injection system,

which refills the reactor vessel in just seven minutes.

The water enters the molten corium and begins to cool it.

It's almost like when you see lava flow from a volcano meet

the ocean, and it's cooled quickly.

When water comes in contact with corium, it produces

an extremely light chemical element that can form

an explosive mixture with air, hydrogen.

But at around 1:30 p.m., nearly nine and a half hours

after the start of the accident,

the hydrogen that built up inside the containment

triggers an explosion.

The final shield holds.

This is what saves the people of Pennsylvania

from a catastrophic outcome.

In the days that follow, cooling systems are

gradually stabilized, the remaining hydrogen is

expelled, and the corium begins to cool at the bottom

of the reactor vessel.

On March 28, 1979, at 8:00 p.m.,

the immediate crisis is over.

[Rob] Nobody died as a direct result of the situation

at Three Mile Island, but it forced a new perspective

for the general population on how it saw nuclear power,

slowing down progress with this technology for years to come.

Reactor number 2 is a total loss --

700 million U.S. Dollars gone just four months

after it started.

And yet, against all odds, reactor 1 returns

to operation and continues to produce electricity

for nearly 30 more years.

It is finally shut down in 2019, seemingly marking the end

of the nuclear era at Three Mile Island.

Then, in 2023, a surprising new chapter begins.

A giant tech company buys the site to build a massive

data center, partially powered by nuclear energy.

A new life for a site where the current of history

never truly faded.

At Three Mile Island, poor control room design led

to human error.

At Windscale, a basic lack of understanding in fire

containment was evident.

In 1945, as the fires of World War II

were finally beginning to die down,

and the world entered a long process of reconstruction

a new era was emerging...

...the nuclear era.

[Rob] The Americans have already developed their own technology

and metre producing atomic bombs.

And the British were involved in that development.

But now the race is on as the Russians are also

developing their technology.

Britain asked its ally, the US, to share

their manufacturing secrets.

But President Eisenhower and the US Congress

were reluctant to share information about a weapon

that cost them today's equivalence

of 28 billion US dollars.

A US law was passed making it illegal to share

nuclear knowledge outside the United States.

[Rob] Britain is undeterred by this shirking

of the United States, and so decided to develop

their own means, their own technology,

their own facilities to produce their own atomic weapons.

The British had to act quickly, fearing that the Russians

would develop the bomb first.

[explosion blasts]

The British authorities decided to build their nuclear facility

on the site of the former Sellafield Munitions Factory.

And this site will become known as Windscale, a name that

will go on to be associated with the first nuclear accident

that would change the way the world viewed

nuclear technology.

It was composed of two air cooled reactors known as

Pile 1 and Pile 2.

Designed to transform uranium rods into weapons-grade

plutonium, the core of each reactor consists of thousands

of small channels drilled into a massive

2,000-ton graphite block.

[Rob] Graphite is a really important material used

at Windscale reactors.

It serves to moderate the speed of neutrons as they're fired

out of the fission reactions taking place.

[Shini] And secondly is able to absorb large amounts of excess heat

and energy created during the nuclear reaction.

As the uranium rods move through the channels,

a nuclear reaction takes place.

This is how plutonium is created.

When each rod reaches the end of its channel, it drops

into a water tank where it is cooled.

The problem with the process is the immense amounts of heat,

if not evacuated, can build up and to cause a disaster.

To evacuate the heat, massive fans are activated

to continuously blow cold air over all the channels.

They expel the heat through 120-meter-high chimneys

on the other side of the reactor core.

British Nobel prize-winning physicist Sir John Cockroft

designed a protective shield specifically for the plant

to be placed at the top of these chimneys in the event

of radioactive leaks outside the reactor.

A 50-ton filter composed of successive layers

of gravel and sand.

It became known as Cockroft's follies in the end

because many didn't feel that they were needed.

Seven years after the nuclear plant's opening,

these filters would prove to be lifesaving.

Monday, October 7, 1957. 1:13 a.m.

The Windscale plant operators must carry out

a delicate operation to the core of Reactor Pile 1.

[Emmanuelle, in French]

This is what is known as Wigner energy.

If not gradually released, it can dangerously accumulate

and cause severe overheating of the core.

[Shini] To release the potentially critical build up of

Wigner energy, engineers and on site physicists

develop an empirical method known as annealing.

[Emmanuelle, in French]

[Shini] Instead of triggering a gradual release

of Wigner energy, the increased power

causes a sudden and rapid spike in temperature.

On Wednesday, October 9, at 10 p.m., the graphite

reaches an abnormal temperature of 405 degrees Celsius.

Scientists attempt a second nuclear heating process

to release the energy, but it does not have

the intended effect.

The graphite accumulates too much energy and quickly reaches

critical levels.

The situation is unprecedented, but the worst is yet to come.

[Rob] At this point, the decision is made

to evacuate the site with only the key workers left

to try and bring the situation back under control.

Thursday, October 10, 4:00 p.m.

The reactor supervisor, Tom Hughes, and another staff

member put on protective clothing and visit the site

to assess the graphite's condition.

As they approach the reactor core, they are horrified

by the situation.

He inspects the core himself and understands that one

of the cartridges has split open.

The aluminum cladding surrounding the uranium is

a crucial protective layer that ensures a safe

nuclear reaction.

If it is ruptured, it means the uranium is exposed

to open air.

This is where the Cockcroft filters prove their worth.

Thanks to their design, no radiation escapes

from the Windscale plant, at least for now.

But the situation is deteriorating fast.

At Windscale

a fire in the reactor core threatens to trigger

the greatest nuclear disaster the world has ever known,

with radioactive releases potentially spreading across

the United Kingdom and Europe.

In an attempt to cool the core, the fans are set

to their maximum speed.

Tom Tuohy, the deputy site manager at Windscale,

takes over.

He begins trying to push the burning cartridges out

of the core using scaffolding poles.

However, at that moment, the core is burning hot,

and the fuel cartridges cannot be moved as the heat has

caused them to deform.

What started out as an incident where one cartridge

is on fire develops into an uncontrollable catastrophe

where multiple cartridges are on fire and the temperature is

raising out of control.

This is not normal,

and it's never been seen before.

On Friday, October 11, at midnight, the fire reaches

temperatures up to 1,300 degrees Celsius.

The men on site need to wait to extinguish the blaze,

but all options carry risks.

[Emmanuelle, in French]

With time running out his team have two options left.

The first option is to smother the flames

with carbon dioxide.

They hope this will cut off the oxygen supply

and suffocate the fire.

Again, it doesn't work.

The temperatures are now out of control, and even the CO2 has

no effect.

Their last hope to extinguish the fire turns out to be

the most controversial -- water.

Water is well known for putting out flames, but when used

on a heat source exceeding 400 degrees Celsius in such

a confined space, it could trigger

an unprecedented explosion.

11 tons of uranium have already burned.

Tom Tuohy and the fire chief are the last men on site

to handle the situation.

Despite the risks involved, they pump huge volumes of water

through the reactor core, trying to bring

down the temperature.

8:55 a.m.

1,400 liters of water per minute are injected

into two channels above the top of the fire.

With no significant effect, the flow is increased

to 3,600 liters per minute.

Outside, staff can now see smoke rising from the chimney.

Tom Tuohy climbs into the reactor again to check if

the water is having the desired effect.

Once again, there's no effect.

The core is still radiating with heat.

The situation seems hopeless.

Tom Tuohy makes a final decision.

It's 10:10 a.m.

The decision's made to switch off the fans because they're

not having an effect on the core.

[water swooshing]

To their complete surprise, the very fans installed to keep

the core safe from overheating have been one of the main

reasons they've been unable to get control of the situation.

The fans have been fueling the fire all along.

like bellows on a wood fire.

By turning off the fans, they've reduced the oxygen

that was supplied.

The fires went out, and the temperatures eventually

became back under control.

By noon, two more hoses are installed, and the water flow

is increased to 4,500 liters per minute.

This flow continues until 3:00 p.m.

the following day, and the pile is finally cooled.

The site was decontaminated over the following weeks

and months, but the reactor is left with 1,700 damaged

cartridges within its core.

[Shini] The reactor is sealed off and remains to this day sealed

with approximately 15,000 tons of uranium still inside it.

Pile 2 is also shut down as a precaution.

And air cooled reactor have not been built since.

Following this accident, the British nuclear industry

underwent significant reorganization,

with the creation of national agencies to oversee operational

and safety measures.

[Rob] The accidents at Windscale, Three Mile Island,

and Chernobyl is clear evidence that there are risks involved

with nuclear energy.

[Shini] Thankfully, every disaster has contributed to global reform

within the sector and lessons continue to shape

the nuclear industry.

Today, as awareness of the devastating impact

of fossil fuels continues to grow,

nuclear energy remains a promising alternative,

but it's still dangerous if not properly controlled.

we must remain constantly vigilant if we don't want

our nuclear power plants to turn into another case

of fatal engineering.