NOVA

Today, in the heart of Ukraine,

an extraordinary race is underway

to construct a $1.5 billion megadome.

At 40,000 tons, it will be the largest structure

ever moved on land.

Its job... to entomb the crumbling remains

of the Chernobyl nuclear reactor.

It's just fantastic.

Really, really an amazing structure.

In 1986, in what was the Soviet republic of Ukraine,

the Chernobyl nuclear reactor exploded,

releasing 400 times more radioactive material

than the Hiroshima bomb.

30 workers died,

50,000 people fled the nearest city,

and the radioactive fallout spread over Europe.

It was the world's worst nuclear accident.

30 years later,

its hastily built enclosure is crumbling.

In a race against time,

engineers are struggling to prevent

another catastrophic release of deadly radioactive debris

into the environment.

It's extremely dangerous.

Everything we touch, everything we do

is completely crazy.

They're battling freezing weather

and lethal radiation

in a desperate race to build "Chernobyl's Mega Tomb."

Right now, on NOVA.

Major funding for NOVA is provided by the following...

Each day, on trains like this, over 1,000 people travel to work

at one of Europe's most dangerous, and high-stakes,

construction sites.

They're heading to Chernobyl, in North-Central Ukraine,

once part of the Soviet Union.

Here, at the Chernobyl nuclear power station,

workers are racing against the clock

to build a massive structure.

It will enclose the highly radioactive ruins

of a nuclear reactor that exploded here

over 30 years ago,

and that remains extremely dangerous today.

The right half of this building

contains the reactor that exploded.

The makeshift containment structure over it

is collapsing,

threatening to release radiation into the environment.

To prevent another deadly catastrophe,

engineers will entomb it inside a completely new shelter.

But the site is still too radioactive

for builders to construct a new cover

directly over the old reactor.

The only solution is to build the new shelter

to the side of the reactor

and slide it into position.

It will be the largest structure ever moved on land.

The vast scale of the project

was what attracted French engineer Nicolas Caille.

It's a big project with people from everywhere in the world.

So, yeah, it is very, very challenging.

Over 40 countries, including the United States,

have contributed a total of $1.5 billion

to build the new shelter,

and finally close a chapter on a disaster

that occurred on April 26, 1986.

On that day, technicians were carrying out a new test

on this Soviet-built water-cooled nuclear reactor.

The reactor design was flawed,

and the test caused it to run out of control,

resulting in a massive steam explosion.

The blast killed two workers

and blew radioactive uranium fuel onto nearby buildings.

Radiation from the debris and the reactor itself

soon killed another 28 people.

Soviet authorities have been trying to downplay the incident,

claiming that there are only two dead...

...conflicting reports coming out of the Soviet Union.

We do know that a zone of deadly radiation is being released...

The explosion sent six tons of radioactive fuel particles

high into the atmosphere above Europe.

Where it settled,

it contaminated both the land and livestock.

In the vicinity of the reactor,

the radioactive fallout forced a third of a million people

to evacuate, never to return.

It remains the world's worst-ever

nuclear power plant disaster.

Kak ty poluchil travmu?

It left the Soviet authorities with a monumental problem.

Around 200 tons of shattered uranium fuel rods

and other radioactive debris

remained inside the damaged reactor building.

Left uncovered,

it would continually release radioactive dust into the air,

a poisonous cloud to threaten the surrounding area.

Over the next six months,

workers braved extreme radiation to seal the reactor

inside a 300,000-ton shelter made from steel and concrete.

It came to be called "the sarcophagus."

But it was flawed from the start.

The extreme radiation prevented the workers

from completing the welds needed to seal

the prefabricated sections of the sarcophagus together.

Nikolai Steinberg was one of the engineers

who built the sarcophagus.

Every day, every hour, every minute,

it was improvisation.

Even worse, the sarcophagus had no foundations.

It simply stood on the ruins of the destroyed reactor.

Even as they built it, engineers knew its days were numbered.

Now, time is up.

Yeah.

These are the world's most uncomfortable boots.

I think they preserve these just for occasional visitors.

Simon Evans is in charge

of financing the complex international project.

With funding tight,

he wants to have the shelter in place as soon as possible,

so he needs to know firsthand

the condition of the sarcophagus.

Inside the damaged reactor building,

Simon's team wears masks to protect them

from inhaling radioactive dust.

Don't stop, don't stop.

He stops to check some critical devices.

This equipment monitors the stability

of the existing shelter,

which is absolutely essential to ensure

that we know precisely what's going on here.

We know that it's way beyond its design life.

It's crumbling.

We've already had a collapse in one part of the turbine hall

about 2 1/2 years ago, with a very heavy snow load,

and see some very, very major structural damage there.

In the main reactor hall lies the most deadly debris:

the remains of the reactor's uranium fuel rods,

radioactive dust,

and molten material from the core of the reactor.

These give off so much radiation

that it shows up as white flashes on video.

None of this must escape.

But the sarcophagus enclosing this material

is falling apart.

If its roof collapses,

it will throw another lethal cloud of radioactive dust

out into the atmosphere,

where the wind could blow it over the surrounding area.

The collapse of the shelter itself over the old reactor hall

is the apocalyptic scenario which we must avoid.

Certainly it would release

another major release of radiation into the environment.

Six years after the sarcophagus was built,

the Ukrainian government held a competition for ideas

for a new containment structure

to make the Chernobyl reactor safe.

They received probably 200 different ideas.

And they selected five of them.

And it took approximately ten years to decide

which idea will be the best, which idea can be constructed.

And in 2007, we won the project,

and we signed the contract 20 years after the accident.

The winning plan,

from the French construction consortium Novarka,

is very ambitious.

About 300 yards away from the reactor,

where radiation levels are low enough

for builders to work safely,

they will construct two halves of a giant steel arch

30 stories high.

They will mount them on two concrete runways

that extend alongside the crumbling sarcophagus.

Next, they will slide the two halves of the arch together

to make one enormous structure.

Inside the ceiling of the arch,

they will attach two giant robotic cranes.

Once complete,

engineers will slide the vast arch over the reactor.

Then, in the future,

the remotely controlled robot cranes inside the arch

will dismantle the old sarcophagus

and remove the remains

to a yet-to-be-built storage facility.

This part of the plan has not been worked out.

In spring 2012, workers start to assemble the steel tubing

that will form the giant skeleton of the arch.

They will use over half a million bolts

to join it together.

It will be a complicated task.

To erect 36,000 of tons, it's quite challenging.

They will build the first half of the arch from the top down,

using jacks mounted on towers to raise it

in a series of massive lifts.

You have a lift of 30 meters, and then you build below,

and the structure is going up, up, up, up, up, up.

It takes three lifts to raise the arch

to its full height of 350 feet.

Now they must bolt on the steel tubes

that will form the sides of the structure.

Once they have built the first half of the arch,