NARRATOR: Today, in the heart of Ukraine, an extraordinary race is underway to construct a $1.5 billion megadome.

NARRATOR: 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.

NARRATOR: 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.

JEAN-PHILIPPE GARDEUR: Everything we touch, everything we do is completely crazy.

NARRATOR: 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... (laughing) NARRATOR: 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.

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

So, yeah, it is very, very challenging.

(man giving instructions) NARRATOR: 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.

REPORTER: Soviet authorities have been trying to downplay the incident, claiming that there are only two dead... REPORTER 2: ...conflicting reports coming out of the Soviet Union.

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

WOMAN (speaking Russian): NARRATOR: 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?

NARRATOR: 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.

(Geiger counter clicking) 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.

STEINBERG (speaking English): Every day, every hour, every minute, it was improvisation.

NARRATOR: 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.

NARRATOR: Now, time is up.

SIMON EVANS: Yeah.

These are the world's most uncomfortable boots.

I think they preserve these just for occasional visitors.

NARRATOR: 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.

EVANS: Don't stop, don't stop.

NARRATOR: He stops to check some critical devices.

EVANS: 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.

NARRATOR: 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.

EVANS: 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.

NARRATOR: 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.

CAILLE: 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.

NARRATOR: 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.

CAILLE: It will be a complicated task.

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

NARRATOR: 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.

CAILLE: You have a lift of 30 meters, and then you build below, and the structure is going up, up, up, up, up, up.

NARRATOR: 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, they will slide it to one side of the worksite to make enough space to construct the second half.

Building it this way enables the construction crew to keep a safe distance from the destroyed reactor, where the radioactive materials left inside present the most danger to the workers.

The remains of the uranium fuel emit deadly radiation.

The most dangerous are gamma rays.

Gamma rays, like X-rays, are made of high-energy photons and can travel long distances.

Some of these pass through the reactor walls and stream out into the surrounding space.

Most will pass straight through a human body.

But some will interact, damaging cells and fragmenting DNA, which can cause cancer.

The dangers are very real.

In 1986, when the Chernobyl reactor exploded, it exposed workers and firefighters to high levels of gamma radiation.

In the following weeks, 28 workers died from acute radiation sickness.

(bell ringing slowly) Every year, on the anniversary of the disaster, the people of Slavutych-- the town where most Chernobyl workers now live-- remember those who lost their lives.

Viktor Ivkin was working at the reactor that night.

Like many others, he received a large dose of radiation.

(translated): By this time, I had already vomited a couple of times.

And if a person shows this reflex, that means they've absorbed over 100 roentgens.

NARRATOR: 100 roentgens is over 900 times the maximum amount of radiation considered safe to receive over an entire year.

(translated): Many of the chaps from that shift died.

I wish there were more of us left.

Every year, we're becoming fewer and fewer.

It's very sad.

NARRATOR: Today, inside the old sarcophagus, 95% of the uranium that was in the reactor before the explosion lies scattered and exposed.

Here, close to the sarcophagus, the high number of gamma rays makes it too dangerous to work for the long periods of time needed to construct the arch.

But the engineers have one thing on their side.

As the gamma rays leave the reactor, they get further apart from each other and some are absorbed in the air.

So for every 1,000 gamma rays passing through a person standing 30 yards from the radioactive source... only about one will pass through a worker standing 300 yards away, where they are building the arch.

Radiological engineer Nicolas Guilcher measures the radiation across the site.

GUILCHER (speaking English): NARRATOR: A special unit of 50 people checks the daily amount of radiation every construction worker receives.

We provide everybody with dosimeters.

I have the French national dosimeter, I have the Ukrainian national dosimeter, and then we also have an electronic dosimeter that is our operational dosimeter.

NARRATOR: The dosimeters ensure no worker receives more than their annual permitted dose of radiation.

So every aspect of the construction must be designed with this in mind, including the arches.

CAILLE: We have decided to build 300 meters away because we can work as much as we want: ten hours a day, eight hours a day, the full week, the full year.

NARRATOR: There's another challenge that makes this already complex project even tougher: the lack of time.

The hastily built sarcophagus covering the reactor could collapse at any moment.

JEAN-PHILIPPE GARDEUR: Good morning, all.

This is a jacking and lifting.

So I will ask you to not stand under the load if you don't need to be under the load.

(interpreter translating instructions) NARRATOR: The lifting team needs to work fast to complete the arch on schedule.

GARDEUR: As soon as you are ready... NARRATOR: Jean-Philippe Gardeur is gearing up for the final lift, to raise the second half of the arched enclosure to its full height.

GARDEUR: Everything on this field is huge, is enormous, you know.

We don't have small things.

Everything we touch, everything we do, is completely crazy.

(machinery humming) Nice to hear this noise, huh?

NARRATOR: It will take 40 huge jacks to raise this metal monster.

That's going to be very tight, that's for sure.

But we will see.

NARRATOR: Each jack has enough power to lift over two loaded jumbo jets.

Okay, guys, so we start jacking now.

MAN (on radio): Okay.

GARDEUR (speaking English): Quickly.

Quickly, quickly!

Seven-eight.

MAN: Yes.

MAN 2: Five millimeters.

MAN (on radio): Okay, let's continue.

(speaking softly) (man responds on radio) GARDEUR: Okay, perfect.

(cheering) NARRATOR: Success.

But the team can't afford to relax.

If the sarcophagus collapses before the arch is in place, the fallout would contaminate the arch and the whole worksite.

It would undo the years of work spent cleaning up the area, a process which began immediately after the accident, and that came with a heavy loss.

After Chernobyl exploded, it spewed radioactive dust onto the surrounding countryside.

The Soviet authorities declared a 19-mile-radius exclusion zone around the reactor.

They drafted 350,000 people to clean up the radioactive fallout.

They were called liquidators.

At the center of the hotone, they cleared the radioactive debris from the roof of the exploded reactor.

Some only had 45 seconds to perform their task, before their dose of radiation became too great.

In the surrounding area, they washed down surfaces to remove the radioactive dust.

They bulldozed and buried the most contaminated homes, along with over a million tons of contaminated soil.

Among the liquidators was Ivan Martynenko.

MARTYNENKO (translated): Our first assignment was to build a special decontamination center for washing vehicles and people.

They told us that if we were exposed to the level of ten roentgens, we would be honorably discharged.

It was a lie.

Even when we had that sort of exposure, they said that there are no units to replace us.

OFFICER: V raion provedeniya spetsialnoi rabotki naselyonogo punkta, po marshrutu: Avtopark, naselyonniye punkty Ordzhonikidze... NARRATOR: The World Health Organization estimates that more than 2,000 liquidators have died or will die as a result of the radiation they received.

No one wants a repeat of this disaster.

(horn beeps) To prevent one, it's essential the arch is completed as quickly as possible.

2 1/2 years after the construction began, it's late fall, and engineers are anxious to entirely cover the shelter's enormous roof before the winter storms hit.

IAN CARLING: The biggest problem we have is the weather.

It's the biggest, biggest problem on here.

For the winter period, we can lose three months, four months of the year.

NARRATOR: British engineer Ian Carling is in charge of constructing the roof of the arch.

In Ukraine's harsh climate, that's a daunting challenge.

Winter temperatures here can plunge to -20 degrees Fahrenheit.

CARLING: Today we have the wind problem.

Yesterday we had ice on the roof, so we could not work.

We lost a whole morning yesterday just through the ice on the roof alone.

NARRATOR: Ian's team must work 350 feet above the ground.

That's more than 30 stories high.

(shouts) CARLING: For this job, it's extremely dangerous-- the winds, the rains.

It makes the surfaces that we're working on very slippy.

NARRATOR: The roofers are all trained rope-access technicians, but the dangerous conditions here require both skill and absolute concentration.

CARLING: Some of the materials that we're using, they can act like a kite, if you like.

You can imagine a sail in the air, and the material is razor-sharp.

So it's very, very dangerous, and...

So yes, we have to be very, very careful.

NARRATOR: Even when its roof is complete, the arch will be far from finished.

Before they slide it over the reactor, engineers must transform it into a machine designed to deal with Chernobyl's radioactive debris.

The radioactive material inside the reactor will remain dangerous for at least 20,000 years.

If engineers simply covered the reactor with the arch and did nothing else, they would only be adding to the problem.

In time, like any building, the arch would eventually collapse.

A future generation would need to build another even bigger structure to keep the reactor safe.

To prevent this, engineers must design a system within the arch to clean up the destroyed reactor.

The process will require a series of difficult steps.

CAILLE: We have to provide tools to enable the deconstruction.

We have to remove the sarcophagus built by the Russians.

So first of all, it's to remove the roof over the exploded reactor.

And then, after, they will have to break the concrete.

And at the end, remove the fuel, the heart of the reactor.

It will take a long time.

NARRATOR: No one has attempted to dismantle an exploded nuclear reactor before.

Radiation makes the job too dangerous to do by hand, so the workers must rely on tools operated at a distance.

(machine humming) In Shoreview, Minnesota, Rob Owen leads the team building a special remote-control crane to dismantle the reactor.

OWEN: The fuel that was there is still there, but when they start to dismantle the sarcophagus, you're going to expose that fuel.

And as it gets exposed, the level of radiation will get much, much higher than it is today.

NARRATOR: Rob is testing a quarter-scale model of the remote-controlled crane that will be installed inside the new shelter.

It uses an ingenious system of wires to carry a platform holding a robotic arm.

The configuration of the wires is crucial.

If the platform was supported by vertical wires, it would swing.

But using three pairs of wires arranged in triangles and adding a heavy weight makes it rigid.

It has one drawback.

A strong side force could move the platform up and slacken some of the wires.

But if the weight on the platform is heavy enough, all the wires will stay tight and the platform will remain rigid enough to hold the robot arm that will dismantle the sarcophagus.

OWEN: We kind of had to really go back and study the design.

Would it work here?

Could we make it big enough?

It had to be considerably larger than anything that had ever been built.

But the idea was really intriguing because of all the pluses.

NARRATOR: To drill into walls or pull a beam, the robotic arm must be able to push and pull horizontally.

OWEN: We have the six wire ropes and a lot of weight here, as you can see on the bottom.

All the cables remain in tension.

It provides that stiffness that allows you to do pushing, pulling.

(whirring) NARRATOR: The model demonstrates that the concept works, but it must be tested at full scale.

If successful, this special crane will be deployed alongside conventional cranes to dismantle the damaged reactor.

The job will take decades.

And this creates another major design challenge for the engineers building the arch.

CAILLE: It must last 100 years.

The metallic structure cannot last 100 years.

You have to protect the structure and repaint it.

I mean, as I'm French, I take the example of the Eiffel Tower, which is repainted every seven to ten years.

NARRATOR: Painting the steel arch protects it, for a time.

But eventually the paint will degrade, exposing the metal to moisture in the air, causing it to rust.

DAVID COULET: The steel structure has been painted in the factory.

This paint will last 15 years.

Unfortunately, we will not be able to renew it once the arch will be in final position, because the radiation conditions at that location are too severe for a painter.

NARRATOR: In its final position over the reactor, the arch will be impossible to repaint.

To solve this problem, they're engineering the steel to be inside a special climate-controlled environment.

The gap between the arch's exterior roof and interior ceiling will be sealed, creating a vast enclosed space around the steelwork.

Powerful fans will pull in air from outside, channel it through massive dehumidifiers to remove moisture, then blow the dried air along two miles of aluminum ducts into every corner of the enclosed space.

The ducts will constantly recirculate the dry air to make sure that the atmosphere in the enclosure remains dry so the steelwork doesn't rust.

For the plan to work, Ian Carling must ensure the interior ceiling is completely sealed.

(speaking Russian): CARLING: We have some junctions here, so because of this gap, we have to do a compressed sealant, which makes the air seal tight.

NARRATOR: But the siding here, known as cladding, has another important job.

When the cranes dismantle the old reactor, they will throw up clouds of radioactive dust.

CARLING: This cladding, it's stainless steel, and it's designed for the purpose of containing any airborne contaminated particles from escaping into the environment during the dismantling of the reactor number four.

NARRATOR: Spring 2016.

The massive cranes have arrived from America.

Before engineers slide the arch over the reactor, they must install them and the carriage that will carry the robotic arm.

Baptiste Briois is the engineer in charge.

Up, up, up, and up.

NARRATOR: Today he has a 26-story climb to the control room.

Three hundred sixty-nine.

369 stairs.

Twice a day.

Everyone in position?

Everything's ready?

We can start.

NARRATOR: The team relies on 13 hydraulic jacks fixed near the top of the arch to lift the 900-ton crane into position.

Inside the jacks, hydraulic jaws grip the wires attached to the crane and slowly hoist them up.

It's coming closer and closer-- I like it.

NARRATOR: But just as the crane lifts off, they hit a glitch.

BRIOIS: Sorry, sorry.

Apparently we have a little problem with the strand carousel.

NARRATOR: As the wires leave each jack, they should coil around a carousel.

But on one, the lifting wires are tangling.

BRIOIS: Down, I'll be right back in a minute.

NARRATOR: Working 300 feet above the ground, it's a precarious operation to wrestle the wires back into place.

(men talking) BRIOIS: It was very quick.

They are very efficient.

So we can still finish today.

NARRATOR: Now engineers can start tests on the full-scale crane.

Now we can lift?

Yes-- we can lift, yes.

NARRATOR: Once the shelter has been moved into position over the old reactor, this crane-- yet to be fitted with its robotic arm-- will start dismantling the ruins of the Chernobyl reactor.

It will be the final step in the long operation to seal the reactor away and make the site safe.

But then they will face a problem that bedevils the entire nuclear industry: where to store the radioactive waste.

Today, more than 30 years after the Chernobyl disaster, the 19-mile exclusion zone around the reactor is still a restricted area.

But aside from the absence of people and a few warning signs, there is little indication that large amounts of radioactive material fell on this land.

Nature has reclaimed the empty human settlements.

Two miles from the reactor lies the abandoned city of Pripyat.

50,000 people once lived here, many of them workers at the nuclear plant.

IVKIN: Ochen mnogo bylo roz.

(translated): There were roses everywhere.

There was a lot of greenery and many young people, and the river flowing right through the city.

It was just great.

We remember the place as clean, beautiful, and bright.

NARRATOR: High levels of radioactive debris contaminated the city.

Like other areas inside the zone, it will remain uninhabitable for hundreds of years.

For decades, the authorities have used the zone as a dumping ground for contaminated machinery from the clean-up operations.

(man whistles) NARRATOR: Now they are building a facility here for storing radioactive fuel and waste from the reactors at Chernobyl and from other plants in Ukraine.

Storing nuclear waste has challenges of its own, and this ambitious plan for the future has yet to be entirely mapped out.

With winter approaching, the team is working to a tight deadline.

The workers have just 11 days to prepare to move the arch.

DRISCOLL: We're coming to the end.

We're preparing for the skidding of the arch over the reactor.

At last, this huge thing is built and it's going to move to where it should be.

NARRATOR: For the engineers, this is the last chance to make sure everything works.

DRISCOLL: There's a lot more activity because we now have many more tasks to complete, all in the same period.

NARRATOR: One remaining job is crucial before they slide the arch-- opening its enormous special doors.

To stop radioactive dust from escaping, the arch must form a perfect seal around the old reactor building.

But parts of the old sarcophagus stick out, which would prevent the arch from sliding to its final position.

So engineers have equipped the arch with tilting panels-- like giant cat doors-- that they must raise and then lower into place when the arch is in position, sealing the small remaining gap with an extra-durable plastic membrane.

Just as the team is raising the heaviest panel, a blizzard strikes.

Despite the weather, the engineers must press on.

They operate the panels remotely from a platform on one side of the arch.

COULET: We have six tilting panels, and the largest one is 320 tons.

So once the arch will have reached its final location, the radiation condition do not allow a manned operation at that location.

So we have designed a system of hydraulic jacks, a system of winches, to close this panel remotely.

NARRATOR: It takes four hours to winch the massive "cat door" open.

4 1/2 years after the work began to build the 40,000-ton shelter, the day finally arrives when the team will attempt to slide it over the reactor.

It will be the largest structure ever moved across land.

CAILLE: Now we are almost at the end.

Yes, we are under pressure.

It's a great challenge because of the size.

I am also thinking about myself.

I have already a lot of white hair, and the sooner it will be finished, better it will be for me.

NARRATOR: Moving this massive structure will be no simple task.

The obvious way to move a monster arch would be on wheels.

But that won't work here.

The arch is so heavy, that it would overload the wheel bearings, which would fracture, leaving the structure stranded.

So instead of wheels, engineers will use 116 stainless steel feet, known as skid shoes.

These will slide on Teflon pads placed on top of the rails.

Geert-Jan Thijssen leads the team that will move the arch.

THIJSSEN: Over these nooks we put the Teflon pads.

And basically, if you put a lot of them, you make your own sliding way for the skid shoes to slide on.

NARRATOR: These pads are made of PTFE, the polymer used on non-stick cookware.

This low-friction surface will help the stainless steel feet of the arch to slide.

To push the arch, engineers will fit each leg with hydraulic pistons.

These move a pair of wedges that grip the steel rail.

Powerful pumps will then extend the pistons to push the arch forward.

More than 200 pistons must work in perfect unison to slowly slide the arch toward the reactor.

CAILLE: This is a one-off skidding.

I mean, you can't go back.

So you should be sure that you have not forgotten anything.

NARRATOR: This is the critical maneuver everyone has been working toward for seven years.

MAN: We have the confirmation that we are ready.

Okay.

(man speaking Dutch on radio) We can go?

Yeah?

Okay, allez, le top, on y va. BRIOIS: On y va, okay.

We go.

NARRATOR: The pistons fire up.

2,000 tons of force pushes against the arch.

MAN (on radio): Okay, going.

NARRATOR: And they're off.

The vast structure, as heavy as 3 1/2 Eiffel Towers, slides toward the reactor at about 33 feet an hour.

Even if all goes well, it will still take six long days.

It's crucial that the pistons move both sides of the arch at the same speed.

To make sure they don't twist the arch, lasers measure the exact position of both sides and display any difference on screens in the control room.

THIJSSEN: If you move one side faster than the other, you will get bending in the arch, which can lead to damage of the arch.

And if we go too far from each other, then, you see, the deviation between the two gets too high, and then we have to adjust and move forward to correct the readings we get on the system.

MAN (on radio): For the south, 383.

Okay.

Okay, thank you.

What is that?

THIJSSEN: That's the difficult part.

You get so much data, you have to act correctly and quickly.

Okay, go.

(man speaking on radio) (man responding on radio) NARRATOR: The closer they get to the reactor, the more difficult the operation becomes.

Sliding the arch over the reactor walls will be tight.

People will follow the sliding itself, of the arch.

80, 90 people will be involved.

We have a lot of watchmen because our clearance is very limited.

The clearance that we have is quite tight.

It's 50 centimeters.

MAN: What could go wrong during the skidding?

Nothing.

(laughs) Everything should be fine.

NARRATOR: But just as the arch approaches the reactor, they run into trouble.

It becomes snagged on a barbed wire fence.

(shouting) NARRATOR: The radiation here is high, so they must act fast.

There is two rebar who are touching the arch.

So we are sending somebody with a saw to remove these rebar to be sure that we are not damaging the arch.

(saw buzzing) NARRATOR: With the wire removed, there's one last task before the final push.

They must partially lower one of the tilting panels.

If they wait until the arch is in position, then, as they close it, it would jam on a chimney.

CAILLE: It will hit the chimney.

So we have to tilt it before.

And then we can restart till the end and finish it tonight.

Oh, Vitaly!

Yes!

Go, go!

NARRATOR: The panel clears the chimney, and the final push begins.

(cheering and applauding) NARRATOR: After seven days of pushing, the arch is finally in position over the reactor.

CAILLE: It's a feeling of pride.

We have achieved something great.

It's a big step for safety.

And Ukraine and Europe will be much safer now.

NARRATOR: In the coming years, the cranes with the robotic arm attached will start the dismantling process.

It has taken 18 years of planning, seven years of construction, and a unique international collaboration of 10,000 men and women from 30 countries, but three decades after the world's worst nuclear accident, Chernobyl is finally sealed away for generations to come.

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