AMNA NAWAZ: Forty years ago this Sunday, the world's worst nuclear power disaster exploded into history at the Soviet Union's Chernobyl nuclear plant north of Kyiv in what's now independent Ukraine.

The ensuing Soviet cover-up and then cleanup operation made Chernobyl a byword for dereliction and mismanagement.

But, decades later, a global energy shock and Russia's war on Ukraine, which has exposed the West's reliance on Russian oil, are changing attitudes towards nuclear energy again.

From the Exclusion Zone of Chernobyl, seen here with the Ukrainian spelling, special correspondents Simon Ostrovsky and cinematographer Amanda Bailly filed this report.

SIMON OSTROVSKY: Nikolai Pobedin arrived at the Chernobyl nuclear power plant in Ukraine one year after the disaster that made this facility infamous 40 years ago.

Almost every year since, he's manned this desk at the plant, managing a critical department that prevents Chernobyl's still radioactive waste from melting down.

It's a demanding job, but, in 2022 it got even harder.

That's when invading Russian soldiers took over the site.

NIKOLAI POBEDIN, Chernobyl Nuclear Power Plant (through translator): They used us like a shield to store their weapons and equipment.

After all, no one would attack a nuclear power plant.

SIMON OSTROVSKY: Were you hostages?

NIKOLAI POBEDIN (through translator): Yes, of course.

SIMON OSTROVSKY: As Pobedin and his colleagues continued to manage the old radioactive debris, they also knew they were confronting a new potential nightmare since Russian troops had turned this place into a military target.

But his critical duties prevented him from ever thinking of leaving.

NIKOLAI POBEDIN (through translator): Sometimes, people ask me, why didn't you run away?

Even if we had a place to run, how could we leave our workstations unattended?

SIMON OSTROVSKY: We'd come to Chernobyl just 70 miles north of Kyiv to see for ourselves what this power plant looked like 40 years after the worst nuclear disaster in history, which sent radiation billowing across Europe.

To get inside today, the first step is to measure your current radiation reading for comparison later.

We changed into a special set of protective clothing to avoid bringing radioactive particles home.

Then our guide, Artur Ovsepian, took us inside.

So how many people work at the plant now?

ARTUR OVSEPIAN, Chernobyl Nuclear Power Plant (through translator): At one time, we have around 600 workers.

SIMON OSTROVSKY: Got it.

So there were 600 people here every day?

ARTUR OVSEPIAN: Yes.

SIMON OSTROVSKY: Two of Chernobyl's reactors remained functional into the late 1990s, but Unit 4, where the disaster occurred, still requires monitoring and cleanup of the radiation, an effort that slated to go on until 2064.

And that's the reactor we were taken to see.

ARTUR OVSEPIAN: Don't lean on anything.

Don't drop anything.

If you drop something, we need to call dosimetrist, and he will check if it's contaminated.

SIMON OSTROVSKY: OK.

We're just going to follow your lead.

Oh, wow.

OK, so this is the control room of the ill-fated reactor No.

4.

It's so spooky in here.

What an eerie, eerie room.

This is the control room of Chernobyl Reactor No.

4.

The decisions that led to the worst nuclear accident in history were made in this room.

It was a flaw in the Soviet-designed reactor that caused it to overheat and blow up just as its operators were trying to shut it down, sending clouds of radiation into the sky and showering the area with chunks of radioactive debris.

Anyone within a nearly 20-mile radius of the plant suddenly found themselves under evacuation orders.

With the Soviet Union offering few explanations and downplaying the scale of the event, here's how the "News Hour" covered the incident two days later.

ROBERT MACNEIL, Former Anchor, "PBS NewsHour": The Soviets today reported an accident at a nuclear plant which damaged a reactor and caused some injuries.

The Soviet news agency TASS said the accident happened at the Chernobyl plant north of Kyiv in the Ukraine.

It gave no more details than to say that measures are being undertaken to eliminate the consequences of the accident, aid is being given to those who have suffered injury.

SIMON OSTROVSKY: Those measures would end up being much more expansive than first reported, with thousands of Soviet soldiers being exposed to radiation as they returned contaminated debris to the damaged reactor before encasing it.

In 2016, this new structure was completed to prevent radiation from leaking out, and it's designed to last 100 years.

It's over 330 feet tall, making it the largest movable structure in the world.

It was assembled next to the damaged structure on rails and moved over to protect it.

I don't think I have ever been in an enclosure this big in my life, but what we're actually looking at is two enclosures.

The cement blocks around Reactor No.

4, those were put there in the years immediately after the accident by the Soviet Union at a great cost to human life.

And then, many years later, the European Union funded this new enclosure that we're standing underneath right now.

That came at a cost of $2.5 billion.

But, in 2025, less than a month after President Donald Trump was inaugurated, Russia struck this containment dome with a Shahed-type drone.

And although the outer shell was quickly patched, the damage compromised the structure's integrity.

This simulation shows how a fire spread inside its inner layers, burning up a membrane meant to seal in radiation.

ARTUR OVSEPIAN: So, the drone caused more than 300 holes, which we couldn't fix.

SIMON OSTROVSKY: And, before, this was all completely sealed, nothing came in, nothing went out?

ARTUR OVSEPIAN: No.

No.

SIMON OSTROVSKY: It's unbelievable that Russia would do this so close to its own border.

ARTUR OVSEPIAN: Yes, yes, exactly.

SIMON OSTROVSKY: I mean, we're very close to the border of Belarus, and Russia isn't much further away, and yet they would send a drone to strike this dangerous contaminated site.

ARTUR OVSEPIAN: Yes, for sure.

Russia does, like, nuclear terrorism.

And the Chernobyl nuclear power plant is the first power plant in the world which was under occupation.

The next one is the Zaporizhzhia nuclear power plant.

SIMON OSTROVSKY: Which is still under occupation.

ARTUR OVSEPIAN: Yes.

Yes.

SIMON OSTROVSKY: The fate of the Zaporizhzhia nuclear power plant, Europe's largest, remains in limbo after Russia captured it in 2022.

ARTUR OVSEPIAN: Their terror has no limits.

SIMON OSTROVSKY: The attack has put cleanup efforts behind schedule, and it's estimated it will cost another half-a-billion to repair the damage, money that will come from an international fund, not Russia.

When we get ready to leave, we're tested again for radiation.

COMPUTER VOICE: Four, three, two, one.

Thank you very much.

No contamination.

SIMON OSTROVSKY: That's a relief.

Just outside the plant is the abandoned town of Pripyat, where 50,000 people once lived before the explosion; 40 years later, it's been reclaimed by the forest.

For decades, this ghostly landscape stoked fears around nuclear energy.

Despite that, Russia went on to become the world's largest exporter of reactor fuel.

But the invasion of Ukraine cost it billions in uranium supply contracts with the West, prompting Europe and the U.S.

to reinvest in their own nuclear power industry.

Volodymyr Kudrytsky is the former head of Ukraine's national energy company.

He says not enough has been done globally to break from Russian energy.

VOLODYMYR KUDRYTSKY, Former CEO, Ukrenergo: I think that not many countries in the world really realize how dangerous it is to depend on Russian energy in any form.

SIMON OSTROVSKY: Gas?

VOLODYMYR KUDRYTSKY: Be it gas, oil, oil products, or uranium.

Russia weaponizes energy.

It did it with Europe.

It can do this with any other big customer like United States.

SIMON OSTROVSKY: Kudrytsky says Russia's targeting of Ukraine's energy sector makes the case for building many smaller energy facilities, including nuclear, and shifting away from centralized large sites that can more easily be destroyed in a conflict.

VOLODYMYR KUDRYTSKY: We experienced thousands of attacks on our large power objects.

And, of course, these power objects are very vulnerable to those attacks because of their scale.

The only answer to this can be decentralization.

This is what they do on the front line.

You do not group 1,000 people in one spot, right?

You disseminate people over a certain area.

The same principle with energy.

SIMON OSTROVSKY: Forty years after the disaster of Chernobyl, the war with Russia has forced Ukraine to rely heavily on nuclear power, as its grid has come under relentless attack.

After nearly four decades at the plant, Nikolai Pobedin understands the risks of nuclear energy as well as anyone, so I asked him if he thought we should continue to develop it.

NIKOLAI POBEDIN (through translator): I think this is the energy of the future.

SIMON OSTROVSKY: But to get to that future, Ukraine must still survive an uncertain present.

For the "PBS News Hour," I'm Simon Ostrovsky in the Chernobyl Exclusion Zone.