When Russia took control of Zaporizhzhia's nuclear power plant, Ukrainian technicians were forced to flee, or work at gunpoint

The warfare taking place around Ukraine's largest nuclear power plant has prompted warnings of possible disaster - but workers say the scenes inside the station are just as alarming.

"Every day [Russian soldiers] walk around armed with machine guns," says Sergii, a long-term employee at the facility, who asked the ABC not to publish his last name.

"One wrong word and you would be tortured."

For months, mostly Ukrainian workers have kept the Zaporizhzhia nuclear power plant in southern Ukraine operating while living and working under Russian occupation.

Many stayed to keep critical functions running despite the dangers of working in a nuclear facility constantly caught in the crossfire of the war on its doorstep.

"This is our work, it's our duty, all the equipment is still operating, and we had to keep monitoring this," Sergii said, almost a year after his ordeal began.

The morning after Russian troops broke through defence lines and arrived to take over the town of Enerhodar, south of Zaporizhzhia, Sergii rose as normal and went to work.

For 16 years he had arrived each day at the power plant on the outskirts of the city.

But this time the Russian flag was hanging over the station - the largest nuclear facility in Europe.

"It was very nerve-wracking, all the scientists did not know what to do, how to behave," he said.

He was allowed through the security checkpoint to resume his role as nuclear engineer in one of the plant's restricted areas.

Almost immediately he said he saw evidence of a bloody battle.

"I saw the remains of bullet-proof vests, and the uniforms of our soldiers," he said.

"Our soldiers, guards and other military personnel defended the nuclear plant.

"[I was told] at least 40 or 50 were killed."

Russians were now in control, and the regular Ukrainian workforce had no choice but to take orders from Moscow's forces, he said.

"People began to withdraw from their workplaces, the [Russians] began to beat, began to intimidate," Sergii said.

"Many of my friends were tortured, they were very badly beaten, there were short-term and long-term abductions.

"I don't know the fate of some."

Overnight, his life became unrecognisable. But he carried on for as long as he could, bound by his duty to his work.

Where do things stand at the site?

Since the start of Russia's full-scale invasion, the United Nations nuclear watchdog claimed shelling and missile strikes around the plant had caused the site to lose power at least six times.

The latest was less than two weeks ago, prompting an urgent warning from the head of the UN's International Atomic Energy Agency, Rafael Grossi.

"This cannot go on. I am astonished by the complacency - what are we doing to prevent this from happening?" Grossi said.

"Each time we are rolling a dice, and if we allow this to continue then one day our luck will run out."

Many experts have said the Zaporizhzhia facility is different, with more safeguards than the Chernobyl plant in Ukraine's north, which was the site of one of the world's worst nuclear disasters in 1986.

Still, "the situation regarding the nuclear plant is extremely critical", according to Dmytro Orlov the Ukrainian mayor of Enerhodar, who fled the city but was still running some operations from the nearby city of Zaporizhzhia.

"One of the worst issues is that the occupiers are using the plant as a nuclear shield while they are shelling other cities," Orlov said.

"They are covering themselves with a nuclear facility knowing Ukraine won't return fire."

Moscow denied these claims, and has said it has no heavy weapons at the site.

Last year, the UN called for the site of the nuclear facility to be demilitarised amid days of dangerous air attacks.

Both Russia and Ukraine accused the other side of plotting attacks around the power plant, but UN inspectors were allowed to enter the site in September.

The UN's nuclear watchdog has since released detailed reports showing the physical integrity of the plant had been compromised due to shelling, and while it addressed staff morale it did not claim Russian authorities were treating workers poorly.

"The reduced staffing levels combined with psychological stress due to the ongoing military conflict and the absence of family members who fled the area have created an unprecedented situation that no [nuclear power plant] staff should have to endure," Grossi wrote in one report.

Inside Zaporizhzhia's nuclear power plant

Another of the station's former workers, Alexey Melnichuk, paints a grim picture of life inside the facility.

He fled after four months of occupation but is proud of the work done by his friends who stayed.

"[My colleagues] show their professionalism - in many situations, without any instructions, they have prevented serious accidents," Melnichuk said.

"Every day it became harder and harder, people disappeared, my colleagues disappeared, some came back, some did not.

"Some people were tortured and they are so sick and unwell after this, having problems with their heart and circulation.

"The consequences are very heavy."

Escape from Bakhmut

On a mud-covered, shrapnel-dented minibus, 21-year-old Svyat unloaded his latest round of civilians evacuated from the besieged Ukrainian city of Bakhmut. Once they are safely off, he will turn around and make the deadly journey again.

Operations at the plant had been wound back, said Orlov, who still has regular contact with those running the facility.

"The nuclear power plant does not work for power generation anymore [due to safety issues] ... but it must maintain cooling of nuclear fuel," he said.

"We worry about the water reservoir, which is low ... this is how we keep spent nuclear fuel cool ... if this suddenly drops it will not be good."

He also holds grave concerns for the workers who stayed to keep the plant operating.

"[We have heard] about torture and kidnapping [of] staff ... the situation is very serious," he said.

After three weeks working under occupation, Sergii said he received an ominous warning that he may become a target for Russian authorities and made the difficult decision to try to escape Enerhodar.

"I have managed to bring only the jeans and the sweater that I am wearing now," he said.

"I am a patriot for my country, and it is impossible to be one there."

At first he joined Ukraine's military, fighting in some counterattacks in the country's east, but now he works with the aid charity helping displaced Enerhodar residents like him in Zaporizhzhia.

He still worries, not just about the people he left behind, but about the power plant he had dedicated much of his life to.

"The people who remain, this is their duty, this is their life's work.

"They understand that the country needs a safe operation of the nuclear power plant.

"If they leave it won't be safe for anyone to work, then there will be a disaster."

The fight brewing in the south

The south of Ukraine has become a critical battleground in this war, and analysts predict that will only intensify in the year ahead.

Battalions of men have been fighting to hold defensive lines here for more than 12 months, but soldiers at a southern front-line command centre told the ABC they were ready to start pushing Russia back.

"They constantly attack, we constantly repel them, they constantly get hit and attack again," the commander of one of these southern units, who wants to be known only as Bohun, said.

"The attacks are becoming fewer and fewer. We will keep putting pressure on them and we will defeat them."

Here they were motivated by Kyiv's successful counteroffensive in the southern city of Kherson late last year.

"The motivation of the fighters remains, because their homes remain there, here is motivation, we have nowhere to go and everything to lose," said Did, a 76-year-old soldier known as the grandfather of the Cossack battalion.

A successful counteroffensive from Ukraine in the occupied area south of the city of Zaporizhzhia could change the tide of the war, said Alina Frolova, an analyst from the Centre for Defence Strategies.

"It's critical because Ukraine could cut the front line in two ... it would isolate Crimea and stop Moscow's supply line onto the mainland."

"This is the start of counteroffensive on the Crimean territory, and Crimea is principal in this war because actually the war started here.

"Crimea has some kind of sectoral meaning for Russia and release of Crimea will have a devastating influence on their political life."

Dmytro Orlov was hopeful his city of Enerhodar would be liberated this year, and with it he hoped to see the successful rejuvenation of the power plant which had sustained so many livelihoods there.

"Before the occupation [the power plant] produced roughly one-fourth of the country's energy, so it will be extremely necessary in its restoration in order to renew the economy, to renew our country," he said.

"We know we will return Enerhodar, and we will rebuild."

-ABC