When Russian troops launched the first-ever armed assault on a nuclear facility, Andriy Tuz became the voice to the West of what seemed a looming disaster.

As spokesman for Ukraine’s sprawling Zaporizhzhia complex, the 33-year-old appeared on local television, Western media and in solemn online updates to describe chaotic scenes of falling shells and gunfire that shocked nuclear-safety experts and governments worldwide.

“Shooting is being continued, from air and tank,” Tuz told CNN’s Anderson Cooper in March 2022. “Any moment, it may result in nuclear accidents.”

In the months that followed, after Zaporizhzhia was taken, Tuz said he was tortured by the Russians and his mother’s life was threatened. And then to get out of prison, he agreed to make a video disavowing his previous statements that the facility wasn’t safe. He said he doesn’t believe that now, and he didn’t believe it then. He worries that the risk of nuclear terrorism remains high at Zaporizhzhia.

He didn’t think anyone would take the video seriously.

To his disbelief, they did. “I’m not an important person,” he said.

After being released by the Russians and fleeing with his mother, Tuz learned from colleagues that they had been told he was a traitor, they shouldn’t speak with him and he had been fired.

Now living in exile in Boston, where he was given a job by the Massachusetts Institute of Technology, Tuz said he is lucky to be in a safe place and with a job he loves. More than anything, he wants Ukraine to prevail so he and others can go home, though he doesn’t know if it will ever be safe for him after being accused of collaborating with the Russians.

Representatives for the Ukrainian security service, the SBU, didn’t respond to requests for comment about whether Tuz is still suspected of collaborating with the Russians.

The SBU is tasked with investigating thousands of allegations of collaboration with the Russians, but it isn’t always easy to distinguish between those who threw their lot in with the Russians for their own benefit and those just seeking to survive. SBU officials said the task is particularly hard when the events took place on territory that is still occupied.

More than five million people are displaced inside Ukraine and 17 million need humanitarian assistance, according to the United Nations. An estimated 6.3 million are refugees globally.

Around 480,000 Ukrainians like Tuz have come to the U.S., according to the Department of Homeland Security.

Tuz is grateful to the U.S. and European allies for the support of Ukraine, but never aspired to the American dream—he was happy in Ukraine. As a child, his mother fed him so well that he didn’t even recognize the feeling of hunger until he went to college.

Tuz received three master’s degrees and lived in Enerhodar, a city that is home to thousands of Zaporizhzhia’s employees and that means “energy’s gift.” He owned a house, had his mom nearby and indulged his love of speed with a 2015 Maserati Ghibli, one of thousands of vehicles that get deemed unfixable after an accident in the U.S. but can be bought cheaply abroad.

The Zaporizhzhia plant is just 300 miles from Chernobyl, the site of the world’s worst nuclear accident, in 1986. When Russia invaded, Tuz thought it would be one of Ukraine’s safer places.

Now Enerhodar is occupied. The reactors at Zaporizhzhia where Tuz had worked for a decade are shut down. Sometimes when he calls friends across Ukraine, he hears bombs in the background.

“The war has broken many lives in Ukraine,” Tuz said.

The reality of living under Russian occupation is hard to explain.

“You cannot say what you think. You cannot do what you want,” he said. “It’s like a rabbit on a farm. You can eat carrots, you can walk around, but anytime the farmer wants to kill you, he can kill you.”

The Russians detained and in many cases tortured hundreds of plant workers. When first asked to make the propaganda video for the Russians, Tuz refused, but he knew he couldn’t avoid it forever.

Tuz and his mother fled together, driving east and south, hoping to circle the Black Sea through countries including Russia, Georgia and Turkey to reach western Ukraine.

Thousands of Ukrainians traveled the same route, but Tuz was stopped at a checkpoint and taken into custody. He said he was beaten, hooded, handcuffed and driven to a prison in Sochi, Russia, where he was held for two days. Tuz said his hands were burned and guards laughed as they hit the back of his head, saying they were helping him by killing a mosquito.

“I can’t think. I can’t breathe,” Tuz said. “At that time, I think I’ll die for sure. I just don’t know how fast.”

Tuz said he was kept in a dark cell, about 2 meters by 2 meters, with another man who had gone crazy from the experience and banged on the door nonstop. Human waste covered the floor.

Tuz said he was told that if he made a video, he might be able to see his mom again. In the video, he said that he was on vacation in Sochi, a resort town on the Black Sea, and that everything was fine at the nuclear plant.

“Only a person who stays in the same situation can understand,” Tuz said of the experience.

Afterward, the Russians returned his passport but warned him not to talk any further about the state of the plant. “Some kind of accident can happen with you. We can do something with you and your mom anywhere,” he said he was told. “Anywhere in Europe, the United States, you have no safe place.”

Terrified of being apprehended again, he and his mom embarked on a 10,000-mile journey, often going off road to avoid military checkpoints. They slept in the car or on the ground in the woods, afraid to speak with other travelers or linger in countries with Russian sympathizers. He and his mom eventually made their way to Switzerland.

Jacopo Buongiorno, a nuclear engineering professor and the director of science and technology at the MIT reactor, had a job opening and wanted to find a former Zaporizhzhia employee.

“We knew that there was talent there and we wondered if we could do some good for at least one person,” Buongiorno said.

Buongiorno’s contact at the Ukrainian Nuclear Society recommended Tuz and confirmed his story. Buongiorno and Tuz emailed and met for lunch when Buongiorno was speaking at Zurich University in February.

Buongiorno’s first impression was that Tuz was serious, composed and “obviously has been through a lot of pain and challenges.”

MIT brought Tuz to Boston through the U.S. government’s Uniting for Ukraine program, which provides a way for Ukrainian citizens displaced by the war to stay temporarily in the U.S. Around 174,000 people had come to the U.S. through the program as of Jan. 3, arranging their own travel.

Tuz, like all employees at nuclear reactors in the U.S., also had to pass a background check.

In July, Tuz joined the staff of MIT’s Nuclear Reactor Laboratory, where he works as an irradiation facilities engineer, helping design, build and operate experiments in the research reactor. Irradiation exposes materials or instruments to ionizing radiation, which is powerful enough to knock electrons out of atoms and can help make radionuclides for cancer treatments, among other uses.

Tuz lives in a small apartment, walks to work and doesn’t own a car. He has adopted one aspect of American life—his days revolve around work. “Go to my work, go back home, go to work, come back home,” he said of his days, but added, “I absolutely love my work.”

Tuz has 3,700 phone contacts but life remains a little lonely. There isn’t a large community in Boston with whom he can speak Ukrainian.

He remains mystified by the frozen foods and out-of-season fruits in the grocery store and the American habit of eating sandwiches for lunch. “This is not food,” said Tuz, who was used to a minimum of three dishes for lunch, often borscht with salo, the cured pork fat popular in Ukraine, along with a side dish of potatoes with steak, and dessert, often syrniki with honey.

Tuz doesn’t know if he will be able to stay in the U.S. after his two-year visa expires. His mother remains in Europe. He stays in touch with friends in Enerhodar, though there is always the fear that someone is listening.

“Inside they support Ukraine,” Tuz said, “but they cannot say the truth.”

—James Marson contributed to this article.

Write to Jennifer Hiller at jennifer.hiller@wsj.com