Harold Denton, left, is shown in the control room at Three Mile Island with President Jimmy Carter and a power plant technician on April 1, 1979. (AP)

At 3:55 a.m. on March 28, 1979, people living near the Three Mile Island nuclear power plant about a dozen miles from Harrisburg, Pa., were awakened by a loud roar that “shook the windows, the whole house,” in the words of one resident.

Within hours, alarm sirens sounded inside the facility, as workers struggled to understand what was happening. Harold Denton, the country’s leading authority on nuclear safety, was summoned from a meeting at Nuclear Regulatory Commission headquarters and told that a “relatively serious sort of event” had occurred.

Mr. Denton, a once-obscure federal regulator who went on to be hailed as a hero for his calm leadership and technical mastery during the most serious nuclear power accident in the country’s history, died Feb. 13 at his home in Knoxville, Tenn. He was 80.

He had complications from Alzheimer’s disease and chronic obstructive pulmonary disease, said his wife, Lucinda Denton.

Mr. Denton had been an inspector of nuclear reactors for 15 years before he became director of the federal Office of Nuclear Reactor Regulation in 1978. Just one week before the Three Mile Island emergency, he happened to watch “The China Syndrome,” a Hollywood movie about an accident at a nuclear power plant. When a technician, played by Jack Lemmon in the film, looked at a gauge, Mr. Denton casually remarked to his wife, “That’s a faulty reading.”

Harold Denton speaks to reporters at the Three Mile Island nuclear plant on April 2, 1979. (Paul Vathis/AP)

As the real-world emergency unfolded at Three Mile Island, there was confusion about the accident and its potential risk. State officials and executives from the utility company, Metropolitan Edison, offered little guidance.

“I live a mile from the plant,” one reporter asked. “What are you going to be doing to protect my family?”

Telephone lines were overburdened, making communications between Three Mile Island and Washington difficult. NRC Chairman Joseph Hendrie said he and Pennsylvania Gov. Richard Thornburgh were “operating almost totally in the blind. His information is ambiguous, mine is nonexistent.”

Thornburgh recommended that everyone within 10 miles of the nuclear facility stay indoors. Officials at the Hershey’s candy company, 11 miles away in Hershey, Pa., wondered if it was safe to make chocolate with local milk.

CBS News anchor Walter Cronkite described the events at Three Mile Island as “the first step in a nuclear nightmare.”

The danger was real: Increased radiation levels had been measured in the atmosphere, and temperatures inside the reactor were abnormally high.

News reports speculated on several apocalyptic scenarios, including the possibility that an explosion could rip through concrete walls four feet thick. The most serious risk was a meltdown, in which the reactor’s superheated core could burn through the building’s base and burrow into the earth.

Mr. Denton was monitoring events from NRC’s headquarters, but President Jimmy Carter said a federal official should be at the scene to take charge. On March 30, two days after the initial accident, Mr. Denton flew to Three Mile Island in a White House helicopter.

He found the power plant to be in “absolute chaos,” he told The Washington Post at the time. He brought in as many as 100 scientists to examine the facility, and a special phone line was installed, connecting Mr. Denton directly to the White House. His son drove to Harrisburg with a suitcase containing extra clothes and a toothbrush.

During his first day, Mr. Denton learned that, in a colossal blunder, radiation had been released from an auxiliary building. A large hydrogen bubble in the building that contained the reactor was in danger of exploding.

Moreover, water pumps intended to cool the reactor’s fuel rods had been turned off, and a relief valve was locked in the open position, which allowed steam to escape. As a result, the 36,000 uranium fuel rods in the reactor’s core overheated, and as many as half were damaged beyond repair. Three Mile Island experienced, in other words, a partial meltdown.

As the face of the federal government at Three Mile Island, he made it clear that the utility company answered to him.

“Since I’m the director of the office of nuclear reactors,” Mr. Denton said at the time, “I can issue, modify or suspend licenses. So I never had any doubt that if I didn’t like the way they were running it, I could issue an order on the spot.”

His news conferences were carried on national television, and the public found his technical expertise and folksy demeanor reassuring. He gave the president a tour of the facility, calming public fear.

Within days, the hydrogen bubble dissipated, the reactor began to cool and the danger passed. There were no injuries at Three Mile Island, but the accident led to reforms and slowed the construction of nuclear power plants for decades.

Mr. Denton was profiled in People magazine, his likeness appeared on T-shirts, and several Pennsylvania colleges awarded him honorary degrees. He was that oft-maligned figure, a $50,000-a-year federal regulator, who managed to be the voice of competence and reason at a time of peril.

“Harold Denton was the true hero of the Three Mile Island nuclear crisis,” Thornburgh, who later served as U.S. attorney general, told the Harrisburg Patriot-News. “His easy manner, apparent candor and ability to speak plain English as well as nuclear jargon quickly made him the world’s most believable expert on the technical situation at TMI.”

Harold Ray Denton was born Feb. 24, 1936, in Rocky Mount, N.C. His father drove a bread delivery truck, and his mother died when he was young.

At North Carolina State University. he studied nuclear engineering and helped pay for his tuition by sweeping floors and by tutoring the football team. After graduating in 1958, he worked at a DuPont nuclear facility in South Carolina.

In 1963, he joined the old Atomic Energy Commission, which was reorganized into other agencies, including the NRC, in the 1970s. After nine years leading the regulatory unit, Mr. Denton took over NRC’s public affairs office in 1987. He retired in 1993.

He was among the first U.S. scientists to visit Chernobyl, the site of a deadly 1986 nuclear disaster in the former Soviet Union. He also visited Japan to review the damage to a reactor after a 2011 earthquake.

Survivors include his wife of 57 years, the former Lucinda Oliver of Knoxville; three children, Liz Baird of Holly Springs, N.C., Harold R. Denton Jr. of Knoxville and Spencer Denton of Winston-Salem, N.C.; seven grandchildren; and a great-grandson.

Mr. Denton spent almost three weeks at Three Mile Island, losing 10 pounds in the process. The accident was the result of compounded human errors — pumps turned off, valves left open, instrument readings wrongly interpreted — but the regulations Mr. Denton helped put in place kept it from being even worse.

“I suspect it was a little bit because of our actions and maybe a bit of serendipity,” he told The Post in 1979. “A little bit of luck and little bit of forethought.”