MANHATTAN PROJECT NATIONAL HISTORICAL PARK, Wash. >> In a spring morning in high, dry southern Washington, a bright yellow bus rumbled to a stop in a lot at the Hanford Site near the Columbia River. The fourth-graders of Orchard Elementary School in nearby Richland, Wash., were about to see one of this nation’s newest historical parks, surrounded by a valley filled with sagebrush, eagles and elk.

When the bus door opened, the kids rushed straight into a metal- and- concrete box of a building, nearly 100 feet tall, neighbored by a 200-foot exhaust stack topped by a wind-whipped American flag. Inside, looming like a Borg ship in “Star Trek,” stood a massive cube of graphite bricks and aluminum tubes.

“Welcome to the B Reactor,” said docent David Marsh. Then he explained how in this room American scientists made “the nuclear weapon that was used to end World War II.”

“Fat Man,” the atomic bomb that detonated on Aug. 9, 1945, over Nagasaki, Japan, originated here. The National Park Service, best known for its stewardship of peaks and valleys, is taking on the job of explaining how and why the U.S. built and used the deadliest weapons ever turned against mankind.

The Manhattan Project National Historical Park, established in November, is a joint effort by the Park Serv-ice and the U.S. Department of Energy. Besides the Hanford Site it includes Oak Ridge, Tenn. (where the enriched uranium that fueled the Hiroshima bomb was produced), and Los Alamos, N.M. (where bombs and components were designed and assembled).

Congress voted in 2014 to create this park, and Park Service leaders describe it as a chance to explore history that shaped not only the end of World War II, but also the advance of science and at least a half-century of geopolitics.

“It changed the world,” said Anne Vargas, an Energy Department docent whose father worked at Hanford.

The B Reactor is the park’s focal point in Hanford and the only structure most visitors will enter. The building had stood idle since 1968 and was slated for closure until the B-Reactor Museum Association, led by retired Hanford scientists and engineers, launched a preservation campaign. The association also built models on display at the reactor and made videos detailing the science and history behind the structure.

To see it, you reserve a seat on an official tour bus and meet at the Hanford visitor center in Richland. The bus ride into the restricted site takes about an hour; visitors typically spend about two hours at the reactor with a docent. (Another tour focuses on remnants of communities that the secret project quietly displaced.) This year, for the first time, all ages are welcome.

“OMG,” said one boy, facing the heart of the reactor, which is known as the pile.

“So,” Marsh asked the fourth-graders, “what does a reactor do?”

“It makes plutonium to make atomic bombs,” said one boy.

“What would you use to make the plutonium?” asked student Gloria Caridad.

“Uranium,” another docent answered.

So eager to learn

The reactor tours, often led by docents retired from jobs at the Hanford Site, have been a hot ticket among local families since the Energy Department started offering them in 2009. This year’s tour season continues until Nov. 19.

“Does that red light always flash?” asked a mom, Colleen Lane, eyeing the equipment. (The answer was yes. The reactor is monitored full time to make sure radiation remains at “background levels.”)

“Do you know what nuclear fission is?” asked docent Marty Zizzi.

Another boy raised his hand, then froze.

“I forget,” he said. “We just learned it yesterday.”

“We’ve been talking about it for a week now,” teacher Liz Cronin said later. “They wanted to know why Japan bombed Hawaii in the first place. And they wanted to know why we needed plutonium when we had so many other bombs.”

Enthusiasm for this trip was so high, Cronin said, that from her class of 26 kids, 18 parents volunteered to chaperone. She had room for four.

Hanford’s Manhattan Project story started in 1943, when Gen. Leslie R. Groves of the U.S. Army Corps of Engineers chose the site for its remote location; the pure, cool water of the Columbia River; and the ample electricity generated by the nearby Grand Coulee and Bonneville dams.

Within weeks federal officials took over more than 600 square miles of riverside land, emptied the small towns of Hanford, White Bluffs and Richland, and evicted members of several Native American tribes.

Then DuPont, the military contractor that designed and built the reactor, started construction. By 1944, 45,000 workers from across the country had raised and filled scores of mysterious industrial buildings surrounded by a secret city with barracks, trailers, Quonset hut neighborhoods (segregated by sex and race), baseball fields, an auditorium, eight mess halls and a brewery.

By September — just 11 months after groundbreaking — the B Reactor was built and began operations. By July 1945 Hanford had produced enough plutonium to power a practice bombing, the Trinity Test in Alamogordo, N.M. After the Aug. 6 bombing of Hiroshima, Japan, Hanford’s rank-and-file workers learned they’d been helping to make atomic weapons. Three days later Fat Man landed on Nagasaki, fueled by Hanford plutonium.

During the Cold War, Hanford’s reactors cranked out about 67 metric tons of plutonium, fueling four decades of nuclear brinkmanship and, the Energy Department now acknowledges, creating one of the earth’s biggest radioactive messes.

You might not see signs of it from your B Reactor tour bus, but a guide could mention the Energy Department’s cleanup efforts. The agency has 56 million gallons of high-level radioactive and chemical waste in storage tanks at Hanford, along with more than 80 square miles of contaminated groundwater. There’s a separate Energy Department cleanup tour that takes 4-1/2 hours.

Confronting the B Reactor pile today is like stepping into the orchestra pit of a theater, then gazing up at a metal monster at center stage: 75,000 graphite blocks, 2,004 aluminum tubes running through them. In operation, the tubes were full of immensely hot uranium cylinders — about 64,000 of them, cooled by water from the Columbia, which eventually drained back into the river.

“The power of the place is incredible,” said visiting Park Service ranger Denise M. Shultz, chief of interpretation and education at Washington’s North Cascades National Park Complex. “I had goose bumps all over.”

Just down the hall from the pile is the control room, with a central seat for the reactor operator, surrounded by dials, monitors and wiring.

“You guys know ‘The Simpsons’ on TV?” asked Marsh. “You know how Homer Simpson operates his nuclear reactor from his seat? This is the seat he would be in.”

Later, somebody pulled the kids together for a group picture and hollered, “Smile and say, ‘B Reactor!’”

Having conversations

Nobody asked about the atomic bombs’ effects in Japan. Nor were death or injury statistics offered. In fact, the 28-page document that docents use as their main source doesn’t include information on deaths and destruction.

But now, said Kirk Christensen, manager of B Reactor preservation for Energy Department contractor Mission Support Alliance, “we’re going to have these conversations.”

With about 12,500 visitors expected this year, the Energy Department is footing the costs of the Hanford tour program while the Park Service waits to see how much funding the next federal budget will include. Tracy Atkins, interim superintendent of the Manhattan Project park, said she would make her first hires soon.

The Park Service will spend the next 18 to 24 months developing the park’s first round of interpretive materials, drawing on input from scholars and community members in New Mexico, Tennessee, Washington and Japan. A separate approach for kids younger than 12 will probably be included, Atkins said.

The park service might also print some materials in Japanese, Atkins said, but “we can’t change everything overnight.”