YAKIMA — Workers at south-central Washington’s Hanford nuclear reservation have finished the process of closing and “cocooning” the longest-running of nine nuclear reactors built there for the U.S. atomic weapons program.

With the completion of work at N Reactor, six reactors at the nation’s most contaminated site have been dismantled and cocooned. That involved removing extra buildings around the reactors, demolishing all but the shield walls surrounding the reactor cores and sealing them in concrete.

Much work remains to be done, but the conclusion of the $65 million project marks a historic moment for a facility that generated significant attention at the height of the Cold War. The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb.

Over the next 40 years, nine nuclear reactors were built to produce plutonium for the nation’s nuclear weapons arsenal.

Some 37,000 people traveled to Hanford on Sept. 26, 1963, to see President John F. Kennedy dedicate N Reactor next to the Columbia River, noting its role in changing “the entire history of the world.” The reactor operated until 1987.

N Reactor was unique because it operated on a closed-loop cooling system. That enabled the reactor to recirculate water for cooling, which led to less river contamination, and to produce steam for electricity, making it the only dual-purpose reactor in the U.S.

At its peak, the 860-megawatt reactor was producing enough power for 650,000 homes.

In the 1960s, N Reactor represented the future of energy in America, and was successful in both its defense and civilian duties for more than two decades, said David Huizenga, senior adviser for the U.S. Department of Energy’s Office of Environmental management.

“Now, the cocooned N Reactor stands as a symbol of what we can achieve with large-scale cleanup at Hanford,” Huizenga said in a statement Thursday.

The federal government shut down the reactor for routine maintenance, refueling and safety upgrade following the Chernobyl nuclear accident in 1986, but it never operated again. It was decommissioned when the Cold War ended in 1989, and focus turned to cleaning up Hanford.

The first reactor built there, B Reactor, has been designated a National Historic Landmark. The reactor produced plutonium for the first atomic blast, the Trinity Test, and for the bomb dropped on Nagasaki, Japan, during World War II.

Work still to be completed along Hanford’s scenic river corridor includes two reactors, K East and K West, which must be permanently closed up and cocooned; many waste sites and contaminated soil need to be dug up; and tainted groundwater must be treated.

The Energy Department planned to complete much of that work by 2015 in hopes of shrinking the overall footprint of the Hanford site. But hundreds of new waste sites have been uncovered in recent months and could delay that effort.

“Completing the cocooning process is the culmination of years of detailed planning and safe, disciplined operations by workers dedicated to protecting one another, the environment and the river,” said Carol Johnson, president of the Hanford contractor handling the work, Washington Closure Hanford.

Hanford’s cocooned reactors are scheduled to remain in interim safe storage for up to 75 years to allow the Energy Department, regulators and other stakeholders to determine the final disposal method and to allow the structures’ high radiation levels to decay to safer levels.