MIDDLETOWN, Pa. (WHTM) – The clouds of steam that billowed over Three Mile Island for more than 40 years are no more.

TMI’s Unit 1 reactor was taken offline and retired from service at noon Friday.

“The world-class commercial nuclear power facility now enters a new chapter,” Exelon Generation said in a news release. “Over the next few weeks, workers will remove the reactor’s fuel supply and store it safely in the station’s used fuel pool. After that, workers will begin preparing the station for long-term decommissioning.”

Decommissioning the plant is expected to take decades.

Three Mile Island Unit 1 began operating in September 1974 and was licensed to operate through 2034. TMI’s Unit 2 has not operated since the 1979 accident that severely damaged the reactor core.

Exelon estimates that during TMI’s 45 years of operation, the station and its employees contributed more than $3.5 billion to the local economy. More than 600 employees worked at the plant.

About 300 workers will stay for the first phase of decommissioning, while others will take on different roles at the Exelon and some have decided to retire or take jobs elsewhere, the company said.

Chicago-based Exelon announced in 2017 that it would shut down Three Mile Island unless the state stepped in with a financial rescue. Exelon complained the plant was losing money and couldn’t sell electricity in competitive markets.

In May, the company announced was moving forward with the shutdown by Sept. 30, saying it was clear a state policy solution would not be enacted in time to reverse the premature retirement of the plant. TMI had a fuel purchasing deadline of June 1.

“At a time when our communities are demanding more clean energy to address climate change, it’s regrettable that state law does not support the continued operation of this safe and reliable source of carbon-free power,” Bryan Hanson, senior vice president and chief nuclear officer, said in a statement after Friday’s shutdown.

“It’s critical that we continue to pursue policy reform to prevent other carbon-free nuclear resources from being pushed out of the market by rules that fail to evenly value clean energy resources and at the same time allow emitting resources to pollute for free,” Hanson added.

Exelon in April filed decommissioning plans with the Nuclear Regulatory Commission. In the federally required Post Shutdown Decommissioning Activities Report, Exelon said used nuclear fuel will be transitioned into a spent fuel pool then moved to dry cask storage by the end of 2022. 

Facility staffing will decrease to 50 full-time employees by 2022, and dismantling of the station’s cooling towers and other large components will begin in 2074, Exelon said.

Spent fuel pools are where spent fuel assemblies are kept under at least 20 feet of water to shield anyone near the pool from radiation. The assemblies are moved into the water pools from the reactor along the bottom of water canals so that the spent fuel is always shielded to protect workers, according to the NRC’s website.

Dry cask storage allows spent fuel that has already been cooled in the spent fuel pool for at least one year to be surrounded by inert gas inside a container. The containers, or casks, are typically steel cylinders either welded or bolted closed to confine the spent fuel, according to the NRC. Each cylinder is surrounded by additional steel, concrete, or other material to shield workers and the public from radiation.

TMI’s used nuclear fuel in dry cask storage will be protected in a hardened facility with multiple layers of structural, human and electronic security, Exelon said.