Where will the nuclear waste go after Three Mile Island shuts down?

After the infamous Three Mile Island nuclear accident 40 years ago, most of the reactor’s partially melted uranium fuel was hauled away to the Idaho National Lab, where the radioactive waste now slowly decays in steel and concrete containers, awaiting long-term disposal.

But the formal decommissioning of the damaged Unit 2 reactor near Harrisburg, site of America’s worst commercial nuclear disaster, has not yet really begun. Its owner, FirstEnergy Corp., has said that the plant would remain dormant until the surviving reactor, owned by a different company, shuts down.

FirstEnergy, in a 2013 filing with the U.S. Nuclear Regulatory Commission, said that both reactors would be decommissioned simultaneously "to achieve economies of scale, by sharing costs between the units, and coordinating the sequence of work activities."

The timing of the final dismantlement and interment of Three Mile Island plant was thrown into uncertainty last week when the owner of the operating reactor, Exelon Generation, announced that it would take nearly 60 years to decommission its unit if it prematurely shut down operations in September. Exelon says it is losing money on the plant and has no option but to shut it down without a state rescue.

The prolonged decommissioning of the operational reactor — and by implication, the damaged reactor — could push back the final cleanup and remediation of Three Mile Island to 2079, a century after the meltdown.

“We’ve been living with this for 40 years," said Eric Epstein, chairman of Three Mile Island Alert, a Harrisburg nuclear watchdog group. “Out of a sense of fairness, we need to have this cleaned up.”

The fate of the damaged reactor is further complicated because FirstEnergy Solutions, an Akron company that operates FirstEnergy Corp.'s power generation plants, including the Beaver Valley Power Station in Pennsylvania, last year filed for bankruptcy.

The Unit 2 decommissioning costs, which FirstEnergy last year estimated at $1.26 billion, would be paid out of a trust fund. (Exelon estimates its reactor, TMI Unit 1, would need an additional $1.2 billion to decommission, paid from a separate trust fund.)

Jennifer M. Young, a spokesperson for FirstEnergy, which owns the damaged reactor through its GPU Nuclear subsidiary, said the company is evaluating how Exelon’s announcement would affect its cleanup timetable.

“We are currently looking at all of our options in light of the recently-released TMI-1 decommissioning schedule, which reflects a potential earlier shutdown for the unit than originally planned," Young said in an email. "While we have not yet made any decisions, we will work with Exelon and follow any required regulations should an adjustment to the TMI-2 decommissioning schedule be needed.”

FirstEnergy and Exelon have a memorandum of understanding to coordinate decommissioning of the two reactors, which are located in separate buildings. But FirstEnergy also faces an NRC-imposed deadline to complete the decommissioning 60 years after shutdown, which puts it on an earlier timeline than the neighboring reactor.

From a regulatory standpoint, the damaged reactor did not formally cease operations on March 28, 1979, when the accident ended its productive life. The formal “cessation of operations” happened 14 years later, in September 1993, when the NRC accepted it into “post-defueling monitored storage.”

FirstEnergy has until 2053 to decommission the site, 60 years after operations ceased. It contracts Exelon to maintain the dormant reactor, and provide security.

FirstEnergy can request an exemption to push back its decommissioning date if it appeared the task could not be completed within 60 years, said Neil Sheehan, an NRC spokesman.

FirstEnergy, in its 2013 filing, anticipated that Exelon would run its reactor through to the expiration of its license in 2034, and then the companies would jointly complete decontamination and dismantlement of both reactors in less than 20 years.

By decommissioning both reactors simultaneously, FirstEnergy said, it can use Exelon’s fuel storage equipment to contain the “small quantities of core debris and fission products” that still remain from Unit 2′s partial meltdown, which occurred after a series of mechanical and human errors led to a loss of coolant, allowing the uranium fuel to overheat.

Any spent fuel from the operating reactor, or any remaining radioactive debris collected during decontamination of the damaged unit, could be stored in dry casks on the reactor site, at federal expense, until the federal government builds a long-term underground disposal facility.

The government had planned and designed a repository at Yucca Mountain, Nev., to store spent fuel, which includes some isotopes that remain lethally radioactive for tens of thousands of years. The Obama administration abandoned the plan in 2009. The Trump administration last year requested Congress fund efforts to renew licensing activities for the Yucca Mountain site.

Until the issue is sorted out, most decommissioned U.S. reactors will be forced to keep their spent fuel in canisters on the former reactor sites.

But 99 percent of the fuel from Three Mile Island’s damaged reactor was already packed up and shipped to the Idaho National Laboratory after the TMI accident.

The U.S. Department of Energy stores 2,750 tons of nuclear waste at four sites in South Carolina, Washington, Colorado and Idaho from an assortment of commercial, naval, and weapons-related reactors.

The site in eastern Idaho, which holds 358 tons of spent fuel, is kind of a mausoleum of nuclear detritus. It contains 46.9 tons of waste from the Shippingport Atomic Power Station in Beaver County, Pa., the nation’s first commercial reactor. It’s also home to 1.8 tons of thorium‐uranium carbide fuel from Philadelphia Electric Co.'s Peach Bottom Unit 1, an experimental reactor in Delta, Pa., that shut down in 1974.

The 90.8 tons of ruined nuclear fuel from Three Mile Island’s damaged reactor is the biggest contributor of waste to the Idaho site. It is contained in 29 steel canisters encased in concrete containers.

But it is an unwelcome long-term resident in Idaho.

Under a 1995 agreement between the state of Idaho and the federal government, the energy department agreed to remove all spent fuel from the state by Jan. 1, 2035.

The agreement specifically includes Three Mile Island’s waste.