OMAHA, Neb. (AP) — Omaha Public Power District officials say it will cost $5 million to end what was supposed to be a 20-year-contract with the company that has managed day-to-day operations at the Fort Calhoun nuclear power plant since 2012.

The power district began permanently shutting down the nation’s smallest nuclear power plant on Monday. It’s one of the first steps of a decommissioning process that could last decades.

Exelon Generation has managed Fort Calhoun since September 2012, when it stepped in to bring the nuclear plant back from a nearly three-year outage. The utility would have paid $400 million to Exelon if it had completed its licensing term in 2033.

A utility spokeswoman says it is unclear when the contract will be terminated.