Xcel Energy Inc. on Wednesday lifted the secrecy about cost overruns at its Monticello nuclear power plant, revealing that recently completed upgrades are estimated to cost twice as much as originally budgeted.

The Minneapolis-based electric and gas utility said in a regulatory filing that a five-year project to extend the life and increase output at the reactor, originally budgeted at $320 million, had climbed to nearly $640 million.

The Star Tribune on Monday reported that the project had experienced significant cost overruns, and that the latest estimate had been classified by Xcel as a trade secret. That estimate had been submitted to state regulators in January.

Xcel has previously said it kept the latest overrun estimate secret so that vendors submitting their bills for work on the project wouldn’t know “how much contingency was built into the budget.”

Xcel is about to restart the Monticello plant after a four-month shutdown that allowed workers to complete the replacement of aging pumps and other equipment. The upgrades are intended to extend the life of the 43-year-old reactor another two decades and boost its output.

In a filing with the Minnesota Public Utilities Commission, Xcel said Wednesday that it reached a point where the previously protected information could be made public and “contribute to a full and complete discussion of the project before the commission.”

Xcel released the information through the PUC’s electronic docket system, and executives were not immediately available to comment on the figures.

Xcel’s 1.2 million electric customers in Minnesota are being asked to pay for the cost overruns because nuclear-related expenses are among the reasons the utility says it needs a rate hike. An administrative judge recommended that Xcel get a 4.7 percent increase, less than half what the utility originally requested. The commission, which has the final say on the rate hike, is expected to act this fall.

Xcel has defended the cost increase for upgrading the Monticello plant, citing schedule changes, vendor issues, evolving regulations and unexpected work.

“[It] is a large, complex project with many intricate components that required changes from original plans,” Xcel’s chief nuclear officer, Timothy O’Connor, said in earlier written testimony submitted to state regulators.

The newspaper had petitioned the PUC to reveal the secret cost overrun data, but the agency’s staff had not yet made a ruling and the issue is now moot.