A top U.S. Nuclear Regulatory Commission official said Monday that Xcel Energy’s Monticello nuclear power plant needs to improve its “degraded” performance in light of a serious lapse discovered last year in the reactor’s flood-response plans.

“It is imperative that the licensee identify the depth and breadth of their performance issues and take corrective action,” Cynthia Pederson, the NRC’s regional administrator, said in an interview with the Star Tribune.

Pederson, who is based in Lisle, Ill., spoke Monday in Monticello at the plant’s annual regulatory meeting for community members and plant workers. Such meetings usually are low-key events. Regional administrators typically attend only when a plant has slipped into a “degraded” status on one of NRC’s “cornerstone” or significant performance issues.

Pederson said Monticello remains a safe nuclear reactor. However, she said Xcel managers need to look at how decisions are made and at “multiple examples of inadequate procedures or use of procedures.’’

Last June, NRC inspectors faulted the Monticello plant, which is on the banks of the Mississippi River, for being unprepared for worst-case flooding. The finding was classified as having “substantial safety significance,” which is one step below the most serious safety shortcoming in NRC’s rating system.

Xcel says it has corrected the problem by placing dike-building materials on site in case of a catastrophic flood. The plant is 40 miles northwest of the Twin Cities.

“There is always a surface-level answer,” said Pederson, a native of Bemidji, Minn., who was appointed to the top regional post last August. “But you have to ask multiple ‘why’ questions. … They need to get to the depth of the issue such that when they formulate their corrective action, they make sure they are dealing with the fundamental performance issues.”

Karen Fili, Xcel’s site vice president at Monticello, said the company “has performed a root-cause evaluation” and is implementing performance improvement plans.

“The NRC is aware of the progress being made and will continue monitoring the actions we take to continue to improve,” Fili said in a statement.

The inadequate flood preparedness finding was the Monticello plant’s most serious safety shortcoming since the NRC adopted a color-coded, four-step ranking system for inspection results. That flood preparedness problem was ranked “yellow,’’ just below “red,’’ the most serious, level. (The two lowest levels are “green” and “white.”)

Pederson said the NRC also is concerned about lesser white and green findings at the plant that revealed weaknesses in human performance. She said the NRC will conduct extra inspections this year to assure that Xcel, the state’s largest utility, is addressing regulators’ concerns.

The federal scrutiny comes as the Minnesota Public Utilities Commission also is investigating whether it was prudent for Xcel to spend $665 million to extend the Monticello plant’s life and boost its output. The upgrade cost more than double the original estimate.

In a separate action, the NRC on Friday issued the second and final amendment to the plant’s license allowing it to operate at 671 megawatts, up from 600 megawatts. Xcel, which completed the plant upgrade last year, has been operating at the old power level. The company said it is still testing the systems, but expects to ramp up to the new power level by midyear.