Two Minnesota nuclear power reactors that were abruptly shut down Tuesday by Xcel Energy pose no safety hazards, federal regulators said Wednesday.

The reactors, one at Prairie Island just north of Red Wing and the other in Monticello, were shut down for unrelated reasons.

At Prairie Island, two emergency backup generators failed. A second reactor at Prairie Island was unaffected and continued to operate. A water or steam leak was reported in Monticello.

The Prairie Island Indian Community said Wednesday that it wants assurances that Prairie Island's critical emergency generators would not fail in an actual emergency. While the nuclear reactors and their cooling apparatus are normally powered by the outside electric grid, the emergency generators are designed to take over in case outside power is lost.

The Chicago office of the federal Nuclear Regulatory Commission (NRC) said that the clouds seen emanating from the closed reactor at Prairie Island were not dangerous and are a routine side effect of cooling the reactor with an auxiliary water-circulation system.

During normal reactor operations, the steam would be used to turn a turbine to create electricity, but while the reactor is idle, the steam is vented to the air instead, said Prema Chandrathil, an NRC spokeswoman. The water forming the steam was never in contact with the reactor, she said.

A failure of both emergency generators "is not common," Chandrathil said. "But if the situation arises, the reactor must be shut down."

Ron Johnson, secretary of the tribal council of the Prairie Island Indian Community, said he believed the NRC's explanations but was concerned about what will be done to prevent future failures of the emergency generators.

"When you live 600 yards from the nuclear plant, it changes all scenarios of how you would react to an incident there," Johnson said. "We have to react in seconds or minutes, not hours or days. We want assurances that the emergency generators are not going to fail in case the plant suffers a loss of outside power."

His comments followed a Tuesday statement from the Prairie Island Indian Community that the reactor shut down and subsequent clouds were "ominous reminders of the fact that the 40-year-old Prairie Island Nuclear Generating Plant operating a half-mile from our homes relies on aging technology."

The Monticello nuclear plant's single generating unit, which had been operating at 10 percent capacity since last weekend, was shut down because a connection between two pipes inside a concrete building was leaking either water or steam, Chandrathil said. On-site NRC inspectors were overseeing the repairs, she said.

Such a leak "is not unusual but doesn't happen frequently," Chandrathil said.

NRC officials and Xcel did not say when the two nuclear reactors would be restarted.

"Operators at both facilities are currently making repairs and performing maintenance on the units," Xcel said in a statement. "They are making excellent progress, and the units should be back online in the next couple of days."

Xcel confirmed unofficial reports that one of the two flawed Prairie Island emergency diesel generators was operable and that the other was repaired and awaiting testing.

Steve Alexander • 612-673-4553