Xcel Energy said Tuesday it has shut down two of Minnesota's three nuclear reactors for what it called minor repairs.

Neither plant released any radiation or posed any danger to citizens, the company said. The Monticello nuclear plant's single generating unit, which had been operating at 10 percent capacity since last weekend, was shut down because of a leaking pipe inside the plant's concrete containment structure, the company said.

One of the two nuclear generators at the Prairie Island plant was shut down because its emergency diesel generators suffered exhaust leaks.

"It's unusual for us to shut down both units in [the] same half day, but it's not unheard of," Xcel spokeswoman Mary Sandok said.

The company is compensating by buying more electricity from other providers on the Midwest power grid, Sandok said. Although additional electricity costs are passed to ratepayers, the effect is not likely to be large enough for most to notice, she said.

The 600-megawatt Monticello generator and the 550-megawatt Prairie Island unit account for 20 percent of the power Xcel generates for the Upper Midwest. Both are expected to be on again in a few days, Sandok said.

The Prairie Island Indian Community expressed concern about that plant's age and condition.

"Today's unplanned shutdown -- and the unusual white steam clouds released throughout the day during the reactor shutdown -- are 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," Tribal Council President Johnny Johnson said in a statement.

Johnson wrote that "over the past couple of years, there have been more than 30 reported incidents like failing equipment, security breaches, human performance problems and operating errors."

STEVE ALEXANDER