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The shutdown of Xcel Energy's nuclear power plant in Monticello to fix a leak of mildly radioactive water caused some collateral environmental damage: a fish kill in the Mississippi River.

Xcel Energy reported that 230 fish died from thermal shock, not the tritium that has seeped into groundwater. The plant uses river water for cooling, and then sends warmed water back to the Mississippi. As the plant powered down and the flow of warm water was cut off, the sudden change in temperature proved fatal for bass, channel catfish, carp and sucker fish.

Pat Rivers, the deputy director of fish and wildlife for the Minnesota Department of Natural Resources, said the event was "a pretty small fish kill in relative terms, but it's still unfortunate."

Theo Keith, a spokesman for Xcel, wrote in an email that the utility tries to avoid shutdowns "in winter months where temperature differences are greatest." He said the utility powered down the plant slowly to try to lessen the impact.

The radioactive tritium that previously leaked at the plant has not reached the river, according to a joint statement from the Minnesota Pollution Control Agency and DNR.

Xcel and state agencies first reported on March 16 that 400,000 gallons of tritiated water had seeped into the groundwater under the Monticello plant, and that a temporary fix had been put in place. Last week, Xcel said its patch had failed, so it was going to shut down the plant immediately for a repair, rather than waiting for a planned outage next month.

Tritium is a radioactive form of hydrogen that occurs naturally and as a byproduct of nuclear power generation. It doesn't pose a health risk unless ingested, and state agencies have repeatedly said the tritium has not reached any drinking water sources.

Keith wrote that the leak has been permanently fixed, and that the plant would begin producing power again in the next week, before powering down again April 15 for refueling.

There have been far larger fish kills in Minnesota — last summer, 2,500 dead fish, mostly brown trout, were found in a creek in the southeastern part of the state. The state later determined that manure and pesticide runoff were the most likely causes of that die-off.

Rivers said that thousands of fish can also die in winter kills, when thick snow covers the ice on lakes and blocks sunlight to the aquatic plants below. That stops photosynthesis, cutting off oxygen.