The Monticello, Minn., nuclear power plant was shut down suddenly on Monday after an equipment problem, causing a fish kill in the Mississippi River from thermal shock.

In a report to the U.S. Nuclear Regulatory Commission, Xcel Energy said it shut down the plant while operating at 100 percent power after a problem arose with a reactor pump. The utility said shutdown happened safely, with no release of radiation and no risk to the public.

But the sudden drop in temperature in the discharged cooling water resulted in a fish kill. Xcel said it counted 59 dead fish. The fish were crappies, sunfish, bass, catfish and carp, according to the state Department of Natural Resources, which was notified of the incident.

During unexpected shutdowns, the water temperature near the plant can drop from about 65 degrees to 40 degrees in a few hours, said Harland Hiemstra, a DNR information officer. The fish can’t cope with the sudden change in temperature, he said. “It is thermal shock,” he added.

DNR is monitoring the situation with Xcel’s onsite biologist, he said. There is no evidence that the fish kill exceeded Xcel’s count, he said. The utility will be assessed a penalty for each dead fish.

Fishing continues below the power plant late into the season because the warmer water keeps fish active. The plant lies just upstream of the Montissippi Regional Park, which has a boat ramp.

On Tuesday, after the plant had shut down, a second issue arose with another pump, triggering a second report to the NRC, but no additional action, Xcel said.

The plant, 45 miles northwest of the Twin Cities, has a single boiling-water reactor. It supplies electricity to about 500,000 customers in Minnesota.