Meanwhile, a state agency said a fish kill that claimed at least 230 fish in the Mississippi River near Xcel Energy’s Monticello Nuclear Generating Plant was caused by a water temperature change resulting from the shutdown of the plant, not by any chemicals leaking into the river.

The nuclear plant, about 38 miles (60 kilometers) northwest of Minneapolis, was fully powered down around 7 a.m. Saturday to fix the leaky pipe discovered last week, Xcel Energy spokesman Theo Keith said.

“Crews have already repaired the leak, which did not pose any risks to public health, safety, or the environment,” Keith said in an email. “The pipe was confirmed as the only source of the leak.”

The plant will return to service “in the next week,” Keith said, but will temporarily close in mid-April for an annual maintenance project.

In November, a leak was discovered. There was a monthslong delay in announcing the initial leak that raised questions about public safety and transparency, but industry experts said there was never a public health threat.

Tritium occurs naturally in the environment and is a common by-product of nuclear plant operations. It emits a weak form of beta radiation that does not travel far and cannot penetrate human skin, according to the Nuclear Regulatory Commission.

Last week, the new leak was found coming from a temporary repair to the original leak, the company said. Chris Clark, president of Xcel Energy–Minnesota, North Dakota and South Dakota, said the new leak also posed no risk to the public or the environment.

The Minnesota Pollution Control Agency and the state health department have sampled groundwater wells and found “no evidence that the tritium has reached the Mississippi River or contaminated drinking water sources,” the MPCA said.

The MPCA announced the fish kill on Monday. A statement from the agency said that in normal operations, warm water from the plant enters the river and the fish get used to it.

“The fish kill is unfortunate but not unexpected given the significant temperature change that can occur when warm water from the plant stops flowing to the river during a shutdown in operations,” the statement said.

Fish found dead included bass, channel catfish, common carp, and one or more species of sucker fish.

Tyler Abayare, who regularly fishes near the plant, said he has noticed dead fish near the plant in previous years, when the facility closes for maintenance in April. Then, the dead fish drift downstream to shallow water and decompose. He typically avoids fishing the area until July.

“It just starts to stink and makes your eyes water,” Abayare said.

March 29, 2023: This story has been corrected to reflect that radioactive material never leaked into the Mississippi River.