US Xcel Energy says workers have repaired the leak at the Monticello NPP, which will resume power production shortly. The company temporarily shut down the plant, after monitoring equipment detected a second lead of radioactive tritium in groundwater. A short-term repair failed to fix an earlier, larger leak last November.

The plant will close again in mid-April for a previously scheduled refuelling outage, Xcel spokesperson Theo Keith confirmed. Xcel and state officials have said the leak did not pose any threat to public health, safety or the environment.

The Minnesota Pollution Control Agency (MPCA) 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,” MPCA said. The agency further explained that the discovery of some 230 dead fish was due to temperature changes in the river and not contamination.

It said in a statement 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.