The recently revealed leak of radioactive tritium at the Monticello Nuclear Generating Station was the sixth highest recorded among U.S. nuclear power plants.
A monitoring well at the site registered a tritium level of 5 million picocuries per liter, according to a Nov. 22 report from the plant to the state. Only five other plants across the country have registered leaks of higher concentrations, according to a 2021 report from the Nuclear Regulatory Commission, the most recent available.
Tritium, a mildly radioactive form of hydrogen, mixes easily into water but presents no health risk unless it is ingested. It occurs in nature but more commonly as a byproduct of nuclear power production. Xcel has been pumping up and collecting contaminated water, and is also pumping to keep the underground plume of tritium from spreading.
By last week, the concentration in the monitoring well had decreased to 2.47 million picocuries per liter, said Kirk Koudelka, an assistant commissioner at the Minnesota Pollution Control Agency.
The federal limit for tritium in drinking water is 20,000 picocuries per liter, but none of the contaminated water has left the site or reached any drinking water source, including the Mississippi River, state officials and Xcel have said.
The lack of risk to the public was one reason state officials gave for waiting months before publicizing the leak last week. After high levels of tritium were found in November, the leak was patched about a month later — and the public wasn't told the extent of the event until March 16.
"Tritium is a very low risk, and none of that has left the site," said Daniel Huff, assistant commissioner for health protection at the Minnesota Department of Health. "If there had been the potential of any imminent risk, we would have notified folks immediately."
He said he knows some people have criticized the delayed announcement by health officials and the Minnesota Pollution Control Agency.