Trouble on the Water

Monticello Nuclear Generating Station. Source: Xcel Energy.

Xcel Energy owns the Monticello reactor on the Mississippi River, 40 miles upstream of Minneapolis. This 54-year-old General Electric reactor, one of the three oldest in the country, was originally designed and built last 40 years. It should have been decommissioned in 2010.

On Nov.22, 2022, Xcel reported to the Nuclear Regulatory Commission (NRC) that radioactivity was found in groundwater from an onsite monitoring well. Xcel’s notice said nothing about the leak’s volume, its source, or its level of radioactivity, but it did claim without evidence that, “There was no impact on the health and safety of the public or plant personnel.” The public was not informed of this “harmless” radioactive pollution until March 2023, four months later.

Xcel said then that 400,000 gallons contaminated with radioactive tritium had leaked from underground pipes into the groundwater. Alarmingly, this negligent estimate would be more than doubled in December 2023.

The company said the concentration of tritium in its leakage was “five-million pico-curies-per-liter,” a very high concentration at the level of primary reactor coolant. Very few tritium leaks from U.S. nuclear reactors (2/3 of them leak) ever had more tritium-per-liter.

A “pico-curie” — a small amount of radioactivity — is one-trillionth of a curie. A curie, which is a lot of radioactivity, amounts to 37 billion atomic disintegrations-per-second.

At 400,000 gallons (1,761,953 liters), with 5-million pico-curies of tritium per-liter, the leak contained roughly eight curies of radioactivity. This is no small matter. The partial reactor meltdown at Three Mile Island, one of the worse in U.S. history, released 14 curies of radioactive iodine-131. As noted below, 14 curies of tritium is now Xcel’s latest radioactive pollution estimate.

This tritium is a direct threat to the Mississippi River and the people and animals that rely on it, because, as the NRC says: “After a radioactive leak or spill … tritium travels as a form of water through the soil faster than other radionuclides.”

On July 20, 2023, WCCO television reported for CBS Minnesota: “Xcel Energy: Small amount of leaked water may have reached the Mississippi River.”

The city of Minneapolis proudly declares on the web, “Our water comes from the Mississippi River. Roughly 21 billion gallons of water are pumped from river each year, and 57 million gallons of drinking water delivered every day.


On Dec. 18, 2023, Xcel reported to the NRC that its first leak estimate was grossly in error. Xcel now estimated that 829,000 gallons of radioactive water had leaked into the ground — not 400,000. Xcel also reported that leaking had started earlier than first thought, and that the leak’s volume was far greater than it had earlier estimated. Xcel’s noted that its estimate of radioactivity leaked into the groundwater was now 14 curies.

In a May 10, 2023 letter to the NRC, Xcel also confirmed that more radioactive materials than tritium were in the 829,000-gallon leak. Xcel wrote that samples from Monticello’s laboratory “detected Iodine-131, Iodine-133, Iodine-135, Xenon-133, and Xenon-135 in MW-9A [monitoring well-9A].” The May 10 letter also said, “The highest I-131 concentration seen” in the laboratory “was 61.6 pCi/l on 12/20/2022.”


The wastewater leaked from two old underground pipes between the reactor building and the turbine building, which were later replaced. The company announced in March 2023 that the leak had been stopped. Later, it admitted that the fix had failed when a collection system overflowed, dumping tritium-contaminated water into the ground again.

In a Nov. 9, 2023 letter to the NRC, Xcel reported that “The visual examination of the removed [carbon steel] piping sections showed that both of the CRD [control rod drive] pipes experienced severe corrosion of the external surfaces in the regions exposed to the ground water present between the buildings” and “[b]oth CRD pipes … were replaced with uncoated stainless-steel piping.” The same letter reports on Xcel’s plans for inspecting thousands of Monticello’s other underground pipes. Xcel says it, “will inspect the underground stainless-steel piping … once every 10 years.”

In January 2023, 12 months before the 829,000-gallon leak estimate was made public, Xcel applied to the NRC for an operating license extension to run Monticello until it is 80. This GE Mark I reactor is identical to the three melted and exploded Fukushima units in Japan. With Xcel’s stellar record of never endangering anyone’s health and safety, the NRC ought to rubber stamp the application, right? 

John LaForge is a Co-director of Nukewatch, a peace and environmental justice group in Wisconsin, and edits its newsletter.