Radioactive Leaks from Monticello Reactor in Minnesota Threaten the Mississippi

A leaking pipe at Xcel Energy’s Monticello reactor on the Mississippi River in Minnesota is causing a radioactive pollution problem.

Last November, radioactive tritium from the 52-year-old General Electric reactor was found in an on-site groundwater “monitoring” well. Xcel, formerly Northern States Power, waited until mid-March to report the month-long 400,000-gallon leak, but inspectors from the Nuclear Regulatory Commission (NRC) winked at the four-month delay, saying reporting wasn’t required by law. Xcel publicly claimed to have plugged the leak.

The concentration of tritium was “about 5 million picocuries per-liter” in the groundwater, according to Xcel.   This is 250 times the amount of tritium contamination legally permitted in drinking water (20,000 picocuries per liter). After saying the leak was fixed, Xcel reported that another several hundred gallons of tritium-tainted water had spilled from an overflow tank used to collect some of the initially poisoned groundwater.

The Monticello reactor is a GE Mark I design identical to the three Fukushima units that melted down and partially exploded in Japan in 2011. There are 23 other reactors just like them still operating in the United States. Monticello’s reactor’s old and mostly uninspected pipes are worn out and corroded — a chronic, nationwide problem across the country which is reason enough to retire the whole fleet. The Minneapolis StarTribune reported June 17, 2023 that, “Tritium leaks unfortunately have been relatively common in the nuclear industry, and the Monticello spill was among the nation’s 10 largest.”

“Radioactive tritium leaks found at 48 U.S. nuke sites,” blared the headline in an lengthy Associated Press investigative series (“Aging Nukes” by Jeff Donn), originally published in June 2011.  “You got pipes that have been buried underground for 30 or 40 years, and they’ve never been inspected, and the NRC is looking the other way,” engineer Paul Blanch told the AP. Blanch, who had worked for the industry and later became a whistleblower, added, “They could have corrosion all over the place.”

NRC public affairs officer Prema Chandrathil replied August 10 to some questions I posed, writing, “The now-stopped leak was from a pipe that ran between two buildings on site where the water had already been processed, filtered, and demineralized.” Asked if there were other radioactive materials in the wastewater, Chandrathil wrote, “When the leak was going on there were low levels of xenon and iodine detected near the leak. They decay away and become non-radioactive quickly due to their short half-life. Therefore, tritium is the only type of radioactive material currently present in the groundwater.”

The reply raised more questions than it answered. Iodine-125 has a 60-day half-life, and, because it takes ten half-lives to “decay away,” its gamma radiation spews for 600 days. Iodine-129 has a half-life of 16 million years. If the NRC official meant Iodine-131 — Chandrathil didn’t specify — that isotope decays for 80 days. Nor does Xenon-137 just “decay away” as the NRC public affairs officer said. It decays to Cesium-137 which takes 300 years to “decay away.” Again Chandrathil didn’t report which isotope of Xenon was leaking, Xe-133, Xe-137, or some other.

The Minn. Department of Health web site report on the radioactive leaks says: “A conservative assumption in radiation protection is that any radiation exposure could result in an increase in cancer occurrences in the population, with the risk increasing as exposure increases.” However Xcel has said there is “no” health risk to the public as the affected groundwater contains “very low levels” of tritium.

This reassurance is untrue. Even “very low levels” of radiation exposure create a risk as the state health department noted. Radiobiologists all agree that no one can speak of a “safe” radiation dose level. Every federal agency that regulates industrial releases and medical uses of ionizing radiation warns that exposure to radiation, no matter how small, increases one’s risk of cancer and other illnesses. For example, the U.S. Environmental Protection Agency says, “There is no firm basis for setting a ‘safe’ level of exposure above background ….”[1] “Based on current scientific evidence, any exposure to radiation can be harmful (or can increase the risk of cancer). ….no radiation exposure is completely risk free.[2]  “[T]here is no level below which we can say an exposure poses no risk. … Radiation is a carcinogen.[3]

Tritium emits beta radiation in the form of fast-moving particles. The U.S. EPA and other authorities say beta particles are “more penetrating than alpha particles,” and “are capable of penetrating the skin and causing radiation damage.” In her book No Immediate Danger, Dr. Rosalie Bertell says that if beta particles are inhaled or ingested they can inflict biological damage more severe than an external exposure, because the beta particles can penetrate cell membranes.

Xcel attempts to protect Mississippi

In a practical admission that it has lost control of the plume of radioactive groundwater moving toward the Mississippi, Xcel announced August 17, 2023 that it would build an “underground metal barrier” between the leaking facility and the great river. Xcel said the steel wall — 40-feet deep and 600 feet long — would take four to eight weeks to install “along the edge of the plant’s boundary with the river,” and that it is intended to keep contaminated groundwater from reaching the Mississippi River. Yet it is axiomatic that water moving underground can be deeper than 40 feet.

One grim irony of Xcel’s “groundwater wall” is that at Japan’s Fukushima — where three identical GE reactors were destroyed by damage from the shattering 3/11/11 earthquake and follow-on simultaneous meltdowns — the owners also tried to stem the flow of groundwater. Tokyo Electric Power Co. built a deep “ice wall,” to keep groundwater from gushing through cracks in reactor foundations, but the $250 million effort has failed.

Originally licensed in 1971 to operate for 40 years, Monticello was designed to close in 2010. The Nuclear Regulatory Commission issued Xcel an extension in 2006, allowing it to produce rad waste until 2030. In 2011, the Japanese catastrophe of three simultaneous meltdowns of Monticello-like GE Mark I reactors proved how reliable the design is. Now the risk-takers at Xcel have applied for a second operating extension, and the owners of this leaker want to drive it for 80 years — until 2050 — twice the distance it was designed to run. Boom!


[1] ; updated July 8, 2011.

[2] U.S. EPA, “Ionizing Radiation Series,” No.2, Air & Radiation, 6601J, EPA 402-F-98-010, May 1998.

[3] U.S. EPA, “Radiation: Risks & Realities,” Air & Radiation, 6602J, EPA 402-K-92-004, August 1993.

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