US power company Xcel Energy has shut down its Monticello NPP after an additional water leak was detected following previous reports about the leak of 400,000 gallons of water containing tritium. The Minnesota Pollution Control Agency (MPCA) and Minnesota Department of Health MDOH) said in a statement they were “encouraged that Xcel Energy is taking immediate action to address the recurring issue”. Xcel Energy said the new leak, located near the spot of the earlier release, is still ongoing.
The statement added: “State agencies have no evidence at this point to indicate a current or imminent risk to the public and will continue to monitor groundwater samples. Should an imminent risk arise, we will inform the public promptly. We encourage the US Nuclear Regulatory Commission (NRC), which has regulatory oversight of the plant’s operations, to share ongoing public communications on the leak and on mitigation efforts to help residents best understand the situation.”
MPCA and MDOH said they will continue to review Xcel’s response to the groundwater contamination and oversee the recovery, storage, and disposal of the impacted groundwater. “We will also continue to coordinate with city, county, and other local officials to make sure the public is kept informed of developments.”
Xcel Energy said it does not anticipate any impacts to customers’ electric service from powering the plant down early. After identifying the source of the leak late last year, Xcel Energy implemented a short-term solution to capture water from the leaking pipe and reroute it back into the plant for re-use. This was originally designed to prevent any new tritium from reaching the groundwater until the company could install a replacement pipe during its regularly scheduled refuelling outage in mid-April.
However, monitoring equipment recently indicated a small amount of new water from the original leak had reached the groundwater. Upon investigation, operators discovered the temporary solution was, over the past two days, no longer capturing 100% of the leaking water. Xcel said the new leakage, which is estimated at hundreds of gallons, will not materially increase the amount of tritium the company is working to recover and does not pose any risk to health or the environment.
Ongoing monitoring from around 25 on-site monitoring wells confirmed that the leaked water was contained on-site and has not been detected beyond the facility or in any local drinking water. Xcel Energy added that it has so far recovered about 32% of the tritium released and will continue recovery over the course of the next year.
“The schedule for resuming operation at the plant is still to be determined,” the company noted. “The plant is also scheduled to begin its refuelling process this spring, with maintenance and construction projects taking place that can only be completed when the plant is not operating. Refuelling and plant maintenance will improve the reliability of equipment at the plant and help to ensure that Monticello continues to provide safe, clean, and reliable power to the region.”
Christopher Clark, Xcel's Minnesota President, told reporters at the Monticello Community Centre that the company is voluntarily shutting down the plant and had not been requested to do so by state or federal regulators. "We could have continued to operate the plant safely," he said. MPCA confirmed it did not ask for the plant to be shut down. "We want to put this behind us," he added.
The plant may not be reopened before the 25-day planned outage in April. However, Clark said the shutdown is occurring during a "shoulder season" when power demand is relatively low. "Our other plants can fill in pretty easily for Monticello coming offline," he explained.
He stressed that neither the original leak nor the new one pose a threat to the environment or drinking water. The tritium leak has not moved beyond Xcel's property or into the Mississippi River, he said, which the MPCA confirmed.
Xcel originally reported the leak to NRC in November 2022. The company then discovered that a 3-inch pipe between the plant's reactor and turbine buildings had been leaking. There is only a half-inch space between the buildings, which made the leak hard to detect, Xcel officials said. The leak was remedied about a month after it was found. Basically, Xcel put a container under the pipe to catch the leaking water and send it back into the plant's water processing system. However, the leak was large enough to cause the container to overflow, Clark said, allowing more tritium-laced water to seep into the ground. "Within the last couple of days, we realised the catchment was no longer capturing the water," he said.
Once the plant is completely shut down and the reactor has cooled, Xcel will remove the pipe and conduct a battery of tests. "What we want to do is cut that pipe out and do a full root cause analysis," Clark said. The plant opened in 1971 and the pipe is original equipment, he explained. "We don't know the cause of the leak," he said. "Obviously, with a pipe of that age, we can all speculate." Clark said Xcel will investigate to see if the leak is part of a larger problem, but added: "I think we are dealing with an isolated situation here."
Edwin Lyman, director of nuclear safety for the Union of Concerned Scientists, said Xcel "tried a quick patch and it failed”. He added that he was unaware of any other instance where a plant had to shut down to address leaking tritium which he described as a widespread problem for the nuclear industry. "The industry has always played down the significance of these things and obviously, shutting the plant down in response would be a pretty extreme action," Lyman said. “The shutdown “likely suggests that there may be a deeper issue" at Monticello.
Tritium levels found in a groundwater monitoring well at the Monticello plant have been well above standards set by the Environmental Protection Agency. But "in the quantities being discharged, it actually dilutes very quickly," Clark said. "Even if tritium-tainted water hit the river, it would very quickly be diluted so as to not pose a danger," he said. "It is very unlikely that this tritiated water could reach the drinking water supply."
This was confirmed by Dan Huff, assistant commissioner for the state Department of Public Health. "If it reached the river, all 400,000 gallons of it at once, the volume of the river would dilute it so much, that it would be well below the limits of our concern, and almost to the limits of our detection," he said. NRC agrees that the leak is not a danger and resident inspectors were aware of it, said spokeswoman Viktoria Mitlyng.
Clark said Xcel does not yet have a cost estimate for leak-related repairs. "Our belief now is that we will be able to absorb the costs in our normal budget," he said. "That could change." If Xcel's budget is unable to cope, the company would have to ask Minnesota utility regulators to pass the costs to ratepayers.
Image: Monticello nuclear power plant (courtesy of Xcel Energy)