ST. PAUL, Minn. — Minnesota regulators said Thursday they’re monitoring the cleanup of a leak of 400,000 gallons of radioactive water from Xcel Energy’s Monticello nuclear power plant, and the company said there’s no danger to the public.

“Xcel Energy took swift action to contain the leak to the plant site, which poses no health and safety risk to the local community or the environment,” the Minneapolis-based utility said in a statement.

While Xcel reported the leak of water containing tritium to state and federal authorities in late November, the spill had not been made public before Thursday. State officials said they waited to get more information before going public with it.

“We knew there was a presence of tritium in one monitoring well, however Xcel had not yet identified the source of the leak and its location,” Minnesota Pollution Control Agency spokesman Michael Rafferty said.

“Now that we have all the information about where the leak occurred, how much was released into groundwater, and that contaminated groundwater had moved beyond the original location, we are sharing this information,” he said, adding the water remains contained on Xcel’s property and poses no immediate public health risk.

The company said it notified the federal Nuclear Regulatory Commission and the state on Nov. 22, the day after it confirmed the leak, which came from a pipe between two buildings. Since then, it has been pumping groundwater, storing and processing the contaminated water, which contains tritium levels below federal thresholds.

“Ongoing monitoring from over two dozen on-site monitoring wells confirms that the leaked water is fully contained on-site and has not been detected beyond the facility or in any local drinking water,” the Xcel Energy statement said.

When asked why Xcel Energy didn’t notify the public earlier, the company said: “We understand the importance of quickly informing the communities we serve if a situation poses an immediate threat to health and safety. In this case, there was no such threat.” The company said it focused on investigating the situation, containing the affected water and figuring out next steps.

The Monticello plant is about 35 miles (55 kilometers) northwest of Minneapolis, upstream from the city on the Mississippi River.

Tritium is a radioactive isotope of hydrogen that occurs naturally in the environment and is a common by-product of nuclear plant operations. It emits a weak form of beta radiation that does not travel very far and cannot penetrate human skin, according to the NRC. A person who drank water from a spill would get only a low dose, the NRC says.

The NRC says tritium spills happen from time to time at nuclear plants, but that it has repeatedly determined that they’ve either remained limited to the plant property or involved such low offsite levels that they didn’t affect public health or safety. Xcel reported a small tritium leak at Monticello in 2009.

Xcel said it has recovered about 25% of the spilled tritium so far, that recovery efforts will continue and that it will install a permanent solution this spring.

“While this leak does not pose a risk to the public or the environment, we take this very seriously and are working to safely address the situation,” Chris Clark, president of Xcel Energy–Minnesota, North Dakota and South Dakota, said in the statement. “We continue to gather and treat all potentially affected water while regularly monitoring nearby groundwater sources.”

Xcel Energy is considering building above-ground storage tanks to store the contaminated water it recovers, and is considering options for the treatment, reuse, or final disposal of the collected tritium and water. State regulators will review the options the company selects, the MPCA said.