Xcel Energy, operator of the nuclear plant northwest of Minneapolis in the Midwest state of Minnesota, did not say why it waited more than three months to acknowledge the leak to the public.

The company said it notified state officials and the federal Nuclear Regulatory Commission (NRC) once it learned of the leak on November 22.

"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, the utility's president, said in a statement.

The Minnesota Pollution Control Agency said the company told it some 400,000 gallons of water containing tritium leaked at the site, but none "reached the Mississippi River or contaminated drinking water sources."

State officials "are actively reviewing data" from the site and "overseeing remediation efforts," the agency said.

The company said it has "recovered about 25 percent of the tritium released and will continue recovery over the course of the year."

The leak originated in "a water pipe between two buildings" at the Monticello nuclear plant.

Tritium is a radioactive isotope of hydrogen that is a byproduct of the production of electricity at nuclear plants. It can also occur naturally in the environment.

Monticello is 63 kilometers (39 miles) northwest of Minneapolis, the largest city in the state, and also where Xcel Energy has its headquarters.

Xcel said it detected the spill while doing routine groundwater testing.

It said it contained the leak by diverting water to an in-plant treatment facility, and will need to build "large storage tanks... to store recovered water until it can be treated and reused."

The company said it is conducting more frequent tests from some two dozen groundwater monitoring wells in and around the site.

The US has suffered one major nuclear accident in its history -- the meltdown of the Three Mile Island reactor in Pennsylvania on March 28, 1979.

Some 92 nuclear reactors provide power to tens of millions of US homes. Smaller accidents have occurred over the years but usually contained with localized impacts.