Clean-up of radioactive water leak ongoing at Minnesota nuclear plant

The radioactive spill came from a pipe between two buildings at the Monticello nuclear power plant

Officials are monitoring the clean-up of a leak of 400,000 gallons (1.5m litres) of radioactive water from a local nuclear power plant in Minnesota.

Xcel Energy, the utility company that runs the plant, said the spillage was "fully contained on-site and has not been detected beyond the facility".

State officials said there was no immediate public health risk.

The leak was first discovered in late November, but state officials did not notify the public until Thursday.

The water contains tritium, a common by-product of nuclear plant operations.

A naturally occurring radioactive isotope of hydrogen, tritium emits a weak form of beta radiation that does not travel very far in air and cannot penetrate human skin, according to the federal Nuclear Regulatory Commission (NRC).

Tritium spills occur at nuclear plants on occasion, but are typically contained on-site and rarely affect public health or safety, the NRC says.

Xcel first discovered the leak on 21 November, from a pipe between two buildings at its Monticello plant. The plant is about 35 miles (56km) from the state's most populous city, Minneapolis, upstream along the Mississippi River.

The Minnesota Department of Health has said the leak did not reach the Mississippi River.

The company said it had notified the state and the NRC the next day.

"If at any point there had been concern for the public safety, we would of course, immediately have provided more information. But we also wanted to make sure we fully understood what was going on before we started raising any concerns with the public around us," Chris Clark, president of Xcel Energy-Minnesota, North Dakota and South Dakota, told CBS News, the BBC's US partner.

"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 Minnesota-based utility said in a statement on Thursday.

State officials said they had waited to get more information before going public with the leak, but confirmed that the water remains contained on Xcel's property and poses no immediate public health risk.

On Friday, the mayor of Monticello defended the city's response, saying that city officials did not learn of the leak until "the end of February" and that the city does not have any authority over the power plant.

"Though the Xcel plant is within our community, the City of Monticello does not have the authority to govern the nuclear plant," Mayor Lloyd Hilgart said in a statement saying that oversight is done by state and federal officials.

"If state or federal oversight agencies determine that there is any potential or actual impact to the City's drinking water supply or infrastructure, the City will immediately notify the public with assistance from these agencies," he said.

Xcel Energy said crews inspected the plant at all points where leaks could occur and will be examining the leaked pipe in a lab.

Roughly one-quarter of the spilled tritium has been recovered so far, and Xcel has said it may build above-ground storage tanks to store the contaminated water.

It is also weighing how to treat, re-use or dispose of the collected tritium and water.

The company previously had a small leak at its Monticello plant in 2009.