The authorities in the US state of Minnesota recently announced that the Monticello Nuclear Generating Plant near Minneapolis had suffered a radioactive water spill involving more than 1.5m litres of tritiated water. Plant owner Xcel Energy is working to clean up the spill and says there is no danger to the general public. The Minnesota Pollution Control Agency (MPCA) said that the treated water had leaked from a broken pipe at the facility. The was first discovered on 21 November but not made public. The company said it notified the Nuclear Regulatory Commission NRC and the state the next day. The source of the leak was found on 19 December was plugged “soon after”.

The authorities agreed not to publicise the incident while Xcel Energy and the state were “actively managing” the situation to prevent the underground plume of irradiated water from spreading to the nearby Mississippi River, MPCA assistant commissioner for land and strategic initiatives Kirk Koudelka told the Minneapolis Star-Tribune. “We are monitoring the area. Xcel is taking actions to remove contaminated groundwater from one series of wells, and in addition using other wells to control the contamination to prevent it from going offsite, whether that’s the river or outside its property boundaries,” Koudelka said.

“Our top priority is protecting residents and the environment, and the MPCA is working closely with other state agencies to oversee Xcel Energy’s monitoring data and clean-up activities,” Koudelka noted, “We are working to ensure this clean-up is concluded as thoroughly as possible with minimal or no risk to drinking water supplies.”

The steel pipe that leaked is about four inches in diameter and carries condensate water away from the steam turbine that drives the plant’s generators. According to Pat Flowers, Xcel’s manager of environmental services, the damaged pipe was in an inaccessible spot.

“The leak took place in that tiny little space and it wasn’t really visible until you drilled a hole through two feet of concrete to get to it to physically see what was leaking,” he explained.

Xcel will analyse the pipe to try to identify the cause of the break. "In order to really understand what happened to this pipe, we're going to have to take out several feet of concrete around the pipe so that we can get access to it,” Flowers noted. “That's not going to happen until our [routine refuelling] outage that starts here in mid-April, and we've got plans in April to remove that pipe so we can do the metallurgy, we can repair it, and we can also understand what took place, what caused the failure."

MPCA spokesman Michael Rafferty added: “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.”

"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," he said.

Xcel’s president for Minnesota Christopher Clark insisted that “this does not present a public health or drinking water issue”. However, he admitted the tritiated water was “well above” the 20,000 picocuries standard mandated by the Environmental Protection Agency (EPA), and was “in the millions” directly below the plant. "If at any point there had been concern for the public safety, we would of course, immediately have provided more information," he told CBS Minnesota. "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."

The Minnesota Department of Health stated on its website that the leak did not reach the Mississippi River. "The groundwater beneath the facility, it's been determined that it moves in the direction of the Mississippi River, slowly, but that's the direction that it flows, or moves, underground," Doug Wetzstein an industrial division director with the Minnesota Pollution Control Agency, told CBS Minnesota.

The city of Monticello issued a statement, saying its drinking water supply was not affected by the leak, which happened outside the area from which they draw groundwater for municipal wells. No contaminated water has been detected beyond the plant’s perimeter, Xcel told KMSP-TV. "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," an Xcel Energy statement said.

An estimated 25% of the water has been recovered and pumped into a treatment system on site. The company is considering building storage tanks or a retention pond for the project, which could take a year or more. Xcel said recovery efforts will continue and that it will install a permanent solution this spring. The company 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, MPCA said.

US Nuclear Regulatory Commission (NRC) spokeswoman Viktoria Mitlyng said the leak was well below the NRC safety threshold and that the plan is not in violation of regulations. Tritium leaks “are not uncommon for nuclear plants,” she noted. In January, Xcel applied to NRC to extend Monticello’s operating licence for another 20 years beyond 2030, when its current permit expires. The company argued that the extension was “critical” in order to comply with a new Minnesota law mandating 100% carbon-free electricity by 2040.

Previously the NPP suffered a leak of around 7,500 litres in 1981, when some of the water reached the river. However, state health officials at the time said it posed no danger to wildlife or public health. Xcel also reported a small tritium leak at Monticello in 2009.

Tritium is a mildly radioactive isotope of hydrogen, often found in NPP cooling water. Daniel Huff, an assistant commissioner at the Minnesota Department of Health said it can only affect people if they breathe it in or drink tritiated water. Nevertheless, he added that the public’s exposure from a NPP “should be zero”.

Image: The Monticello nuclear power plant near Minneapolis (courtesy of Xcel Energy)