US power utility Xcel Energy has reported another, smaller, spill of radioactive water at its Monticello NPP in Minneapolis. Workers have been pumping and storing water from the ground at the plant since a faulty pipe caused a leak of about 400,000 gallons of water contaminated with tritium in November. Xcel temporarily shut down the plant and repaired the leak in March. However, Xcel has now notified the Minnesota state duty officer that 300 to 600 gallons of pumped groundwater had overflowed a holding tank and returned to the ground.

Xcel spokesperson Kevin Coss said the water – about an hour’s worth of pumping – will be recollected. Overall, Xcel said it had made “substantial” progress in recovering the contaminated groundwater. It has now pumped over 1.1m gallons of water and has recovered more than half of the tritium released, Coss said. He added that the concentration of tritium in the groundwater has declined. The highest measurement, directly under the plant near the leak, was about 5m picocuries per litre but this is now down to 1.5m picocuries per litre. The US Environmental Protection Agency considers 20,000 picocuries per litre to be safe for drinking water. State officials have said the contamination has not left the plant site, and does not pose a threat to any public or private drinking water wells.

The US Nuclear Regulatory Commission will hold its annual public meeting in Monticello to discuss the plant's safety performance. NRC previously determined the plant operated safely in 2022, and said all inspection findings and performance indicators were of very low safety significance. More significant findings can trigger increased federal oversight and increased inspections. Xcel is seeking federal approval to continue operating the Monticello plant beyond its current licence, which expires in 2030 and is also requesting to store additional radioactive nuclear waste at the plant.

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