After officials discovered radioactive material in the groundwater, a Minnesota nuclear power plant was temporarily shut down on Friday.

At least 400,000 gallons of water contaminated by radioactive tritium were leaked from Monticello’s nuclear power station of Xcel Energy. It is located along the Mississippi River. Last week was the first time that the leak was made public. After monitoring equipment discovered more radioactive water, the plant had to be shut down.

Minnesota Public Radio

Tritium, a radioactive and mildly radioactive hydrogen form, is found naturally in the environment as well as during nuclear power generation. It reacts with oxygen to make radioactive water. Federal and state officials agree that it is not dangerous if it is ingested in large amounts.

Xcel Energy put in a container to capture the contaminated water after the initial discovery was made last November. It was intended to be a temporary solution, but it was discovered that the container had sunk.

According to the company, “hundreds of gallons” of contaminated water were not released into the environment this time.

“Ongoing monitoring of over two dozen monitoring wells on-site confirms that the leakage water is fully contained on-site,” Xcel Energy stated in a statement.

Xcel Energy had removed approximately 32% radioactive water as of Thursday. According to Xcel Energy, radiation is not a threat to residents or the environment.

Minnesota Pollution Control Agency (MPCA) and the Minnesota Department of Health confirmed the assessment. They stated that there was no evidence to suggest a public risk and will continue to monitor groundwater samples.

Chris Clark, President of Xcel Energy’s Minnesota and Dakota operations, stated that although it wasn’t clear exactly what caused the leak, the pipe that caused the spillage will be removed and sent for testing to a laboratory.