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State and Xcel officials announced earlier this month that 400,000 gallons of water contaminated with tritium leaked from a water pipe between two buildings at the the Monticello nuclear power plant.

The leak was detected nearly four months ago and reported to state and federal regulators. The federal Nuclear Regulatory Commission 

, but the company and state agencies did not notify the general public until March 16.

Xcel Energy said last week it will power down the plant after monitoring equipment detected more radioactive tritium was leaking into groundwater.

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 about tritium, water safety and nuclear power. Here’s what we found out.

Xcel held an open house last Friday to give residents a chance to ask questions about the leak. A second open house was held Monday at the Monticello Community Center.

Tritium is a colorless, odorless gas with a half-life of 12.3 years, meaning it takes just over 12 years for half of the radioactive atoms to decay.

Tritium is a relatively weak source of radiation. It emits beta particles at such a low level they are unable to penetrate human skin. Tritium can be hazardous, but only if it’s ingested in large quantities.

Assistant Minnesota Health Commissioner Dan Huff said if you drank water contaminated with tritium at the EPA limit, over a year you'd be exposed to about 4 millirems of radiation. In comparison, a cross-country airplane flight results in an exposure of about 3 millirems, and an X-ray can be 10 to 25 millirems or higher.

How much radioactive material was spilled?

State officials said 400,000 gallons of water contaminated with tritium leaked from a water pipe between two buildings at the plant. (For comparison, an Olympic-sized swimming pool holds about 660,000 gallons of water.)

Initially, when the release was first detected, it was measured at 5 million picocuries per liter in the groundwater. The U.S. Environmental Protection Agency's limit for tritium in drinking water is 20,000 picocuries per liter.

Since then, that amount has been reduced to about 2.5 million picocuries per liter right at the plant, through the pumping Xcel has been doing. Farther away from the site, the amount that shows up in monitoring wells drops. 

After identifying the source of the leak in November, Xcel implemented a short-term solution to capture water from the leaking pipe and reroute it back into the plant for reuse. The fix was focused on preventing any new tritium from reaching groundwater until Xcel could install a replacement pipe during a scheduled refueling outage in April.

In a March 23 statement, the utility said the short term fix was no longer capturing 100 percent of the leaking water. Xcel officials said the new leakage amounted to hundreds of gallons, a much smaller amount of water than previously leaked.

“While the leak continues to pose no risk to the public or the environment, we determined the best course of action is to power down the plant and perform the permanent repairs immediately,” said Chris Clark, president of Xcel Energy–Minnesota, North Dakota and South Dakota in a statement.

The schedule for restarting the plant has not been determined. As of late last week, Xcel had recovered about 32 percent of the tritium released.

If I live near the plant or downstream, does this impact my drinking water?

The underground plume of contamination has not spread beyond the plant boundaries or gotten into drinking water supplies or the Mississippi River, according to state officials, so there’s no risk to people who live downstream.

The Minnesota Department of Health says there’s no risk to communities that draw their water supply from the Mississippi River, because even if the contamination were to reach the river, it would diffuse very rapidly in such a large volume of water.

Map of Monticello nuclear plant in Minnesota

If the contamination plume were to move off Xcel’s property, the health department would notify any property owners who have nearby wells. The closest well is owned by Xcel Energy and is on the plant property. The closest public well is at a church about 2 miles away, but on higher ground, so the tritiated water isn’t flowing toward it.

The health department does routinely monitor the air around the nuclear plant, the groundwater and the Mississippi River downstream from the plant and precipitation during rainfalls, said Assistant Health Commissioner Dan Huff.

“That is another way for us to know if there's any radiation from any source that has leaked from the plant,” Huff said.

What effect does the leak have on local flora and fauna?

The contamination has not left Xcel’s property or reached the Mississippi River, so there’s no risk to birds or wildlife, state officials said. If it were to enter the river, the risk would still be very low, because tritium is a low-energy radioactive element that only poses a health risk if ingested in large amounts.

However, authorities say fluctuating water temperatures in the Mississippi River following the shutdown of Xcel Energy’s Monticello nuclear power plant have killed more than 200 fish. 

Company and state officials said the fish kill is not the result of tritium.

“In Minnesota, our nuclear and thermal plants use once-through cooling. This process continuously draws water from nearby rivers for cooling steam and equipment and then returns the water,” Xcel said in a statement Monday night. “When a plant is running, it warms the water in the nearby river, keeping fish active. When a plant is taken offline, the river cools down. Fish are affected by the change in water temperature.”

As of Monday afternoon, at least 230 fish had died in the Mississippi River near the plant, including bass, channel catfish, common carp and suckers.

Why didn’t we know about the leak sooner?

The U.S. Nuclear Regulatory Commission did post a notice publicly at the time of the leak, but Xcel and state agencies didn't notify the general public until March 16.

Kirk Koudelka, an assistant MPCA commissioner, said had there been an immediate threat to human health or the environment, they would have notified residents immediately.

“The situation right now does not present that imminent risk to residents’ health,” he said. “It is a plume that is contained on site underneath the plant right now, and there's not that direct exposure."

Koudelka said it's a balancing act to make sure they have enough information to share, but not to give incomplete information and create unnecessary concern. He said officials are always looking to improve, and will take the feedback into consideration in the future.

Xcel’s Chris Clark Clark has acknowledged criticism that the company should have informed the public sooner about the original leak. 

“I think one of our lessons here is even though we followed the proper formal notifications, we have an opportunity to do a better job being transparent with our neighbors,” he said. “That's certainly a lesson we take from this.”

The Associated Press contributed to this report.

Kirsti Marohn

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Anna Haecherl

State and Xcel officials announced earlier this month that 400,000 gallons of water contaminated with tritium leaked from a water pipe between two buildings at the the Monticello nuclear power plant. MPR News asked what questions you have about tritium, water safety and nuclear power. Here’s what we found out.

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What we know about the Monticello nuclear plant tritium leak

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