USA: Hundreds of fish have died after a nuclear power plant in Minnesota was shut down because of a radioactive leak, according to a state regulator.

A previous breach at the same location caused hundreds of thousands of gallons of contaminated water to spill into the environment.

The shutdown, implemented last Friday after another leak at the Xcel Energy Monticello Nuclear Generating Plant near Minneapolis was discovered instead to contain radioactive contamination, according to the Minnesota Pollution Control Agency (MPCA), which killed an estimated 230 fish.

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The fish become accustomed to the warmer water entering the river as part of normal operations at the Monticello plant. The affected species include bass, channel catfish, common carp and one or more species of sucker fish, the regulator said in a statement on Monday.

The fish die is unfortunate but not unexpected as significant temperature changes can occur when warm water from the plant stops flowing into the river during a shutdown in operations," the statement continued.

The plant was shut down on Friday to give maintenance workers time to fix the leak. After the problem was discovered last year, a temporary fix was put in place, but it did not work, causing additional contamination in the area around the facility.

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Monitoring equipment at the location showed that hundreds of gallons of contaminated water had been released since the initial repairs.

While officials claim the radioactive water has not yet reached the nearby Mississippi River, Xcel Energy insists the leak poses no threat to public safety and has since been fixed.

The owner acknowledged that some of the material was discovered in groundwater near the plant, which eventually empties into the Mississippi.

Tritium, a radioactive isotope, is carried in contaminated water. Before the leak was discovered last November, 400,000 gallons of tritium leaked into the ground from a ruptured pipe.

The public was not made aware of the problem until mid-March, despite the fact that plant operators and the local government had known about it for months.

A mildly radioactive isotope of hydrogen called tritium is often found in the water used to cool nuclear reactors. According to Daniel Huff, an assistant commissioner with the Minnesota Department of Health, people can only be exposed to it if they breathe it in or consume treated water.

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Even though Xcel claims to have fixed the most recent leak, company president Chris Clark was unable to give an exact timeline during a press conference last week.

As a result, it is unclear when the plant will be operational again. However, he said he does not anticipate any material impact on electric service, claiming that Xcel has "sufficient margin" to supply customers.