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NRC: Nuclear plant failed to evaluate flood risk

Prairie Island Nuclear plant operators knew of the potential for flooding in the plant's Unit 1 and Unit 2 turbine buildings, but failed to understand the implications on important safety-related equipment, according to a preliminary finding submitted to the plant Thursday by the U.S. Nuclear Regulatory Commission.

The failure to identify and correct the potential safety issues in a timely manner is a significant human performance issue and cause for further review by the agency, according to NRC inspectors. Plant officials have 10 days to respond to the findings before the NRC decides whether to take enforcement action.

"We're waiting now for their response," said NRC spokesperson Viktoria Mitlyng.

The agency's preliminary findings are tied to a 2009 violation of low to moderate safety significance - called a "White finding" - involving the facility's failure to provide adequate protection of piping against natural events such as tornadoes and earthquakes.

Later, when plant operators were evaluating piping in the turbine building for similar issues, they found that a rupture of piping caused by a natural event could result in the flooding of the building. At that time, they didn't know what the extent or volume of that flooding would be, according to Xcel Energy site Vice President Mark Schimmel.

Schimmel said that it took time for plant operators to determine whether flood damage to important safety equipment such as emergency generators and feedwater pumps could occur in the event of a piping rupture.

When operators found that there was a risk of flood damage, Schimmel said they took immediate action to correct the problem - establishing a flow path to lead water out of the turbine building and setting up flood barriers to protect important safety-related equipment.

"We said, 'We have to look at this flooding because it's more significant than we had thought,'" he said.

Mitlyng said the agency's concerns stemmed from the plant's failure to evaluate the flood risk before being prompted to do so by agency officials. She pointed to a report by NRC inspectors that claimed plant operators had repeatedly identified potential problems with piping in the turbine building, but failed to prioritize them.

"The facility had opportunities to identify the problem, but they didn't," she said. "The safety equipment should not have been in a compromised position."

The NRC has given Xcel officials the option to provide a response to the preliminary findings in writing, or to present its response at a public regulatory conference. Site Vice President Schimmel said Xcel officials have not yet decided on a course of action.

Once it receives a response, the NRC will conduct a final evaluation of the issue, said Mitlyng. At that point, she said, the agency will determine whether to flag the plant with a violation of "greater than very low safety significance," and what enforcement actions, if any, to take.

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