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NRC Technical Staff and Industry Differ Over Filtering Alternatives

The following news article originally appeared in NEI’s Nuclear Energy Overview.

NRC staff told the agency’s Advisory Committee on Reactor Safeguards last week it believes that installing external containment vent filters on some reactors could be beneficial. The industry, however, said that filtering strategies inside containment that take advantage of the FLEX strategy are the key to mitigating radioactive releases during an accident.

“This is not a simple ‘add a filter, no filter’ decision,” said Tony Pietrangelo, NEI’s senior vice president and chief nuclear officer. “It’s much more complex than that. Ultimately, you’re trying to prevent land contamination.”

John Monninger, associate director of the NRC’s Japan lessons learned directorate, told the ACRS it was the staff’s belief that external filters would improve reactor operators’ confidence to vent containment if needed.

The industry said that filtering strategies that occur inside containment—such as sprays or immersion (also known as containment flooding)—are just as effective at reducing radioactive releases as external filters.

“We believe the NRC erred in asking its technical staff whether facilities should install filters on containment vents to prevent significant radiation releases,” said Pietrangelo. “We have focused on answering a related, but different, question: What is the most effective way … to reduce radiation releases?”

Filtering strategies inside containment accomplish this goal, Pietrangelo said. That is because the water that must be injected into containment if there is core damage—which would be required even if there is an external filter—also retains radioactive particles in containment, preventing the particles from being released into the environment. Including controlled venting in the filtering strategies also reduces particle releases, he said.

According to a recent report from the Electric Power Research Institute the best way to minimize the release of radioactive materials in an extreme event is through a comprehensive strategy that includes installed equipment, operator actions and the capabilities brought by the industry’s diverse and flexible (FLEX) severe accident prevention and coping strategy.

A filtering strategy would not occur in isolation. The FLEX strategy now being implemented at U.S. plants is designed to provide the capability to respond to an extended loss of power by relying on backup emergency equipment such as generators, battery packs, pumps, air compressors and battery chargers.

“The best way to reduce the risk of radiation releases is by preventing a severe accident from occurring in the first place,” Pietrangelo said.

A decision to vent would occur after fuel inside the core had been damaged and in the context of emergency operating procedures.

“Maintaining containment integrity is critical to preventing significant radioactive releases. This can only be achieved by active fuel cooling and controlled containment venting,” Pietrangelo said.

He added that past studies have shown that adding external filtered vents is of questionable value.

“Of the several analyses that have been done of potential filtration requirements over the years, none has passed the regulatory standard for providing significant safety benefits,” Pietrangelo said. “Use of a filtration system outside the containment structure may not provide the desired level of protection, because certain sized aerosols containing radioactive particles might not be filtered by the external filtration system.”

Later this month, NRC staff plans to issue its recommendations on whether boiling water reactors with Mark I and Mark II containments should use external filters in a staff paper. The five-member NRC commission will vote on the staff’s proposals.

Thirty-one of the country’s 104 operating reactors are boiling water reactors with Mark I and II containments. The Advisory Committee on Reactor Safeguards is an independent forum of technical experts that advises the NRC on a range of issues.

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