Representatives of the nuclear energy industry once again this week asked the NRC to adopt a performance-based approach to minimize the release of radioactivity from damaged fuel in a severe accident, while agency staff reiterated its earlier recommendation to require external filters for the containment vents of early-model boiling water reactors.
“A performance-based approach to filtration provides the greatest overall improvement in plant safety with sufficient filtration,” said Maria Korsnick, Constellation Energy Nuclear Group’s chief nuclear officer and chief operating officer. “The industry approach has the advantage of retaining the filtered radionuclides in containment. The desire is to prevent land contamination.”
Korsnick added that a performance-based approach that would specify results rather than methods to attain them would require plant-specific analyses. She noted that the NRC’s independent Advisory Committee on Reactor Safeguards “supports this approach”.
NRC staff told the commissioners it believed filtered vents should be required for boiling water reactors with Mark I and II containments, similar to those involved in the 2011 accident in Japan.
“There is general agreement of all stakeholders that filtration is important to mitigate potential radioactive releases,” said Bill Borchardt, executive director for operations at the NRC. “However, it’s the staff’s view that it is prudent to require the installation of [external] filters on the containment vents.”
Preston Swafford, chief nuclear officer at Tennessee Valley Authority, said he agrees with the industry position as put forward by Korsnick, adding that his organization is conducting analyses on the effectiveness of filtered vents.
“We are supporting Maria and the industry on [the performance-based approach to filtering],” Swafford said. “A dry filter … might be of value. We haven’t concluded one way or another yet, but there may be some additional coping time that filter would give us. We have to finish that [analysis].”
NRC Chairwoman Allison Macfarlane asked whether the decision to manually vent containment during an accident would be easier for operators to make if they knew the vent had an external filter.
“I feel very confident that operators would vent containment as necessary. I don’t think a filter changes that, quite frankly,” Korsnick said.
Neil Wilmshurst, vice president and chief nuclear officer at the Electric Power Research Institute, gave a technical explanation of the performance-based approach. He said that a performance-based filtration strategy should target a 1,000-fold reduction in the amount of fission products released, also known as a decontamination factor of 1,000.
According to a recent EPRI analysis, that level could be reached using a combination of strategies. These would include spraying or immersing damaged fuel within the containment, along with a specifically designed hardened vent that could reliably open and close at appropriate times. This combined approach would not necessarily require a filtered vent.
“A [decontamination factor] of 1,000 is the benchmark we set in our analysis,” Wilmshurst said. “If utilities can come to a position of spraying and flooding and have a controlled reliable, hardened vent, our analysis shows that DFs of over 1,000 can be achieved without a filter.”
Wilmshurst added that the performance-based approach would require site-specific analyses and that filters may play a role in certain scenarios.
“[The EPRI analysis] doesn’t mean filters might not be needed in certain circumstances,” he said. “This is a generic analysis. Specific plant evaluations would be required.”
The five-member commission is continuing its study of filtering strategies and will likely vote on the matter in the near future.