NRC staff has recommended requiring engineered filters to the containment vents for early-model boiling water reactors as a post-Fukushima response.
A staff paper (SECY-12-0157) released this week for the commission’s consideration said an alternative performance-based approach to filtering preferred by industry and by the NRC’s Advisory Committee on Reactor Safeguards would work but take too long to implement.
The commission had directed agency staff (SRM-SECY-11-0137) to consider whether adding external filters at the same time as reliable hardened containment vents for Mark I and II boiling water reactors would help limit significant radiation releases from severe accidents.
The staff response said the option to require the installation of vent filters “would provide the most regulatory certainty and the timeliest implementation.”
The staff paper noted that quantitative considerations alone did not demonstrate that the benefits would exceed the costs of its recommended approach. However, it said if qualitative factors such as the importance of the containment function within the NRC’s defense-in-depth philosophy were included, its recommendation for external filters could be justified.
The staff also considered an option for a more comprehensive strategy to maintain cooling and control containment pressure that would combine the use of installed equipment, operator actions and improvements to plant-specific severe accident management guidelines. The industry considers this combined approach the most effective way to reduce radiation releases in extreme situations where reactor fuel may be damaged and consistent with the diverse and flexible “FLEX” coping strategy that is being implemented at every nuclear energy facility.
Steven Kraft, NEI’s senior technical adviser, said the industry believes that use of filters may be unnecessary. In addition, because the filters would be external to the containment building, the radioactive particles generated during an accident would not remain trapped in containment.
The industry’s comprehensive filtering strategy was developed by the Electric Power Research Institute. A report released by EPRI in September said, “Combining containment sprays or immersion of damaged fuel with a specifically-designed vent that can reliably open and close at appropriate times would provide a more than 1,000-fold reduction in the amount of fission products released” (see Nuclear Energy Overview, Oct. 1).
EPRI also commended the industry’s FLEX strategy, observing that “the best way to avoid radiological release and potential land contamination is to prevent an accident from occurring by improving and augmenting the strategies for preventing core damage.”
The NRC’s Advisory Committee on Reactor Safeguards concurred with the industry’s filtering approach. The ACRS said it preferred more general “performance-based standards” that would give plant operators scope to decide on innovative strategies to mitigate radiation releases, while allowing for the alternative technologies and procedures to be measured against objective outcome-based criteria (see Nuclear Energy Overview, Nov. 15).
The staff paper acknowledges that the industry approach is “consistent with NRC policy to encourage the use of performance-based requirements.” However, it said it was recommending vent filters because the combined strategy would “involve a longer-term effort” to complete a rigorous regulatory analysis including a cost/benefit assessment to comply with the agency’s backfit rule. The staff also said the performance-based approach “would likely extend the resolution of this issue by several years,” which it believes runs counter to the commission direction to consider the filtering issue “without delay,” along with the reliable hardened vent issue.
The NRC staff is also recommending that the reliable hardened vents ordered for Mark I and II containments be upgraded to vents that will operate following a severe accident.
NEI’s Kraft said that the NRC staff’s concern about the length of the effort is largely misplaced. Many of the questions the staff appears to have about the filtering strategies option also apply to the use of external filters, he said. For example, injecting water into containment to cool a damaged core is needed in all cases, including if an external filter is installed.
Kraft added, “The industry looks forward to continuing to work with the NRC and other stakeholders to identify the most appropriate way to retain radioactive material inside containment following a severe accident that damages the nuclear fuel and on scheduling and timeline issues related to the regulatory analyses required for a performance-based approach.”
The commissioners will be briefed by the industry, other stakeholders and the staff on Jan. 9. NRC spokesman Scott Burnell said the commissioners are expected to vote on the staff’s recommendations sometime in January.