20 Mar (NucNet): The US Nuclear Regulatory Commission’s (NRC) commissioners have directed the agency’s technical staff to follow “a two-track approach” for further improvements to systems for safely venting pressure during potential accidents at 31 US reactors.
The directive calls for upgrading a March 2012 order requiring “hardened” venting systems at 31 boiling-water reactors (BWRs) with Mark I and Mark II containments.
The NRC said the directive also starts rulemaking activities to produce requirements for those reactors to cool core debris and retain radioactive material in conjunction with venting during severe accidents.
NRC chairwoman Allison Macfarlane said the directive had been issued because the venting issue was one of the most important in the NRC’s post-Fukushima accident review process.
The NRC has given the staff 60 days to finalise the improved order, which will require the vents to handle elevated pressures, temperatures and radiation levels from a damaged reactor. The order will also ensure plant personnel can operate the vents safely under accident conditions.
The NRC has also given the staff a year to produce a technical evaluation to support rulemaking on the filtering of radioactive releases.
The NRC directed the staff to consider both the use of a filter to be placed on the vent, as well as a more performance-based approach using existing systems to achieve a similar reduction in radioactive release during an accident. The staff must develop a final rule by March 2017.
In March 2012, the NRC began to implement several recommendations for improving safety based on lessons learned from the accident at Fukushima-Daiichi in March 2011.
One of those recommendations, specifically for BWRs with Mark I and Mark II containments, was to improve venting systems or install new venting systems that help prevent or mitigate core damage in the event of a serious accident.
In a memorandum issued in March 2012, the NRC said operators at Fukushima-Daiichi were unable to successfully operate the containment venting system during the early part of the accident. The inability to reduce containment pressure inhibited efforts to cool the reactor core.
“If additional backup or alternate sources of power had been available to operate the containment venting system remotely, or if certain valves had been more accessible for manual operation, the operators at Fukushima may have been able to depressurise the containment earlier,” the NRC said.
“This, in turn, could have allowed operators to implement strategies using low-pressure water sources that may have limited or prevented damage to the reactor core.”
The NRC said the events at Fukushima-Daiichi showed that reliable hardened vents at BWR facilities with Mark I and Mark II containment designs are important to maintain core and containment cooling.
