7 Jun (NucNet): US Nuclear Regulatory Commission (NRC) has ordered additional improvements to containment venting systems at most boiling water reactors, further strengthening a requirement that was first issued last year in response to the Fukushima-Daiichi accident.
The NRC said its latest order supersedes a March 2012 order for the 31 reactors with Mark I and Mark II containments to install or improve their “hardened” venting systems. The new order requires the vents to handle the pressures, temperatures, hydrogen concentrations and radiation levels from a damaged reactor.
The improvements also ensure plant personnel can operate the vents safely if the reactor core is damaged, the NRC said.
The ability to vent containment successfully could be important to preserving containment integrity and preventing release of radioactive material, the NRC said.
The order requires upgrading of vents from one section of containment, known as the wetwell, to be completed starting in June 2014. Owners of affected units must analyse whether they need to add vents from a second section of containment, the drywell, and if needed, complete that work as soon as June 2011.
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.”
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