US Mark I and II BWRs directed to harden venting

7 June 2013

The Nuclear Regulatory Commission has directed 31 U.S. reactors to further improve their systems for safely venting pressure from their containment buildings during potential accidents.

The agency's order today 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 enhanced order requires the vents to handle the pressures, temperatures, hydrogen concentrations and radiation levels from a damaged reactor. The enhancements also ensure plant personnel can operate the vents safely if the reactor core is damaged.

"Strengthened vents will help these plants continue to protect the public and the environment even if emergency systems can't immediately stop an accident," NRC Chairman Allison M. Macfarlane said. "By safely releasing built-up pressure and hydrogen, the plants will preserve the buildings that contain radioactive material."

The order covers venting scenarios for both "wetwells," structures meant to condense accident-generated steam and control pressure, and the larger "drywell" structures that surround the reactor. The order requires the 31 plants to complete wetwell venting improvements starting in June 2014 (depending on a plant's refueling schedule). The plants must also analyze their drywell venting scenarios and if necessary, install a drywell venting option starting in June 2017.

The NRC staff is also following Commission direction to develop a rule for the 31 plants to implement strategies to enhance filtering of radioactive material from any vented gases, as well as improve procedures for preserving containment integrity. The staff expects to hold public meetings later this month to discuss both the technical basis for the rule and the preparation of guidance for complying with the order.

Image: Simplified Mark I BWR from 1996 Sandia National Laboratory report, via Wikimedia Commons

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