A recently issued Nuclear Regulatory Commission (NRC) order will require the Monticello Nuclear Generation Plant to improve two pressure venting systems.
Following the March 2011 earthquake and tsunami at the Fukushima Daiichi, the NRC established a senior-level task force that conducted a methodical review of the agency’s regulations and processes to determine if the NRC should have plant operators make plant safety improvements in light of the events in Japan.
The enhanced NRC order requires nuclear power plants to make sure certain vents can safely handle the pressures, temperatures, hydrogen concentrations and radiation produced by a damaged reactor.
The enhancements also ensure plant personnel can operate the vents safely if the reactor core is damaged.
At Fukushima, operators were unable to successfully operate containment venting systems early in the event.
The inability to reduce containment pressure inhibited efforts to cool the reactor core. The NRC’s June 6 order calls for “enhancing requirements” at 31 plants, including Xcel’s nuclear plant in Monticello.
Terry Pickens, Xcel Energy’s director of nuclear regulatory policy, explained the significance of the NRC’s order covering venting scenarios for “wetwells,” internal plant structures meant to condense accident-generated steam and control pressure, and the larger “drywell” structure that surrounds a nuclear reactor.
“The NRC has been having a lot of discussions about venting,” Pickens said.
“Previously, they issued a different order and said we needed a reliable, hard-pipe vent system. We thought we were in good shape on that and had done things that Fukushima had not, like installing pneumatic vent valves instead of electric.”
Pickens said the Monticello plant’s vent comes off the wetwell, a donut-shaped structure underneath the reactor core. “We’ll go through the process of checking the vent off the wet portion of the containment and make sure it works,” he said. “The NRC also said that we need a hardened vent off the dry containment.”
Pickens said the NRC wants to make sure proper venting can take place in the unlikely event that severe accident conditions occur.
“We don’t have cost estimates at this point in time for what’s it going to cost for the vent off the dry part of the containment,” Pickens said, adding NRC has given the Monticello plant until its spring 2017 refueling outage to complete installation of wet containment vent. The dry vent must be installed before the 2019 refueling outage.
“There’s the additional issue mixed up in this whole thing of whether or not we’ll have to install an external filter on the vents,” Pickens said.
The most recent NRC order did not require plants to address that issue. “They will do that via a rulemaking process, and that’s a completely different schedule,” Pickens said.
According to Pickens, the vent off the wet portion of the containment is about 8 inches in diameter. “It’s not huge. It’s less than a foot,” he said. “The drywall vent will probably be smaller because it goes from the basement of the plant all the way up. An existing point of penetration, will be used.
“Going through a 3-foot thick steel and concrete wall and inch-and-a-half thick steel plate that make up the containment would be a big chore,” he said. We’ll look for spare penetrations.”
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