Nuclear Energy Insight
Fall 2012—In recent months, experts from the nuclear industry and the NRC have turned their attention to two less obvious aspects of plant operations: vents and filters.
The importance of venting became clear during the early days of the Fukushima Daiichi accident last year. If containment vents at the Japanese facility had been opened in a timely manner, the worst consequences of the accident might have been avoided.
“The major problem at Fukushima was getting the containment vents open,” said Doug True, president of ERIN Engineering and Research. “Significant delays in the decision to open the vents, combined with the Japanese decision-making process and difficulties in opening the vent valves, delayed the venting action, which in turn led to hydrogen explosions.”
Containment vents can be opened to reduce the pressure and temperature building up inside containment during a reactor accident and allow hydrogen generated during the accident to harmlessly escape.
“The purpose of venting in an accident condition is to protect the integrity of the containment structure,” True said. “A vent would only be opened and closed during an accident when it is necessary to preserve the containment.”
At Fukushima, units 1 and 3 containments released hydrogen not through the vent but directly into the secondary containment where it reached an explosive concentration.
Containment vents are pipes that lead from the reactor containment to the outside environment, although not all containments have vents.
“A vent generally refers to a pipe connected to the primary containment of a nuclear power plant and leading to the plant’s [exhaust] stack,” said True. “The vent is designed to allow gases inside the containment to be removed.”
In contrast to Japan, in the U.S. the decision to vent containment is dictated by emergency operating procedures and would generally be made by the on-site emergency director.
“In general, plant procedures call for the use of the vent at a point when the pressure in containment is at or above the design pressure of the containment,” True said.
FILTERING STRATEGIES HOLD PROMISE
Vents are just part of the story. Venting coupled with an appropriate filtering strategy can greatly reduce the amount of radioactivity that could be released in the event of an accident.
“Filtering strategies like sprays and immersion can significantly reduce radioactive releases,” True said. “For example, water sprays inside containment combined with control of the containment vent can reduce the release by a factor of 1,000.”
Filtering strategies can accomplish this goal because water retains radioactive particles, preventing the particles from being released into the environment.
There are three common filtering strategies around which a facility can design a solution: an external water tank (also known as a “wet filter”), water immersion and water sprays.
“There are a variety of [wet filter] designs, but the most prevalent design is a tank of water such that the air from the vent would be bubbled up through the water,” said Maria Korsnick, chief nuclear officer and chief operating officer, Constellation Energy Nuclear Group. “As the water bubbles up it creates a scrubbing action which contains the radioactive materials in the water, significantly removing radioactivity from the air.”
Wet filters are used in Europe, but not the United States. In an external wet filter, the water tank is placed outside of containment and the scrubbing action occurs there.
Filtering with water immersion and water sprays uses the same basic principle—water retains the radioactive particles, greatly reducing the amount of radioactive release. The key difference is that both immersion and sprays keep radioactive particles inside the containment chamber, with water falling from the top of the chamber in the case of sprays and water coming up from below in the case of immersion.
Containment vents at U.S. nuclear power plants do not have wet filters, but the industry and the NRC are investigating which filtering strategy might be most appropriate.
“Recent industry work has pointed toward the use of water sprays in containment, combined with strategies to control the opening and closing of the vent, as highly effective,” True said. “Such strategies retain the radioactivity inside containment, as designed, rather than collecting them in a tank somewhere else.”
Importantly, a filtering strategy would not occur in isolation. The decision to vent, and the use of sprays or immersion, would only occur in the context of emergency operating procedures. Furthermore, US plants are implementing the FLEX strategy that provides the capability to respond to an extended loss of power by relying on backup emergency equipment—generators, battery packs, pumps, air compressors and battery chargers.
“In addition to all the systems we already have in containment that could do flooding and spraying, we will also have FLEX equipment that will be able to provide the same service,” said Korsnick. “This isn’t a question of a containment filter or no filter. It’s a question around filtering strategy and what’s the best, most efficient way to achieve the goals of a filtering strategy.”
—Read more articles in Nuclear Energy Insight and Insight Web Extra.