Over the objections of the nuclear industry, the staff of the Nuclear Regulatory Commission is planning to recommend the adoption of a new rule requiring American reactors similar to the ones at Fukushima Daiichi to install emergency vents with filters on them.

The filtered vents would be required on two of the oldest reactor designs sold by General Electric. The idea is that their containments could be opened early in an accident to vent a puff of slightly radioactive gas and explosive hydrogen and thus prevent a buildup in pressure or explosions as an accident unfolds. The reactors did not have such vents originally, but most of the oldest models, equipped with Mark I containments, added vents in the early 1990’s.

After the Fukushima accident of March 2011, the commission ordered that vents be added to Mark II reactors as well but told its staff to quickly study whether filtered ones were necessary.

The United States has 23 Mark I reactors, all of which now have vents, and eight Mark II reactors, none of which have vents. None have filtered vents.

The question to be considered is whether the releases would have enough particles of radioactive materials, called radionuclides, to contaminate surrounding areas or result in radiation doses to the population.

At a cost of at least $16 million per reactor, the changeover to filtered vents would be one of the most significant ordered on plants that already have operating licenses. Some plants that installed vents in the early 1990s would probably have to remove those and substitute vents with filters.

The commission usually applies a cost/benefit analysis, calculating the potential damage to be avoided by a hardware improvement by taking into account the severity of an accident and the probability of the accident’s occurring. But through a technique called probabilistic risk assessment, it calculates the probability of an accident in which a vent would be useful as very low.

On that basis, the filtered vents cannot be justified, the staff said on Thursday as it presented its position to a committee of senior nuclear experts. But installing filtered vents does fit in with the commission’s philosophy of “defense in depth,” meaning several layers of protection, the staff said. It is due to report to the five-member commission at the end of November.

At a meeting conducted by the commission’s advisory committee on reactor safeguards, a panel of senior nuclear experts that writes letters to the commission endorsing or contradicting staff positions, a representative of the nuclear industry, Steven Kraft of the Nuclear Energy Institute, said the industry was “committed to mitigating releases of land-contaminating radionuclides through a performance-based approach.”

“Performance-based” means setting a goal and letting each license holder meet it in whatever way is most appropriate at that site.

Mr. Kraft said the $16 million estimate for installing filtered vents in a reactor was too low because it counted what outside vendors said they would need to do the job but not the costs to plant owners should other modifications be needed to accommodate the vents.

When the reactors were designed, planners assumed that in the event of an accident, excess steam would be bubbled through a reservoir near the reactor vessel to scrub out radioactive particles. Now the industry is discussing whether it would also flood parts of a reactor building that are normally dry, providing yet more water to filter out radioactive materials. The industry hopes it can demonstrate that existing vents are thus adequately filtered.

If filters were added outside the reactor building, they could take the form of tanks of water or beds of sand, through which the gas would be bubbled; one design uses a venturi, which has a shape similar to an hourglass. When gas is passed through the constricted waist of the structure, particles are flung to the sides, allowing them to be collected.

One reason to favor a filtered vent, said John D. Monninger, associate director of the commission’s Fukushima “lessons learned” group, was to remove any hesitation in an operator’s mind about whether the best course was to vent the containment. “We believe adding the passive system there takes some of that burden from the decision-maker away,’’ he said. Others disagreed, suggesting that operators would feel safest following established procedures.

The commission staff’s position puts it in the unusual position of winning praise from nuclear opponents. Jim Riccio of Greenpeace told the advisory committee, “This is the only country except for perhaps Slovenia that is not moving to put these filters in place.’’

In fact, an N.R.C. document he cited shows that Mexico and India are also in that nonadopter category. Others are installing them.

Another opponent, Mary Lampert of Pilgrim Watch in Massachusetts, addressed the group by speakerphone. “We have seen three core melt accidents in real time,’’ she said, referring to Fukushima, Chernobyl and Three Mile Island. Speaking of the staff’s use of probabilistic risk assessment, she said, “It’s time to learn from real experience, not P.R.A. theoretical games.’’

A filtered vent is a “no-brainer,” she said.

The staff is somewhat less emphatic; it will present the commission with a range of options, including a performance-based approach that could leave some plants installing filters and others demonstrating that the water provides adequate filtration.

Article source: http://green.blogs.nytimes.com/2012/11/02/a-hard-look-at-u-s-reactor-hardware-after-fukushima/?partner=rss&emc=rss