The head of the Nuclear Regulatory Commission's Fukushima inquiry task force said yesterday his panel is concerned that the severe threats that Japan's massive earthquake and tsunami posed to the Fukushima Daiichi nuclear complex reveal gaps in the voluntary guidelines that protect U.S. plants against incidents deemed unlikely.

Task force leader Charles Miller, who briefed NRC commissioners yesterday, said the panel is also considering whether older nuclear plants should be held to more demanding standards that have been applied to newer reactors, based on evolving safety insights.

Miller did not disclose how the task force will come down on these issues. It is scheduled to make its final report to the commission on July 12, with a public release of its recommendations on July 19.

NRC Chairman Gregory Jaczko and Commissioner George Apostolakis also pointed to potential weaknesses in the commission's voluntary policy toward low-probability but extreme threats to nuclear plants.

"As we look at this event," Jaczko said of the Fukushima plant catastrophe, "it's been a moment of reflection, because I think deep down there was a belief that you would never see an event like this, that we had done everything to basically take this type of event completely off the table. And obviously, we haven't."

"And we have over the years done things maybe halfway, and not all the way, to try to address these things," Jackzo said.

Jaczko suggested the differences in safety standards applied to older and newer plants might need to be addressed when nuclear plant owners ask the NRC to renew the initial 40-year operating licenses.

"Obviously, at license extension time we have an opportunity -- although the commission has not availed itself of that opportunity -- to in a sense re-baseline everybody ... so that everybody at that point kind of has a consistent understanding and basis for what is the definition of safety, what is the definition of external hazard," Jaczko said.

A focus on older plants
Miller said the task force had not looked at that. "We can't go back and rebuild" older plants, but this is an opportunity to consider reasonable actions to cover vulnerabilities, he said.

NRC's executive director for operations, Bill Borchardt, stepped into the discussion at that point to advise that changing the current requirements for license renewal "would clearly be a major policy decision." He said that the NRC had acted too hastily in making some ill-advised regulatory changes following the Three Mile Island nuclear accident in 1979, and that the agency should move deliberately this time.

Apostolakis, a former professor at the Massachusetts Institute of Technology, said he was puzzled by the line between mandatory regulation and voluntary guidance. "I'm wondering, what does voluntary mean?" he said.

Miller said that at some plants, the operators' attention to voluntary guidelines falls below its level of compliance with mandatory regulation. "I think the industry has come to those same conclusions, also," Miller said.

NRC policy has not considered the risk that a natural disaster could cause an extended loss of outside electric power from the grid, and also backup power within a plant, shutting down cooling systems that prevent core melting or exposure of spent fuel in storage pools, said Miller. He is a senior NRC official who delayed his retirement to head the short-term review of the Fukushima accident.

Extreme threats were considered so remote that they were covered by voluntary "severe accident management guidelines" (SAMGs) to plant operators, Miller noted. But NRC inspections since the Fukushima disaster in March revealed failures at some U.S. plants to keep the emergency guidelines and operators' training up-to-date.

"As a voluntary initiative, the SAMGs did not get rigorous oversight by many licensees. This was confirmed by our inspection results. The inspections revealed inconsistent implementation" by some of the licensees, Miller said.

Borchardt said the inspections revealed that "some equipment, mainly pumps, were not operable when tested or lacked acceptance criteria. Some equipment was actually missing from the locations or dedicated to other activities, and in some cases, plant modifications had rendered the strategies that were developed at the time unworkable." But, he added, "we didn't find any cases where ultimately the [safety] function could not be performed."

Miller said the Fukushima disaster has focused the task force on the impacts of multiple threats to nuclear plants, such as a major fire and an earthquake, or an earthquake and flooding, that could knock out power for a plant's emergency equipment. Backup battery power for crucial safety equipment is only adequate for four to eight hours of emergency use at U.S. plants, Miller noted. The Fukushima accident forces a re-evaluation of whether that is enough protection, he concluded.

The current NRC regulations, called the station blackout rule, do not consider the impact of disasters that could cut off both on-site and off-site power simultaneously, Miller said.

New look at generators and vents
"As a result, the regulatory guidance assumed that the event causing the off-site power disturbance does not impact on-site power sources," he said. "And therefore the availability of on-site power is based on historic reliability of emergency diesel generators to start and perform their safety function when called upon."

The four- and eight-hour battery backup capabilities assume that outside power can be promptly restored. "In the case of an extreme external event, it might take days to restore [outside] AC power, as was the case at Fukushima," Miller said.

Miller also raised the issue of the vents that lead out of the wetwell, or torus, at the bottom of the Mark I reactors at Fukushima. When reactor fuel assemblies melted during the accident, hydrogen gas built up at high pressures inside the torus. Instead of exiting the reactor containment structure, hydrogen is presumed to have leaked from the venting system and exploded.

After the Three Mile Island accident, U.S. Mark I owners were required to install hardened exhaust vents, Miller noted. But the regulation did not specify a design for the vent configurations, nor did the regulations require a specific inspection program for the vents, he said. The designs in the United States vary, he added, and there are differences in the number and location of valves that might have to be operated in an emergency situation.

The venting systems "were not specifically designed for operation during a long-term station blackout," he added. "Therefore, depending on a plant-specific design, it may be a challenge to open the vent path in a scenario like the Fukushima accident."

He summed up, "in evaluating potential new [safety] requirements, the [NRC] staff tends to lean more toward the quantitative cost-benefit aspects of the regulatory analysis rather than the qualitative defense-in-depth consideration. This can result in more weight to protection strategies and less balanced approach to defense in depth."

An NRC requirement that plant owners make major modifications to reflect new safety standards is called the "back-fit" rule, and it includes a balancing of safety benefits and regulatory costs.

Commissioner Kristine Svinicki said Miller's discussion suggested that "you could quickly find yourself swimming in the waters of back-fit. ... That is something the task force is having to confront."

"We are confronting it," Miller said. "It is an awkward time."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500