US nuclear regulators should focus on extreme events: NAS

Washington (Platts)--24 Jul 2014 431 pm EDT/2031 GMT

US nuclear regulators and industry officials must do more to protect reactors from extreme, but unlikely, events like the earthquake and tsunami that caused the accident at Japan's Fukushima nuclear plant, the National Academy of Sciences recommended in report issued Thursday.

In its report on the causes and lessons from the Fukushima nuclear accident, NAS also urged regulators and industry to incorporate "modern risk concepts" to increase reactor safety in the US.

In addition, the study concluded that Fukushima plant operator Tokyo Electric Power Co. and Japan's nuclear regulator at the time failed to heed mounting evidence that the plant was not properly protected from flooding.

The report raised questions about the approach to nuclear safety used in the US and other countries, which relies on studies of the most likely accident scenarios for a site, the so-called "design-basis accident," and holds plant operators accountable for being able to withstand them. Less likely events are considered "beyond design-basis" and are regulated differently.

That approach may not consider accident varieties widely enough, said John Garrick, a nuclear engineer and consultant who was vice-chairman of the committee that undertook the study. "There's some new evidence now that some of these events are not as rare as perhaps we thought," he said on a conference call with reporters.

Congress asked the National Academy of Sciences to examine the causes of the March 2011 accident in Japan and report on any potential lessons for US plants. The report was completed under a contract with the US Nuclear Regulatory Commission. A committee of 21 specialists held 39 meetings over two years to gather information for the report and recommendations.

The report did not make specific policy proposals, limiting recommendations to technical areas.


An earthquake on March 11, 2011, caused a large tsunami that killed thousands and disabled key safety systems at the Fukushima I and II stations. Three reactors at Fukushima I suffered what the report said was "severe core damage," releasing radionuclides and causing a series of hydrogen explosions. Nuclear plant operators and regulators should "actively seek out and act on new information about hazards" that could threaten nuclear plants, the report said.

NRC and the nuclear industry should do more to assess the risks from events that could lead to a loss of safety functions at plants, the report continued.

While NAS said nuclear plant operators and regulators in the US and elsewhere are "taking useful actions" to upgrade systems, procedures and training at reactors, it recommended that US regulators focus on protecting power for instrumentation and control systems that would allow operators to better monitor conditions in reactors and spent nuclear fuel pools.

The NRC and US industry should improve the ability to identify and manage risks from "low frequency, high magnitude" events, such as occurred at Fukushima, the report said. That includes preparing workers to take "ad hoc" actions for the most severe external events, the report said.

NRC and the US nuclear industry should focus on low probability events that could affect large geographic regions and multiple nuclear plants, the report said, citing earthquakes, widespread floods and geomagnetic disturbances.

Further, the agency and industry should use "modern risk assessment principles" instead of NRC's traditional "deterministic" regulatory approaches in regulating nuclear power safety, the report said. NRC has started to incorporate the use of computer-based risk modeling into its regulations, but should increase its use, the report said.


NRC is reviewing the report and agency staff will report to commissioners "in the near future," spokesman Scott Burnell said Thursday. "An initial review of the report shows many areas of agreement between lessons identified by the Academy and those identified by the NRC," he said.

The Nuclear Energy Institute, which represents nuclear companies, said the NAS report "affirms the culture of safety adhered to by the US nuclear industry." The report validates steps the nuclear industry has taken and NRC has required in the wake of Fukushima, NEI said in a statement.

Those measures include adding portable equipment to provide backup cooling water and power to handle extreme external events, and re-assessments of the vulnerability of US plants to earthquakes and flooding. The report said such measures were important steps, but said it was too soon to evaluate their effectiveness.

The report concluded that workers at the Fukushima plant "responded with courage and resilience" and likely reduced the severity and consequences of the accident.

But several factors prevented workers from being able to do more, the report said.

Tepco and the nuclear regulator, then the country's Nuclear and Industrial Safety Agency, failed to protect critical safety equipment from flooding "in spite of mounting evidence that the plant's current design basis for tsunamis was inadequate," the report said.

The tsunami waves disabled power distribution equipment and the lack of main and backup power kept plant workers from providing cooling water to reactors, taking other safety measures and communicating effectively, the report said. In addition, there were not adequate procedures and training for dealing with an extended loss of all power, the report said.

Because there were six units at the Fukushima I plant, workers at different units competed for resources and the attention and help of the on-site emergency response center, which coordinated accident mitigation efforts, the report said.

The accident revealed vulnerabilities in Japan's emergency management planning, the report said and recommended that agencies with responsibilities for emergency management in the US assess their plans for dealing with a nuclear accident, especially a "regional-scale disaster."

Congress also requested that NAS study the safety and security of spent nuclear fuel and high-level nuclear waste. That topic will be the subject of a future study that is likely to be released in 2015, Committee Chairman Norman Neureiter said.

--William Freebairn, --Edited by Lisa Miller,

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