US report urges focus on 'extreme' events

The "overarching" lesson learned from the 2011 Fukushima Daiichi accident is that nuclear plant licensees and their regulators "must actively seek out and act on" new information about hazards with the potential to affect the safety of nuclear plants. That is the conclusion of a congressionally mandated report from the National Academy of Sciences (NAS) published yesteday.

A committee of 21 specialists gathered information over two years for the report, which was sponsored by the US Nuclear Regulatory Commission (NRC). 

"Until now, US safety regulations have been based on ensuring plants are designed to withstand certain specified failures or abnormal events, or 'design-basis-events' - such as equipment failures, loss of power, and inability to cool the reactor core - that could impair critical safety functions," NAS said. "However, four decades of analysis and experience have demonstrated that reactor core-damage risks are dominated by 'beyond-design-basis events'," it said.

The Fukushima Daiichi, Three Mile Island, and Chernobyl accidents were all initiated by beyond-design-basis events.

The committee found that current approaches for regulating nuclear plant safety - which have been based traditionally on deterministic concepts such as the design-basis accident – "are clearly inadequate" for preventing core-melt accidents and mitigating their consequences.

"A more complete application of modern risk-assessment principles in licensing and regulation could help address this inadequacy and enhance the overall safety of all nuclear plants, present and future," NAS said.

The Fukushima Daiichi accident raised the question of whether offsite emergency preparedness in the US would be challenged if a similar-scale event - involving several concurrent disasters - occurred in the USA.

The committee "lacked time and resources" to perform an in-depth examination of US preparedness for severe nuclear accidents, the report said. It recommends therefore that the nuclear industry and organizations with emergency management responsibilities assess their preparedness for severe nuclear accidents associated with offsite regional-scale disasters.

Emergency response plans, including plans for communicating with affected populations, should be revised or supplemented to ensure that there are scalable and effective strategies, well-trained personnel, and adequate resources for responding to long-duration accident/disaster scenarios, NAS said.

In addition, industry and emergency management organizations should assess the balance of protective actions - such as evacuation, sheltering-in-place, and potassium iodide distribution - for affected offsite populations and revise the guidelines as appropriate.

Recommendations

The report's recommendations concern technical areas only and not policy.

These include "paying particular attention" to improving the availability, reliability, redundancy, and diversity of specific nuclear plant systems. This includes, for example, DC power for instrumentation and safety system control; tools for estimating real-time plant status during loss of power; reactor heat removal, reactor depressurization, and containment venting systems and protocols.

NAS also makes four main recommendations aimed at further improving the resilience of US nuclear plants.

First, the US nuclear industry and the NRC should give specific attention to improving resource availability and operator training, including training for developing and implementing ad hoc responses to deal with unanticipated complexities.

Second, they should strengthen their capabilities for assessing risks from events that could challenge the design of nuclear plant structures and components and lead to a loss of critical safety functions. Part of this effort should focus on events that have the potential to affect large geographic regions and multiple nuclear plants, including earthquakes, tsunamis and other geographically extensive floods, and geomagnetic disturbances, the report said. NRC should support these efforts by providing guidance on approaches and overseeing rigorous peer review, it said.

Third, NRC should further incorporate "modern risk concepts" into its nuclear safety regulations using these strengthened capabilities.

Finally, NRC and the US nuclear industry must continuously monitor and maintain a strong safety culture and should examine opportunities to increase the transparency of and communication about their efforts to assess and improve nuclear safety.

NRC said it has scheduled a public meeting on 31 July to discuss recommendations the report makes.

Researched and written
by World Nuclear News