Mitigation of Beyond-Design-Basis Events Proposed Rule issued for Public Comment (ARCHIVED)

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Enhancements Under the Proposed Mitigation Strategies Rule

The March 11, 2011, accident at the Fukushima Dai-ichi nuclear power plant in Japan was caused by an earthquake and tsunami that significantly exceeded the design basis of the facility.  The tsunami flooded important safety systems at the plant, resulting in an extended loss of electrical power.  This loss of electrical power prevented operators from adequately cooling the reactor cores, which led to overheating, core damage, and the release of radioactive materials to the environment.

The NRC took a number of actions following the accident to strengthen the protection of U.S. nuclear plants against events that could exceed a plant’s design basis.  These included issuing orders and requiring licensees to conduct several evaluations. Now the NRC is moving forward in making permanent some of the lessons we've learned from the Fukushima nuclear accident through the development of a new regulation.  The Commission has directed the staff to seek public comments on a proposed rule for mitigating "beyond design basis events," which can be stronger than a plant's current design. The Commission made a few changes to the proposed rule, which consolidates several of the most safety significant recommendations of the NRC's task force report from shortly after the events at Fukushima.

The public can comment on the proposed rule until February 11, 2016. To view the proposed rule or submit comments, go to and search for Docket ID NRC-2014-0240. You may also e-mail comments to The staff is also planning a public meeting during the comment period, and we'll post the meeting notice on our public website.

The staff will consider the public's comments on the proposed rule and expects to provide a final rule to the Commission for its consideration in December 2016. The NRC staff expects the rule, if approved, would require plants to fully comply within approximately two years (2019-2020), with the exact date varying from plant to plant. Although that seems far away, much of the safety benefit from the rule is already achieved by two orders that are discussed below. Nearly all U.S. plants will comply with those orders by the end of 2016. Safety is being enhanced well before the final rule.

This proposed rule would apply the requirements of two existing orders, Mitigating Strategies (EA-12-049) and Spent Fuel Pool Instrumentation (EA-12-051), to any operating or future U.S. nuclear power plant. The Mitigating Strategies Order ensures that a plant will have sufficient procedures, strategies, and equipment to indefinitely cool the reactor core and spent fuel, as well as protect the reactor's containment, if it loses power. The Spent Fuel Pool Instrumentation Order requires that the plants ensure they can monitor spent fuel pool water levels. These two orders are already being implemented across the nuclear fleet.

The proposed rule also incorporates many other task force recommendations. It also address the concerns of several Fukushima-related petitions that were submitted by members of the public to change the NRC's regulations. These additional issues include:

  • Mitigating Strategies (Recommendation 4) and Spent Fuel Pool Instrumentation (Recommendation 7):

The previously-issued orders would be written into the NRC's permanent rules, so that their requirements would apply to any future reactors.

  • Onsite Emergency Response Capabilities (Recommendation 8):

Plants have a variety of response guidelines (FLEX support guidelines, emergency operating procedures, etc.), each of which would be used by licensees depending on the specific situation. The proposed rule would ensure plants can smoothly transition between the different types of response guidelines.

  • Emergency Plans to Address prolonged Station Blackout and Multiunit Events (part of Recommendation 9):

The proposed rule clarifies requirements for plants to be able to monitor and assess potential radiation doses from events involving more than one reactor or spent fuel pool at once. Plants are currently implementing this ability through a voluntary initiative. The proposed rule also simplifies technology requirements for the Emergency Response Data System (ERDS), one method for plants to communicate with the NRC during an event. Licensees voluntarily modernized the system (Recommendation 9.4).

  • Command and Control Structure and Qualification (Recommendation 10.2)

The proposed rule ensures plants' decision-makers are properly qualified and have the proper level of authority and oversight.

  • Enhanced Onsite Emergency Response Resources (Recommendation 11.1)

The proposed rule requires plants to have enough onsite resources to address all units at a site during a prolonged station blackout and also that they be able to deliver equipment to the site despite damage to offsite roads, etc., and infrastructure.

  • Petitions for Rulemaking from the Natural Resources Defense Council (NRDC) (PRM-50-97, PRM-50-98, PRM-50-101, and PRN-50-102) and Mr. Thomas Popik (PRM-50-96)

Collectively, the NRDC petitions for rulemaking rely solely on Near-Term Task Force Report Recommendations 4, 7, 8, and 9, discussed above. The rulemaking addresses Mr. Popik's petition to the extent that it establishes a means of assuring long-term cooling and unattended water makeup to spent fuel pools. Separate from this rulemaking, the NRC will further consider whether requirements are needed for licensees to address geomagnetic storms, which was another part of Mr. Popik's petition.

Furthermore, the proposed rule incorporates information from the plants' reevaluated earthquake and flooding hazards. Each plant's mitigating strategies, which are used to meet the rule's requirements, must address the reevaluated hazards.

One of the changes the Commission directed the staff to make in the proposed rule involves the treatment of Severe Accident Management Guidelines, or SAMGs, which a plant would use in responding to very unlikely accidents that involve damage to the reactor. The Commission directed that the SAMGs should continue to be implemented voluntarily.  Based on the Commission's direction, the NRC will provide periodic oversight of SAMGs through its Reactor Oversight Process.  Another Commission change to the proposed rule removes proposed design requirements for new reactor applicants.  Instead, the new reactors would be subject to the same performance-based criteria that applies to the currently-licensed fleet.

Process Overview

Related Documents:

Learn more about the NRC's rulemaking process.

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Page Last Reviewed/Updated Monday, July 27, 2020