A panel of nuclear safety experts convened to discuss the how the safety of U.S. nuclear plants have been addressed following a detailed analysis of the Fukushima Daiichi accident in Japan. The panel focused mainly on changing regulations from the U.S. Nuclear Regulatory Commission (NRC), new performance standards from several professional engineering societies, and improvements in technology related to mitigating the effects of a Fukushima-type accident – also known as a station blackout (SBO).
William Reckley, a member of the NRC’s Risk Management Task Force, explained that the new NRC regulations would require all plants to prepare mitigation strategies which would be implemented in the event of a SBO. These Station Blackout Mitigation Strategies (SBOMS) are evaluated using a three phase approach, with an end goal for maintaining core cooling and containment and spent fuel pool cooling for an indefinite amount of time. The three phases, which occur in increasing time periods after reactor shutdown are:
- An Initial phase which must be survived with installed equipment.
- A Transition phase which must by survived with portable, onsite equipment.
- A Final phase which must be survived with resources obtained from offsite.
The panel, which convened at the 2014 American Nuclear Society (ANS) Annual Meeting, also featured representatives from the Institute of Electrical and Electronic Engineers (IEEE) and the American Society of Mechanical Engineers (ASME). The professional society representatives discussed several standards, which would supplement regulations developed by the NRC. Concerns were expressed over the consideration of “beyond design basis accidents” (accidents of either type or magnitude considered to be out of the realm of possibility for a nuclear plant) in the present standards.
The technological improvements presented at the panel, which help mitigate issues observed at the Fukushima Daiichi accident, were related to the over-pressure and over-temperature of the core containment and subsequent leakage of gasses. One of the places of concern for containment are the penetrations which allow electronics cables to carry information over the containment boundary. The development of “glass-to-metal seals” greatly increase the pressure and temperatures that can be handled by these containment penetration points. Additionally, the development of better hydrogen detectors aid in the mitigation of hydrogen buildup external to containment and the prevention of an associated hydrogen explosion.