The importance of withstanding earthquakes and their consequences has proven to be critical for the safety of nuclear power plants, and seismic re-evaluation has been identified as an important step towards reducing the risks facing these plants. After the Fukushima Daichii accident in 2011, which was caused by a tsunami following an earthquake, many countries performed comprehensive safety and risk evaluations of their nuclear power plants against external hazards.
The specific seismic risk for each nuclear facility is vital to identify when developing and implementing the safety requirements for these plants, and two recent IAEA publications provide assistance to national experts on implementing improved seismic safety.
“The IAEA is contributing to continued enhancement of nuclear safety globally by promoting best international practices and experience. From this perspective, we should consider the increased likelihood of rare natural events, which have been more common in recent years, and postulate that they may occur during a facility’s lifetime,” said Greg Rzentkowski, Director of the IAEA’s Division of Nuclear Installation Safety.
The IAEA’s Safety Report No. 103 Methodologies for Seismic Safety Evaluation for Nuclear Installations and TECDOC-1937 Probabilistic Safety Assessment for Seismic Events support implementation of the IAEA Safety Standards based on updated technologies and state-of-the-art practices. Both provide detailed technical guidance for the assessment of seismic safety for nuclear installations.
“Seismic hazard is a key contributor to overall plant risk evaluation. At the same time, observations regarding extreme seismic events are rather limited, and predictive models are subject to considerable uncertainties,” Rzentkowski said. “Yet, we have to anticipate such events and consider corresponding risks in the design process of nuclear power plants to ensure adequate protection of people and the environment.”
These publications will help regulators develop Probabilistic Safety Assessments (PSA) that correspond to the current international practices for seismic safety assessment of nuclear installations. Important areas covered include seismic fragility analysis, plant response, seismic risk quantification, as well as the identification of potential risk significant components.
Safety Report 103 covers the safety evaluation of the seismic design and seismic margin adequacy that are essential for nuclear power plants, while TECDOC-1937 describes the most advanced tools for seismic risk assessment, in relation to potential initiating events that can be caused by seismic induced failures and assessment of the accident sequences.
“Highlighted in these publications are the two most advanced methodologies: the seismic margin assessment (SMA) method and the seismic PSA method,” said Paolo Contri, Head of the External Events Safety Section at the IAEA, adding that “these methodologies are currently used to assess safety of operating and new nuclear power plants against seismic hazard, incorporating lessons learned in the last three decades of seismic safety evaluation.”
To strengthen the robustness of a nuclear power plant’s design, these publications cover best practices in data collection and methodology for investigations, instrumentation and technology used to assess seismic safety, and provide guidance on the assessment of the safe shutdown of reactors in the face of seismic risk, Contri explained.
Readers will also get an overview of lessons learned from the Fukushima Daiichi accident, based on the IAEA Director General’s 2015 Report and the updated IAEA methodologies for seismic safety evaluation of nuclear installations.
“Protecting nuclear facilities against strong earthquakes is of high interest in Japan. We are developing methods for quantifying seismic risk and, thus, enhancing the seismic safety of the operating nuclear power plants. We expect that the IAEA TECDOC-1937 will be very valuable to our efforts,” said Masato Nakajima from the Japanese Nuclear Risk Research Center (NRRC) of the Central Research Institute of Electric Power Industry (CRIEPI).
Through its technical publications and safety review services, the IAEA continues to play a significant role in providing concrete guidance and relevant recommendations to improve and protect the safety of nuclear installations against external hazards.
“Safety analysts are now routinely faced with the challenge of evaluating the adequacy of older nuclear installations' seismic design,” said Robert Budnitz of the Lawrence Berkeley National Laboratory in the United States, adding that “when these two recent IAEA publications are used together, the safety analysis community worldwide can be confident that the seismic evaluation of existing facilities will be done competently and using the latest methodologies.”
Dennis Henneke from the Chief Engineer's Office of General Electric-Hitachi added: “The most advanced reactors, including small modular reactors, require either a design phase seismic PSA or a plant and site-specific seismic PSA. TECDOC-1937, when used in combination with other IAEA documents in this specialized area of nuclear safety, provides a comprehensive approach that can be used to support these advanced plant seismic PSAs. These documents fill a gap in the industry methodology.”
Through its technical publications and safety review services, the IAEA continues to play a significant role in providing concrete guidance and relevant recommendations to improve and protect the safety of nuclear installations against external hazards.
