Fukushima Response: Seismic Engineering to Assist Plant Evaluations
The Magnitude 9 Great Tōhoku Earthquake and subsequent tsunami that struck the northeast coast of Japan on March 11, 2011 caused extensive damage, injuries, and fatalities. In addition to the profound human tragedy, there were four nuclear power plants on the northeast coast, and reactors at the Fukushima Dai-ichi nuclear power plant suffered severe damage when the tsunami overtopped the seawall and disabled emergency backup generators and batteries. Loss of the ability to cool the fuel led to hydrogen explosions in Units 1, 2 and 4, destroying the reactor buildings and releasing radioactive materials.
In response, the NRC has studied the performance of the plants on Japan’s northeast coast as well as the country’s response to the unfolding situation. The NRC’s Near Term Task Force made a number of recommendations to improve the safety of the U.S. fleet of reactors and to enhance our ability to respond to beyond-design-basis events. As a result, all U.S. reactor sites have been required to reevaluate the earthquake hazard at their site with respect to the seismic design basis for the plant construction. Based on new information regarding the seismic hazards in the central and eastern U.S., some plants have found that seismic design loads have increased since the time of construction. At those plants, additional engineering is required to determine whether structures, systems, and components require retrofit.
Exponent offers structural, geotechnical, seismic, and flooding expertise to assist plants to formulate technically sound and defensible responses to updated seismic hazards.
Exponent services include:
- Calculation of fragility curves for structures, systems and components
- Structural earthquake performance of reinforced concrete and steel elements based
- Soil-structure interaction
- In-structure response spectra
- Peer review of seismic and flood evaluations
- Design of seismic base isolation for equipment and structures.
Exponent can provide the following consulting services to help your plant respond to the events like those at Fukushima. These services are in direct response to the NRC’s Near Term Task Force lessons learned from their Fukushima investigation. Exponent’s services would typically be offered on a fixed-price basis, with the fee determined once the work scope and deliverables are well defined.
- Assist plant operators assemble and interpret design documents to define and document the design basis earthquake ground shaking (Safe Shutdown Earthquake).
- Develop seismic hazard curves for your region based on the most recent CEUS seismic source characterization, using the current EPRI attenuation curves.
- Calculate the geotechnical amplification effect, if any, due to resonance of site soils. Exponent uses the software package SHAKE, taking into account the uncertainty in soil measurements and modeling techniques to provide realistic bounds for site soil properties. In addition, we can evaluate site geo-hazards such as landsliding, liquefaction potential, seismically induced settlement, etc.
- Evaluate the design basis earthquake in comparison with the GMRS from site seismic hazard curves to determine whether additional Probabilistic Seismic Risk Assessment or Seismic Margins Analysis is warranted.
- Should a full seismic risk assessment be triggered, calculate fragility curves for structures, systems and components, considering both aleatoric and epistemic uncertainty. Curves can be established using state-of-the-art analysis software and/or shake table testing.
- Assist plants in modifying their existing event trees to incorporate the seismic and flood risks.
- Compile all hazard and risk information into a single, defensible package defining the probabilistic seismic risk at a plant.
- Recommendation 4.2 requires licensees to provide “reasonable protection” for emergency equipment being stored on/near site in the event of Station Black-Out. Exponent can evaluate structures housing this equipment and provide retrofit designs, if required, such that the reliability of these structures is rigorously demonstrated to be consistent with NRC seismic performance objectives.
- Provide independent peer review of any of the tasks above.
In addition, Exponent can function as an independent, third-party reviewer for any of the technical work described above. Comprehensive peer reviews can substantially reduce costs by identifying issues requiring more work prior to NRC audits.