On June 28, 1991, the U.S. Nuclear Regulatory Commission (NRC) issued Supplement 4 to Generic Letter (GL) 88-20, "Individual Plant Examination of External Events (IPEEE) for Severe Accident Vulnerabilities, 10 CFR 50.54(f)," and NUREG-1407, "Procedure and Submittal Guidance for the Individual Plant Examination of External Events (IPEEE) for Severe Accident Vulnerabilities." Specifically, the NRC requested that each licensee perform an IPEEE to identify and report to the NRC all plant-specific vulnerabilities to severe accidents caused by external events. The external events to be considered in the IPEEE included seismic events; internal fires; and high winds, floods, and other (HFO) external initiating events including accidents related to transportation or nearby facilities and plant-unique hazards.
Nuclear power plant licensees were required to conduct either a probabilistic risk analysis (PRA) to determine a point estimate of core damage frequency (and plant HCLPF) or to conduct a seismic margin assessment (SMA), which could be either deterministic or probabilistic. All of the currently operating U.S. nuclear power plants completed their assessments and submitted their analyses for NRC review.
Following the earthquake and tsunami that struck the Fukushima Daiichi nuclear power plant in Japan on March 11, 2011, the Nuclear Regulatory Commission (NRC) established a comprehensive set of new Requirements developed to ensure the safety of America's currently operating nuclear power plants.
Information gathered during safety risk assessments, mandated as part of the new requirements, would be used to determine whether or not the NRC would re-issue a power plant's operating license and also determine whether any currently operating plant was "at risk" or needed improvements or upgrades to its existing structures or systems.
The NRC addressed the comprehensive re-establishment of the design basis for existing plants. (See "Individual Plant Examination of External Events (lPEEE) for Severe Accident Vulnerabilities." Or NUREG/CR-4334 at http://www.sc-nuclear.com ). The document issued by the NRC that detailed the re-evaluation of the seismic, tsunami, flooding, and other external hazards that could impact the safe operation of plants and provides for the requirements for dealing with these issues is known as Generic Issue 199 (GI-199).
Nuclear utility operators and power plants are required to address the mandates specified in GI-199. Since many of the nation's nuclear power plants were built in the 1970's, when current computer software modeling and analysis of possible significant events (such as earthquakes and tsunamis) did not yet exist.
Decades ago, stick modeling was the only method to evaluate and perform structural and seismic risk assessments.
Soil-structure interaction (SSI) has a significant impact on nuclear power plant (NPP) structures, especially for massive and rigid structures found on soils, such as containments. Over the past several decades, the scientific and engineering communities have conducted extensive research on SSI and development analytical methods and software (such as SASSI) for adequately incorporating the SSI impact into the seismic design and analysis of structures. The NRC requires incorporation of the SSI effect in the seismic design and analyses of NPP structures. Today, many engineers and utility managers utilize SASSI or another technology to accurately compute seismic induced soil pressures on deeply embedded nuclear power plant structures, a frequent and critical factor in risk assessments and critical to meeting the mandates of GI-199. By utilizing the latest technologies, combined with the new Requirements created by GI-199, the NRC is making a strong effort to ensure that a "Fukushima" type event never happens on American soil.
SC Solutions is a highly recognized leader in the nuclear utility industry. SC solutions provides seismic risk assessments, SSI analysis, nuclear power plant walkdowns and complete modeling, analysis and consulting services. They have worked with the nation's largest NPP and utility operators. Located in Northern California with offices in Seattle, WA and Charlotte, NC, SC Solutions provides spot-on analysis and has created a Presentation geared for NPP personnel interested in addressing the GI-199. For information on this Presentation, or to receive our White Paper on GI-199, email Alex
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