Use of seismic margins methodology for DOE

The US Nuclear Regulatory Commission (NRC) has been sponsoring research to assess the safety of nuclear power plants with respect to earthquakes since the 1970s. Similar research has also been sponsored by the Electric Power Research Institute (EPRI). These programs were motivated by a perceived need to better assess safety margins inherent in plant designs. The occurrence of several medium intensity earthquakes in the eastern US within the last ten years contributed to the feeling that seismic margins had to be better quantified. Another reason was the possibility that the large earthquake which occurred in Charleston, South Carolina in 1886 could occur elsewhere on the eastern seaboard of the US If this is found to be the case it could result in a reevaluation of the design basis earthquake level at a number of plant sites. Finally the results of a number of probabilistic safety assessments have indicated that seismic events can be major contributors to plant risk. Although these results have large uncertainties associated with them, they have caused more attention to be focused on seismic safety. Probabilistic analysis has shown that earthquakes larger than the design basis of the plant have a small but finite chance of occurrence. Thus it is important to know that the plant will be capable of shutting down safely even if earthquakes somewhat above the design basis were to occur. A margins assessment will provide this assurance and even though it is generally accepted that current nuclear power plant designs are capable of withstanding earthquake induced motion substantially greater than their design basis, methods are needed to systematically demonstrate this. This paper describes the review method developed to assess seismic margins of operating commercial nuclear power plants and trial applications of the method. The extension of the seismic margins approach to Department of Energy facilities is discussed