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Faced with the lack of historical data or access to specific plant maintenance records, the analyst can estimate failure rates for plant‐specific vessels and pipes using the Thomas model. The model uses available information regarding the vessel shape, size, and thickness along with the construction details of the vessel or pipe (number and type of welds, etc.). Maintenance and inspection history of the equipment can also be factored into the model, allowing the model to reflect the analyst's state of knowledge about the impact of these observed praictices on predicted failure rates. The model was orginally developed using a large database for predicting vessel and pipe failures for the nuclear industry but can be equally applicable to the chemical industry. Any additional information about plant‐specific failures can be easily factored into the analysis by using Bayesian techniques. In the present application, the Thomas model was used to predict failure rates of process vessels storing corrosive and hazardous chemicals. Engineering judgement was used in making the model reflect the corrosive properties of the stored chemicals and the effectiveness of the inspection programs. Finally, the results of the model were compared with available industry wide data sources and were found to be reasonable.

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This methodology begins by quantifying the fragility of all key components and structures in the plant. By means of the logic encoded in the plant event trees and fault trees, the component fraghties are combined to form fragilities for the occurrence of plant damage states or release categories. Combining these, in turn, with the seismicity curves yields the frequencies of those states or releases. Uncertainty is explicitly included at each step of the process.KEY WORDS seismic; nuclear; methodology; dependency.The purpose of this paper is to describe a methodology for calculating the risk to nuclear power plants from seismic events. The methodology is an outgrowth, or further development, of ideas described in Ref. 1. Other aspects of the methodology have been discussed in Refs. 2 and 3. Sections 11.1 and 11.2 of the PRA Procedures Guide (4) give an extensive bibliography and historical review and also a comparison with the Seismic Safety Margins Research Program method of the Lawrence Livermore Lab~ratory.'~)Although the methodology has been used so far, to our knowledge, mainly in nuclear power it is equally applicable to other types of facilities and to other types of "external" events (e.g., high winds@) and floods, where the severity of the event is characterized by one or more parameters). For simplicity, however, our presentation will be cast only in terms of seismic events in nuclear plants and will make use of the definition of risk and the matrix formulation described in Refs. 3,5,6,8,and 9. 'Pickard, Lowe and Garrick, Inc., Inine, California 92714. STEPS IN A SEISMIC RISK ANALYSISA seismic risk analysis of a nuclear plant con-1. Seismicity. Description of the seismic activity;i.e., the likelihood of various sized ground motions at the location of the plant in question. Fragility ofComponents. Identification of key components (equipment, systems, people, and structures) in the plant and description of the vulnerability of these items; i.e., the likelihood of their failure under various postulated ground motions. 3. Plant Logic. Identification of the combinations of component failures which would lead to plant level failures; e.g., core melt or radioactivity release. This is done from the logic of the plant's design and the physical phenomena involved. 4. Plant Level Fragilities. From the component fragilities and combination logic, compute the fragility of the plant level events (melt, release, etc.) being analyzed.