Journal of Energy Engineering

This introduction provides a background of the evaluation methods for earthquakes larger than design basis. Seismic probabilistic risk assessment (SPRA) and seismic margin assessment (SMA) methods are introduced to the reader. The basic parameters used to define seismic capacity for each method are explained. The objectives of both evaluation methods and how they can be used to evaluate the adequacy of a seismic design are discussed. General issues related to computing seismic capacity are reviewed relative to SPRA and SMA. Four companion papers presented in the journal following this introduction discuss the types of information (i.e., dynamic tests, earthquake experience, and analytical data) that are used to determine the real capacity of structures and equipment in nuclear power plants to resist earthquakes. The motivation for discussing these three types of information is presented as an introduction to these papers that follow in this journal edition. The purpose of the present paper is to lay the groundwork and provide motivation for these papers.

Flat‐bottom vertical storage tanks are vital components in nuclear power plant safety systems. Potential failure modes of such tanks due to earthquake ground motion and current methods available to evaluate the seismic capacities of tanks against these failure modes are identified. Substantial data on tank failure modes, seismic response, and seismic capacity are cited. These data are distinguished into three types: (1) Earthquake experience data from the performance of tanks in actual earthquakes; (2) test data from laboratory specimens or tanks in the field; and (3) analytical data from theoretical derivations or analyses. It is concluded that these data provide clear guidance on probable failure modes. Limited analyses have been performed to benchmark current seismic evaluation methods for tanks against test and earthquake experience data. In general, these studies demonstrate that current analytical evaluation methods provide sufficiently accurate means to predict the seismic capacities of flat‐bottom tanks at nuclear power plants.

Under internal over‐pressure loading, experimental studies have demonstrated that the most probable ultimate failure mode of concrete containments of nuclear power plants is leakage instead of a catastrophic break. For steel containments, scaled model tests have shown instantaneous catastrophic failure. However, due to simplistic representations of scaled models, the data base is not conclusive for interpreting full‐scale steel containment behavior. Rather it serves primarily as a technical basis for method validation. Structurally, analytical and experimental data have shown that both concrete and steel containments can withstand two to three times internal design basis pressures without failure. Experimental data on external seismic loading plus internal overpressure loading is limited. Most of the findings are based on analytical studies. Because of geometric complexity (3‐D features, local stiffness discontinuities, etc.) and nonlinear behavior (geometric nonlinearity and material nonlinearity) under extreme loadings, containment capacity and failure‐mode prediction requires qualified and validated analytical tools, together with sound engineering judgment.

A summary of available data concerning the structural response of low‐rise shear walls is presented. These data will be used to address two failure modes associated with shear wall structures. First, the data concerning the seismic capacity of the shear walls are examined, with emphasis on excessive deformations that can cause equipment failure. Second, the data concerning the dynamic properties of shear walls (stiffness and damping) that are necessary for computing the seismic inputs to attached equipment are summarized. This case addresses the failure of equipment when the structure remains functional.

A summary of available experimental expansion anchor bolt test data is presented. These data were collected as part of programs by the nuclear industry to address generic issues related to verification of seismic adequacy of equipment in nuclear power plants. Some of the data presented are suitable for use in seismic probabilistic risk assessments. For example, mean values of ultimate strength, along with their standard deviation and coefficients of variation, for a range of most typical expansion anchor bolt sizes are presented. Effects of interaction between shear and tension, edge distance, spacing, and cracking of the concrete are presented in a manner that is more suitable for use in deterministic evaluations. Related industry programs to derive anchor bolt capacities are briefly discussed. Recommendations for areas of further investigation are also presented.