12.173. Commercial nuclear power plants are designed to withstand the ground motion caused by the most severe earthquake that is likely to be experienced. From historical records of seismic events in the plant vicinity, the earthquake that would be expected to produce the largest ground motion at the reactor site is predicted. This is called the “safe shutdown earthquake.” Analysis must show that the reactor can be tripped and the

engineered safety features will function properly if such an earthquake should occur. Furthermore, the plant must be capable of remaining in full operating condition should a specified ‘‘operating basis earthquake” be experienced. Licensing requirements are specified in 10 CFR 100, Appen­dix A.

12.174. In developing the design necessary to meet these requirements, dynamic analysis based on expected ground acceleration spectra is applied to those components and structures (called Class I) vital to safe shutdown. There must be assurance that the resulting stresses will comply with ap­plicable codes. Furthermore, special attention must be paid to the pre­vention of common-mode failure (§12.18) in the event of an earthquake. If the reactor plant is designed to withstand a maximum horizontal accel­eration of 0.3 of gravity as the result of a safe shutdown earthquake, it is considered to be satisfactory for most sites in the United States.

12.175. The design philosophy used in the United States to accom­modate possible earthquake motion is to render the structure and its com­ponents as rigid as required to prevent damage. In addition to planning for ground motion in the basic structural design, various devices such as struts and springs are used to manage stresses. In the case of piping which must expand and contract with temperature changes, mechanical shock arrestors, or snubbers, are used to attach the pipes to rigid structures. In some other countries, a design philosophy of decoupling the entire plant from the surrounding soil by the use of energy absorbing materials is favored [19].

12.176. The seismic design of nuclear power plants has become a spe­cialized engineering subdiscipline. Computer models are available for the dynamic analysis of proposed structures and their components [20]. Such analysis is supported by various experimental programs that employ com­ponents and scale models on shake tables. An important design step is to characterize the damage produced by an expected earthquake to a can­didate design and make changes as necessary. However, the prediction of the ground acceleration to be expected at a specific site is a technical challenge. For example, during the 1994 Los Angeles earthquake, unex­pected high accelerations were measured, which varied considerably over short distances.