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Seismic Design for Nuclear Installations
Seismic Design for Nuclear Installations
1INTERNATIONAL ATOMIC ENERGY AGENCY, Seismic Design and Qualification for Nuclear Power Plants, IAEA Safety Standards Series No. NS-G-1.6, IAEA, Vienna (2003).
2A cliff edge effect is an instance of severely abnormal conditions caused by an abrupt transition from one status of a facility to another following a small deviation in a parameter or a small variation in an input value.
3For seismic events, it is assumed that early warnings are not possible and that there is a high probability of combinations with other seismic induced hazards (e.g. internal fires, floods).
4The definition of ‘rock’ varies between States. In some States, a site is considered to be a rock site when the average shear wave velocity is larger than about 2000 m/s.
5For example, some States recommend not using Type 3 (soft soil) sites.
6In some States, SL-2 corresponds to an earthquake level often denoted as the ‘safe shutdown earthquake’.
7In some States, SL-1 corresponds to an earthquake level often denoted as the operating basis earthquake.
8In some States that use a performance based approach to define a site specific SL‑2 level, the ground motion level is calculated by scaling the site specific mean uniform hazard spectrum by a design factor greater than 1.
9For low to moderate seismicity where the seismic margin is used to assess the robustness of the design, some States define a factor of 1.4, 1.5 or 1.67.
10This implies an annual frequency of exceedance lower than the one used to define the SL-2 level. In some States, mean values for the annual frequency of exceedance in the range 1 × 10⁻5 to 1 × 10⁻4 are used.
11In this Safety Guide, acceptance criteria are specified bounds on the value of a functional or condition indicator for an SSC in a defined postulated initiating event (e.g. an indicator relating to functionality, leaktightness or non-interaction).
12An example is the containment vessel and the surrounding internal concrete structures: if they are connected, they could interact during the earthquake.
13The P-Delta effect is a second order bending moment equal to the force of gravity multiplied by the horizontal displacement a structure undergoes when loaded laterally.
14The natural frequency is the frequency of vibration of a linear dynamic system when it is not disturbed by any external dynamic forces.
15Vibration isolation devices not designed for earthquake loads have failed during earthquakes affecting industrial facilities.
16For the most common tray designs, it is good practice for the span of cable trays between adjacent supports not to exceed 3 m in the direction of the run, as an average. When the cable tray extends beyond the last support in a run, it is installed such that the tray does not cantilever out (overhang) beyond this support by more than 1.5 m.
17For the most common duct designs, it is good practice for vertical support spans not to exceed 4.5 m, for supports to be set within 1.5 m of fittings such as tees in each branch of the fitting, and for duct cantilever (overhanging) lengths to be less than 1.8 m.
18Ducts with slip joints without pocket locks, rivets or screws could experience joint separation due to the differential displacement between supports.
19Seismic capacity is the highest seismic level for which the necessary adequacy has been verified, expressed in terms of the input or response parameter at which the structure or the component is verified to perform its intended safety function.
20Typical values used by States are ±15%.
21Heavy, stiff structures founded on soft ground might experience significant differences in their seismic response than the same structures founded on rock. These differences may be important even for ground with an intermediate stiffness. This effect is the result of phenomena that are jointly designated as ‘soil–structure interaction’.
22Structure–soil–structure interaction refers to a phenomenon by which the seismically induced motion of a structure is transmitted to an adjacent structure through the foundation medium. A typical effect of this phenomenon is that, in the in-structure spectra of the affected structure, peaks appear at the natural frequencies of the adjacent structure.
23Structural integrity is the ability of an item, either a structural component or a structure consisting of many components, to hold together under a load, including its own weight, without breaking or deforming excessively.
24A simplified component in this context is one that has been reduced to just those parts necessary to deliver the safety function.
25For distribution systems (e.g. piping, cable trays, conduits, tubing and ducts, and their supports), modal response spectrum analysis may be used for the seismic design of large bore piping (e.g. diameter greater than 60 mm) for safety classified systems, while static methods are usually applied for the analysis of small bore piping. Spacing tables and charts based on generic analysis or testing are also used in the evaluation of small bore piping and are typically used to evaluate cable trays, conduits, tubing and ducts.
26The use of industry standards will depend on national regulations. In some States, standard IEEE/IEC 60980344  is used. Other national or international industry standards endorsed by the national regulatory body could also be used.
27The core damage frequency is an expression of the likelihood that an accident could cause the fuel in a nuclear reactor to be damaged. It is a term used in probabilistic safety assessment that indicates the likelihood of an accident that could cause severe damage to the fuel in the reactor core.
28The large early release frequency is the frequency of accidents that could lead to a radioactive release prior to the implementation of protective actions such that there is the potential for deterministic effects.
29To demonstrate adequate seismic margin (for nuclear power plants), the reference review level earthquake in seismic margin assessments is typically defined by a factor of 1.4, 1.5 or 1.67 based on a peak ground acceleration corresponding to SL-2.
30Seismic instrumentation is an array of strong motion accelerographs installed at and around the site of the installation and in defined locations in safety related structures.
31Three triaxial strong motion recorders at the basemat will allow the translation motion corresponding to the horizontal and vertical directions to be evaluated and the rocking corresponding to both horizontal directions to be estimated.
32The limit state defines the limiting acceptable deformation, displacement or stress that an SSC might experience during, or following, an earthquake and still perform its safety function. SSCs are graded on the basis of the unmitigated consequences of SSC failure or of an SSC’s reaching its limit state. Deformation related failures resulting from other, non-seismic natural phenomena hazards are defined by the design codes and criteria used to design the SSCs.
33In this section, the term ‘performance goal’ is used instead of typical reactor based risk parameters (e.g. core damage frequency, large release frequency) since nuclear installations include a large variety of non-reactor facilities. Therefore, the performance goal is associated with the definition of accident conditions for these facilities (mainly losing barriers and controls of the confined nuclear materials).
Tags applicable to this publication
- Publication type:Specific Safety Guide
- Publication number: SSG-67
- Publication year: 2021