Design basis earthquake exceedance criteria

It is discussed in Section 4.1.1 that both first excursion damage and cumulative fatigue damage caused by seismic vibrations need to be considered, and AJMA and standardized CAV are proposed as DIPs. The former is a peak parameter for first excursion damage, and the latter is an integral parameter for cumulative fatigue damage.

FIG. 64. Peak parameter and integral parameter of the observed earthquake motion.

Observed Earthquake Motion

TABLE 36. TYPICAL TIME-HISTORY OF CAV VALUE DURING EARTHQUAKE (KASHIWAZAKI-KARIWA UNIT 1 REACTOR BUILDING BASEMAT IN 2007 NIIGATA-KEN CHUETU-OKI

EARTHQUAKE)

Acceleration Time-History (Gal)

CAV (g-sec)

Standardized CAV (g-sec)

The relation between AJMA and standardized CAV is shown in Fig. 41 and their capacities to characterize earthquake motion seem to be different. In other words, the existence of seismic damage will be judged from the position in a 2D space (plane) defined by two DIPs, one peak parameter (typically AJMA) and one cumulative parameter (typically standardized CAV), as shown in Fig. 64, rather than evaluating seismic damage with single DIP.

A challenge for determining the region within the 2D space where damage is expected is that, doing so, requires an extremely large number of tests and research work. It is possible that such a region is dependent on the type of component, the seismic design methods and even on the judgments made in actual design. Thus, it is considered more realistic to compare the DIPs of observed earthquake motion at the installation points of SSCs with that of design basis earthquake input motion to the SSCs, considering that the margins included in the design methods (seismic response analysis, load and stress calculation, allowable stress, etc.) and in the actual design are on the conservative-side to evaluate threshold DIPs.

4.6.3.2. Equivalent DIP to static seismic coefficient

The threshold DIP values (standardized CAV and AJMA) for the design basis earthquake input motion can be calculated with the equations shown in Section 4.1, if the acceleration time-histories are available. And this is applicable to both design basis ground motions and in-structure motions as to compute the threshold DIPs. However, it may be difficult to apply this method to mechanical equipment, because seismic design conditions for mechanical equipment, in particular, are often given in terms of static seismic coefficients or acceleration response spectrum.

For example, floor acceleration response spectrum is often used for the seismic design of mechanical equipment. Usually, however, artificial processing (e.g. the envelope of several seismic waveforms and smoothing against the natural frequency) has been applied to the floor response spectrum used for design. It takes time and effort to calculate artificial earthquake motion time-histories from the floor response spectrum in order to compute AJMA or Standardized CAV. Depending on the seismic design practice, moreover, non-safety-related equipment of low seismic importance is often designed using a static seismic load defined by a static seismic coefficient.

In the Japan’s seismic design practice, dynamic design and static design are used together depending on the seismic importance. Every structure, system and component in a nuclear power plant is classified into three levels, depending on its seismic safety importance. These levels are Class S, Class B and Class C, and the ratio of static seismic coefficient for the design earthquake load calculation is 3.0, 1.5 and 1.0, respectively. The minimum design load value for Class C component is defined as 0.2×0.8×1.2=0.192 (g).

The calculation process of JMA instrumental seismic intensity, shown in Section 3.3.2.2, suggests that the physical meaning of AJMA may be close to that of static seismic coefficient.

The acceleration calculated from static seismic coefficient and acceleration response spectrum is a design value against first excursion damage, and it is considered to be a momentary parameter such as the ZPA or the AJMA. Fig. 65 shows the correlation between AJMA and ZPA with horizontal lines corresponding to the static seismic coefficients used in the Japan’s seismic design practice. Although, no seismic damage to safety-related components (seismic design class S) has ever been experienced, damage has occurred to class B and C components, which are designed using static seismic coefficient, and shown with solid markers in Fig. 65. The lines representing static seismic coefficients on the vertical axis (AJMA) look like indicating the threshold of damage occurrence.

As discussed in Section 3.3.2.2, the calculation process of AJMA considers a frequency filter which cuts off the high frequency contents having less influence on damage of ductile mechanical equipment and it also considers the velocity and accumulation effect (energy). In

FIG. 65. AJMA and seismic static design load.

other words, AJMA is considered to be one of parameters so-called as ‘effective acceleration’.

On the other hand, static seismic coefficient corresponds to an acceleration with infinite duration period and affects SSCs as a seismic inertial force.

4.6.3.3. Exceedance evaluation chart

Fig. 66 is proposed as a chart to determine whether the observed earthquake motion exceeds the design basis, in which either acceleration time-histories or static seismic coefficients are specified. The shaded area in this figure corresponds to both the peak parameter and the integral parameter being below the values calculated for the design earthquake. When the point of observed earthquake remains in this area and the calculated AJMA is below the static seismic coefficient line, the observed earthquake motion can be judged as not exceeding the design basis.

It needs to be noted that the actual exceedance determination needs to consider both the assessment by DIPs as mentioned above and the assessment by the response spectrum, because the vibrational effect of SSCs in the seismic response needs to be taken into account as discussed in IAEA Safety Report Series No. 66 [1] and others. In addition, consideration to some kind of margin needs to be given when performing evaluations with DIPs like peak parameters, especially with the relatively flexible SSCs, because of lack of information about their seismic response.

FIG. 66. Diagram to assess exceedance of seismic design bases based on computed DIPs.