Damage level, earthquake level and post-earthquake actions in IAEA Safety Report

2.1.2.1. Earthquake motion severity and post-earthquake actions

As mentioned earlier, IAEA Safety Report Series No. 66 defines post-earthquake actions (i.e.

the ‘action level’), taking into consideration the severity of the observed earthquake motions (i.e. the ‘earthquake level’), and the extent of equipment damage identified during post-earthquake inspections (i.e. the ‘damage level’).

The ‘damage level’ has four categories, as listed below, depending on the extent of damage identified mainly by visual inspection during operator walkdowns performed immediately after the earthquake (or during the initial focused inspection in the event of a plant shutdown), and the expanded inspection performed as the need arises [1].

“Damage Level 1: No significant damage or malfunction to SSCs important to safety and those not important to safety.

Damage Level 2: No significant damage or malfunction to SSCs important to safety.

Significant damage or malfunction to SSCs not important to safety (NRPG: not required for power generation).

Damage Level 3: No significant damage or malfunction to SSCs important to safety.

Significant damage to or malfunction of SSCs not important to safety (RPG: required for power generation).

Damage Level 4: Significant damage to or malfunction of SSCs important to safety (it is highly likely that SSCs not important to safety will experience significant damage at this damage level).”

In addition, the earthquake motions that have been observed by the seismometers installed in the nuclear power plant are to be compared with the earthquake motions envisaged in seismic design. The IAEA’s seismic design standards specify two levels of design motion: SL-1 and SL-2. These two design levels are used to set the experienced ‘earthquake level’ from among the three levels listed below [1]:

“Earthquake Level 1: Instrumental records indicate that the earthquake motion is less than or equal to the SL-1 earthquake level.

Earthquake Level 2: Instrumental records indicate that the earthquake motion is greater than the SL-1 earthquake level and less than or equal to the SL-2 earthquake level.

Earthquake Level 3: Instrumental records indicate that the earthquake motion is greater than the SL-2 earthquake level.”

In IAEA Safety Reports Series No. 66 post-earthquake actions are determined by the combination of ‘damage level’ and ‘earthquake level’.

It should be noted that comparison of observed earthquake motions with the design earthquake motions in terms of their impact on equipment may not be straightforward. At first sight, the simplest way would be to compare the recorded peak of acceleration with the maximum acceleration of the earthquake motion considered at the design stage. However, it is well known from past experience that the peak ground (or floor) acceleration of an earthquake motion is a parameter that does not correlate well with seismic damage. If it were possible to predict equipment damage using indicating parameters calculated from motion records, then plant locations to inspect and the inspection methods would be easier to determine.

Consequently, it is important to determine what kind of equipment DIPs to use in order to set the ‘earthquake level’ based on the observed earthquake motions and to determine as well what kind of seismic instrumentation system to install for that purpose, thereby improving the reliability of post-earthquake actions.

2.1.2.2. Damage indicating parameters

At the seismic design stage, the response spectrum of absolute acceleration is often used to calculate the seismic loads acting on equipment due to design earthquake motions. Thus, it is a general practice to calculate the response spectrum using the acceleration records of the observed earthquake motion (in many cases, a damping ratio of 0.05 is used) and compare it with the design response spectrum to determine the potential impact. However, the response spectrum only shows the maximum response acceleration value as a function of natural frequency and it does not include any information about cumulative fatigue failure, for example.

As mentioned earlier, the peak value of the observed acceleration waveform is not necessarily related to equipment damage. Moreover, the acceleration response spectrum is influenced by the peak acceleration of the acceleration record. This may lead to an erroneous judgment, especially in the high frequency region in which such influence tends to be significant.

In one Member State, where no automatic reactor shutdown is performed based on earthquake motion observations, it is required for operators to perform manual reactor shutdown when the observed earthquake motions exceed the SL-1 level (sometimes designated as the operating basis earthquake). A possible set of exceedance criteria are presented below for the purposes of illustration [3, 4]. Both criteria need to be met in order to consider that the operating basis earthquake was exceeded.

1. Response spectrum check: The 5% damped ground response spectrum for the earthquake motion at the site at frequencies between 2 and 10 Hz exceeds the corresponding operating basis earthquake design response spectrum or 0.2 g, whichever is greater; and

2. Standardized cumulative absolute velocity (CAV) check: The computed standardized CAV value from the earthquake record is greater than 0.16 g-sec.

Here, CAV stands for cumulative absolute velocity, which is one of DIPs to be discussed later.

Note that the response spectrum check criterion does not consider frequency contents higher than 10 Hz, by limiting the frequency range of response spectrum comparison to 2 to 10 Hz.

The intent is that earthquake motions that do not affect equipment are eliminated by using both the response spectrum and CAV criteria.

In IAEA Safety Report Series No. 66 [1], Section 3.3.1, the response spectrum check is described in paragraph (a) (details of which are omitted in this publication) and the DIP check is described in paragraph (b), which is quoted below.

“Exceedance of a damage indicating parameter. An additional check needs to be part of the SL-1 or SL-2 exceedance criteria, utilizing a parameter that suitably describes damage from earthquake motions (damage indicating parameter). One such parameter is the CAV, which has been correlated with observed damage to ductile components that have experienced earthquake motions. Another potential damage indicating parameter is the JMA [Japan Meteorological Agency] intensity.”

IAEA Safety Report Series No. 66 recommends performing a check combining (a) and (b), that is, not using only one of the conditions.

As it can be seen in the quoted material above, IAEA Safety Report Series No. 66 points out CAV and the JMA intensity as candidate DIPs, but specific studies are yet to be completed. In

this publication, several DIPs are discussed as parameters that can suitably describe the damage caused by earthquake motions (Section 4).