overall seismic capacity of the plant is lower than SL-1 (e.g. during the seismic re-evaluation), the lower trigger level should be referred to the actual seismic capacity of the plant.”
In KTA 2201-05 (Germany), this setting level is specified in connection with walkdown inspections as shown below.
“5. (3) The threshold values for alarms shall be adjusted to the acceleration limit values that correspond to the maximum accelerations specified or calculated for the inspection levels at the respective placement locations.”
There are some cases, for instance in China and France, in which both functions of SAS are performed by treating an SDAS trigger signal as an alarm. On the other hand, Table 6 shows some cases in other Member States in which an alarm is activated at an earthquake level different from a trigger signal.
ASTSs have been installed in Japan. In this regard, when an earthquake motion exceeds a pre-set trigger level, based on the design basis earthquake motion, ASTS activation is indicated as an alarm in the main control rooms. Furthermore, utilities set the main control room alarms and annunciations separately depending on the level of seismicity at their sites. Some instances are shown in Table 7. In Japan, as discussed in Section 2.3.1 (c), on-site inspection classes have been specified according to the level of earthquake motions observed to notify operators that the SDAS has started up and have them recognize the earthquake motion intensity. As shown in the column for Utility Z in Table 7 there is an instance in which earthquake early warnings are used as alarms during earthquakes, which is detailed later in Section 2.3.5.
2.3.3. Important considerations for seismic alarm/annunciation systems
TABLE 6. MAIN CONTROL ROOM ALARM LEVELS (INSTANCES IN WHICH ALARM LEVELS ARE DIFFERENT FROM SDAS TRIGGER LEVELS)
Member States
Nuclear power plant Main control room alarm set levels
Remarks
Hungary Paks 0.05 g Sensor on the containment building
foundation mat
India TAPS-1&2 0.05 g The trigger level of the seismic switch will
not exceed 50 % of PGA (SL-1 level) and the maximum value will be 0.1 g and below.
(Applicable standard): AERB/SG/S-11
RAPS-3&4 0.025 g
MAPS-1&2 0.04 g
KAIGA-1,2,3&4 (with
ASTS) 0.05 g
KAPS-1&2 (with ASTS) 0.1 g
United States of America
North Anna Horizontal: 0.06 g,
Vertical: 0.04 g
Sensor on the containment building foundation mat
TABLE 7. EARTHQUAKE-RELATED INFORMATION IN MAIN CONTROL ROOM (INSTANCES IN JAPAN)
Utilities Utility X Utility Y Utility Z
Indication in the main
control room An instrumental seismic intensity meter has been installed in Unit 1 (two-unit site). When acceleration equal to 1 Gal or higher is detected, the seismic intensity and maximum acceleration will be printed out in the main control room (without sounding an alarm). Observed values will be displayed on the data recorder installed in the main control room.
When the earthquake warning system installed on the second floor of the auxiliary building detects acceleration equal to 3 Gal or higher, the JMA instrumental seismic intensity, intensity grade, and maximum acceleration will be printed out and an alarm will go off in the main control room and three other locations, and observed values will be recorded in the data recorder connected with the earthquake warning system.
A seismometer assigned for safety assessment is installed on the representative reactor building foundation mat and the maximum acceleration and JMA seismic intensity will be indicated in the main control rooms.
When it is expected that an earthquake of seismic intensity 3 or higher will occur (early warning), the estimated intensity grade will be announced on-site (including the main control rooms) with a paging system.
accidental impact is prescribed by the regulation in some Member States [6]. It is important to prevent the effects of non-seismic events and frequency filters are recommended to be applied (see Section 2.3.2.1).
As mentioned in Section 2.1.2.2, the acceleration response spectrum within the bandwidth of 2 to10 Hz is used to determine operating basis earthquake exceedance. It is considered that
mechanical equipment made of ductile metallic materials will not be damaged by accelerations at 10 Hz or higher frequencies.
Protection against external impact is also effective in protecting seismometers from noise. In particular, it is considered as an essential measure when the seismometers are part of a SAS.
An example of external impact prevention measures in a Japanese plant is shown in Fig. 6.
In Member States where importance is given to main control room alarms (plants without an ASTS), redundancy is considered for the seismic switch in order to assure the objectives of the SAS. An example of redundancy in China is shown in Table II-5 of Annex II. Two accelerometers are selected as SAS sensors for redundancy.
2.3.3.2. Multi-unit sites
The concept of seismic instrumentation system at a multi-unit site is provided in Section 2.1.5.
When seismic instrumentation is installed at one representative unit on a multi-unit site, however, it is necessary to send alarm/annunciate signals from the seismic switch to the main control room of all units, which is prescribed in the quoted texts shown below.
Regulatory Guide 1.12 [6]
“Triggering of the free-field, downhole, or any foundation-level time-history recorder should be announced in the control room. If there is more than one control room at the site, annunciation should be provided to each control room.”
Règle Fondamentale de Sureté (France) I.3.b [8]
“The starting of the records will be based on triggers set to a threshold corresponding to a significant earthquake. Exceeding this limit will generate an alarm in the control room of every unit of the site.”
Information about the observed earthquake motion intensity is needed not only in the main control rooms, but also in administrative buildings. In instance Z of Table 7, for example, this information is announced not only in administration buildings, but also across the plant, which is considered particularly important at multi-unit sites.
2.3.3.3. Other considerations Testability
Highly reliable maintainability and testability are required for a SAS. It is desirable that functional testing and on-site calibration can be performed. From this perspective, the following description is very suggestive [12].
“The system should include capability for remote, in place functional testing to verify performance during routine maintenance procedures. The sensors should be designed to allow on-site calibration using simple procedures and tools. Sensors with DC response usually offer simpler calibration procedures (tilt vs. shake table testing) and are recommended for this reason. Procedures for zero offset adjustments of DC sensors should be easily undertaken by plant personnel without specialized training.”
c
Appearance/Seismometer sensor inside the cover
FIG. 6. Example of external impact prevention measures for seismometer sensors in Unit 2 reactor building of Shika nuclear station. (Courtesy of Hokuriku Electric Power Company)
Power supplies
A loss of off-site power is anticipated in the event of an earthquake. It is recommended that uninterruptible power supplies need to be used to fill the window time until the emergency AC power supply system starts up.
On the other hand, especially in low seismicity areas like France, there are no requirements on electric power supply. However, the regulation requires mechanical records of peak acceleration instead, saying that some complementary devices, autonomous, simple, without any requirement of power supply, need to provide approximate maximum values of felt accelerations at various locations of the specified structures [8].
Seismic and quality requirements
See Table 1 in Section 2.1, General Considerations on Seismic Instrumentation for Decision Making.
Arrangements
Depending on the seismic instrumentation system configuration, the SDAS may also function as seismic switch. In those cases, the followings the following considerations apply to the SAS seismic switch:
The SDAS accelerometer and the SAS seismic switch will normally be oriented such their axes are parallel to the axes of the coordinate system used for the seismic analysis of the building [7].
(When the SDAS also functions as SAS) One SAS seismic switch need to be specified for each building from among several SDAS acceleration meters.
The reliability of seismic switches is important, and maintainability needs to be taken into account:
Seismic switches need to be accessible as needed for operation and maintenance [7].
Digital or analogue
For SDAS, digital systems are recommended because of the necessity of prompt data processing. On the other hand, SAS functions as a switch, for which reliability and maintainability are important. Thus, for SAS, mechanical systems (analogue) can be employed as well as digital systems.
Area network information
Information from regional emergency preparedness networks is important as well as on-site earthquake observation. It is required to obtain accurate earthquake information by actively using off-site network information.
2.3.4. Lessons learned and other observations