Types and characteristics of seismic alarm/annunciation systems

Here, the functions of the seismometers that are used for operators to take emergency actions and their background are discussed.

As a function of seismometers used for emergency actions, a seismic switch is installed to trigger a signal indicating that the pre-set value has been exceeded. This seismic switch does not have to record the acceleration time-history. A seismic switch generally consists of a triaxial accelerometer, equipped with sensors to close relay contacts, power cables and relays inside the seismic instrumentation panel. It has the function of instantaneously indicating at a remote location that a preset acceleration value has been exceeded. Depending on the seismic instrumentation system configuration, the seismic switch may constitute a SDAS, rather than being an independent device. Here, the intent is to clarify the function of a SAS by discussing its functional requirements considering the seismic switch as independent. However, the ideas may also be applicable when an SDAS functions as a SAS.

The functions of the seismic switch can be summarized as follows:

 Generation and transmission of trigger signals. The SDAS will be started up when the preset seismic acceleration value is exceeded. In general, the trigger signal is indicated in the main control room, and the time of occurrence is recorded for analysing its relations with plant transients resulting from the earthquake event. The data recorded by the SDAS, started up by the trigger signal, is to be used for automatic or manual DIP calculations to decide post-earthquake actions.

 Main control room alarm and indication of seismic parameters. The peak acceleration value or seismic parameters will be indicated as a control room alarm and printer output to initiate the emergency response procedures according to the earthquake levels thereof.

As seen from the above, the function of a seismic switch is to trigger the SDAS and provide indications and alarms in the main control room. Although, its basic performance is similar to that of SDAS sensors, it is strongly required from a SAS to prevent malfunctions. In IAEA Safety Guide NS-G-1.6, for example, spurious signal prevention is emphasized as described below [2]:

TABLE 4. SCOPE OF POST-EARTHQUAKE INSPECTIONS AND EARTHQUAKE LEVELS (EXAMPLE)

Check main control room alarms, instruments, and others to see whether plant anomalies have occurred

JMA seismic intensity degree 3 or lower

Check main control room alarms, instruments, and others to see whether plant anomalies have occurred and take emergency steps as necessary

25 Gal or higher ~ below 50 Gal

Check main control room alarms, instruments, and others to see whether plant anomalies have occurred

Perform round inspections in the scope of normal patrol

JMA intensity degree 4 or higher

Check main control room alarms, instruments, and others to see whether plant anomalies have occurred and take emergency steps as necessary

Perform round inspections on plant equipment (excluding special

Perform round inspections in areas under special control with locks and the like in addition to class II inspections

Personnel responsible for equipment will perform round inspections on plant equipment (including areas under special control with locks and the like)

Perform radiation control assessment

Check to see whether chemicals, instruments, tools, and others have fallen or scattered in permanent materials storage areas and job sites under control

JMA intensity degree 5-lower or higher (without reactor scram)

Check main control room alarms, instruments, and others to see whether plant anomalies have occurred and take emergency steps as necessary

Perform round inspections on plant equipment (including special access-controlled areas except for reactor containment vessel interiors)

Check leak detection systems, radiation monitors, and tank levels for radiation control inspection inspections

Perform performance testing of engineered safety features and the like in accordance with the surveillance test procedure 125 Gal or

higher ~ below 250 Gal

Perform class III inspections Perform safety function check I Perform reactor safety assessment

Check main control room alarms, instruments, and others to see whether plant anomalies have occurred and take emergency steps as necessary

Perform round inspections on plant equipment (including reactor containment vessel interiors)

Check leak detection systems, radiation monitors, and tank levels for radiation control inspections

Perform performance testing of engineered safety features and the like in accordance with the surveillance test procedures Perform functional testing of safety critical equipment in accordance with the surveillance test procedures or the functional test procedures

“7.11. Both post-earthquake operator actions and automatic scram should be based upon a proper set of parameters derived from the recorded data and suitably processed, with two main goals:

(1) To avoid spurious signals;

(2) To provide an indicator of damage for comparison with the assumptions made at the seismic design phase.”

While the SDAS is designed for post-earthquake data analysis, the SAS and the ASTS are characterized in that they need to be capable of processing data in real time and transmitting accurate information during the earthquake.

Matters to pay attention to, with regard to triggers and main control room alarms/indications, are discussed in the following sections.

2.3.2.1. SDAS start-up trigger Trigger setting level

The trigger signal to start-up the SDAS is also related to the definition of a ‘felt earthquake’. It may also be used as an alarm or annunciator signal to indicate in the main control room that the SDAS has started up.

In general, seismic switches detect acceleration, and the regulatory requirements related with trigger acceleration setting values are different from one Member State to another, as shown below.

United States of America

In the United States, 0.01 g is specified as the maximum setting value for the seismic trigger [6]. The frequency range of the seismic trigger needs to include the range of 1 to 10 Hz.

Germany

In Germany, the regulations on settings are [7]:

 The trigger in the reactor building is to be set to 0.1 m/s² or below. The trigger in the free field will be set to 0.2 m/s² or below. When the SDAS starts up frequently, the trigger location needs to be altered. (Changing the setting value is the last resort to spurious signal prevention).

 The limit value of a seismic switch will be set to the acceleration corresponding to the maximum acceleration value analysed or specified for the inspection of the setting location.

India

Depending on the following requirement on the trigger value, it is commonly set to 0.01 g at all plants, according to the results of investigations.

Regulations on setting values (Atomic Energy Regulatory Board (AERB) /SG/S-11):

 The trigger level will not exceed 0.02 g (free field ground surface). One trigger will be able to start-up all instruments.

China

As a trigger to start-up the SDAS, the same seismic switch used for sending an alarm to the main control room is used, and the trigger level is set to 0.01-0.02 g. The seismic instrumentation system configuration of a typical nuclear power plant is described in Table II-5 of Annex II.

France

Règle Fondamentale de Sureté I.3.b requires an alarm in every unit control room and time-histories recorded after the onset of a significant earthquake (greater than 0.01 g acceleration).

Triggers are implemented [8]:

 At the floor of the reactor building

 At the floor of another building containing systems important to safety and whose foundations are different from those of the reactor building

Japan

Since an ASTS is installed for safety purposes, SDAS is considered as a voluntary system for structural analysis purposes. Although, the start-up (trigger) level is not particularly specified, the setting range of a trigger built in conventional seismic instrumentation systems is variable between approximately 0.1 Gal and 100 Gal. Utilities make decisions based on the intended use and the level of seismicity. As shown in Table 5, it is clear that these setting levels are generally lower than those of other Member States. As mentioned in Section 2.1.4, in Japan the reference seismic motion is defined on a (hypothetical) free surface of baserock, and great importance is attached to measuring points within buildings. Because the SDAS is intended to record the dynamic behaviour of the ground and the structures, it can be seen that SDAS will start-up upon a lower acceleration level when obtaining data from inside the instrumented boreholes in the ground.

Simultaneity of SDAS start-up timing

As shown in Section 2.2.3.1, SDAS sensors are installed at various locations. To examine dynamic behaviours of structures, it is desirable that these sensors are synchronized. The following functions are required from the seismic switch as a trigger of the SDAS:

 The SDAS’s horizontal and vertical acceleration time-history recording need to be started up simultaneously. To this end, one or more seismic switches are to be installed [6].

 The SDAS is to be trigged by both the vertical as well as the horizontal seismic excitation.

Data recording will start as soon as the data recording threshold is exceeded. In addition, recording need to be continued for at least 30 seconds after the last exceedance of this threshold. A medium capable of storing 30 minutes of data after the trigger start-up is to be used [7].

 The circuit design is to be able to start-up all acceleration sensors and recorders with one trigger. For SDAS acceleration sensors, one alarm-sending seismic switch and one triggering seismic switch is to be designed for each building [7].

TABLE 5. EXAMPLE OF TRIGGER SETTING LEVELS AND OBSERVATION POINTS IN JAPAN Utilities Utility L Utility M Utility N

Trigger setting level

Indications in main control rooms

Operators in the main control room are to be announced that the SDAS has started up (start-up signal from the SAS).

 When time-history acceleration data comes from a free ground surface or from a foundation level, the trigger needs to be announced in the control room. When a multi-unit site has more than one main control room, it needs to be announced in every control room [6].

Recording of trigger signals

As mentioned in Section 2.2.3.1, SDAS data is important for the clarification of the plant’s transient phenomena after an earthquake. Thus, the function to clarify the connection between SDAS data and transient phenomena is required.

 The time of trigger occurrence is to be recorded [4].

Dynamic characteristics

In view of the purpose of seismic switch installation, frequency band-pass filter is advisable for protection against noise.

 In the frequency range from 0.1 to 30 Hz no resonances are allowable in the sensors [7]. The amplitude frequency response shall not deviate by more than ± 1 % from the amplitude setpoint. the phase frequency response may not deviate from the set point by more than ± 2 % [7].

 To suppress influences not caused by earthquakes, the seismic trigger needs to incorporate amplitude attenuation above 10 Hz (e.g. a low-pass filter with a cut-off frequency of 10 Hz [7]).

 Bandwidth of triggers may be limited to the range of 0.1 to 10 Hz to avoid spurious tripping by phenomena other than earthquakes [8].

2.3.2.2. Control room alarms and annunciations

In IAEA Safety Standard NS-G-1.6, seismic alarms for operators at a plant without ASTS are described as shown below [2].

“(b) For seismic monitoring: to provide alarms for alerting operators of the potential need for a plant shutdown depending on post-earthquake inspections.”

The following description in Kerntechnischer Ausschuss (Germany) 2201-05 is important for the function of SDAS and it is considered applicable not only to nuclear power plant without ASTS, but also with ASTS [7].

“5. (4) The following alarms shall be documented in the main control room or in a control room annex:

(a) Actuation of data measurement and recording, (b) Actuation of any of the alarm triggers,

(c) Loss of the external power supply to the instrumentation specified in Section 3.

These alarms shall be interconnected to initiate a group alarm that shall be optically and acoustically announced in the main control room”.

To this end, it is recommended that the SDAS panel is placed in the main control room or in its vicinity. Procedures need to prescribe that operators will check the DIPs provided by the SDAS (for instance, CAV or instrumental seismic intensity), when there is a seismic alarm.

Both functions required of SAS, that is, starting up SDAS and giving warning to operators, use the mechanism of a seismic switch relying on acceleration sensors instrumentation. On the other hand, as detailed later in Section 3 and 4, the peak acceleration value is not necessarily related to seismic damage to SSCs. To allow operators determine the ‘earthquake intensity’

more precisely, alarms/annunciations based on acceleration may be combined with DIP indications. In Japan where JMA instrumental seismic intensity prevails, instrumental seismic intensity meters and the like are used for general industrial purposes.

In the event of an earthquake, an alarm or annunciation will be activated in the main control room upon the three earthquake levels listed below.

 SDAS trigger signal level (See Section 2.3.2.1)

 Earthquake level requiring a decision for manual reactor shutdown (see Section 2.3.1.2 for a plant without ASTS)

 ASTS activation level (at a plant with ASTS)

The second alarm level is particularly applicable to nuclear power plant without ASTS and it is specified as described below in IAEA Safety Standard NS-G-1.6 [2].

“7.7. The lower trigger level (alert) should be close to SL-1 (usually associated with operational limits), at which significant damage to safety items is not expected. If the

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