Important considerations for seismic data acquisition systems

Instrument locations need to be selected by the operator of the nuclear installation to obtain adequate information to meet the overall regulatory and mission critical requirements.

Instrument locations need to be selected taking into account the following considerations:

(a) Will the recorded data be used to calculate motions at other locations in the structure? If so, are translational inputs at a single location adequate or does a small array of instruments need to be placed to permit definition of rotations? Are multiple triaxial instruments needed on the base mat to define rotations (torsion and rocking)? This issue may be important for new nuclear power plant designs that implement very large basemats.

(b) Will the recorded data be used to compare with in-structure design basis data? This will guide the locations of the instruments within the structures of interest.

(c) Are the instruments located such that equipment-structure interaction effects are minimal? Generally, structural response is desired without the complication of local effects like flexible slabs, equipment-structure interaction, etc.

(d) Are results from a site-specific probabilistic risk assessment used in the decision-making process? The location of instruments needs to be based on the site-specific probabilistic risk assessment insights (which buildings, which elevations within the building, etc.). An alternative is to use results from other probabilistic risk assessments on similar designs coupled with engineering judgment to account for differences in design basis.

Instrument locations need to be selected with appropriate consideration given to items (a) through (d) above, and to obtain data that can be directly related to ‘input’ and ‘output’ vibratory motion used in the seismic design. Locations of sensors to capture free-field ground motion, basemat motion and in-structure motion are further discussed below:

 Free-field ground motion: Especially for Case A in Section 2.1.4, the free-field is defined as those locations on the ground surface or in the site soil column that are sufficiently distant from the site structures to be essentially unaffected by the vibration of the site structures. To the extent practical, the location of the instruments needs to be on a soil profile similar to the soil profile used in the seismic design process. Practical definitions of ‘sufficiently distant’ to satisfy the requirements vary from Member State to Member State. In France, the Règle Fondamentale de Sureté I.3.b [8] considers as free field any point with a minimum distance of 100 meters from any heavy building (buildings of the nuclear island, turbine hall, cooling towers). A sensor located in the free-field records essentially the free-field ground motion. This free-field motion is used to determine whether the design bases earthquake ground motion (SL-1 or SL-2) have been exceeded.

Instruments placed in the free field may directly determine a DIP; alternatively, the DIP

may be calculated from the recorded acceleration time-histories. The free-field motion may be the starting point for confirmatory seismic analyses to be performed of the nuclear power plant structures, if deemed necessary.

 Foundation or basemat motion: One or more sensors located on the foundation or basemat may serve multiple purposes, for instance, comparison of the seismic design motion with that of the recorded data in the form of response spectra comparisons, input to the calculation of the detailed response of the structure (if deemed necessary to do so) etc.

 In structure motion: Time-history accelerometers located at key locations in safety-related structures provide data for direct comparison with the seismic design parameters for SSCs, benchmarking of the dynamic models, input to supported subsystems at these locations, etc. The locations selected for these instruments need to adhere to principles such as locations important to overall soil-structure response, locations where secondary effects are minimal (flexible sub-structures like floors or slabs, equipment-structure interaction, etc.), locations where systems and components important to safety are located, as determined from risk importance concepts such as those contained in US NRC Regulatory Guide 1.201 on Guidelines for Categorizing Structures, Systems, and Components in Nuclear Power Plants According to Their Safety Significance, or from a seismic probabilistic safety assessment or from a seismic margin assessment. Examples inside the containment are reactor equipment supports, reactor piping supports, and other safety-related equipment and piping supports.

Four examples of guidelines on the number and placement of seismic instruments are presented below:

IAEA safety standard on Seismic Design and Qualification for Nuclear Power Plants [2]

It recommends that a minimum amount of seismic instrumentation is to be installed at any nuclear power plant site as follows:

“One triaxial strong motion recorder installed to register the free field motion;

One triaxial strong motion recorder installed to register the motion of the basemat of the reactor building;

One triaxial strong motion recorder installed on the most representative floor of the reactor building.

The installation of additional seismic instrumentation needs to be considered for sites having an SL-2 free acceleration equal to or greater than 0.25g.”

LE 3. COMPARISON OF NUCLEAR POWER PLANT SEISMIC INSTRUMENTATION PERFORMANCE IN THE UNITED STATES OF AMERICA (COURTESY PROF. NIGBOR) Performance Parameter 70s-Vintage, Kinemetrics SMA-3 RG 1.12 Rev. 2 1997 ANS 2.2 – 2002 2009 Commercial NPP System, Kinemetrics or Syscom

Advanced National Seismic System Class B Advanced National Seismic System Class A

RG 1.12 Rev. 3 2017 amic range, dB4060 60 86-108 >86>110 >=110 amic range, equiv. bits 7 10 10 16-18 >= 16 >= 20 >=18 ll-scale, g 1 1 1 3-43.5 3.54.0 equency range, Hz0.2-30 0.2-50 0.02-50 0-500.1-350.02-500-100 mple rate, samples/second Analog 200 200 200 (min) 200 (min) 200 (min) 250 (min) ross-axis sensitivity, g/g 0.03Not specified0.030.01 0.010.01< 0.01 lute timing accuracy, msec No timingNot specified Adequate to differentiate main <1 with GPS <1 <1 5 e-event memory, seconds0 3 3 99 or greater >=60>=60>=60 ecording capacity, minutes102525>100 >=60 >=60>=120

US NRC Regulatory Guide 1.12 on Nuclear Power Plant Instrumentation for Earthquakes [6]

It states that triaxial acceleration sensors need to be provided at the following locations:

1. “Free-field.

2. Containment foundation.

3. Two higher elevations (excluding the foundation) on a structure inside the containment.

4. A Seismic Category I structure foundation included in a certified standard design or facility where the expected response is different from that of the containment structure.

5. An elevation (excluding the foundation) on a Seismic Category I structure selected in 4 above.

6. An elevation in a non-containment Seismic Category I structure supported by the containment foundation.

7. A Seismic Category I structure foundation not included in a certified standard design or facility.

8. A higher elevation in the instrumented Seismic Category I structure selected in (7).

9. Alternatives to sensor locations (2) through (8) may be necessary in order to support other instrumentation criteria in this regulatory guide.”

Other Considerations in US NRC Regulatory Guide 1.12 are as follows:

 Instruments operational in all modes of plant operation including periods of plant shutdown (Regulatory Guide 1.12, C 3).

 Instruments at multi-unit sites (Regulatory Guide 1.12, C 2).

 Control Room notification (Regulatory Guide 1.12, C 7).

Further to selecting sensor locations, a design review of the location, installation, and maintenance of proposed instrumentation, including considerations for keeping exposures as-low-as-reasonably-achievable needs to be performed.

Kerntechnischer Ausschuss (Germany): KTA 2201.5 on Design of Nuclear Power Plants against Seismic Events, Part 5: Seismic Instrumentation [7]

It states the following requirements regarding number and location of seismic instruments.

They apply to plant sites for which the maximum acceleration of the design basis earthquake does not exceed 0.25 g.

For single unit plants:

“3.2 (1) Accelerographs shall be provided for in the free-field and inside the reactor building.”

“3.2 (3) The placement location of the accelerograph in the free-field shall be chosen such that any influence of buildings on the data to be measured can be ruled out. The accelerograph shall be placed at a distance away from the reactor building that it is equal to at least twice the largest length of the reactor building foundation and, also, away from other buildings by at least the largest ground-plan dimension of the respective building”

“3.2 (5) At least three accelerographs shall be installed in the reactor building. Two of these shall be installed in the lowest building level and one in an upper level of the reactor building (e.g., pool floor level); the horizontal distance between the lower acceleration sensors should be as large as possible. The placement locations of the accelerographs should be chosen such that a direct comparison of the measured data with the corresponding design quantities is possible. At their placement location, there should be only negligible operation-related effects on the measurements.”

“3.2 (6) The acceleration sensors of the accelerographs should normally be oriented such that their axes are parallel to the axes of the coordinate system used for the seismic analysis of the reactor building.”

“3.2 (7) The accelerographs shall be accessible for the necessary operating and maintenance procedures. The accelerographs shall be designed and installed such that an evaluation of the recorded data is not adversely affected, e.g., by damages to components or civil structures that fail during an earthquake nor by a superposition of the measured seismic signal with seismically induced oscillations of neighbouring components.”

“3.2 (8) The acceleration sensors of the accelerographs shall be mounted such that no movements relative to the mounting support can occur.”

Règle Fondamentale de Sureté (France) on location of instrumentation devices (Ref. [8], I.3.b 2.2.3)

The implementation of seismic sensors depends on the type of site:

Homogeneous sites

For a homogeneous multi-unit site, only one unit may be equipped with a seismic instrumentation. Triaxial accelerometers need to be located:

 At the basemat of the reactor building;

 At one or more floors of the reactor building, with sufficient elevation to have significant amplifications in acceleration and also to be able to estimate with better accuracy the effects on some important safety-related components located above the base mat. These accelerometers are located approximately in the same vertical;

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

 In the free field.

These triaxial accelerometers are arranged so that their respective three orthogonal directions coincide with each other along the principal axes of the buildings of the instrumented unit.

Heterogeneous sites

For a heterogeneous site, in addition to instrumentation installed on a uniform site, the following devices need to be installed:

 An additional triaxial accelerometer in the free field placed in an area of geological and mechanical characteristics or topography different from that which is already instrumented;

 A triaxial accelerometer at the basemat of the reactor building of each unit.

2.2.3.2. Other considerations

 Seismic qualification: The SDAS needs to be seismically qualified to perform its functions during and after the earthquake ground motion. The minimum level of seismic qualification needs to be to the SL-2 level. It is preferable for the seismic qualification to be at a level greater than the SL-2 to assure operability if an earthquake produces ground motion greater than the SL-2 at the site. Seismic qualification needs to include any support systems necessary for system operation (e.g. emergency power, battery backup).

 Maintenance and testing: To reasonably assure operability of the SDAS, appropriate periodic maintenance needs to be performed. In many instances, maintenance may be sub-contracted to a third party (often the system supplier) for an extended period of time, for instance, 10 years. The operating organization needs to verify that maintenance is performed even after such maintenance agreements have expired. In addition, testing of the operability of the system needs to be performed on a regular schedule, for example, quarterly. Schedule maintenance to keep maximum number of instruments in service.

 Operability of SDAS in all operational modes of the nuclear installation: Operational modes of the installation include low power states and plant shutdown (scheduled and unscheduled outages).

 Multi-unit sites: Provisions need to be made to coordinate the SDAS for multi-unit sites.

In some cases, one set of sensors may serve multiple purposes on-site (e.g. free field sensors). For sites, where there are common systems for more than one unit, these common systems may be monitored by, and the recorded data may be announced in multiple control rooms. Immediate post-earthquake actions for multi-unit sites need to be closely coordinated between units.

 Control room notification: Appropriate notification of recorded earthquake motions needs to be announced in the control room. These data include necessary information for operator actions, such as manual shutdown of the installation. In addition, notification to the control room needs to be made if earthquake records data storage is approaching the

storage maximum capacity and additional storage devices need to be added. The operational data of the SDAS, such as outage due to loss of the electric source or malfunction are also recommended to be notified.

 Installation and configuration control: Installation of all elements of the SDAS needs to be such that all seismic systems interaction concerns are resolved. Seismic systems interaction concerns are the consequence of failure of not-important to safety, SSCs causing failure or malfunction to important to SSCs. The latter could include the SDAS.

These concerns are sometimes referred to as ‘II/I interaction’. The phenomena associated with seismic systems interaction are: falling of items impacting the important to safety item, proximity meaning impact of adjacent not important to safety components causing malfunction or damage to important to safety components, and spray/flooding.

Installation and on-going configuration control procedures need to assure that potential II/I issues do not prevent the SDAS from performing its functions. In some cases, enclosures may be used to prevent inadvertent damage to portions of the system (or accidental impact by plant personnel), for instance, for sensors that initiate the ASTS.

 Anchorage of sensors: It is advisable that when an acceleration sensor is installed on structures, it is installed in a safe location, taking account protection from accidental impact. When installed on ground surface, lightness and rigidness of its foundation structure (concrete pad) need to be enhanced to the extent possible to avoid interactions with the supporting ground. Fig. 3 shows the configuration recommended in Ref. [12].

When the sensor not installed on rock, a pile foundation needs to be used to guarantee a minimum bearing capacity. Exposure to water needs to be prevented as well (e.g. by a rainproof shed).

FIG. 3. Example of foundation when sensors are installed on the ground surface [12]. (Courtesy of EPRI)

 Electric power supply: A SDAS needs to have a power supply system anticipating a loss of off-site power in the event of an earthquake. It should be noted that a backup power supply from an emergency diesel generator has a window time until the diesel generator starts up upon a loss of off-site power. In this context, SDAS needs to be also energized by an uninterruptible power supply. In the US Regulatory Guide 1.12, for example, it is required to install a battery that allows for more than 120 minutes of recording, and is rechargeable with an uninterruptible power supply or another power supply. Even in the event of a loss of off-site power, it is recommended that 24-hour recording, and 10-minute continuous recording are possible and verifiable.

 Data processing time: At a plant without ASTS, the time available to analyse SDAS data for the decision on the necessity of a reactor manual shutdown is limited (within four hours after an earthquake in the United States regulation, for example). It is necessary to install a solid-state digital SDAS capable of processing data in a short time or to develop a system that will promptly export the recorded data to a computer (see the lessons learned in Section 2.2.4).

 Maintenance and management: Similarly, a plant without an ASTS needs to enhance the reliability of SDAS. It is required that online maintenance and repairs are performed when the maximum number of SDAS instruments are available; system channels need to be checked every other week during the first three months from the start of plant operation, followed by monthly checks (including batteries). Thereafter, channel functions need to be checked every six months and channel calibration needs to be checked during plant refuelling outage [12].

 Logging of start-up signals: Many transients in plant systems may occur in the event of an earthquake, even at a plant without ASTS. To analyse these transients, their timing in relation with seismic motions becomes very important. It is recommended that the initiation of the SDAS (start-up trigger signal) be recorded on the general plant transient recorder, so as to make it possible to relate in time the earthquake motion signals with the transients recorded in plant systems.