ML21050A156
| ML21050A156 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 10/21/2020 |
| From: | Vladimir Graizer, Dogan Seber, Scott Stovall Office of Nuclear Regulatory Research |
| To: | |
| T. Weaver | |
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| ML21050A140 | List:
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Text
Analysis of TVAs Watts Bar Nuclear Power Plant Strong-Motion Records of the M 4.4 December 12, 2018 Decatur Tennessee Earthquake Vladimir Graizer, Dogan Seber, and Scott Stovall Vladimir.Graizer@nrc.gov 1
DOE/NRC Natural Phenomena Hazards Meeting October 20-22, 2020
Disclosure
- Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S.
Nuclear Regulatory Commission.
- Any
- opinions, findings, and conclusions expressed in this presentation are those of the authors and do not necessarily reflect the views of the U.S. Nuclear Regulatory Commission.
- This presentation is based on paper published in journal Seismological Research Letters (SRL) 91, 1579-1592.
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4 Earthquake Information from the USGS https://earthquake.usgs.gov/earthquakes/eventpage/se60247871/executive Tectonic Summary The December, 12, 2018 9:14 (UTC) earthquake occurred in the Eastern Tennessee Seismic Zone (ETSZ). The ETSZ extends across eastern Tennessee and northwestern Georgia into northeastern Alabama. It is one of the most active earthquake areas in the Southeast. Although the zone is not known to have had a large earthquake, a few earthquakes in the zone have caused slight damage.
Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. The December, 12, 2018 M4.4 Decatur, Tennessee earthquake was felt over 310 miles from Southern Kentucky to Fort Benning, Georgia. Strong shaking capable of causing slight damage have been reported near the epicenter.
The Eastern Tennessee seismic zone is laced with known faults, but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths. Accordingly, few, if any, earthquakes in the ETSZ can be linked to named faults.
Watts Bar Nuclear Plant
The plant is located in Rhea County, Tennessee, near Spring City, between the cities of Chattanooga and Knoxville. The plant has two Westinghouse pressurized water reactor (PWR) units: unit 1 was completed in 1996, and unit 2 was completed in 2015. Both units are the newest operating civilian reactors to come online in the United States, and unit 2 is the first and only new reactor to enter service in the twenty-first century.
- Hypocenter of the December 12, 2018 moment magnitude (M) 4.4 Decatur, Tennessee, earthquake was located at a depth of 7.9 km. The epicentral distance to the plant was 4.9 km. The earthquake occurred on an unknown fault and has a predominantly strike-slip mechanism.
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6 M4.4 Decatur earthquake ShakeMap and Watts Bar NPP location Location: 35.612° N and 84.732° W Hypocenter depth 7.9 km Epicentral distance to NPP 4.9 km The earthquake fault mechanism was predominantly strike slip
Safe Shutdown Earthquake (SSE) and Operating Basis Earthquake (OBE) 7 U. S. Code of Federal Regulations (CFR) requires designing nuclear power plant structures, systems, and components important to safety to withstand the effects of natural phenomena, such as earthquakes, without loss of capability to perform their safety functions. Specific requirements include the establishment of Safe Shutdown Earthquake (SSE) and Operating Basis Earthquake (OBE) ground motions which are used to assess the integrity of the plants structures, systems, and components as well as shutdown and restart procedures following an earthquake. Both SSE and OBE ground motions are characterized by response spectra.
The US regulations define the SSE as the vibratory ground motions for which certain structures, systems, and components must be designed to remain functional. The OBE ground motions are associated with plant shutdowns and inspections. Watts Bar NPP Unit 1 operating license was issued in February 1996. In plants licensed before January 10, 1997 OBE is usually set up as one-half or less of the SSE (earthquake engineering criteria in appendix A to 10 CFR part 100). If vibratory ground motions at a NPP site exceed the OBE spectra, a plant shutdown and specific inspections are required as described in Regulatory Guide (RG) 1.166 (1997). To assess whether earthquake ground motions have exceeded the OBE or SSE, it is important that a reliable seismic instrumentation system be in place.
NPPs licensed before 1997 are not required to have a free field instrument. The free-field requirement first appeared in RG 1.12 Rev. 2 published in March 1997. However, the most recent regulatory guide, RG 1.12 Rev. 3 (2017), suggest new NPPs to have more extensive seismic instrumentation including a free-field accelerograph to allow direct comparisons with the current ground motion models and calculation of the cumulative absolute velocity (CAV), and a borehole instrument for the direct comparison to the plants design SSE and OBE ground motions.
Seismic Instruments and Recordings at Watts Bar NPP The existing seismic instrumentation system at the NPP unit 1 was installed in September 1999 and consists of the Kinemetrics CONDOR digital recorder connected to three triaxial accelerometers and an ETNA triaxial strong motion accelerograph.
The earthquake was recorded in Unit 1 by the CONDOR recording system at three locations:
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Elevation 702.78 ft. in Unit 1 Reactor Bldg. on the base (75A) 2.
Elevation 756.63 ft. in Unit 1 Reactor Bldg. on the floor slab (75B) 3.
Elevation 742 ft. in Diesel Generator Bldg. on the base slab (75D)
And an independent accelerograph ETNA at the 4.
Auxiliary Bldg. at elevation 757 ft. (BA)
The CONDOR recording system is located in the Control Building. First three transducers are interconnected and have full scale range of 0 to 1.0 g with a bandwidth of 0 to 50 Hz, and 3-sec pre-event and 10-sec post-event memory. ETNA accelerograph at the Auxiliary Building have a range of 0 to 2.0 g and 20-sec of pre-event and 10-sec post-event memory.
An internal seismic trigger actuates the recording systems when a threshold acceleration level is sensed. The trigger threshold is set to initiate recording when the acceleration at the containment foundation exceeds 0.01g. Sampling rate is 200 samples per second.
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9 CONDOR Seismic Monitoring System for NPPs Downloaded from Kinemetrics archives.
Triaxial Accelerometers in Unit 1 Containment 10 Shear wave velocity profile at control point 64 ft below plant grade
11 We read the Kinemetrics EVT files using their software to get uncorrected acceleration files, and reprocessed the files to get corrected accelerations, velocities, displacements and Fourier and 5% damped response spectra.
Processed data have a bandwidth of 0.65 to 53 Hz.
Accelerations Velocities Displacements
12 Fourier Spectra 5% damped Response Spectra
13 Accelerations Velocities Displacements
14 Fourier Spectra 5% damped Response Spectra
15 Accelerations Velocities Displacements
16 Accelerations Velocities Displacements
17 Location/Channel Acceleration, g Velocity, cm/sec Displacement, cm Reactor, El.703 ft, Long 0.0264 1.011 0.0853 Reactor, El.703 ft., Vert 0.0206 0.508 0.0493 Reactor, El.703 ft, Tang 0.0216 0.925 0.0802 Reactor, El.757 ft, Long 0.0482 1.434 0.0993 Reactor, El.757 ft., Vert 0.0417 0.578 0.0480 Reactor, El.757 ft, Tang 0.0290 1.229 0.1097 Diesel Gen., El.742 ft, Long 0.0527 1.741 0.0879 Diesel Gen., El.742 ft, Vert 0.0206 0.502 0.0487 Diesel Gen., El.742 ft, Tang 0.0443 1.135 0.1034 Auxiliary, El.757 ft, N-S 0.0642 1.435 0.110 Auxiliary, El.757 ft, Vert 0.0354 0.5367 0.0455 Auxiliary, El.757 ft, E-W 0.0529 1.579 0.108 Table 1. Peak amplitudes of acceleration, velocity and displacement recorded by seismic sensors in Unit 1.
18 Average Horizontal Spectral Accelerations Fourier and Response Spectral Ratios Log-Normal scale Log-Log scale
- Structural resonance frequencies depend upon a number of factors including the elevation of seismic instrument, its proximity to the specific structural component and the depth of the Reactor building embedment. Multiple finite element model calculations of the seismic response of PWR result in a variety of resonance frequencies depending upon the specifics of the plant.
- ~5 and ~14 Hz structural resonances are consistent with modeling results shown in the papers of Kang and Lee (14WCEE, Beijing, China, 2008) and Lee et al. (15 WCEE, Lisbon, Portugal, 2012) for the PWR NPP.
19 Structural Resonances in PWR Containment
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.1 1
10 100 Spectral Acceleration, g Frequency, Hz Horizontal Spectral Accelerations Compared to SSE, OBE and EPRI_2013 GMPE Reactor Base, El.703, Long Reactor Base, El.703, Tang Diesel, El.742, Long Diesel, El.742, Tang Reactor, El.757, Long Reactor, El.757, Tang Auxiliary, El.757, E-W Auxiliary, El.757, N-S SSE OBE Min OBE (EPRI 1988 Criterion)
EPRI 2013, Rjb=4.9 km Rjb=4.9km +1sigma Rjb=4.9km -1sigma Horizontal OBE = 0.09g SSE = 0.18g 5% damped response spectral accelerations recorded at 4 locations are below the OBE.
Small exceedance at Auxiliary bldg at one horizontal component at frequencies of 45-50 Hz is beyond the limits of consideration.
Availability of records helped making quick decision of ground motions non-exceeding OBE and no need of plant shutdown.
Comparisons with Horizontal OBE and SSE 20
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.1 1
10 100 Spectral Acceleration, g Frequency, Hz Vertical Spectral Accelerations at Watts Bar Compared to SSE, OBE and EPRI_2013 GMPE Reactor Base, El.703, Vert Diesel, El.742, Vert Reactor, El.757, Vert Auxiliary, El.757, Vert SSE Vertical OBE Vertical Min OBE (EPRI 1988 Criterion)
EPRI 2013, Rjb=4.9 km Rjb=4.9km +1sigma Rjb=4.9km -1sigma Vertical OBE=0.06g SSE=0.12g 5% damped response spectral accelerations recorded at 4 locations are below the OBE at frequencies < 10 Hz.
Availability of records helped making quick decision of ground motions non-exceeding OBE and no need of plant shutdown.
Comparisons with Vertical OBE and SSE 21
22 Comparing Displacements in Containment at Base at El. 703 ft and El. 757 ft Tangential Vertical Longitudinal
Animation of Absolute Motion of Containment 23
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25 Longitudinal Tangential Vertical
26 Fourier Spectra 5% damped
Response
Spectra
27 RDs between base and the higher elevation in the containment structure are higher than those of between the containment base and the diesel generator building. RDs between the two structures are in phase and have similar shapes.
Vertical RDs between the reactor base and a higher elevation floor in the containment structure, as well as between the reactor base and the diesel generator building as expected, are a few times lower than the horizontals. Vertical RDs are similar in shape and predominantly only high-frequency 5 Hz motions.
Resonance frequencies observed in the longitudinal direction are at 1 and 4.5-5 Hz and at 0.6-0.7 and 5 Hz in the tangential direction. High-frequency peaks near 5 Hz can most likely be associated with the natural frequencies of the reactor building floor or internal structures whereas low-frequency peaks at 0.6-1 Hz can most likely be associated with a rocking motion of the containment building.
Observations Based on Relative Displacements
Animation of Relative Motion of Containment 28
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Deformation Deformation (simple shear) in the structure along the x-axis where xn(t) and xn-1(t) are amplitudes of horizontal ground motions at the same time t at different elevations in the structure, and L is the distance between those elevations measurements (base).
Simple shear strain with the rate is the combination of pure shear strain with the rate /2 and rotation with the rate of =/2.
1
( )
( )
( )
n n
x t x
t t
L
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Lessons Learned
- Availability of recordings helped making quick decision of ground motions non-exceeding OBE and no need of plant shutdown.
- Availability of multiple synchronized seismic records allowed to identify structural resonances at 5 and 14 Hz within the containment and other important features of structural response.
- Setting up trigger level at 1% g, while RG 1.12 Rev.2 (1997) required not more than 2% g allowed to have clean and complete recordings not missing any key portions of the waveforms.
- Absence of a free-field recording limited the seismic analysis not allowing to calculate CAV and estimate some input motion parameters for direct comparison with ground motion models. Having more dense seismic instrumentation in the free-field and in structures as recommended by the RG 1.12 Rev. 3 (2017) can help with post-earthquake engineering analysis.
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Acknowledgments
- The authors are grateful to the NRC project manager Robert Schaaf and Resident Inspectors Jared Nadel and Matt Thomas for their help in obtaining unprocessed recordings and other information necessary to interpret seismic data from the plant.
- The authors appreciate support of the TVA Watts Bar Nuclear plant staff helping to get technical information related to seismic recordings.
- We appreciate help of our coworker Jinsuo Nie.
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