ML20207D990
| ML20207D990 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 03/02/1999 |
| From: | Cloninger T HOUSTON LIGHTING & POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NOC-AE-000452, NOC-AE-452, NUDOCS 9903100113 | |
| Download: ML20207D990 (20) | |
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! Nuclear Operating Company E
South TcasPrtyctEkctrkGeneratingStatkm 20 Ax239 hhmxth Teus77483 m March 2, 1999 NOC-AE-000452 STI: 30821299 File No.: G20.01 10CFR50.12 j l
U. S. Nuclear Regulatory Commission i Attention: Document Control Desk Washington, DC 20555 South Texas Project ,
. Units 1 and 2 Docket Nos. STN 50-498, STN 50-499 Draft Request for I
An Exemption From Seismic Instrumentation Reauirements '
The STP Nuclear Operating Company (STPNOC) plans to submit a request for exemption from the requirements of 10 CFR 50.34(b)(11),10 CFR Pan 50, Appendix A, General Design Criterion (GDC) 2, and 10 CFR Part 100, Appendix A, Section VI(a)(3) to the extent that they require the maintenance of seismic instrumentation. This request will not affect the design or qualification of structures, systems or components to withstand the effects of canhquakes.
STPNOC's licensing condition for maintaining seismic instrumentation is specified in the Technical Requirements Manual. STPNOC's scismic instrumentation is described in Section 3.7.4 of the Updated Final Safety Analysis Report. Seismic instrumentation provides the capability to determine the magnitude of a seismic event and to evaluate the response of those features important to safety. This capability is required to permit comparison of the measured response to that used in the design basis of the facility to determine whether the plant can continue to be operated safely and to permit such timely action as may be appropriate pursuant to Appendix A of 10CFR100.
STPNOC is located in an area of very low seismic activity. The STPNOC seismic design margins are documented in the Updated Final Safety Analysis Report. STPNOC's Probabilistic Risk Assessment estirnates that the risk of core damage due to a seismic event at the South Texas Project is not significant (i.e.,7.4 x 10'8 per year), and the seismic instrumentation does not contribute to this low core damage frequency. The Operating Basis Earthquake (OBE) for STPNOC is 0.05g. The annual probability of exceedance of this acceleration level at the South Texas Project site is on the order of 5 x 10E-5. Therefore, the probability of an earthquake affecting the safe operation of the South Texas Project units is extremely low.
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l NUC-AE-000452 File No.: G20.01 10CFR50.12 Page 2 The seismic instrumentation at the South Texas Project con;ists of ec,mpment that is no langer being manufactured, and that is becoming increasingly dif ficult and expensive to :
maintain. As a result, STPNOC is considering replacement of this equipment. It has been determined, however, that the safety benefits of the seismic instrumentation are negligible at the .
South Texas Project and do not justify the cost of replacing this equipment. Since the probability of an earthquake affecting the safe operation of the South Texas Project units is extremely low, we believe alternative measures can be used to detect a seismic event and obtain information necessary for assessing whether the STPNOC units can continue to be operated safely. The proposed alternative measures result in no increase in risk and meet the principles of risk-informed regulation discussed in NRC Regulatory Guide 1.174, dated July 1,1998, "An Approach to Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Current Licensing Basis."
The draft request for exemption from seismic instrumentation requirements is attached for the Nuclear Regulatory Commission's consideration. STPNOC would like an onportunity to meet with the NRC staff in the near future to discuss this issue.
If you should have any questions concerning this matter, please contact Mr. Ken Taplett at (361) 972 8416 or me at (361) 972-8787.
/ -
. H. C nin -
Vice P esir nt Engi e ng and echnical Services KJT/
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Attachment:
Draft Request for Exemption from the Seismic Instrumentation Requirements of 10 CFR 50.34(b)(11),10 CFR Part 50, Appendix A, General Design c riterion 2 and 10 CFR Part 100, Appendix A, Section VI(a)(3) l 1
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NOC-AE-000452 l File No.: G20.01 Page 3 Ellis W. Merschoff Jon C. Wood Regional Administrator, Region IV Matthews & Branscomb j U. S. Nudear Regulatory Commission One alamo Center ;
611 Ryan Plaza Drive, Suite 400 106 S. St. Mary's Street, Suite 700 '
Arlington, TX 76011-8064 San Antonio,TX /8205-3692 Thomas W. Alexion Institute ofNuclear Power Project Manager, Mail Code 13H3 Operations - Records Center U. S. V" dear Regulatory Commission 700 Galleria Parkway Wasa.::gton, DC 20555-0001 Atlanta, GA 30339-5957 Comelius F. O'Keefe Richard A. Ratliff Sr. Resident Inspector Bureau of Radiation Control c/o U. S. Nuclear Regulatory Commission Texas Department of Health P. O. Box 910 1100 V/est 49th Street Bay City, TX 77404-0910 Austin, TX 78756-3189 J. R. Newman, Esquire D. G. Tees /R. L. Balcom Morgan, Lewis & Bockius Houston Lighting & Power Co.
1800 M. Street, N.W. P. O. Box 1700 Washington, DC 20036-5869 Houston,TX 77251 M. T. Hardt/W. C. Gunst Central Power and Light Company City Public Service ATTN: G. E. Vaug's'C.
t A. Johnson P. O. Box 1771 P. O. Box 289, Mail Code: N5012 San Antonio,TX 78296 Wadsworth,TX 77483 A. Ramirez/C. M. Canady U. S. Nuclear Regulatory Commission City of Austin Attention: Document Control Desk Electric Utility Department Washington, D.C. 20555-0001 721 Barton Springs Road Austin, TX 78704 l
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l Attachment NOC-AE-000452 Page 1 of 8 Draft Request for Exemption )
from the Seismic Instrumentation Requirements of I 10 CFR 50.34(b)(11),10 CFR Part 50, Appendix A, General Design Criterion 2 and 10 CFR Part 100, Appendix A, Section IV(a)(3) i Pursuant to 10 CFR 50.12, STP Nuclear Operating Company (STPNOC) requests an I exemption from the requirements of 10 CFR 50.34(b)(11),10 CFR Part 50, Appendix A, General i Design Criterion (GDC) 2, and 10 CFR Part 100, Appendix A, Section VI(a)(3) to the extent that they require the maintenance of scismic instrumentation. This request does not affect the design or qualificatidn of structures, systems or components to withstand the effects of eanhquakes.
Thus, except with respect to the seismic instrumentation, the South Texas Project will continue to meet applicable design criteria and the requirements of 10 CFR 50.34(b)(11), GDC 2 of 10 CFR Pan 50, Ann. A md 10 CFR Part 100, App. A. I A. Bac h ad Appendix A to 10 CFR Pan 100 provides Seismic and Geologic Siting Criteria for Nuclear Power Plants (Siting Criteria). These criteria, in addition to addressing site suitability, also include in Section VI provisions on application of seismic and geologic information to engineering design.Section VI(a)(3) describes required seismic instrumentation. In accordance with the provisions of Section VI(a)(3) the South Texas Project has installed seismic l instrumentation to measure the response of plant features to an earthquake, and has incorporated l into its Technical Requirements Manual, requirements for surveillance testing of this
' i instrumentation. I I
10 CFR Part 100 does not explicitly require licensees to comply with the Siting Criteria.
Rather, the purpose of Pan 100 is to establish approval requirements for proposed sites for stationary power and testing reactors. 10 CFR 100.1, "The Siting Criteria", however, cite GDC 2, and state that the purpose of the Criteria is to set forth the considerations which guide the Commission in its evaluation of both site ;-uitability and the suitability of the plant design basis.
Thus, the Siting Criteria provide considerations for assessing compliance with GDC 2. In addition,10 CFR 50.34(b)(11) states that the provisions of Appendix A to Part 100 apply to holders of operating licenses for nuclear power plants.
The seismic instrumentatLn the South Texas Project installed in accordance with 10 CFR Part 100, App. A, f VI.(a)(3) consists of equipment that is no longer being manufactured, and it is becoming increasingly difficult and expensive to maintain. As a result, STPNOC is considering replacement of this equipment. Appendix A to this attachment provides a basic Wp\nrmrc-wk\ mise.7N00452
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NOC-AE-000452 i I
Page 2 of 8 I
description of STPNOC's Seismic Monitoring System. We have determined, however, that the safety benefits of the seismic instrumentation are negligible at the South Texas Project and do notjustify the cost of replacing this equipment. The estimated cost of upgrading the seismic instrumentation at the South Texas Project is $105,000. Recent maintenance history indicates l the cost of maintaining the current seismic instrumentation at the South Texas Project to be on l the order of $25,000 annually. A recent failure of the system cost the station approximately j
$30,000. The cost of maintaining the upgraded equipment is expected to be less than the current l maintenance costs yet a substantial cost nevertheless since surveillance testing and equipment performance monitoring would still be required as a minimum.
Instead of continuing to maintain the instrumentation, STPNOC proposes to inspect and test plant equipment upon confirmation of an earthquake. The South Texas Project would follow the guidelines of NRC Regulatory Guide 1.167, dated March 1,1997, which endorses Electric Power Research Institute " Guidelines for Nuclear Plant Response to an Earthquake", EPRI NP- l 6695, December 1989. This inspection and testing will assure that appropriate actions are taken l in the unlikely event that an earthquake affects the South Texas Project. This approach is !
acceptable because (1) the South Texas Project is located in an area of very low seismic activity, (2) the plant incorporates substantial seismic design margins,(3) the Probabilistic Risk j Assessment for the South Texas Project (STP PRA) estimates that the risk of core damage due to a seismic event at the South Texas Project is not significant (i.e.,7.4 x 10' per year), and (4) the j seismic instrumentation does not contribute to this low core damage frequency. Since the l seismic instrumentation does not help prevent or mitigate accidents,it is not considered in this l estimate of core damage frequency.
Instead of continuing to maintain the seismic instrumentation, STPNOC requests an ,
exemption from the requirements for such instrumentation. No change or exemption is being proposed regarding the seismic equipment quaufication requirements in the South Tex.
Project's current licensing basis for structures, systems and components. Thus, the South Texas l
Project will continue to meet applicable design criteria, the requirements of GDC 2 of 10 CFR l Part 50, and the requirements in Part 100. including Appendix A, except for the requirement of seismic instrumentation.
B. Request for Exemption l
10 CFR 50.12(a) provides that the Commission may, upon application by any interested j person or upon its own initiative, grant exemptions from the requirements of the regulations in i Part 50, which are authorized by hw, will not present an 'mdue risk to the public health a id j safety, and are consistent with the common defense Nd se urity. Additionally, Section 50.12(a) l Wp\nnnrc-w k\ misc.99%fXJ0452 l
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Attachment NOC-AE-000452 Page 3 of 8 states that the Commission will not consider granting an exemption unless special circumstances are present.
- 1. The proposed exemption is authorized by law.
The Atomic Energy Act authorizes the Commission to grar;. exemptions from the q
Commission's regulations. No law requires that nuclear power plcas have seismic '
instrumentation installed at the facility. Consequently, the proposed exemption is authorized by ,
law. '
- 2. The proposed exemption will not present an undue risk to public health and :
' safety. ;
The proposed exemption will not present an undue risk to the public health and safety since it involves monitoring equipment only which has no accident prevention or mitigation safe shutdown function. In addition, there are adequate alternative means to determine if a significant earthquake has occurred so that the facility may be inspected to determine if it can be safely operated. The bases for these conclusions are discussed further below. Appendix B provides information regarding the probability of an earthquake being felt at the South Texas Project site.
- 3. The proposed exemption is consistent with the common defense and security.
The proposed exemption does not pertain to the security or safeguards plans. Therefore it does not have any affect on the common defense and security.
- 4. Special circumstances are present.
h Section 50.12 identifies various bases on which the Commission may find that special i circumstances are present, including (a) application of the regulation in the particular ;
circumstances would not serve the underlying purpose of the rule or is not necessary to achieve I the underlying purpose of the rule (@50.12(a)(ii)); and (b) there is present any other material j circumstance not considered when tne regulation wn adopted for which it would be in the j public interest to grant an exemption (Q50.12(a) (vi)). If criterion vi is relied on exclusively for j
=atisfying the requirement for special circumstances, the exemption may not be granted until the l Executive Director for Operations has consulted with the Commission. Both of these {
circumstances are present in this case, as shown below. !
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Attachment NOC-AE-000452 Page 4 of 8 a) application of the reculation in the particular circumstances is not necessary to achieve the underlying purpose of the rule.
l The underlying purpose of the requirement that nuclear power plants install seismic l instrumentation is for use in deciding whether the plant can continue to be operated safely following an earthquake and to provide inforTnation that will permit such timely action as may be appropriate. Information regarding the occurrence of an eanhquake can be determined using alternative methods. Appendix C provides a discussion of the alternative method that STPNOC plans to use in response to an eanhquake in lieu of maintaining seismic instrumentation. In addition, at the South Texas Project there is very little risk that an earthquake will affect the facility and th'e facility design incorporates seismic design margins. As a result, the risk that the South Texas Project will be adversely affected by an earthquake is extremely small. If an earthquake does occur, the purpose of Section VI(a)(3) can be achieved by inspection and testing of plant equipment.
Geotechnical analyses a i research shows that the South Texas Project site is in a region that has very low scismic activity such that the occurrence of a Safe Shutdown Earthquake (SSE) is not considered credible. The design basis ground accelerations are based on conservative extrapolations of seismic activity in regions relatively distant from the South Texas Project that have greater seismic activity than the South Texas Project site. The extrapolation resulted in ground accelerations lower than the 10 CFR Part 100 minimum criterion. Since South Texas Project's design load ground accelerations are based upon the minimal criterion in Part 100 rather than the lower values extrapclated for the site, the South Texas Project has substantial i seismic design margins. l Growth " faults" are found to exist in the site vicinity, but extensive study revealed no evidence of significant seismic activity on growth " faults" at any time. The very low level of seismic activity is in keeping with the conclusion that the growth " faults" are non-tectonic in that they are confined to Cenozoic formations. Cenozoic formations are not sufficiently consolidated and indurated to enable them to store strain energy to the point where sudden rupture can produce an energy release in the form of an earthquake. This negligible seismicity is also consistent with the gravitational mechanism of growth " faults" involving gradual, almost plastic, creep-like slippage along the growth " fault" surfaces as they develop. (UFS AR, Chapter 2.5.1)
The Senth Texas Project design basis for vibratory ground inotion is defined by a SSE having a maximum horizontal acceleration of 0.10 g. Engineering studies conservatively indicate a maximum horizontal acceleration of 0.07 g at the South Texas Project site. The 0.10 g acceleration for a SSE is adopted because this is the minimum ground accelention in Appendix A of 10 CFR Pan 100. Surface faulting ir not a factor in the South Texas Project design.
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Reference:
South Texas Project UFSAR, Section 2.5.1)
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Attachment NOC-AE-000452 Page 5 of 8 The South Texas Project design uses an Operating Basis Earthquake (OBE) with a maximum horizontal acceleration of 0.05g (i.e., % the SSE). However, only three Intensity V Modified Mercalli earthquakes have been reported within 200 miles of the site in the last 100 years and no earthquakes have been reported within 80 miles. Thus, the maximum earthquake reasonably expected within the site vicinity, based on historical records and geology,is an earthquake ofIntensity IV to V modified Mercalli. Acceleration / intensity correlations show the maximum acceleration of an Intensity V modified Mercalli eanhquake to be 0.035 g which would represent a reasonable OBE. However, to comply with Appendix A to 10 CFR Part 100, a minimum acceleration of 0.05 g (% of the SSE) was adopted for the OBE. (
Reference:
South Texas Project UFSAR, Section 2.5.1) Two eanhquakes of Intensity VI have been recorded in the South - Central Texas region, one in 1993 and the other in 1997 (see Appendix B). These earthquakes occurred at a distance of greater than 100 miles from the South Texas Project site and their felt area was relatively small and did not include the South Texas PrcJect site.
The seismic design provisions of Appendix A to Part 100 incorporate conservatism to assure that nuclear power plants are designed with ample safety margins. The safety margins for seismic design of the South Texas Project exist because of the difference between the calculated acceleration due to an SSE and the minimum acceleration of 0.10 g required by Appendix A to Pan 100. Based on these factors, and detailed analysis of the plant design, the STP PRA estimated that the core damage frequency associated with seismic events at STP is 7.4 x 104 per year, which is an insignificant contributor to risk. Since the seismic instmmentation does not help prevent or mitigate accidents,it is not considered in this estimate of a.,m damage frequency.
In lieu of maintaining seismic instrumentation, STPNOC proposes that in the event of a eanhquake as described in Apper: dix C, both units will be shutdown and maintained in a shutdown condition. Sufficient inspection and testing would be conducted to provide an adequate basis for a decision that the units can continue to be operated safely. Appendix C provides proposed earthquake response criteria for determining the need for plant shutdown for use in lieu of maintaining seismic instrumentation. For plant restart, the South Texas Project would follow the guidelines NRC Regulatory Guide 1.167, dated March 1,1997, which endorses Electric Power Research Institute " Guidelines for Nuclear Plant Response to an Earthquake",
EPRI NP-6695, December 1989. This approach is preferable to the requirement for seismic instrumentation because studies of seismic activity in the region of the facility indicate that there is a very low probability of an earthquake that could affect plant operation. In addition, the design criteria for the South Texas Project have resulted in a large margin of safety. Both of these factors are explained further below.
Seismic instrumentation is not needed to determine whether an earthquake has occurred.
An earthquake of sufficient magnitude to have a potential to affect plant equipment will be sensed by plant personnel and the general population in the plant vicinity. Information about the Wp\nl\nrc-wk\ misc-9WXA152
r-Attachment NOC-AE-000452 Page 6 of 8 l earthquake magnitude and epicenter will be readily available from the National Earthquake i
Resource Center. This information will be assessed edng the proposed criteria in Appendix C to i determine the need to shutdown the South Texas Project units in response to a seismic event, l Seismic instrumentation can be utilized to determine whether an earthquake has occurred l
that might be of sufficient magnitude to damage plant equipment. In the event instrumentation indicates that such an earthquake has occurred, the plant would be required to conduct inspections and tests to determine whether the facility has been damaged and whether it can be operated safely. In the absence of such instrumentation, the South Texas Project would make conservative assumptions about the potential effect of an earthquake felt at the site It is estremely unlikely that the South Texas Project will ever need to conduct such
- inspections. The site, as described above, is in an area that,in terms of historical earthquakes,"is l one of the least active areas of the United States." (Ref
- Safety Evaluation Repon related to the constmetion of the South Texas Project, Units 1 and 2, NUREG-75/075, August 1975) Thus,it is highly unlikely that the South Texas Project will ever experience an earthquake that could potentially impact the plant.
For these reasons, administrative controls, derived from Electric Power Research Institute !
" Guidelines for Nuclear Plant Response to an Earthquake", EPRI NP-6695, December 1989, that require appropriate inspection and testing in the event that an eanhquake is felt at the South Texas Project provide a high level of assurance of safety. Consequently,in the circumstances of the South Texas Project, seismic instrumentation is not necessary to achieve the underlying i purpose of Section VI.(a)(3) of 10 CFR Pan 100, App. A.
b) material circumstances, not considered when the regulation was adopted, for which it would be in the public interest to erant an exemption.
l Since adoption of the requirement for seismic instrumentation, there have been advaaces l in the state of the art for probabilistic risk assessment. Based on these advances, the seismic characteristics of the South Texas Project site and the design of the facility, the STP PRA l
estimates the risk of core damage at the South Texas Project due to a seismic event at 7.4 x 10'8 per year or less than 1% of the total core damage frequency.
l This risk level is insignificant. Elimination of seismic instrumentation will not alter this low risk, since seismic instmmentation is not used to prevent or mitigate accidents, and since the l
PRA does not take into consideration any actions based on the information provided by the seismic instrumentation. This new circumstance of having a quantified risk, provides a sound basis for reassessing the benefit of the seismic instrumentation.
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1 Attachment NOC-AE-000452 Page 7 of 8 C. NRC Guidance on Use of Probabilistic Risk Assessment j Utilization of this risk insight to assess the need for a continued requirement to maintain seismic instrumentation is consistent with the NRC guidance in Regulatory Guide 1.174 for risk- 3 informed decision making.
- 1. Current regulations The proposed change to the current licensing basis is an exemption from the requirement to maintain se'ismic instrumentation on site. Upon approval, the requirements pertaining to t seismic instrumentation would be deleted from the Technical Requirements Manual and the l Updated Final Safety Ana'ysis Report would be revised. No other changes to the South Texas Project current licensing basis are being proposed. The seismic qualification requirements for structures, systems and components will continue to be as provided in the current licensing basis for the Somh Texas Project.
- 2. Defense-in depth maintained l 1
The only components and system affected by this change is the seismic instrumentation.
This change will result eliminating the technical requirements to maintain or to perform surveillances on this instrumentation. No physical change or operational practice change is proposed for any structures, systems or components required for maintaining the principal safety barriers, for achieving and maintaining safe shutdown, or for mitigating the consequences of accidents. System redundancy, independence and diversity features are maintained. As described above,if an earthquake does occur, th; South Texas Project will use the criteria proposed in Appendix C to determine the need to conduct inspections and tests of structures, systems and equipment prior to continued operation. Thus the defense-in-depth that is part of South Texas Project's design philosophy described in the South Texas Project Updated Final Safety Analysis Report (UFSAR)is unaffected by this change. j l
- 3. Sufficient safety margins are maintained This change does not affect any accident parameters discussed in the South Texas Project
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UFSAR. The seismic equipment qualification requirements described in the current licensing basis for South Texas Project systems, structures and components will continue to be met.
Equipment functionality, reliability and availability will be una;fected by this change.
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! Attachment NOC-AE-000452 l Page 8 of 8
- 4. Proposed changes in risk, both individual and cumulative, are small and do not cause the NRC Safety Goals to be exceeded As discussed above, the seismic instrumentation would only be utilized after an l earthquake has occuned. It does not prevent or mitigate any accident, and the STP PRA does not I
take credit for the seismic instrumentation. Therefore, its elimination would not cause a change i l in risk, or cause the NRC Safety Gc s to be exceeded.
l 5. Need for Monitoring of Impact of Proposed Change As described above, the South Texas Project is in an area of very low seismic activity, and it is unlikely that an earthquake will affect the plant site. If an earthquake were to occur, in addition to conducting necessary inspection and testing to assess the safety of continued j operation, the South Texas Project would reassess the need for seismic instrumentation. A i i
monitoring program is not needed to provide timely feedback and corrective action related to this design change.
D. Conclusion i
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l The seismic instrumentation requirements of Part 50 and Appendix A to Part 100 do not j serve a significant safety function at the South Texas Project. The plant is in an area that has very low seismic activity and the plant design contains additional safety margins beyond the minimum required by NRC regulations. In the unlikely event that an earthquake affects the l South Texas Project, the =afety of continued operation would be determined on the basis of inspections and tests of plant structures, systems and components. ConscQuently, the South Texas Project should be exempted from the seismic instrumentation requirements. 3 l
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f Appendices:
A: Basic Description of STPNOC's Seismic Monitoring System B: Probability of Earthquakes at the South Texas Project C: Proposed Earthquake Response Plan Wp\nrarc-wk\mhc-99VXX452 I
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l Appendix A to Attachment
.- NOC-AE-000452 Page 1 of 2 Basic Description of STPNOC's Seismic Monitoring System Types ofInstruments
. T_riaxial Time-History Accelerometers (TTHA): These instruments monitor acceleration in three directions (North-South axis, East-West axis, and Vertical axis) and transmit the acceleration signals to the seismic monitoring panel (CP013) in the Unit 1 Control Room.
There are seven TTHAs in the Seismic Monitoring System, six providing signals to be i recorded at CP013 in Unit 1 Control Room (XT-0001 through XT-0006) and one used solely as a system trigger (XT-002A).
- Triaxial Peak Accelerographs (TPA): These are local instruments, requiring no power to operate, which record the peak acceleration in three directions. The acceleration is recorded on plates which must be retrieved and processed with developing solution in order to l determine the peak accelerations. There are three TPAs in the Seismic Monitoring System. !
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- Self Contained Triaxial Accelerograph (SCTAj: The SCTA monitors acceleration in three i directions and records the signals on its own recording unit when triggered. The SCTA uses l its own trigger which is set to 0.02g in each direction. The SCTA is independent of the l remainder of the Seismic Monitoring System, allowing acceleration data to be obtained in the event of a failure af the Triaxial Time-History Accelerometer trigger and recording system.
Recording and Analysis Capability j
- DCA-300 Master and Slave Units: The DCA-300 consists of six tape recorders to record signals from six of the Triaxial Time-History Accelerometers. The DCA Master is the upper recording unit and consists of three tape recorders. The DCA Slave is the lower unit and consists of three recorders.
- RSA-50 Response Spectrum Analyzer: Based on tne calculations of the response spectroms for the OBE and SSE events, each accelerometer is assigned a set of limits that are stored in the internal memory of the RSA-50. The limits consist of OBE and SSE acceleration values for frequencies between 0 and 32 Hz. Each accelerometer is assigned a unique set of limits.
The RS A-50 actuates alarms on CP013 if the inner (OBE) or outer (SSE) acceleration limits a..ceeded at any frequency. The RSA-50 can perform on-line analysis of accelerometer data or perform the analysis of data recorded on tape using the SMR-102 playback unit.
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Appendix A to Attachment
. NOC-AE-000452 Page 2 of 2
- SMR-102 playback unit: Used to plot accelerometer data stored on tape. A tape from one of the DCA-300 Master or Slave tape recorders is played back in the SMR-102, producing output in the form of a four channel plot. One channel on the plot is time and the other channels are the three acceleration signals from the three accelerometer directions (Nonh-South, East-West, and Vertical). The data from playback of a tape on the SMR-102 is also sent to the RS A-50 for analysis.
Automatic System Response The output cf TTHA XT-002A is continuously monitored for acceleration greater than 0.02g in any direction. At 0.02g the Seismic Trigger causes the following:
. The six tape recorders start and record the signals from the TTHAs.
. The RSA-50 starts and performs on-line analysis of two TTHAs (XT-0001 and XT-0006).
- The " SEISMIC SYSTEM TRIGGER" alarm light is illuminated and the audible alarm is actuated on CP013.
. A Seismic Switch (independent of the Seismic Trigger) monitors the output of TTHA XT-002A and actuates the "OBE LIMIT SEISMIC SWITCH" alarm light if 0.05g in either horizontal direction or 0.033g in the vertical direction is exceeded, or the "SSE LIMIT SEISMIC SWITCH" alarm light if 0.lg in any horizontal direction or 0.066g in the vertical
. The RSA-50 actuates the "OBE LIMIT RSA-50" and/or "SSE LIMIT RSA-50" if the applicable acceleration limits stored in internal memory are exceeded at any frequency during on-line analysis.
- The system continues to operate automatically until the signals from trigger accelerometer XT-002A are less than 0.02g for 5 seconds.
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i Appendix B to Attachment )
! NOC-AE-000452 Page 1 of 2 l
Probability of Earthquakes at the South Texas Project l
l l The South Texas Project design basis for vibratory ground motion is defined by a SSE j having a maximum horizontal acceleration of 0.10 g. The South Texas Project design uses an Operating Basis Earthquake (OBE) with a maximum horizontal acceleration of 0.05g (i.e., % the SSE). However, only three Intensity V Modified Mercalli earthquakes have been reponed within l l 200 miles of the site in the last 100 years and no eanhquakes have been reported within 80 miles.
l Thus, the maximum eanhquake reasonably expected within the site vicinity, based on historical records and geology,is an earthquake ofIntensity IV to V modified Mercalli. I Acceleration / intensity correlations show the maximum acceleration of an Intensity V modified Mercalli earthquake to be 0.035 g which would represent a reasonable OBE. However, to comply with Appendix A to 10 CFR Pan 100, a minimum acceleration of 0.05 g (% of the SSE) was adopted for the OBE. Two Intensity VI Modified Mercalli earthquakes have occurred recently at distance of greater than 160 kilometers. However, the Intensity VI was only felt within a few kilometers of the epicenter.
The following represents the approximate annual probability of exceedance of peak ground acceleration at the South Texas Project site. (Ref: Figure 3.4.4-2 of the South Texas Project Individual Plant Evaluation, Revision 0, August 21,1992)
Peak Ground Acceleration Annual Probability of Exceedance
- 0. l g 2 x 10E-5 0.05g 5 x 10E-5 0.0lg 6 x 10E-4 The Nuclear Regulatory Commission published NUREG-1488, " Revised Livermore Seismic Hazard Estimates for Sixty-Nine Nuclear Power Plant Sites East of the Rocky Mountains",in April 1994. This repon is an update of a probabilistic scismic hazard analysis presented in NUREG/CR-5250. Probabilistic evaluations are intended to capture the uncenainties in estimating seismic hazard in the Eastern United States. For the South Texas Project, NUREG-1488 reported that the peak ground acceleration levels of 0.05g and 0.15g had a mean peak ground acceleration hazard estimate of 1.628 x 10E-4 to 3.256 x 10E-5 respectively.
While these probabilities are on the order of half a magnitude higher than the South Texas Project Individual Plant Evaluation, they are still very small.
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Appendix B to Attachment NOC-AE-000452 Page 2 of 2 On August 31,1998, the University of Texas Institute for Geophysics with support provided by the Federal Emergency Management Agency published a document titled
" Earthquake Hazard Identification and Risk Assessment forTexas; A Preliminary Analysis of Counties at Risk". This repor: assessed four regions in Texas where historical earthquake activity (1847 - 1998) indicates there is an earthquake hazard. The closest region to the South Texas Project site assessed is the South - Central Texas region. The closest boundary of this j region is approximately 80 to 100 miles from the South Texas Project site. The Regional Hazard l Assessment from the report is provided as Tab A to this appendix. The assessment concludes the following regarding the hazard level of this region:
" Earthquakes with epicenters within this region are rare and small; perhaps 10-20 earthquakes with magnitudes between 3 and 4.5 will occur each century. A significant l fraction of these earthquakes are induced by human activities, notably petroleum I production. These events pose little or no risk unless their foci are extremely close to i poorly built or very sensitive structures." l Tab A: " Earthquake Hazard Identification and Risk Assessment for Texas; A Prelim.aary Analysis of Counties at Risk", by Cliff Frohlich, Scott Davis, and Jay Pulliam, University of Texas Institute for Geophysics,31 August 1998,2eelonal Hazard Assessment: South Centra! Texas Earthquakes (pp 12S-1 through 12S-4).
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- f. Tab A to Appendix B l NOC-AE-00n/ 52 1
I Regional Hazard Assessment:
l South-Central Texas Earthauakes (Largest City - San Antonio) 1 Counties included (19): Atascosa, Bastrop, Bexar, Brazos, Burleson, Caldwell, '
Comal, Gaudelupe, Grimes, Hayes, Jim Wells, Karnes, Lavaca, Lee, Live Oak, Travis, Waller, Washington, Wilson Hazard Level: Earthquakes with epicenters within this region are rare and small; perhaps 10-20 earthquakes with magnitudes between 3 and 4.5 will occur each l century. A significant fraction of these earthquakes are induced by human l activities, notably petroleum production. These events pose little or no risk unless their foci are extremely close to pooriy built or very sensitive structures.
)
1 l Justification: Many small earthquakes, some of natural origin and others induced I by man's activities, have occurred in these counties.
Significant Historic or Induced Earthauakes Affecting This Region l
Small earthquakes with epicenters in this region occasionally do occur- 1 l some of natural origin and some apparently induced by petroleum production (see Attachment 12S-A). These include:
- A magnitude 3.9 earthquake centered in Travis County south of Austin which ,
occurred on 9 October 1902. This earthquake is clearly of natural origin.
- A magnitude 4.2 earihquake near Fashing in Atascosa County on 9 April 1993. I This earthquake is ons of several in this region which may have been induced by petroleum production.
- A magnitude 3.8 earthquake near Alice in Jim Wells County which occurred on
! 24 March 1997. This earthquake may have been induced by petroleum l production.
l Mitigation Strategy i
- Residents of this region should understand that small natural earthquakes
! occacicnally do occur in this region. However, the most numerous earthquakes are small events associated with petroleum production in some; but not all fields. :
These small earthquakes pose a hazard only in the immediate vicinity of their l
! epicenter; the occurrence of significantly larger earthquakes is unlikely. ]
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' Tab A to Appendix B NOC-AE-000652 1 .
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I Shaded regions indicate areas of intensity IV and above for earthquakes of 1887 (Bastrop County),1902 (Travis County),1910 (Walter County),1993 (Atascosa County), and 1997 (Jim Wells County).
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l Appendix C to Attachment ;
NOC-AE-000452 Page 1 of 2 s
Proposed Earthquake Response Plan 1
This attachment outlines the proposed earthquake respcase plan at the South Texas Project in lieu of maintaining seismic instrumentation. An earthquake of sufficient magnitude to l have a potential to affect plant equipment (i.e. peak acceleration level greater than 0.035g) will be sensed by plaat personnel and the general population in the plant vicinity. Information about the earthquake magnitude and epicenter will be readily available from the National Earthquake Resource Center.
Acceleration / intensity correlations show the maximum acceleration of an Intensity V modified Mercalli earthquake to be 0.035 g which would represent a reasonable OBE at the South Texas Project. (
Reference:
South Texas Project UFSAR, Section 2.5.1) Upon sensing ground motion ,
or upon receiving information that a seismic event had occurred within 200 miles of the South l Texas Project, the station will contact the National Earthquake Information Center (NEIC) in Golden, Colore.do. For events of a magnitude of 3 or greater, the NEIC has the capability to provide a time of occurrence for the event, a location of epicenter by latitude and longitude, a magnitude of the event and the estimated depth below the surface of the epicenter. The location of the epicenter for earthquakes of a magnitude 3 or greater is considered to be very accurate and for sensed activity less than a magnitude 3 is still within an accuracy of a few kilometers.
Based upon the information from NEIC, both South Texas Project units would be shutdown if l one of the following applies:
1
. The earthquake resulted in Modified Mercalli Intensity (MMI) VI or greater within 5 km of the plant, or
+ The earthquake was of magnitude 5.0 or greater and occurred within 100 km of i the plant l l
The shutdown criteria above is consistent with Appendix A of NRC Regulatory Guide 1.166, " Pre-Earthquake Planning and immediate Nuclear Power Plant Operator Post-Earthquake Actions", March 1,1991 for plants for which no free-field or foundation-level instrumental data l are available with the fole ing exceptions.
- U/hereas NRC Regulatory Guide 1.166 discusses an additional application for ,
shutdown for an earthquake felt within the plant and of a magnitude 6.0 or greater, the l criteria " felt within the plant" was considered too open to interpretation. It is believed l
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! Appendix C to Attachment l
NOC-AE-000452 Page 2 of 2 i
the second criteria above for shutdown adequately covers an earthquake of magnitude 6.0 in the South-Central Texas Region. i i
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- A distance from the South Texas Project site of 100 kilometers for a magnitude of 5.0 or greater is considered reasonably conservative based upon the regional geology of the Texas Gulf Coast area. Recent studies of earthquakes in the Texas Gulf Coast area indicate that Intensities of IV to VI have been felt but that the felt area has been small. These recorded earthquakes have occurred greater than 160 kilometers from the South Texas Project site. See Table 1 below.
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TABLE 1.
Compilation of Mat nitudes m,t,, Felt Areas, and Maximum Intensities (MMI) for Earthquakes Along the Texas Gulf Coast Date I.ocation me t, Felt Area Af (km ) ' MMl(max) Source !
6 Nov1981 Jacksonvil" 3.2 800 V Davis etal.(1989) 23Jul1983 Fashing' 3.4 200 V Pennington etal.(1986) 3 Mar 1984 Pleasanton' 3.9 1300 V Pennington etal.(1986) 18 Sep1985 ValleyView 3.3 700 V Dav!s etal.(1989) 20Jul1991 Falls City' 3.6 75-250 IV Olson and Frohlich(1992) l 9 Apr1993 Fashing' 4.3 4000 , VI l Davis elat.(1995) 24 Mar 1997 Alice 3.8 900 V-VI this study
- a. The Falls City and Pleasanton epkenters all lie within about 40 km of of Fashing; I
Ref: Seismological Research Letters, Volume 69, Number 2, March / April 1998 " Felt Reports from the Alice, Texas, Earthquake of 24 March,1997", by Andria Bilich, Stephen Clark, Brian Creighton, Department of Geological Sciencies, University of Texas at Austin; and Cliff Frolich, Institute for Geophysics, University of Texas at Austin)
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