ICONE-22 July 7, 2014 Prague, Czech Republic Slide 1 SPEAKERS Dr. - - PowerPoint PPT Presentation
Forthcoming Seismic PRA for US Operating Plants Following Fukushima Event ICONE-22 July 7, 2014 Prague, Czech Republic Slide 1 SPEAKERS Dr. Sanj Malushte smalusht@bechtel.com Dr. Annie Kammerer amkammer@bechtel.com Dr.
Forthcoming Seismic PRA for US Operating Plants Following Fukushima Event
ICONE-22 July 7, 2014 Prague, Czech Republic
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SPEAKERS § Dr. Sanj Malushte smalusht@bechtel.com § Dr. Annie Kammerer amkammer@bechtel.com § Dr. Farhang Ostadan fostadan@bechtel.com
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10 Minute Break (10:50 - 11:00)
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Fukushima Near Term Task Force (NTTF) Recommendations
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NTTF Recommendations § In July 2011, NRC’s Near Term Task Force (NTTF) provided a report
(http://pbadupws.nrc.gov/docs/ML1118/ML111861807.pdf) containing 12
depth, protection, mitigation and emergency preparedness (encl./SECY 11-0093)
§ SECY 11-0124 (http://pbadupws.nrc.gov/docs/ML1124/ML11245A127.pdf)
contains NRC’s assessment of the NTTF recommendations that can and, in NRC’s judgment, should be initiated, in part or in whole, without delay
§ SECY 11-0137 (http://pbadupws.nrc.gov/docs/ML1126/ML11269A204.pdf)
contains NRC’s prioritization of the NTTF recommendations, including: recommended regulatory actions, implementation challenges, technical and regulatory bases, additional recommendations, schedule and milestones
Focus Areas of NTTF Recommendations
§ Regulatory framework (1 recommendation) § Defense-in-depth philosophy (total 10 recommendations) § Protection from natural phenomena (2 recommendations) § Mitigation for long-term station blackout (5 recommendations) § Emergency preparedness (3 recommendations) § NRC programs (1 recommendation) § Many of the 12 NTTF recommendations were further divided into
more detailed actions. In all, a total of 35 actions were identified
Broad Description of NTTF Recommendations
§ Recommendation 1: Enhance NRC framework for regulating beyond
design basis (BDB) events and severe accidents
§ Recommendation 2: Update seismic and flooding analysis to protect
plants from BDB events*
§ Recommendation 3 (Long Term Evaluation Topic): Evaluate potential
enhancements to prevent or mitigate seismically induced fires and internal floods
§ Recommendation 4: Strengthen coping mechanisms for prolonged
station blackout (SBO) events
§ Recommendation 5: Require reliable hardened vent designs in BWRs
with Mark I and Mark II containments * Items marked in red are the focus of this presentation
Broad Description of NTTF Recommendations
§ Recommendation 6: Identify insights about hydrogen control and
mitigation inside containment or in other buildings
§ Recommendation 7: Enhance spent fuel pool instrumentation and
makeup capability
§ Recommendation 8: Strengthen and integrate onsite emergency
response capabilities (EOPs, SAMGs, and EDMGs)
§ Recommendation 9: Require that facility emergency plans (EP)
address prolonged SBO and multi-unit events
Broad Description of NTTF Recommendations
§ Recommendation 10: Additional EP topics related to multi-unit events
and prolonged SBO (Protective equipment for emergency responders, decisionmaker qualifications, command and control, and Emergency Response Data System (ERDs))
§ Recommendation 11: Additional emergency preparedness issues
(offsite emergency response, EP decisionmaking, radiation monitoring, public education on radiation safety and use of potassium iodide)
§ Recommendation 12: Strengthen regulatory oversight of licensee safety
performance (ROP) by focusing more attention on defense-in-depth requirements
Prioritization of NTTF Recommendations
§ SECY 11-0137 provided detailed prioritization order for all NTTF
actions by organizing them into three tiers
§ Tier 1: Recommendations are those which the staff determined
should be started without unnecessary delay and for which sufficient resource flexibility, including availability of critical skill sets, exists
§ Tier 1 includes eight actions, the seven identified in SECY
11-0124 plus one additional item (7.1 SFP Instrumentation)
§ Tier 1 actions are: 2.1, 2.3, 4.1, 4.2, 5.1, 7.1, 8, and 9.3
§ Tier 2: The second tier consists of those recommendations which
could not be initiated in the near term due to factors that include the need for further technical assessment and alignment, dependence on Tier 1 issues, or availability of critical skill sets
§ Tier 2 recommendations do not require long-term study and can
be initiated when sufficient technical information and applicable resources become available
§ Tier 2 Actions are: 7.2, 7.3, 7.4, 7.5, 9.3
Prioritization of NTTF Recommendations
§ Tier 3: Recommendations that require further NRC study to
support a regulatory action, have an associated shorter-term action that needs to be completed to inform the longer-term action, are dependent on the availability of critical skill sets, or are dependent on the resolution of Recommendation 1
§ Once the staff has completed its evaluation of the resource
impacts of the Tier 1 and Tier 2 recommendations, it will be able to more accurately address the Tier 3 recommendations
§ Tier 3 Actions: 2.2, 3, 5.2, 6, 9.1, 9.2, 10, 11, 12.1, 12.2
Prioritization of NTTF Recommendations
Fukushima Near Term Task Force (NTTF) Recommendations
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NRC 50.54f Request for Information letter
u
Screening
u
Individual Plant Examination for External Events (IPEEE)
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Phase 2 Risk-Informed Upgrade Decisions
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EPRI and Industry Response
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NTTF Recommendations NTTF Recommendations
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R2.1: Seismic Hazard and Risk Reevaluation NRC 50.54(f) Request for Information Letter
§ In August 2010, the NRC issued a report on Generic Issue 199 (GI-199), which demonstrated that the ground motions calculated using current techniques exceeded the ground motions used in design for a significant number of US plants (the report also noted that plants are designed with significant seismic margin) [ Fukushima accident occurs ] § On August 23, 2011, the North Anna Power Station was shut down by beyond-design-basis ground motions from the Mineral VA earthquake § On December 23, 2011, the Consolidated Appropriations Act, Public Law 112-074, was Signed into law. Section 402 of the law also requires a reevaluation of licensees' design basis for external hazards, and expands the scope to include other external events.
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R2.1: Seismic Hazard and Risk Reevaluation NRC 50.54(f) Request for Information Letter
§ Issued March 12, 2012 § Addressed recommendations 2.1:seismic in Enclosure 1: seismic hazard and risk reevaluation § Requested specific deliverables and process
u Hazard evaluation information u Risk evaluation information (if necessary) u Spent fuel pool analysis
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R2.1: Seismic Hazard and Risk Reevaluation Seismic Guidance Development
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EPRI 1025286 Seismic Walkdown Guidance EPRI 1025287 Screening, Prioritization and Implementation (SPID) NRC Seismic Margin Assessment Guidance JLC-ISG-2012-04
R2.1: Seismic Hazard and Risk Reevaluation NRC 50.54(f) Request for Information Letter
§ Phase 1 includes hazard and risk reevaluation § Hazard evaluation information
u Hazard curves, ground motion response spectra (GMRS),
SSE used in design, and comparison of the GMRS and SSE
u GMRS is a probabilistically-based, risk-targeted ground
motion used in current plant design (see Regulatory Guide 1.208 and ASCE 43-05). Basis of the site-specific SSE.
u Any additional insights (such as from walkdowns) gained,
actions planned or taken, selected risk evaluation method (SMA or SPRA), as necessary
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R2.1: Seismic Hazard and Risk Reevaluation NRC 50.54(f) Request for Information Letter
§ Any additional insights (such as from walkdowns) gained, actions planned or taken, selected risk evaluation method (SMA or SPRA), as necessary
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Expedited Seismic Evaluation Process
address seismic hazard while longer-term actions are
Screening Approach R2.1: Seismic Hazard and Risk Reevaluation
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EPRI SPID 1025287 Figure 1.1
R2.1: Seismic Hazard and Risk Reevaluation Screening Approach
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Seismic hazard analysis performed, GMRS developed and compared with SSE No exceedance Only low frequency exceedance in low hazard area
frequency sensitive components Exceedance in 1 to 10 Hz range
exceptions High frequency exceedance (with or without 1 to 10 Hz exceedance)
EPRI testing program
Exceptions include: GMRS<1.3 x SSE at all points (SMA allowed) Very narrow band frequency exceedance Spectrum from demonstrated IPEEE HCLPF >GMRS and adequate quality demonstrated
Screening exceptions
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Only low frequency exceedance Only high frequency exceedance From EPRI 1025287 (SPID)
Screening exceptions
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Narrow band exceedance 1-10Hz
From EPRI 1025287 (SPID)
R2.1: Seismic Hazard and Risk Reevaluation Individual Plant Examination for External Events
§ 50.54(f) program in late 90s to evaluate beyond design basis earthquake (DBE) loads in response to maturing of the PRA methodologies and increased awareness of potential for beyond DBE
§ Procedures detailed in NUREG-1407 (1991). Results and insights detailed in NUREG-1742 (2002) § Approach was generally qualitative with emphasis on developing risk
A generic Review Level Earthquake ground motion spectrum was used § The NRC allows for R2.1 screening based on demonstrated IPEEE plant HCLPF values and a demonstration by licensees that analyses are still valid and are sufficient to meet the objectives of the March 2012 50.54(f) letter. (See the EPRI SPID for details)
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R2.1: Seismic Hazard and Risk Reevaluation Individual Plant Examination for External Events
§ Three levels of review based on plant setting: full scope, focused scope and reduced scope. For R2.1 screening, focused scope submittals needs to be augmented with site geotechnical hazard study. Reduced scope studies cannot be used for screening § Key elements of NRC review are:
u Prerequisites: Commitments made were met (as detailed in R2.3 Seismic
Walkdown report), modifications or changes credited in IPEEE analysis are in place, deficincies or weaknesses in plant SER were properly justified to assure conclusions remain valid, and a review major plant modifications to identify potential for new issues
u R2.1 submittals showing adequacy: structural models, ISRS scaling,
seismic equipment list, component screening, walkdowns, fragility evaluations, systems model and containment performance analysis
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IPEEE comparison
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From EPRI 1025287 (SPID)
HCLPF Description High confidence of low probability of failure
This is intended to be a value of ground motion at which the potential of failure is exceedingly small High confidence – typically the 95% confidence band Low probability – typically the 1% or 5% value (depending on usage) Failure is the inability to perform the intended safety function
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95%
confidence
Peak Ground Acceleration (g) Probability of Failure
R2.1: Seismic Hazard and Risk Reevaluation NRC 50.54(f) Request for Information Letter
§ Risk evaluation information requested (for SPRA)
u List of risk-significant contributors u Summary of methodologies used to estimate SCDF or LERF
u Description of process used to ensure technical adequacy,
including information on peer reviews
u Identified vulnerabilities and actions planned or taken
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Phase 2 Actions and Activities R2.1: Seismic Hazard and Risk Reevaluation
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EPRI SPID 1025287 Figure 1.1
§ Phase 2 actions/activities will be determined by the NRC based on the risk analysis results provided by the licensees
R2.1: Seismic Hazard and Risk Reevaluation Role of the Electric Power Research Institute (EPRI)
§ Co-sponsored studies to develop new seismic source characterization and ground motion characterization models with NRC, DOE, and USGS § Developed key guidance documents for walkdowns and hazard/risk reevaluation with NRC and NEI staff § Developed of new interim ground motion prediction equations (attenuation relationships) § Developed and implemented high-frequency testing program to address issues of (GMRS to SSE) exceedances >10Hz § Developed concept, guidance and technical basis for the Expedited Seismic Evaluation Process § Managed industry-wide seismic hazard reevaluation activities
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Current and Upcoming Efforts
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FLEX/ESEP Work
u
High Frequency Issues
u
Spent Fuel Pool Evaluations
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What is ESEP? § “Expedited Seismic Evaluation Process (ESEP)” augments 2.1: Seismic implementation
§ ESEP identifies the“Expedited Seismic Equipment List (ESEL)”, and determins the need for seismic evaluation and upgrade (if any) of the ESEL equipment prior to the full implementation of R2.1. § ESEL includes permanent plant equipment connected with the FLEX equipment to help deliver extended core cooling capability after a beyond-design-basis event § By focusing on seismic margin of permanent plant equipment necessary to cope with loss of AC Power and/or loss of UHS, ESEP increases confidence in coping capability, and thus helps address public/NRC safety concerns while risk reevaluation is underway § EPRI Report 3002000704 (2013)
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Typical ESEL Items for ESEP Work
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Typical ESEL Items for ESEP Work
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ESEP Activities Flowchart
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Seismic High Frequency Issue
§ SMA and SPRA activities for R2.1 focus on 1-10 Hz range § For rock NPP sites located in the CEUS, the peak of site-specific ground motion is typically in the 20-40 Hz range § Component proof testing is being implemented to verify adequacy of potentially HF-sensitive bi-state devices such as relays, contactors, switches, potentiometers and similar devices whose output signal or settings could be changed due to HF motion § A two-phase test program has been conducted to determine the seismic fragility spectra for hundreds of bi-state devices § Phase 1 of the test program developed the test protocol to be used for Phase 2 to determine the high-frequency vulnerability of a broad set of control devices used in NPPs - 11 sample devices were tested in Phase 1, and 154 more were tested in Phase 2 § EPRI Report 3002000706 provides more information
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Phase 1 High Frequency Test Program § A variety of high-frequency input motions were applied to determine the most appropriate for the Phase 2 effort (because high-frequency seismic behavior of contact type devices had not been studied before)
Ø Sine sweep motions over the range 16-64 Hz Ø Random multi-frequency (wide band) motions Ø Filtered random multi-frequency (narrow band) motions
§ Filtered multi-frequency narrow-band inputs resulted in fragility
values that were 2-3 times the fragilities obtained using the wide- band multi-frequency inputs. This suggests that the clipping factors used for low frequency fragility are valid for high-frequency fragility.
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Phase 1 Input Motions
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Wide Band Random Multi-Frequency Filtered Random Multi-Frequency Normalized Spectra for Sine Beat and Pure Sine
Phase 2 High Frequency Test Program
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The following groups of devices were tested in Phase 2 (in all 154 items):
Phase 2 High Frequency Test Program § Control Switches and Transmitters were tested to table limits (20g, 14g) without chatter or output discontinuity - conclude that these devices are not high frequency sensitive § Energized control relays were tested to table limits (20g) without chatter – conclude that energized control relays are not high frequency sensitive § The remaining components types and de-energized control relays can have chatter or functional fragility limits at varying lower levels but all appear to have capacity greater than that obtained for low frequency testing (still being verified by EPRI) – conclude that there is no unique high frequency sensitivity and that, device capacity is manufacturer and model specific
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Spent Fuel Pool (SFP) Evaluations
§ Not necessary to perform structural integrity evaluation provided that the checklist in NUREG-1738 is followed to show that SFP has a high seismic HCLPF value § Necessary to evaluate failures that could lead to uncovering of fuel: Ø Failure of a connection penetrating the SFP structure below the top of stored fuel (e.g., drain line, cooling water line, etc) Ø Failure of a connection penetrating the SFP structure above the fuel such that significant inventory can be drained off in the absence of makeup due to evaporation and boil-off Ø Extensive sloshing causing loss of significant inventory and fuel uncovering Ø Potential for siphoning inventory due to seismic failure of cooling system discharge line (or its connections) Ø Tearing of steel liner due to seismic movement of fuel assemblies Ø Failures that could lead to draining if in refueling configuration
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GMRS Development
u
Probabilistic Seismic Hazard Analysis (PSHA)
u
New CEUS Models
u
GMRS Development
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Probabilistic Seismic Hazard Assessment
Seismic Source Characterization: SSC Model Ground Motion Characterization: GMC Model Base figure from Reiter (1990) Earthquake Recurrence Source Geometry
§ PSHA approaches detailed in NRC Regulatory
Guide 1.208 and in NUREG/CR-6372 (1989) and NUREG 2117. The NUREGs for the SSHAC guidelines)
§ A new SSHAC level 3 seismic source model for
the CEUS was published jointly by the NRC (NUREG 2115), EPRI and DOE in January 2012.
§ Coupled with existing ground motion
characterization models (the most recent was published by EPRI in 2012), this formed the basis
§ Western plants are performing site-specific
SSHAC Level 3 studies. Western plants have additional time to develop and submit their GMRS.
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Probabilistic Seismic Hazard Assessment
PUBLISHED January 2012
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Uncertainty Definitions
Integration over distribution of expected parameter values
logic tree of technically defensible interpretations
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Uncertainty Definitions
Acceleration (g) Annual Prob of Exceedance
Aleatory variability principally gives the curve its shape. Epistemic uncertainty leads to uncertainty bands
85% Median 15%
§ Development of best estimate
dynamic soil properties is based on available data
§ Both epistemic and aleatory
uncertainties must be recognized
§ 60 randomized profiles are used to
capture uncertainties
§ Soil property, layer thickness and
depth to base rock variation must be considered
§ Advantage of co-located plant sites § Given the rock input motion, soil
amplification analysis is the most important step in GMRS calculation
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GMRS – Soil Amplification
§ RCTS (Resonant Column Torsional
Shear) is most commonly used in US
§ Limited test data are obtained and
nonlinear soil curves
§ Variation of soil nonlinear curves
may be obtained from published data
range of index soil properties
§ Nonlinear soil property and its
variation must be simulated
§ Use of kappa in included for deep
layers
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GMRS – Nonlinear Soil Models
§ Hard rock input motion are de-
aggregated to HF and LF spectra
§ RVT method is commonly used
to get spectral amplification avoiding time history
§ Four Soil amplification methods
(NUREG/CR 6728); mostly methods 2 and 3 are used
§ The choice between method 2
and 3 is a function of variability in the amplification functions
§ GMRS is obtained from UHS
applying the design factors
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1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 0.1 1 10 100 5% Damping Spectral Acceleration Frequency [Hz] Input Rock HF 1E-3 LF 1E-3 HF 1E-4 LF 1E-4 HF 1E-5 LF 1E-5 HF 1E-6 LF 1E-6 HF 1E-7 LF 1E-7 HF 1E-8 LF 1E-8 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.1 1 10 100 AR S Amplificat ion Frequency [Hz] BBM Low PI - Ground Surface HF 1E-3 LF 1E-3 HF 1E-4 LF 1E-4 HF 1E-5 LF 1E-5 HF 1E-6 LF 1E-6 HF 1E-7 LF 1E-7 HF 1E-8 LF 1E-8GMRS – Soil Amplification Analyses
§ SSEs for operating plants were typically based on standard shape spectra scaled to selected PGAs § GMRS is a performance based design spectra that includes site specific amplification effects § The frequency content between SSE and GMRS ground motions are quite different § Nearly 70% of the plants screened into further evaluation
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Typical Soil Site GMRS/SSE Comparison
Validation of Existing Structural/SSI Models
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As per SPID Section 6.3, most existing models are stick models and can be used if they meet the following criteria:
§ Represent overall building response in H and V directions § Significant coupling between H and V must be captured § Total mass and center of mass must be preserved § Models must capture structural frequencies up to 20 Hz to
ensure ISRS is adequate up to 10 Hz
§ Torsional eccentricity must be included
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Validation of Existing Structural/SSI Models
Validation of Existing Structural/SSI Models
§ Most existing plants used Lumped-Mass-Stick-Models (LMSMs)
for their design basis seismic analysis
§ Also, some designs were based on fixed base analysis (i.e., no
SSI) , or simple SSI analysis using equivalent soil springs and dashpots
§ Task 2.1: Seismic requires validation of existing SSI and
structural modeling approaches for ensuring development of accurate ISRS up to 10 Hz frequency
§ SPID Section 6.3 provides guidelines for verifying the accuracy of
existing structural/SSI models
§ Fixed base analysis is considered appropriate if the structure is
supported on layers with shear wave velocity in excess of 5,000 ft/sec (about 1,500 m/s)
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Break
10 Minute Break (10:50 - 11:00)
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SPRA and Fragility
u
Overview of SPRA
u
Fragility methods for systems and components
u
Fragility screening
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Overview of Seismic PRA
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Seismic Motion Parameter Conditional Prob. of Failure
i
Systems Analysis
Event trees, Fault trees, Containment Analysis
Seismic Motion Parameter Frequency of Exceedance
Pi
Fragility Development Approaches
§ Objective of a Seismic Fragility Evaluation is estimation of the
conditional probability of failure of important structures and equipment whose failure may lead to unacceptable damage to the plant (e.g., core damage).
§ Failure is defined as the inability to perform intended safety
function.
§ Plant walkdown is an important activity in conducting the
assessed.
§ Two Key methods are currently used
Ø Conservative-Deterministic-Failure-Margin (CDFM) Hybrid method Ø Separation of variables
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Separation of variables method
§ Am is the median capacity § βR is the random variability § Βu is the modeling uncertainty
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Fragility Development
EPRI 1002988 (2002), 1019200 (2009)
Separation of variables method
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Fragility Development
C
Mean Curve Development
CDFM Hybrid method
§ Can be used for first pass through SPRA. Components that are found to be
potentially important to risk must be analyzed using the separation of variables method
§ CDFM method determines HCLPF, which is combined with an assumed β
(fragility logarithmic standard deviation) to develop the fragility curve
§ β ranges from 0.3 to 0.6
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Fragility Development
EPRI NP-6041-SL, Rev. 1 (1991)
Fragility Screening Approaches
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From NRC ISG SMA Guidance JLC-ISG-2012-04 and EPRI SPID
Fragility Screening Approaches
§ The R2.1 reevaluations focus on both core damage frequency
(CDF) and large early release frequency (LERF), therefore both are considered in the component screening analysis
§ The components identified as “high capacity” SSCs should be
assigned capacities equal to the screening level and retained in the system model (can use tables in EPRI NP-6041- SL Rev.1, other recent refs.)
§ As discussed in the EPRI SPID, The screening level may be set as
either:
§ A screening level consistent with a HCLPF capacity that is 2.5 times the review level ground motion, or § A screening level equivalent to the HCLPF that leads to a frequency of failure
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Fragility Screening Approaches
§ Once the SPRA/SMA analysis has been performed, a check must
be conducted to assure that none of the following conditions exist:
Ø A “high seismic capacity” SSC (which has been assigned a conservative HCLPF equal to the screening level) is identified as a dominant contributor to HCLPF of core damage Ø A “high seismic capacity” SSC is identified as a dominant contributor to HCLPF of large early release
§ Setting the screening level well above the review ground motion
level, and the requirement to review the results, are intended to avoid the “surrogate element” issue that arose in IPEEE
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Structural Analysis
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SSI with RVT
§ Seismic demand must be
develop the fragility curves
§ Now for SSI analysis, time
history can be avoid and RVT can be used to get all responses
§ This is most ideal for both
deterministic and probabilistic SSI analysis
§ Most ideal for scenario EQs
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Advanced Structural Analysis Approaches
Incoherency Effects
§ The hard rock incoherency approach is
approved by US NRC (see NRC Interim Staff Guidance COL-DC-ISG-1 ML081400293)
§ The implementation of the model in the SSI
computer programs have also been approved
§ The new ASCE 4-2014 has long section on
use of incoherency
§ The model is most effective for stiff and rock
sites with large mat foundation
§ Per NUREG-0800 the reduction in ISRS in high
frequency is capped at 30% unless it can be justified
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Advanced Structural Analysis Approaches
Soil-Structure Interface Modeling for Extreme Loading
§ The structure may slide under
extreme loading with small amount
§ The slippage reduces the seismic
load and drift significantly
§ The slippage is likely to occur well
before the structure reaches nonlinear state
§ This is an area that could be
beneficial under appropriate conditions and needs to be pursued and developed more pro-actively
Slide 68
Advanced Structural Analysis Approaches
SPRA and Systems Modeling
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Systems modeling and Accident Sequence Identification
internal events systems model and adds enhancements:
Ø Additional earthquake-induced initiating events Ø Updated SSC fragilities that account for both seismic fragility and random failures Ø Increased likelihood of correlated and increased dependencies between components Ø Loss of offsite power is assume for all accident sequences
trees feed individual components into systems analysis
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Systems Modeling and Accident Sequence Identification
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success failure
SPRA Components
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Event tree System fault trees Hazard input Initiating event development for external events Fragility Accident Sequences
Additional Important Considerations
§ Seismic-induced failures are different from those assumed in internal PRA and must be assessed separately § Sequences must be binned by plant damage state § The containment event tree should be reviewed for consistency with seismic core damage sequence types
§ Increasingly considered in the input to the SPRA § Can be implemented as the initiating event analysis is conducted
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SPRA
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Seismic Risk Evaluation and Deaggregation of Results
§ Using the Boolean-algebra based systems model supported by
fault trees and assigned fragilities, and the hazard curve as input, the risk is evaluated by analyzing each of the accident sequences and summing the results.
§ The results can be deaggregated and reviewed such that the
contributions (importance) of individual or grouped accident sequences and individual components are assessed.
§ Sequences are calculated directly § Components are assessed through Fussell-Vesely Importance,
Risk Reduction Ratio, and Risk Increase Ratio, and Uncertainty Importance.
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Seismic Risk Evaluation and Deaggregation of Results
Ø Overall plant risk is calculated as CDF and LERF, which can be compared with overall risk tolerances. Ø Key risk contributors (SSCs) can be identified. This allows for an understanding of where risk comes from and how risk can be reduced most effectively. Ø A review can be made to assure that no single SSC dominates
depth.
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Time Line and Summary
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R2.1: Seismic Timeline
Slide 78
http://pbadupws.nrc.gov/docs/ML1308/ML13085A008.pdf
Current Status
§ All CEUS operating plants submitted a GMRS report in March 2014 § Based on the GMRS results, 18 sites screened out for further evaluation, the rest
are screened in or considered as conditional in subject to further evolution
§ The sites that are screened in or are conditionally screened in are divided into 3
risk evaluation prioritization groups (1 to 3)
§ All plants that screen in must submit a plan for expedited approach by 12/31/2014 § Deadline for seismic risk evaluation for CEUS plants:
June 30, 2017
December, 2019
December 2020
§ NRC issued their draft version of GMRS for each site in early May 2014, currently
utilities are reconciling the differences among the two sets of GMRS
§ The schedule for 3 WUS plant sites is 1-2 years behind the CEUS sites
Slide 79
Question and Answer
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