Seismic Safety Lecture

8/6/2019 Seismic Safety Lecture

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Seismic Safety ofNuclear Power Plants

Bozidar Stojadinovic, ProfessorCEE Department, UC Berkeley

[email protected]

What is Safety?• The state of being safe

• The state of beingprotected fromconsequences ofundesirable events: – Accidents – Errors

– Failures



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Nuclear Power Plants

• Complex energy conversionmachines

• Engineered: – Not naturally occurring – Made and operated by humans

• Introduce a layer of hazard notpresent before we inventedthem: – RadiaVon hazard – Explosion hazard – Other hazards (environmental,

occupaVonal…)

Safety Goal• Protect the public from empirically detectable

harm: – No more: protecVng against what is not

empirically detectable is near-impossible

– No less: causing a detectable change in the hazardenvironment is not acceptable



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Safety Goal

• Focus on radiaVon dose at exclusion areaboundary:

An#cipated opera#onaloccurrences

Design basis events

Beyond design basisevents

Hazard and Risk

Risk = P(event occurring) x (Impact of event occurrence)



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Seismic Safety of NPPs

• Earthquakes

• CharacterisVcs ofground moVons

• Effect of ground moVonon simple structures

• Measures of earthquakehazard

• Effect of earthquakeground moVon on realstructures

• Demand, damage anddecisions: performance

• Probability-based risk-informed technology –

neutral designframework

Cause of EarthquakesIn Japan, peoplebelieved that acatfish that livedunder the landcausedearthquakes everytime it wiggled.

The people in the

picture are strikingthe catfish to stop itfrom shaking theearth.



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Tectonic Plates

iew of a Geo-ScienVst• An earthquake is the result of the sudden

release of energy in the Earth's crust thatcreates seismic waves



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Earthquake Faults

• Earthquake faultsoccur at theedges of tectonicplates (wherethey slip by eachother)

• These arecomplex rock andsoil fracturephenomena

How Do Faults Slip?• An earthquake is caused by a build up of strain on

the edges of the tectonic plates.• The strain becomes so great that rocks give way and

slipping occur along the fault.



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Seismic Wave PropagaVon

• Waves reect from andrefract through crustlayers of differentdensity

h p://www.uwgb.edu/DutchS/EarthSC202Notes/quakes.htm

Wave PropagaVon Through Soil• Body wave:

– Pressure – Shear

• Surface waves: – Lowe – Rayleigh – They are slowest, but

they do most of thedamage when theyarrive at the site of thestructure



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Ground MoVon at the Site

• Depends on: – Energy released at the

source (fault)

– Path, distance the wavestravel to the site

– Local condiVons at thesite:

• Soil

• Focusing• Other structures

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Ground MoVon at the Siteof an NPP Structure

Surfacewaves



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Types of

Ground MoVon• Great variety!

Principal Types of Ground MoVon• Delineated by:

– Magnitude – Distance between the

hypocenter and the site – Local soil condiVons

• Near-Fault – Pronounced direcVvity

• Perpendicular• Parallel

• Near-Field – Strong pulse, ing – High-frequency content

• Far-Field – Longer duraVon – Moderate and low

frequency content

Samedistance



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Magnitude : Measure

of Energy Release• Many versions:

ObservaVons• Intensity of

ground shakingdecreases withincreasingdistance from theepicenter

(1994 M6.7

NorthridgeEarthquake, USGSShake Map)



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How Ground MoVons Affect

Structures?

Dynamic Equilibrium:EquaVon of MoVon

• Total displacement: – Ground displ.

– RelaVve displ.

• Forces: – InerVa

– Damping

– Structural resistance

• Equilibrium



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ElasVc Structural Response

• Characterizedby: – Natural

period(frequency)of vibraVon

– Damping:energy

dissipaVonduringvibraVon

ElasVc Response Spectrum



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ElasVc

Response Spectrum• Displacement: D

• Pseudo-velocity:

• Pseudo-acceleraVon: A

• Note on effecVve force

• Note on strain energy:

Spectra ary, too!• El Centro 1940,

different damping

• More damping: lessdeformaVon

• El Centro, differentearthquakes



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ElasVc Design Spectra

• Average of a largenumber of similarground moVonspectra

• Smoothing occursalong the way

• Note: PGA on the

T=0 axis• Note: Cap at the

maximum values

• Describes elasVcseismic response ofsimple structures

Seismic Hazard and Risk• Seismic Hazard Analysis:

Describes the potenValfor dangerousearthquake-relatednatural phenomena(such as groundshaking)

• Seismic Risk Analysis:

Assesses the probabilityof occurrence of losses(human, social,economic) associatedwith seismic hazard

Risk = P(event occurring) x (Impact of event occurrence)





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Probability of Recurrence:

Magnitude and Likelihood of Occurrence

Physical Limits on Magnitude



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Ground MoVon A enuaVon

• Reasons: – Geometric spreading of

waves

– AbsorpVon (damping) inthe rock/soil

Empirical A enuaVon RelaVons

h p://peer.berkeley.edu/products/nga_project.html



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Model-Based A enuaVon RelaVons

• Complex, regionalmodels: – Sources

– Faults – Geographic features

– Rock and soil layers

• Huge computer

resources• h p://www.scec.org/

Seismic Hazard Curve

• For a given site,provide theprobability that aground moVonintensity parameterwill be exceeded

• P(IM>im)



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Use of Seismic Hazard Curves

Seismic Risk from NPP Structures• Losses to society due to a large radiaVon

release induced by earthquake ground moVon

JNES web site



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How Ground MoVons Affect

Real Structures

How Ground MoVon Affects Structures



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Measure of Seismic Demand

Engineering Demand Parameters (EDP)Deforma#on duc#lity

• Story dri : relaVve moVonbetween top and bo om ofa story

Energy absorp#on ability

• Quality of structural systemand structural detailing

Seismic Demand Model• A relaVon between ground moVon intensity and

demand(s) on the structure• Method:

– Develop a computer model of the structure – Develop a por olio of site-specic ground moVons

scaled to reect different hazard levels – Conduct (a possibly large number of) analyses to

determine demand on the structure imposed by eachground moVon – Do a staVsVcal t



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Seismic Demand Model:

Log-log Linear• A condiVonal probability: P(EDP>edp|Im=im)

Damage to Structures• Local damage affects

global loss of stability



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Damage to Non-Structural Content

• Affects ability toconVnue using thestructure

Seismic Damage Model• A relaVon between demand on the structure

and structural and non-structural damage

• Method: – Gather data (experimental, empirical, from

manufacturers) on the types of damage, whenthey occur and how they affect the structure

– Do a staVsVcal t – Determine probabiliVes of excessive damage



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Seismic Damage Model:

Fragility Curves• CondiVonal probability P(DM>dm|EDP=edp)

Performance of the Structure• HolisVc evaluaVon of risk



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Discrete Performance Levels

Seismic Decision Model• A relaVon between structural and non-

structural damage and the performance ability(or lack there off) of a structure from thestandpoint of its intended funcVon

• Method: – Establish Decision ariables

– Establish threshold values and associate themwith acceptable recurrence intervals



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Seismic Decision Model:

Fragility Curves• CondiVonal probability: P(D >dv|DM=dm)

Risk EvaluaVon• Given a seismic hazard environment and a structure,

the probability that a performance objecVve is notachieved (D exceeds a threshold) is:

• Consider probability distribuVons of seismic hazard,

of demand, damage and decision variables due to: – Lack of knowledge (epistemic uncertainty) – Record-to-record randomness (aleatory uncertainty)



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Performance ObjecVve:

ProbabilisVc DescripVon of RiskPerformance Level Performance Recurrence

Performance ObjecVve Table

R e c u r r e n c e I n t e r v a l



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Acceptance Criteria

• A comparison of Demand and Capacity: – Failure (to saVsfy a performance objecVve)occurs

when (some staVsVcal expression of) demand islarger than (some staVsVcal expression of)capacity

D m e a n

C m e a n

threshold

DemanddistribuVon Capacity

distribuVon

Risk-Informed Design• Formulate design acceptance/rejecVon criteria

such that there is High Condence in LowProbability of Failure (HCLPF) to saVsfy aperformance objecVve

• Example: – 99% condence that the probability of collapse is

less or equal 1% in any 50-year interval

• Accounts for rst and second moments of theinterim model probability distribuVons



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Structural Engineering of NPPs

ASCE 43-05• Two acceptance criteria:

– Less than 1% probability of unacceptableperformance for the Design Basis ground moVon

– Less than 10% probability of unacceptableperformance for 150% of Design Basis groundmoVon

• Both must be saVsed: – Trying to control the shape of the fragility curve by

these two points

Safety Goal• A ain 10-6 annual probability of seismic core

damage

An#cipated opera#onaloccurrences

Design basis events

Beyond design basisevents



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Common Design Framework

• Performance-based design is, in essence,technology-neutral design: – Dene what we want to achieve, not how

• Acceptance criterion units must be selected toenable use of different technologies: – Ability to model and analyze is crucial

• Design must be risk-informed: – HCLPF to perform as desired – Basis for comparaVve evaluaVon of different

technologies

Seismic Safety ofNuclear Power Plants

• Complex energy conversionmachines

• Engineered by teamsrepresenVng all branches ofengineering

• Engineered to perform theirfuncVon in a manner that issafe even under mostextreme hazards



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Thank you!

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Seismic Safety Lecture

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