Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

CONCEPT FOR DEVELOPING SEISMIC-TSUNAMIPSA METHODOLOGY CONSIDERINGCOMBINATION OF SEISMIC AND TSUNAMI EVENTSAT MULTI-UNITS
Katsumi EBISAWA1, Hiroyuki KAMEDA2 and Mitsumasa HIRANO3
1

Associate Vice-President, Japan Nuclear Energy Safety Organization,
Tokyo, Japan, ebisawa-katsumi@jnes.go.jp
2
Professor Emeritus, Kyoto University,
Tokyo, Japan, kameda-hiroyuki@jnes.go.jp
3
Professor, Tokyo City University,
Tokyo, Japan, hirano-mitsumasa@jnes.go.jp

ABSTRACT: The Tohoku Earthquake occurred and caused a huge tsunami. The Fukushima
Dai-ichi NPP were attacked by the tsunami and core damage occurred. In risk evaluation
practice for seismic and tsunami events, no consideration have been taken on dependency ofground motion effects and tsunami effects. The concept and important issues for developing
seismic-tsunami PSA methodology considering the combination of seismic and tsunami
events at multi-units are described.

Key Words: Fukushima Dai-ich NPP accident, Multi-units, Seismic and Tsunami Events,
Seismic-Tsunami PSA, Common cause failure, Correlation

1. INTRODUCTION
The Tohoku earthquake (Mw9.0) occurred at 14:46 on March 11, 2011, and caused a huge tsunami.
strong seismic motion was observed at the Fukushima Dai-ichi NPP (F1-NPP) and reactors were shut
down after control rods had inserted. While the reactors were shut down normally, they were attacked
by tsunami about 46 minutes after the earthquake occurred. Various equipment of the water intake
system and emergency diesel generators were flooded. External power supply was also lost due todamage by strong ground motions and tsunami. In this situation, station blackout took place. As aconsequence, functions of reactor cooling system was lost, core damage (CD) occurred andradioactive materials were released to the off-site area (Japanese government 2011).
In risk evaluation practice for seismic and tsunami events, no consideration have been taken on
dependency of seismic ground motion effects and/or tsunami effects, and seismic PSA and tsunami
PSA have been developed independently for efficiency. Both of each evaluation methods are

1575

composed of the four items (Hirano 2008), (Sugino 2008):(1) setting of accident scenarios,(2) hazard evaluation,(3) fragility evaluation and(4) accident sequence evaluation.
Based on the lessons learned from the accident of F1-NPPs, development of "seismic-tsunami PSA(S-T PSA)" considering combination of seismic motion and tsunami effects is urgently required.
This paper describes the overview of F1-NPP accident and the outlines of seismic PSA andtsunami PSA. Further the paper represents the concept and important issues for developing S-TPSA
methodology considering the combination of seismic and tsunami events at multi-units.

2. OVERVIEW OF FUKUSHIMA NPP ACCIDENT AND LESSONS LEARNED
2.1 Overview of F1-NPP accident at Tohoku earthquake/tsunami
The F1-NPP station is a multi-units site with 6 BWRs. The NPPs detected strong seismic motions of
the 3.11 Tohoku Earthquake and control rods were inserted automatically. However seawater supply
system (seawater pump, switchboard (switchgear) / signal-processing board (motor control)) forseawater pump and also emergency power supply system (diesel generator (DG), DG switchboard(switchgear) / signal-processing board (motor control)) in the support line were flooded due to thetsunami as shown in Fig.1, and as a result all of seawater supply systems / emergency power supply
systems were lost in function simultaneously. Function of interconnected power supply between
neighboring Units (Units 1 and 2, Units 3 and 4, and Units 5 and 6) was lost. Breakers and emergency
transformer in the switchyard were damaged, and a transmission line tower out of the site wascollapsed, and offsite power supply was lost. These results led to loss of all AC power (station
blackout) (Japanese government 2011).
On the other hand rector core isolation cooling system (RCIC), which is steam-driven coolingsystem, in the front line (FL) was operated during a certain time but it stopped after a short time
operation. Cooling systems in FL other than RCIC (and high pressure coolant injection system (HPCI)
or isolation condenser (IC)) were not operated due to loss of AC power. Failure of reactor core cooling
resulted in core damage (core melt) in about 5 or 6 hours. Temperature and pressure in the primarycontainment vessel rose up, and radioactive materials were released through seals to the on-site andthe off-site. The land in wide area was contaminated by the radioactive materials (Japanese
government 2011). Information relevant to the accidents has not always been provided to the public ina proper manner.

Fig.1 Situation of tsunami and disaster at Fukushima Daiichi nuclear power plant
2.2 Lessons learned from accident
The issues of seismic engineering based on lessons learned from F1-NPP accident are as follows:① Occurrence of gigantic earthquake and tsunami, and combination of seismic hazard andtsunami hazard② Risk evaluation of multi-units1576

③ Combined emergency of both natural disaster and nuclear accident④ Core damage during short time based on functional failure of support systems(seawatersupply, power supply and signal systems)⑤ Common cause failure of multi structures and components⑥ Dependency among neighboring units

3. OUTLINE OF SEISMIC PSA AND ITS USABILITY
3.1 Procedure of the Seismic PSA
The procedure of seismic PSA consists of 5 steps as shown in Fig. 2 (Hirano 2008).
- Step 1: Collection of information related to earthquake and setting of accident scenario- Step 2: seismic hazard evaluation- Step 3: Fragility evaluation- Step 4: Accident sequence evaluation- Step 5: Documentation
3.2 Collection of information related to earthquake and setting of accident scenario
The collection of information related to earthquake and setting of accident scenario is shown in Fig. 2.
At first, relevant information should be gathered. Then, conduct “plant walk-down” based on thegathered information. Finally, set various accident scenario based on gathering relevant informationand the result of “plant walk-down”.
Level 1 PSA
Collection / Analysis of plant information and general analysis of accident scenarios:

Setting of Accident
Scenarios(ASs)Collection / Analysis of plant information

Implement of plant walk-down

List of R/B and Equipment

Fragility curveComp.A
Comp.BPGA at bedrock
PGA at bedrock

Core damage prob. curve
Ha

za
rd

CDF

cu
rv
e

Documentation

Seismic hazard curve

Accident sequence evaluation
Core damage freq.
perGalFragility evaluation
Failure prob.

Seismic hazard evaluation

Exceedance
freq.(/yr)

Clarification of ASs,
Analysis of Initiating Event

PGA at bedrock

Level 2 PSA (FP release rate)

Level 3 PSA (Individual risk)

Fig.2 Procedure of seismic PSA
3.3 Seismic hazard evaluation
The seismic hazard evaluation is shown in Fig.3. In the evaluation of seismic hazard, set “specified
source model” for active faults and set “zone source model” for diffuse seismicity. Strong motions areevaluated for these source models by using propagation model (fault model method and attenuation
model). Then, like this seismic hazard curve should be derived with the uncertainty.
The uncertainties do exist in the source models and propagation models of seismic motion describedabove. The uncertainties are consisted of “aleatory uncertainty” that is accompanied withphenomenology and “epistemic uncertainty” that is accompanied with lacks of recognition and1577

information. Evaluation of uncertainty is conducted by using logic tree (LT) with this epistemic
uncertainty as a target as shown in Fig. 4.
Historical earthquake data
Seismic zone Non-specified source
Seismic source
×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

×

Seismic motion

Historicalearthquake

Active fault data
Seismic
source・Magnitude（M ）・Frequency ・Position

Site

Groundmotionpropagation

Active fault Specified source

・Fault Model・Attenuation Model

Exceedance freq.

××Earthquake occurrence

Seismic hazard curve＋σMedian－σ

Seismic motion at Bedrock

Fig.3 Procedure of seismic hazard evaluation

Fig.4 Outline of logic trees

3.4 Fragility evaluation
3.4.1 Targeted Structures and components
In the evaluation method of fragility, targeted buildings, structures and components are those classified
as “class S” which is the most important class on seismic safety as shown in Fig. 5. Those classified as“class B” and “class C” which would affect safety are targeted as well. These buildings, structures andcomponents include reactor building, outside structures and inside components. Inside components are
categorized as static and dynamic components. Each category are consisted of components ofmechanical system and those of electrical system.
On the Seismic PSA, structures and components targeted to evaluate are about 200 and categorized
as about 50 areas on fault Trees.
Median of Prob. Dist. of Realistic Response
Input:200Gal

Response(Liner): 400 Gal

Input:400Gal-- Structures
Structures && Components
Components
includingincluding into
into fault
fault trees
trees⇒⇒ about
about 200
200-- Structures
Structures && Components
Components
forfor capacity
capacity evaluation
evaluation⇒⇒ about
about 50
50

Fig.5 Structures and components to be evaluatedResponse against
ground motion at
bedrock

Response(liner): 800 Gal

Input:600Gal

Response(Non-liner):
1000GalProb. Dist. of Capacity7～8G以下)
Capacity7～8G
over: constantAcceleration, StressLiner region

Seismic motion at bedrock（200､400,600Gal）

Failure Probability-- S
S classclass components
components-- PartPart of
of B
B and
and C
C class
classcomponents
components(important(important safety-related
safety-related
components)
components)(1) Buildings- Reactor building- Control building, etc.
(2) Outside: Structures and components- Seawater pit, seawater duct,transformers, etc.
(3) Inside: Components- Static components- Mechanical comp.: tanks, heatexchangers, reactor pressure
vessel, containment vessel, etc.
- Electrical comp.: condenser, etc.
- Active components- Mechanical comp.: pumps, etc.
- Electrical comp.: electrical panel,electrical valves, etc.
- Piping (pipe, supports)(4) Others- Power transmit network, Land slope

Non-liner region

1.0
○
Pf(a)
0

○

Fragility Curve○ : Evaluation point○
200 400 600
Ground motion at bedrock (Gal)Fig.6 Procedure of fragility evaluation

3.4.2 Fragility evaluation method
The fragility evaluation method by using components installed in a reactor building is shown in Fig. 6.
In the response evaluation of this component, the seismic motion at the bedrock is set up. Then, theresponse analysis of soil, buildings and components against this seismic motion is conducted andevaluation would be made with including dispersion. On the fragility evaluation, seismic motion levelat the bedrock would be increased like 200 Gal, 400 Gal, 600 Gal, and derive distribution of each
responses. Fragility curves are obtained as the conditional probability that the realistic response ofcomponent exceeded its capacity as shown in figure. The capacity of targeted components is evaluatedby including dispersion.
On the fragility evaluation, the evaluation of functional failure limit of structures and components
on the capacity evaluation is extremely important.

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3.5 Accident sequences evaluation
In case of evaluating accident sequences, the accident sequences are represented by using event tree
based on various accident scenario considered in chapter 4. Then develop fault trees that consists each
event trees as shown in Fig. 7.
Core damage probabilities (CDPs) are evaluated by using event trees, fault trees and fragilities ofcomponents as shown in the Fig. 8. The CDF is estimated by multiplying the seismic hazard curve per
Gal by CDP curve, and it is corresponded to the area of this semicircular shape that is calculated byintegration for seismic motion acceleration (Gal).
In general, the range of seismic motion that might contribute to CDF is from 1 to 2.5 times ofdesign basis seismic motion S2 in Japan. The range of Exceedance frequency on seismic hazard isfrom 10-3 to 10-5 (1/year).
Multiple Protections

BWR Reactor Pressure Vessel
Loss ofAC

DG

StandbyGenerator

PLR

○○○

Shut
Shutdown
down(Sample)Black
Out
Success

○○

RPV

PCV
Core Cooling
Core
Heat
HPCS
LPCSRemoval Damage○

ＳＴｒ

Containment
Containment

Standby
SubcriticalPower

Failure

CRD
RHR

pump
Hx LPCS

Isolation
Valve

HPCS

RCIC

Condensate
Storage
Tank

Failure prob.

Emergency
Emergency
Reactor
ReactorCooling
Cooling
SystemSystemLPCS Functional FailureＡＮＤ
B Functional Failure

C Functional Failure
A Functional Failure

Human errorSupport Failure

Pump Damage(1/Gal/y)

CDF
(1/y)

Max. acceleration α(Gal)

Range
Rangeofofcontributing
contributingof
ofseismic
seismicmotions
motionsto
toCDF:
CDF:--Seismic
Seismicmotion
motionat
atbedrock:
bedrock:SS22～～1200
1200GalGal-5-3--Exceedance
Exceedancefrequency
frequency ::10
10-3(1/y)(1/y)(1/y)～～10
10-5(1/y)

Piping

Cooling
CoolingCore damage prob.
CDP (α)Hazard curve
per GalMax. acceleration α(Gal)ＯＲ
pipe・valve Failure Pump(A) Failure

Hazard curve
per Gal(1/Gal/y)Max. acceleration α(Gal)

Example of safety system

seismographseismograph

Hazard curve
H(α) (1/y)

Frequency (1/y)

Initiating
Event

Frequency (1/y)

Defense-in-depth – Multiple protections

Electric System Damage

Example of components in safety system

Fig.7 Accident sequence evaluation

Fig.8 Procedure of core damage
frequency (CDF) and dominantly
contributing ranges of seismic motion

Table 1 Comparisn table of accident scenario of seismic PSA and Kashiwazaki NPP
Examples for wide range of Accidental Scenario described in
the Standard for Procedure of Seismic PSA 2007 (AESJ)① NoneA. Effect on Core Damage by Main Shock
A1.Direct Effect
A2.Indirect Effect①SSCs having Safety Function（Damage of As, A Facilities）

A2-1 Other than SSCs having Safety Function （Secondary
effects may affect As, A facilities directly to lead Core Damage）

・Indoor Facilities①Effect on CV and PV by falling and dropping of Overhead
Traveling Crane②Effect on As, A facilities by damaging of B, C facilities③Effect on adjacent Building by Turbine Missile
・Outdoor Facilities④Effect on buildings by falling of Stack⑤Effect on Buildings and Surrounding Facilities by Possible
Collapses of the Surrounding Grounds above the foundation⑥External Power Source Loss by Damages of Power Line Towersof Power Grids⑦Function Loss of Cooling by stopping Industrial Water Supply

A2-2 Effect on Human Error⑧Incorrect operation introduced by highly stressed operators andworkers during and after earthquakes⑨Trouble of backup operation by blackouts for Damage of
Insulator of Transformer⑩Trouble of transportation on site by soil liquefaction and damageof retaining wall
⑪Effect to operators by Damage of Secondary members of Sealing⑫Mistakes on Plans and Designs, Selecting Materials,
Manufacturing and Building to Completion of Facilities

B. Effect on Core Damage by Aftershocks

Damage Status of NPP by NCOE

⑬Evaluate Increase of
CDF by Aftershocks

1579① (Unit 6）Damage of connection part ofwheel axis of Overhead TravelingCenter② （Unit 4）Crack on connection part ofCondenser, Sea water Leakage（Unit 7）Degradation of water-tightnessof watertight door on RCIC and RHR
System（Unit 3）Come off of Blow Out Panel of
R/B③ None④ Incline of Stack, Shift of Duct
⑤ Partial land slide of slope in the eastside of Switchyard⑥ None⑦ None. Floods in the basement of
Building by Breakage of Piping for Fire
Extinction⑧（Unit 7） Iodine detected at main exhaust
tower in periodic measurement⑨（Unit 3） Fire broke out at Auxiliary
Transformer 3B（Unit 3,4,6）Oil leakage of Auxiliary
Transformer 3B⑩ Caving in yard
⑪ Dropping of secondary members of
Sealing⑫ None⑬ None3.6 Usability of seismic PSA
The Niigata-ken Chuetsu-Oki earthquake (NCO) was occurred in the vicinity of Kashiwazaki-Kariwa
NPP (KK-NPP) in July 2007. The KK-NPP is consisted with 7 reactor units. Seismometers had been
installed in and outside of buildings and many seismic motions are recorded.
The table 1 shows the comparison between the accident scenario of seismic PSA and accidental
event occurred at the NCO earthquake. The accident scenario of seismic PSA was described in theseismic level -1 PSA Implementation standards that was compiled by Atomic Energy Society of Japanin March 2007 that is one and a half year before the occurrence of the NCO earthquake. The contents
described in the seismic PSA implementation standard identified well about the points occurred at the
NCO earthquake. It clearly showed further usability of seismic PSA method.

4. OUTLINE OF TSUNAMI PSA AND ITS USABILITY
4.1 Procedure of the Seismic PSA
The procedure of tsunami PSA consists of 4 steps as shown in Fig.9 (Sugino 2008).
-Step 1: Collection of information related to earthquake and setting of accident scenario- Step 2: Tsunami hazard evaluation- Step 3: Fragility evaluation- Step 4: Accident sequence evaluation
4.2 Collection of information related to earthquake and setting of accident scenario
The collection of information related to earthquake and setting of accident scenario as shown in Fig.10,
at first, relevant information should be gathered. Then, conduct “plant walk-down” based on thegathered information. Finally, set various accident scenario based on gathering relevant informationand the result of “plant walk-down” as shown in Fig. 10.
The accident scenario should be identified with dividing cases for tsunami run-up and backwash.
Following accident scenario are assumed in case of tsunami run-up as shown in this figure.
-Function loss of sea water pump
- Functional loss of power supply system- Functional loss of DG oil tank
- Function loss of sea water intake pit- Function loss of sea water facilities by debris flow attack- Functional loss of sea water intake function by deposition of sea sand- Turn over of sea water pump cause by tsunami backwash
4.3 Seismic hazard evaluation
The seismic hazard evaluation is shown in Fig. 11. The tsunami hazard evaluation is defined by thetsunami wave height at shoreline and its exceedance frequency.
In case of evaluating tsunami hazard, tsunami source models for both near-field active faults andfar-field earthquakes such as Chile earthquake should be set. Then, ocean floor topographic model
should be set by dividing it for far-field and near-field. In addition, onshore topographic models are set
to evaluate onshore run-ups.
The LTs are developed by considering uncertainties of tsunami source models, ocean floor
topographic models and onshore topographic models. Conduct tsunami simulation for paths of eachLTs, and obtain tsunami hazard curve.
The tsunami hazard curve is needed to obtain in cases of both tsunami run-ups and tsunami
backwash as shown in Fig.12.

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Reactor Building
Reactor Building Closed
Cooling Water System(1) Collection of information related to Tsunami
and Setting of accident scenario

Pressure Vessel
Residual Heat Removal
SystemContainment Vessel

Tsunami Source Modeling
Tsunami Analysis

ExceedanceFrequency (1/yr)

Tsunami hazard curve(3) Tsunami Fragility Evaluation

Tsunami run-up analysis(2) Tsunami Hazard Evaluation

Tsunami inundation evaluation

Heat
Exchanger

RCW Sea Water
Pump

Capacity (height) of SSCs

Residual Heat Removal System

Function Failure Probability evaluation

Reactor Building Closed Cooling Water System(4) Accident Sequence Evaluation

R/B

Accident sequence modeling( Event Tree, Fault Tree )Tsunami height (m)pumpIntake tower
Reference sea levelOccurrence Frequency of
accident

transformer

Intake pit

tankIntake tunnel
0

Fig.9 Procedure of tsunami PSA

Fig.10 Collection of information related to earthquakes
and setting of accident scenario
Earthquake scale +location of fault(segmentation)

Length of Slip length
fault

Strikeangle

Path(weight
factor)

Ma+Fault 1
Ma+Fault 2
Ma+Fault 3
Fault 2

Ma+Fault 4
Fault 4Ma+(Fault 1+2)
Ma+(Fault 1+2)

Fault 1

Logic tree for uncertainty
factor of tsunami source

Fault 3

Uncertainty innumerical
analysis
Consider periodicity
of earthquake
occurrence

Calculation of earthquake occurrence
probability

Earthquake occurrence
model

Tsunami analysis against each path(Max. Tsunami height and occurrence
probability at evaluation point)

Next
Year ofevaluation earthquake
Elapsed years after
previous earthquake ΔT= one year – service period
Average interval of occurrence

Exceedance probability

Example of tsunami
earthquake source faults

Tsunami hazard
Tsunami hazard curve(Total of all paths)
Probability of Tsunami
height against eachpath x in logic tree
Max.Tsunami height at shoreline (m)Fig.11 Procedure of tsunami hazard evaluation
Result of analysis

Max. tsunami heightat shoreline● : analysis

Inundation height at 600m

Function lfailure
height assumed
TP+4.5m

R/B

600m from shoreline

Max. Tsunami height (m)Functional Failure
Probability

Total
number of Realistic
TsunamiR/B assumed at the 600 m fromshoreline

Tsunami fragility curve

Run-up analysis

Inundation heightFragility curve

Tsunami fragility curve
Function failure height

Functional Failure
Probability

Tsunami simulation

Function failure height

Inundation height (m)Realistic Tsunami set
Real：amplitude, period

Inundation height (m)Facility set

Max. Tsunami height at shoreline (m) Max. Tsunami height at shoreline (m)Max. Tsunami height (m)Fig.13 Example of tsunami fragility evaluation

Fig.12 Procedure of tsunami fragility evaluation
4.4 Tsunami fragility evaluation
4.4.1 Procedure of tsunami fragility evaluation
Fig. 12 shows the evaluation procedure of tsunami fragilities. On the fragility evaluation of tsunami,
fragility curves are obtained as the conditional probability that tsunami wave height exceeded theinstallation height of targeted structures and components as shown in figure.

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Tsunami wave heights are evaluated by conducting analysis of onshore run-ups to the area thattargeted buildings, structures and components are installed. Evaluate uncertainties and dispersion of
the wave heights of tsunami wave run-ups.
4.4.2 Damage part, damage mode and its physical quantities for evaluation of tsunami fragilities
It is important to identify the modes on function failure against tsunami in case of targeting facilities
outside and inside of buildings. It is also important to identify their functional failure limits andintensities of tsunami. The intensities of tsunami are consisted of tsunami wave height, tsunami wave
force, scour etc.
The failure parts, failure modes and physical quantities representing functional failure limits aredifferent for each targeted structures and components. For quantitative evaluation of tsunami margins,attention should be paid that physical quantities representing margins would differ as the failure partsand failure modes to be evaluated are also different based on targeted structures and components.
4.4.3 Examples of Fragility Evaluation
Fig. 13 shows an example of fragility evaluation. The results on analysis of onshore run-ups oftsunami are shown for targeted point in the Fig. 13. This figure shows the result of fragility evaluationin case of assuming the installation point of reactor building is 600 m distant from the shoreline, andfunction failure is occurred when tsunami run-ups reaches to the building.
4.5 Accident sequence evaluation
In case of evaluating accident sequences, the accident sequences are represented by using event tree
based on various accident scenarios considered in chapter 4. The evaluation method as shown in Fig.
14 is the same as that of Seismic PSA.
The accident scenarios of Tsunami are developed in cases of both tsunami run-ups and tsunami
backwash. Fig. 15 shows the event tree based on an accident scenario for tsunami run-ups. The
accident scenario starts from the function failure of facilities outside buildings and spread out tofunction failure inside buildings, and finally reaching core damage. The ET of tsunami is very simple
as shown in the figure. The ET based on an accident scenario for tsunami backwash is also developed.
The event tree in case of tsunami backwash is simpler than the case of tsunami run-ups.
4.6 Usability of tsunami PSA

Failureprobability

The Tohoku earthquake was occurred off the coast of Tohoku district in March 2011 as shown in Fig.
1. The tsunami of the Tohoku earthquake attacked the F1-NPP with the wave height that is about 2times larger than the design basis tsunami wave height.
Tsunami Fragility curve

Table 2 Comparison results between
tsunami PSA and Fukushima NPP

Evaluation of tsunami accident sequence・ Loss of offsite power ⇒ Failure of emergency DG start-up
・ Loss of sea water supply for emergency coolingMax. Tsunami height (m)[ET analysis for tsunami]
Initiating
event

Accident
sequence

S
S
F

ET1：ET of scram state・Scram shutdown due to earthguake motion
・Scram shutdown in preparation for tsunami
ET2：ET of plant operating state・Case without earthquake scram(Plant damage only by tsunami)F★ET and FT are very simple in comparison with the seismic PSA

CDF
Max. Tsunami height (m)Tsunami hazard curve
ExceedanceFrequency

Max. Tsunami height (m)Evaluation of CDF
Frequency/
probability

Failureprobability

Core damage probability

Max. Tsunami height (m)0

Fig.14 Procedure of tsunami accident sequence evaluation

1582

Accident Scenarios at Tsunami
PSA

Accidents at
Fukushima
Daiichi NPP(1) Failure of Sea Water Pump

Occurred(2) Failure of Power supply

ditto(3) Failure of emergency DG

ditto(4) Failure of Transformer at
outdoorditto(5) Inundation of reactor building

ditto(6) Inundation of turbine building

dittoTable 2 shows the comparison between the assumed accident scenario of tsunami PSA developed
by JNES and accidental event occurred at the Tohoku earthquake. The assumed accident scenario of
tsunami PSA was identified by JNES from 2006 as a part of developing tsunami PSA method.
The contents of tsunami PSA developed more than a year ago identified well about the points
occurred at the F1-NPP. It clearly showed further usability of Tsunami PSA method
Initiating
event

OutsideOffsitePower

Tsunamirun-upInside Building

Support System

StartupSeawaterTransforPump
mer

EmergencyPower

DG Oil
Tank

DG

○
○

Core State

Motordriven
pump
LPCI,CS

Steamdriven
pump
RCIC
HPCI

○

×

×

VesselCoolingCore Cooling

×

Short
Term

○
○

○

Long
Term

Recoveryof RHR○○○×

×Damage

Outside
○

○

○

○

×

×

×

○
○

○

×
×

InsideBuilding○：Intact
×：Function loss

×

×

×

×

○

○

×

×Damage○○○××Damage○○××Damage×Damage

DG：Diesel Generator, LPCI：Low Pressure Coolant Injection, CS：Core Spray,
RCIC：Reactor Core Isolation Cooling, HPCI：High Pressure Coolant
Injection, RHR:Residual Heat Removal System

Fig.15 Example of accident sequence by run-up tsunami

5. CONCEPT OF SEISMIC-TSUNAMI PSA METHODOLOGY
5.1 Concept of development
The concept and important issues for developing seismic-tsunami PSA methodology considering the
combination of seismic and tsunami events at multi-units are described..
5.2 Setting of accident scenarios
Four cases of seismic and tsunami accident scenario are shown in Fig.16;
Case 1: Seismic (no CD) and Tsunami (no CD),
Case 2: Seismic (no CD) and Tsunami (with CD),
Case 3: Seismic (with CD) and Tsunami (no CD) andCase 4: Seismic (with CD) and Tsunami (with CD).
Of the four cases shown herein, Case 1 is the safe case, while other three are cases with coredamage occurrence. While it is clear that the nuclear accident at Fukushima I NPS corresponds to Case2, general framework of seismic-tsunami PSA should incorporate all the four cases.
5.3 Seismic-tsunami hazard evaluation
The hazard for the external event at the multi-unit NPPs is evaluated. Seismic and tsunami hazard
evaluations are practiced by developing hazard curves for seismic motion and tsunami height,
respectively as shown in Fig.17. They are plotted against annual frequency of exceedance. Seismic
hazard curves and tsunami hazard curves are not independent because they are based on common
seismic events. But different nature of strong seismic motion (period range: 0.1~1sec) and tsunami risetime (period range: 10~120sec) requires careful consideration of their source characterization. Becauseof such difference in period ranges, correlated seismic motions at multi-unit locations should beconsidered, while tsunami height can be treated as more or less uniform within a single site.

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5.4 Seismic-tsunami fragility evaluation
In fragility evaluation, structures and components of multi-unit reactors are covered and correlation of
their failures should be considered. The functional failure probability (fragility: FiS−T (α, h ) ) withconditions that an arbitrary component“i” will secure its function against seismic motion α but failagainst tsunami h is represented as equation (1) and shown in Fig.18;()

FiS−T (α, h ) = FiS (α ) × FiT (h ) = 1 − FiS (α ) × FiT (h )( )Where FiS α

and FiT(1)(h ) are functional failure probability of component “i” against α and h,respectively. FiS (α ) is functional success probability of component “i” against α.
In seismic-tsunami fragility evaluation, it is important to consider the fatigue effect of structure andcomponent under seismic motions by main shock and gigantic aftershock.
5.5 Seismic-tsunami accident sequence evaluation
In accident sequence evaluation involving CD, all of the Cases 2~4 should be incorporated. The total
CDF is represented by CDF(S-T) = CDF(Case2)+CDF(Case3)+CDF(Case4). In case of considering
combined seismic and tsunami effects, CDF(S-T) will be higher than CDF(S), the cases of seismic
actions alone, because of contributions of Case 2.
Exceedance Frequency ν (1/Yr.)

Tsunami Hazard

Seismic Hazard
Seismic Motion at
Bedrock α(Gal)FiS(α)

Failure/Success
Probability (-)

Failure/Success
Probability (-)

Fig.16 Cases of accident scenarios on seismic-tsunami PSA Fig.17 Definition of hazard on
seismic-tsunami PSA

FiT(h)
Seismic Motion at Bedrock α (Gal)
Tsunami High at Shoreline H (m)FiT(h)
Seismic Motion at Bedrock α (Gal)
Tsunami High at Shoreline H (m)Fig.18 Concept of fragility evaluation on seismic-tsunami PSA

6.

CONCLUSIONSThe summarizations of this paper are as follows.
(1) Usability and application of PSA
1) PSA is a usable method to identify important accident sequence, system and components forsafety and these results are candidates to take countermeasures as AMs.
2) PSA is an effective measure to evaluate AM’s effectiveness.
(2)Improve IAEA/ISSC’s guideline on PSA of external events
1) JNES will develop and improve seismic hazard method considering gigantic aftershock.

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2) JNES will also develop and improve seismic-tsunami PSA method considering combination ofseismic and tsunami events.
3) JNES will contribute improvement of IAEA’s guideline on PSA of external events through new
EBP of IAEA/ISSC.
REFERENCES
Japanese government. (2011). “Report of Japanese government to the IAEA ministerial conference on
nuclear safety”
Hirano, M., Nakamura, T. and Ebisawa, K. (2008). “Outline of seismic PSA implementation standards
on the atomic energy society of Japan”, The 14th World Conf. on Earthquake Engineering, Oct.
12-17, Beijing, China.
Sugino, H., Iwabuchi, Y., Nishino, M., Tsutsumi, H., Sakagami, M. and Ebisawa, K. (2008).
“Development of probabilistic methodology for evaluating tsunami risk on nuclear power plants”,
The 14th World Conf. on Earthquake Engineering, Oct. 12-17, Beijing, China.
Kameda, H. (2012). “Engineering agenda on nuclear safety from the 2011 Tohoku-Pacific
Earthquake”, Proceeding of the One Year after 2011 Great East Japan Earthquake – International
Symposium on Engineering Lessons Learned from the Giant Earthquake-, Tokyo
Takamatsu, N.,Kameda, H., and Ebisawa, K. (2012). “Fragility enhancement in seismic-tsunami risk
reduction for nuclear power plants”, Proceeding of the One Year after 2011 Great East Japan
Earthquake – International Symposium on Engineering Lessons Learned from the Giant Earthquake-,Tokyo.

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