New structural engineering approach

to enhance resilience against BDBE

Naoto KASAHARA and Takuya SATO

The University of Tokyo

4th IAEA Power Plant Life Management (PLiM) conference – 23-27 Oct. 2017

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Contents

�Introduction

�Structural engineering approach to BDBE

�Fracture control and base technologies

�Failure modes under BDBE

�Development of failure mode maps

�Conclusions and future plan

2

Introduction

As the lessons learned from the Fukushima nuclear power plant accident, it is

recognized very important that evaluation and countermeasure for Beyond

Design Basis Events (BDBE) including Design Extension Conditions (DEC, ref.

IAEA, SSR-2/1).

A philosophy and criteria for BDBE are strongly related to the forth layer of

Defense in Depth (DiD) and are clearly different from ones for Design Basis

Events(DBE), which are categorized as lower layers than the third one of DiD.

Technical approaches are also different between BDBE and DBE according to

their philosophies and criteria.

Many structural research issues may exist to realize the best estimate

approaches for BDBE.

3

4

Large accident

Rare

（Meteorite collision, etc.)

Medium accident

Infrequent

（Excessive

earthquake, etc.）

Small accident

Frequent

（Components

malfunction, etc.）

Efforts to

prevent

Ignore

（Safety myth）

Evacuate

Actual accident frequency Binary approach Continuous approach

Different approach between small accident and Large accident

DB

EB

DB

E

Efforts to

prevent

Efforts to

mitigate

Operational states Accident conditions

Normal operation

Anticipated

operational

occurrence

Design basis

accidents

(*Conservative

evaluation)

Design extension

conditions

(**Best

estimation)

Plant status (considered in design)

IAEA SSR-2/1 Safety of Nuclear Power Plants: Design Specific Safety Requirements

DiD 1st〜3rd Layer DiD ４th Layer

Design Basis Enents Beyond Design Basis Events

DiD 5th

What is suitable categorization of events and countermeasures ?

Actual risk is continuous. However detailed categorization has difficulties to be applied.

Event categories

Defense layers

5

DBE BDBE

Fundamental Safety

PrincipalTo protect people and environment from harmful effects of ionizing

radiation.

Basic

philosophy

DiD From 1st layer to 3rd layer From 4th layer to 5th layer

Basic

purposes

To prevent any failure under

assumed conditions.

To avoid catastrophic failure which

leads to safety function loss under

beyond design conditions.

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Structural engineering approach to BDBE

Various failure modes

Large

deformation

Collapse/

Break

Example: Design and countermeasure of piping

Prevention of all

failure modes

Cracks

Prevention of

catastrophic failure

DBE BDBE

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き裂の貫通によるリーク

（先に生じる）

破断（ブレイク）

き裂の貫通によるリーク

（先に生じる）

延性破壊（ブレイク）

LBB of piping

In the case of fracture accidents, large scale and catastrophic failures can be avoided

by energy release due to prior small scale failure.

Prior leak BreakPrior leak Rupture

LBB of vessels

8

Fracture control and base technologies

Example of process plants

Cone roof tank

Reference ：Takuya Sato, Failure modes and stress of pressure componens, Nikkann kogyo(2013)

9

sidewall

Bottom plate

roof roof

rafter

rafter

Example of racing cars

Monococqe body to protect cockpit

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(1) Clarification of failure modes

Failure modes of BDBE, which are different from DBE

(2) Identification of dominant parameters of failure for best estimation

Design criteria sometimes adopt conservative parameters

(Ex. Peak Ground Acceleration for seismic design)

(3) Relative strength evaluation among failure modes and locations

It can identify order of locations with failure modes . Absolute

strength is not necessary.

Base technologies of fracture control

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[Experimental observation]

Fatigue failure by cyclic strain

[Conventional assumption]

Collapse and brake by the

maximum acceleration

[Conventional assumption]

Ductile fracture of PV and CV by high

temperature and pressure

Failure modes after core damage

[Experimental observation]

Local failure of CV by high

temperature and pressure

[Fukushima accident analysis]

Buckling and Ductile failure by

excessive outer pressure

Failure modes at excessive earthquake

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Failure modes under BDBE

D u c t i l e f r a c t u r e

E l a s t i c P l a s t i c

b u c k l i n g

C o l l a p s e

L a r g e p l a s t i c

d e f o r m a t i o n

F a t i g u e f a i l u r e

Postulated failure modes under DBE

Exc

essi

ve

def

orm

atio

n

Lo

cal b

ou

nd

ary

failu

re

B r i t t l e f r a c t u r e

C r e e p r u p t u r e

D u c t i l e f r a c t u r e

L o c a l f a i l u r e

E l a s t i c P l a s t i c

b u c k l i n g

C r e e p b u c k l i n g

C o l l a p s e

L o c a l d u c t i l e

f r a c t u r e

Very low cycle fatigue

Possible failure modes under BDBE

DBE:

Conservative evaluation against all failure modes

BDBE：

Best estimation for each failure mode

Tran

siti

on

fro

m d

efo

rmat

ion

to

bo

un

dar

y fa

ilure

Tran

siti

on

acco

rdin

g t

o lo

ad

con

dit

ion

s

Gro

ss b

ou

nd

ary

failu

re

13

Temperature and internal

pressure are controlled

Reactor vessel model with the

simulation material(Pb)

Failure at 50℃ 0.57MPa 4.2h Failure at 50℃ 0.57MPa 3.8h

Ductile fracture at the plane

portion

Failure modes are different between two vessels under same loadings.

Failure of vessels under high pressure

Local failure at the structural

discontinuity

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Development of failure mode maps

Strain limit equation（ASME*）

αsl : Material constant m2 : Material constant εLm : Strain limit under multiaxial state

εLu : Strain limit under uniaxial state Tr : Triaxial factor（Hydro static stress／Mises stress）

1/3 ＞1/3

Uniaxial state Multiaxial state

εLuεLm

StrainεLuεLm

σuσ1 for Tr1

Stress－strain curve

σy

A

B

σ1

Tr1

Hydrostatic stressσH1

Uniaxial state

Tr = 1/3

Fracture surface

Hydro static

stress limit

B

Ductile fracture

Local failure

*ASME Section VIII-Division2 Criteria and Commentary

A

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Proposal of failure mode map for high pressure vesselRelative strength evaluation between ductile fracture and local failure

with dominant parameters

AccelerationAcceleration

Ratchet Deformation Collapse

Failure modes under excessive seismic load

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Nakamura, Izumi, and Naoto Kasahara. 2016. “Trial Model Tests With Simulation Material to Obtain Failure Modes of Pipes Under

Excessive Seismic Loads.” In Volume 8: Seismic Engineering, V008T08A019. ASME.

Ratcheting

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• Ratcheting is the cycling accumulation of plastic deformation due

to combination of primary and secondary load

2

1

Progressive deformation which

may lead to failure

Dead load no

progressive

deformation

Seismic ground

acceleration no

progressive

deformation

1 2

1 2

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The dynamic responses of amplification factor have some relation with the

frequency effect on ratchet diagram

Proposed ratchet diagram

0

4

8

12

0 0.5 1 1.5 2 2.5 3

20gal

200gal

400gal

600gal

800gal

1000gal

1500gal

Dyn

am

ic d

isp

./st

ati

c d

isp

.

Amplification factor

graph

Identification of dominant parameters of failure for best

estimation

Proposal of failure mode map for excessive seismic load

Md Abdullah Al BARI, Sakemi RYOTA, Takuya SATO and Naoto KASAHARA, “Numerical study on piping failure

modes under seismic loading - ratcheting and collapse”, Japan Society of Mainenology, 13th Annual conference (2016)

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Relative strength evaluation between ratcheting and collapse with

dominant parameters

[Controlled failure mode]

Ratcheting and Fatigue damage

with plastic cycles

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[The worst failure mode]

Ductile fracture of large parts

of vessels

Countermeasure against high

temperature and pressure

[Controlled failure mode]

Local failure at limited area of

structural discontinuity portions

[Resilient counter measure]

Limitation of a location and a leak

rate by identification of failure

portion and mode

Proposals of fracture control system for nuclear plants

Countermeasure against

excessive seismic loads

[The worst failure mode]

Collapse or break of hole section

of pipe

[Resilient counter measure]

Maintain cooling function

Early recovery by replacing deformed pipes

Conclusions

� As structural engineering approach to BDBE, application of

fracture control concept was proposed.

� To realize fracture control, next base technologies were

studied. � Failure modes under BDBE were clarified

� Dominant parameters for best estimation were identified

� Failure mode maps were proposed to evaluate relative strength

among different failure modes.

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