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The use of Fragility Analysis in Seismic Safety Cases for Nuclear Power

Stations

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LAYOUT

• BACKGROUND• OBJECTIVES• OUTLINE METHODOLOGY• FRAGILITY CURVE DEVELOPMENT• ALARP ANALYSIS• CONCLUSIONS

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BACKGROUND

• LTSR and PSR completed to deterministic principles

• Regulator driven requirement to confirm that the risk posed is ALARP

• Not a Seismic PSA

EQE InternationalOBJECTIVES

• To confirm that the current level of qualification renders the risk of unacceptable seismically induced damage as ALARP

• To understand the relative contributions of different items of plant, equipment and structures to the risk profile

• To identify areas of plant, the modification of which would decrease the annual failure frequency to the lowest reasonably practicable level

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OUTLINE METHODOLOGY

1 Identify claimed safety functions and associated equipment/structures

2 Identify deterministic margins, and the methods of assessment used. Normalise margins. Characterise Variance in Capacity. Calculate “median” margins

3 Convolute the hazard and fragility curves and screen out items with an annual failurefrequency below a defined cut off

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OUTLINE METHODOLOGY cont

4 Identify modifications to increase the margin

5 Associate a bounding fault categorisation to a loss of safety function. Convert the fault category to an associated environmental cost as defined in the Safety Review Guidebook

6 Calculate Cost Benefit Ratio (CBR)

7 Review results, and provide information to the Client to assist in the decision making process

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FRAGILITY CURVE DEVELOPMENT

• Identify Assessment Types

1 Code Based eg BS 5950, 8110, 5628, 806, ACI 349

2 Other Codes eg R6, SQUG

3 Comparison with available experimental data

4 Engineering Judgement

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FRAGILITY CURVE DEVELOPMENT

• Use EPRI and other research work previously undertaken to derive fragility curve parameters

• Development of the application of standard curves for other assessment types eg R6

Illustrative Family of Fragility Curves

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0 1

Peak Ground Acceleration (g)

Fa

ilu

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rob

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ilit

y

MEDIAN

95% Confidence

50% Confidence

5% Confidence

Mean

HCLPF

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DERIVATION OF MEDIAN FRAGILITY

The median fragility â is a function of the following parameters:

a) the normalised assessed margin Ma

b) the Code factor Fc (default value 1.0)

c) the HCLPF factor

d) the combined logarithmic standard deviation c

e) the peak ground acceleration for the site’s

10-4 p.a. URS event, å

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NORMALISATION OF MARGINS• Aim to place all margins on a common basis• Examine basic input for all calculations, and

acknowledge degree of sophistication applied to assessments

• Identify appropriate fragility parameters• This process is to a certain degree iterative,

particularly for items with apparently anomalous margins

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NORMALISATION OF MARGINS- KEY FACTS• Margins have been derived over c 15 years using a

variety of approaches

• The current state of the art is more advanced than when the LTSR work began

• For some cases, seismic margins cannot be readily extracted, for example elements with high working stresses, where only margins against total load have been assessed

• Margins are often calculated against eg a code allowable, not a true measure of the loss of functionality

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Typical URS Seismic Hazard Curve

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 0.2 0.4 0.6 0.8 1 1.2

Peak Ground Acceleration (g)

Ex

pe

cte

d A

nn

ua

l P

rob

ab

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f E

xc

ee

da

nc

e H

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Convolution of Hazard Gradient and Fragility Curve For a Normalised Median Fragility of 1.5. c = 0.42

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Peak Ground Acceleration (g)

Haz

ard

Gra

dien

t & F

ailu

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roba

bilit

y

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

Ann

ual F

ailu

re F

requ

ency

Den

sity

Fun

ctio

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Hazard Gradient

Fragility Curve

Density Function

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ALARP ANALYSIS

Quantifiable Cost Elements• Basic time and materials costing• Costs as an overrun of outage• Costs of safety case preparation

Non-Quantifiable Cost Elements

eg• Worker Dose (qualitative ALARP)

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CALCULATION OF COST BENEFIT RATIO

Annual Safety Function Failure Frequency =

Annual Equipment Failure Frequency X Probability of Safety Function Failure

Risk cost (valuation) =

Accident cost x Remaining Station Life X Annual Safety Function Failure Frequency

C.B.R. = Modification Cost for the Plant Item in QuestionCurrent Risk Cost - Risk Cost Post Modification

• Rank• Review• Report

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CLOSURE

• The approach has been very useful in acting as a ranking tool to better understand the key contributors to the risk profile

• The approach used has shown the absolute necessity for normalisation of margins

• The approach used allows judgment to be applied in a quantifiable manner. This has overcome the limitations inherent in alternative, purely mechanical evaluation methods