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

EQE International LAYOUT BACKGROUND OBJECTIVES OUTLINE METHODOLOGY FRAGILITY CURVE DEVELOPMENT ALARP ANALYSIS CONCLUSIONS

EQE International BACKGROUND LTSR and PSR completed to deterministic principles Regulator driven requirement to confirm that the risk posed is ALARP Not a Seismic PSA

EQE International OBJECTIVES 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

EQE International OUTLINE METHODOLOGY 1Identify claimed safety functions and associated equipment/structures 2Identify 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 failure frequency below a defined cut off

EQE International OUTLINE METHODOLOGY cont 4Identify modifications to increase the margin 5Associate 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 6Calculate Cost Benefit Ratio (CBR) 7Review results, and provide information to the Client to assist in the decision making process

EQE International FRAGILITY CURVE DEVELOPMENT Identify Assessment Types 1Code Based eg BS 5950, 8110, 5628, 806, ACI 349 2Other Codes eg R6, SQUG 3Comparison with available experimental data 4Engineering Judgement

EQE International 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

EQE International

DERIVATION OF MEDIAN FRAGILITY The median fragility â is a function of the following parameters: a) the normalised assessed margin M a b) the Code factor F c (default value 1.0) c) the HCLPF factor d) the combined logarithmic standard deviation  c e) the peak ground acceleration for the site’s p.a. URS event, å EQE International

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

EQE International 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

EQE International

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)

EQE International 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 Question Current Risk Cost - Risk Cost Post Modification Rank Review Report

EQE International 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