ML081190368

MSPI - How It Works

ML081190368
Person / Time
Site:	Salem
Issue date:	04/30/2008
From:	Jacqueline Thompson NRC/NRR/ADRO/DIRS/IPAB
To:
Thompson J, NRR/DIRS, 415-1011
Shared Package
ML081190655	List: ML081190368
References
Download: ML081190368 (10)

Text

Salem Yellow Performance Indicator MSPI - Emergency AC Power)

BACKGROUND

• MSPI is the first risk-informed Performance Indicator.

• Five indicators monitoring five systems are included under MSPI.

• Adds the risk-weighted average of unreliability and unavailability over a three year period to derive one number.

• Unreliability (failures) typically drives the risk in MSPI.

• MSPI has several design features that increases stability and confidence in results.

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MSPI - How it Works

• MSPI = unavailability (UAI) + unreliability (URI).

• URI = risk constant (B) * (change in unreliability)

• Risk constant (B) = CDF * (Fussell-Vesely /unreliability) 4/30/2008 2

MSPI - How it Works (cont.)

Front Stop Attributes:

• Limits highest worth component failure to 5E-7 to prevent a single failure from causing the PI to exceed a performance threshold.

• Is only invoked if the calculated MSPI value (UAI + URI) is in the white band.

• Is not used if the calculated MSPI value (UAI + URI) is in the yellow band.

Comment: The risk cap can allow high risk worth systems to appear much better than they really are, and can cause apparent large jumps in MSPI value in high risk worth systems under some circumstances.

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Development of MSPI

• Prior to the 2006 MSPI implementation, several industry workshops were held to explain mechanics of MSPI and how it works. The front stop and other features of MSPI were explained in detail. All licensees participated in these workshops.

• All licensees were encouraged by the staff and by NEI to set up ways to monitor MSPI margins so that the number of failures to white, yellow, etc could be predicted (see sample industry reports attached).

• There was agreement among the staff and industry to prohibit changes to MSPI risk and coefficient values during the current quarter.

• These rules were designed to prevent reactionary changes of licensees PRAs and MSPI values and would provide stability and consistency of the indicator across the industry.

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Salems Approach

• During the 2006 MSPI implementation effort, Salem chose the most conservative (but easiest to calculate) option (Option 1) for determining the EDG component risk coefficient values for failure-to-start (FTS), failure-to-load/run (FTL), and failure-to-run (FTR) events.

• Option 2 would have allowed the licensee to calculate and utilize the risk coefficients based on each basic event risk worth, instead of always using the highest worth coefficient for any basic event type failure, as done in Option 1.

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Salem EDG Failures

• Licensee experienced 3 EDG failures between 8/30/05 and 12/10/07.

• 1 run and 2 start failures.

• Use of Option 1 meant that highest worth risk coefficient would be used for all basic failure events.

• Based on the Option 1 approach, the risk cap was invoked on the first failure.

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Salem EDG Failures

• Without risk cap, the first failure would have resulted in a white MSPI. In this case, MSPI stayed green.

• After second failure (without the risk cap) MSPI was calculated to be white. With the risk cap in place, MSPI was calculated to be just below the white threshold, so MSPI remained green.

• With the third failure, MSPI was calculated to be in the yellow without risk cap, per the PI guidance, the risk cap was not applied.

• Result was that MSPI went from a high green to a low yellow on the third failure.

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Salem Performance Synopsis

• Three EDG failures (one run and two start failures) in a three year period is considered a significant negative performance trend.

• Licensee made results appear more significant by adopting the Option 1 approach.

• Licensee could have foreseen performance threshold crossings if they were monitoring margins IAW recommendations of staff/industry.

• Licensee requested (via FAQ) that the NRC-industry task group change the PI guidance to allow them to revise MSPI parameters mid-quarter as a result of these events. The ROP Task Group concluded this would have a detrimental impact on the ROPs Performance Indicator Program.

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Sample Margin Management 4/30/2008 9

Conclusions

• Three EDG failures in a three year period indicates a performance trend that is significant for a system that is not expected to experience many failures.

• The yellow performance band entry was either preventable or predictable if the licensee had chosen not to use the highest worth risk coefficient for all basic event failure modes and/or margin management was in place. Both of these areas were discussed in detail w/industry and were strongly encouraged to adopt the approach that best suited their situation.

• Allowing the industry to change any PI parameter mid-quarter would create inconsistency and instability and provide an incentive to the industry to revise PI outcomes if they are not deemed acceptable.

This was acknowledged by the staff/industry ROP Working Group early on, and therefore, was prohibited.

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