ML17340B105

Meeting Presentation, Improving Realism Through the Use of Conditional Probabilities for Plant Trip Due to Operator Discretion

ML17340B105
Person / Time
Site:	Nuclear Energy Institute
Issue date:	12/13/2017
From:	Stiles H Duke Energy Corp, Nuclear Energy Institute
To:	Document Control Desk, Office of Nuclear Reactor Regulation
Shared Package
ML17340B096	List: ML17340B097 ML17340B098 ML17340B100 ML17340B101 ML17340B102 ML17340B103 ML17340B104 ML17340B105 ML17340B106
References
Download: ML17340B105 (6)

Text

Improving Realism Through the Use of Conditional Probabilities for Plant Trip due to Operator Discretion Harold Stiles - Lead Engineer PSA

Use of Conditional Probabilities for Plant Trip due to Operator Discretion Potential Realism Improvement A plant trip is associated with less than 14% of the approximately 400 fires in the FEDB on which the ignition frequencies are based, but some kind of plant trip is the initiating event for every fire scenario.

Assuming a fire-induced plant trip (i.e., automatic, manual or LCO forced trip),

when one is neither required nor likely, tends both to inflate the total risk and to skew the relative risk contributions.

Soliciting operator input on Conditional Trip Probabilities (CTPs) due to operator discretion can reduce uncertainty and improve realism.

Megawatts Core Damage

Use of Conditional Probabilities for Plant Trip due to Operator Discretion Relevant Guidance Section 2.5.3 of NUREG/CR-6850 states that a Fire Compartment need not have an initiating event assigned to it when a review of associated equipment (that could be affect by a fire) concludes that a plant trip (i.e., an automatic, administratively required manual, or LCO forced trip) could not occur.

Both pilot plants incorporated some treatment of CTPs with the process becoming more structured and better documented as it was extended to other plants in the fleet.

(NFPA 805 Safety Evaluation, ML101750604; PRA RAI 1C, ML13205A016; PRA RAI 21, ML15079A025)

Use of Conditional Probabilities for Plant Trip due to Operator Discretion Proposed Approach to Enhance Realism Input From Component Selection (Task 2)

Step 2.5.3 identifies Fire Compartments containing equipment whose failure would:

1) Cause an automatic trip;

2) Cause a procedure-driven manual trip; or

3) Invoke an LCO requiring a shutdown. Step 1: For Fire Compartments retained from Task 2, Input From Cable Selection (Task 3) Set Conditional Trip Probability to 1.0 Step 3.5.6 compiles the FPRA Cable List Input From Qualitative Screening (Task 4)

Step 2: For remaining Fire Compartments, interview Step 4.5 screens Fire Compartments having no:

1) FPRA equipment (and associated circuits)

Operators on probable response to fire

2) Equipment causing a plant trip - Consider fire effect on equipment and cables

- Bin Trip Likelihood: Likely, Not Likely, Very Not Likely

- Document a Basis for the Binning Step 3: Assign Conditional Trip Probability

- Likely=1.0, Not Likely=0.1, and Very Not Likely=0.01 Output To Fire Risk Quantification (Task 14)

Use the Conditional Trip Probabilities to adjust the fire Uncertainty:

scenario frequencies to obtain: - Binning of Fire Compartment

1) Final Fire CDF in Step 14.5.1.2 - Magnitude of Conditional Trip Probabilities

2) Final Fire LERF in Step 14.5.2.2

Use of Conditional Probabilities for Plant Trip due to Operator Discretion Anticipated Realism Improvements Expect typical risk improvements of about 10%.

Usually only a few Fire Compartments are affected.

Fires related to Diesel Generators represent a typical improvement For those few Fire Compartments, the risk impact can be more significant.

Better alignment between the model and Operator expectations increases overall confidence in the risk results of the Fire PRA.

Use of Conditional Probabilities for Plant Trip due to Operator Discretion Questions???