ML17340B105
| 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: |
| 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
Core Damage Megawatts 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.
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)
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.
Use of Conditional Probabilities for Plant Trip due to Operator Discretion Step 2: For remaining Fire Compartments, interview Operators on probable response to fire
- 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 Step 1: For Fire Compartments retained from Task 2, Set Conditional Trip Probability to 1.0 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.
Input From Cable Selection (Task 3)
Step 3.5.6 compiles the FPRA Cable List Input From Qualitative Screening (Task 4)
Step 4.5 screens Fire Compartments having no:
- 1) FPRA equipment (and associated circuits)
- 2) Equipment causing a plant trip Output To Fire Risk Quantification (Task 14)
Use the Conditional Trip Probabilities to adjust the fire scenario frequencies to obtain:
- 1) Final Fire CDF in Step 14.5.1.2
- 2) Final Fire LERF in Step 14.5.2.2 Uncertainty:
- Binning of Fire Compartment
- Magnitude of Conditional Trip Probabilities Proposed Approach to Enhance Realism
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???