ML18213A096

Joint EPRI/NRC-RES Fire PRA Workshop - Task 11b: Main Control Room Fire Analysis and Appendix L_2015

ML18213A096
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Issue date:	07/31/2018
From:	Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute
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Module III - Fire Analysis Task 11b: Main Control Room Fire Analysis and Appendix L Joint EPRI/NRC-RES Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)

Main Control Room Fire Analysis Objectives The objective of this presentation is:

Describe the recommended approach for detailed fire modeling in the main control room. Specifically:

- Differences between the main control room and other compartments

- Criteria for abandonment due to fire generated environmental conditions

- Description of how to analyze:

Conditional probability of damage to a target set Forced control room abandonment time 2

Main Control Room Fire Analysis What is Different in the MCR?

The control and instrumentation circuits of all redundant trains for almost all plant systems are present in the control room.

- Redundant train controls can be within a short distance of each other

- Small fires within control panels could be risk-significant

- Related SR: FSS-A6 The room is continuously occupied, which provides the capability for prompt detection and suppression.

Evaluating control room abandonment conditions is necessary

- Abandonment refers to situations in which control room operators are forced to leave due to untenable fire generated conditions (temperature, toxicity, and visibility).

- Related SRs: FSS-B and its two SRs 3

Main Control Room Fire Analysis Recommended Steps Step 1: Identify and characterize main control room features Step 2: Estimate control room fire frequency Step 3: Identify and characterize fire detection and suppression features and systems Step 4: Characterize alternate shutdown features Step 5: Identify and characterize target sets Step 6: Identify and characterize ignition sources Step 7: Define fire scenarios Step 8: Conduct fire growth and propagation analysis Step 9: Fire detection and suppression analysis and severity factor Step 10: Estimate failure probability of using alternate shutdown features Step 11: Estimate probability of control room abandonment Step 12: Calculate scenario frequencies Step 13: Document analysis results 4

Main Control Room Fire Analysis Step 1: Identify and Characterize MCR Features The specific features of the control room and the control board are identified.

Control room dimensions Other adjacent compartments included in the MCR proper Location, shape, dimensions and special features of the control panels and other electrical panels Main control board layout and location of various controls and displays Cable penetration into the control room and into the control panels Ventilation system characteristics False ceiling features and the ceiling above it 5

Main Control Room Fire Analysis Step 4: Characterize Alternate Shutdown Features The features of alternate shutdown capability vary widely among NPPs In general, a control panel is installed at a location away from the control room where the operators can control and monitor key core cooling functions and parameters independent of the MCR.

In other plants, alternate shutdown capability is achieved through a set of control points and control panels located at various points of the plant requiring coordinated actions of several operators.

It is necessary for the fire risk analysts to understand the alternate shutdown capability of the plant.

- For example, the analyst may select safety-related target sets on the panel that are not backed up by an alternate shutdown control or instrumentation circuit.

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Main Control Room Fire Analysis Step 5: Identify and Characterize Target Sets The target sets can be identified by systematically examining combinations of control and instrumentation items found on the control panels, electrical cabinets, wireways, and cable raceways inside the MCR.

Examine the control panels from one end to the other Groups of adjacent controls and instrumentation Cursory and conservative estimation of the CCDP/CLERP as the basis Elements of a set are located within the reach of a potential fire Exposure fire affecting multiple cabinets Corresponding PRA Standard SRs: FSS-A2 through A4 7

Main Control Room Fire Analysis Step 6: Identify and Characterize Ignition Sources The final product of this step is a list of ignition sources, their relevant characteristics, and fire ignition frequencies associated with each source Similar to Step 3.a of single compartment analysis Type, quantity, dimensions and heat release rate profile of each source Main control board as ignition source Assume fire might occur at any point on a control panel Other control panels, electrical cabinets, wireways, and cable raceways Kitchen appliances and other electrical devices?

Transient combustible fires 8

Main Control Room Fire Analysis Step 7: Define Fire Scenarios Four types of fire scenarios are specifically recommended for evaluation Fire inside the main control board and stand-alone electrical cabinets that open into each other, Fires affecting two adjacent electrical cabinets that do not open into each other, Fires affecting two non-adjacent electrical cabinets, and Transient fires Corresponding PRA Standard SR: FSS-A6 9

Main Control Room Fire Analysis Steps 8 and 9: Non-Supp Prob & Severity Factor The non-suppression probability and severity factors are calculated as recommended in the approach for single compartment fires For fires inside a control panel, use the method described in Appendix L (covered previously) d H

h

w W

10

Main Control Room Fire Analysis Step 10: Estimate Failure Prob Using ASP Two approaches may be followed:

An overall failure probability is estimated representing the failure of successful usage of alternate shutdown means.

The alternate shutdown procedure is integrated in the plant response model (i.e., the fault trees and event trees). The core damage sequences are adjusted to include failures associated with alternate shutdown means, and the human error probabilities are reevaluated based on the alternate shutdown procedures.

This is an area of ongoing research!

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Main Control Room Fire Analysis Step 11: Estimate Prob of Control Room Abandonment The final decision to abandon the control room is assumed to depend on habitability conditions.

The analyst may postulate that the alternate shutdown procedure would be activated The time to activate the alternate shutdown procedure is suggested to be established based on plant operating procedures rather than control room habitability conditions Abandonment possibility should be examined for all postulated target damage scenarios 12

Main Control Room Fire Analysis Step 11: Estimate Prob of Control Room Abandonment Abandonment criteria based on habitability conditions Temperature, or heat flux

- The heat flux at 6 above the floor exceeds 1 kW/m2. This can be considered as the minimum heat flux for pain to skin. A smoke layer of approximately 95°C (200°F) could generate such heat flux.

q& = T sl4 1.0 kW m 2 The smoke or hot gas layer descends below 6 from the floor Visibility

- Optical density of the smoke is less than 3.0 m-1. With such optical density, a light-reflecting object would not be seen if it is more than 0.4 m away. A light-emitting object will not be seen if it is more than 1 m away.

A panel fire affects two target items 2.13 m (7) apart 13

Main Control Room Fire Analysis Step 11: Estimate Prob of Control Room Abandonment The conditional probability of abandonment can be estimated based on the calculated evacuation time Determine the heat release rate generating abandonment conditions Calculate the severity factor for fires of this size Determine the time for abandonment

- Time to reach untenable conditions such as 200oF hot gas layer or smoke density conditions of 3.0 m-1 Calculate non-suppression probability Multiply the severity factor and non-suppression probability to determine conditional abandonment probability Corresponding PRA Standard SRs: FSS-B1 and FSS-B2 Note that this is typical HRR distribution discretization approach 14

Main Control Room Fire Analysis Step 11: Estimate Prob of Control Room Abandonment Probability of abandonment due to habitability can be determined based on:

- The criteria for abandonment due to fire conditions

- Fire modeling analysis using field models, zone models, or hand calculations. (Field models and zone models are the tools usually selected for this)

NUREG-1934 includes a detailed example of a control room evaluation (Discussed in Module 5) 15

Main Control Room Fire Analysis Example Credit prompt detection Inputs Suppression by fire brigade Ambient temperature [C] 20 Duration [sec]

- Pns from CR suppression curve Opening area [m2] 4 Height of opening [m] 2 SF from probability Room length [m] 20 distribution for vertical Room width [m]

Room height [m]

15 6

cabinets with unqualified Thermal conductivity [kW/mK] 0.0014 cable and fire propagating to Density [kg/m3]

Specific heat [kJ/kg]

2000 0.88 more than one bundle. Wall thickness [m] 0.15 Temperature for abandonment [C] 93 16

Main Control Room Fire Analysis Example - (alternate discretization approach based on abd. time) 2.5E-01 900 800 2.0E-01 700 600 1.5E-01 500 Prob 400 [kW]

1.0E-01 300 5.0E-02 200 100 0.0E+00 0 0 10 20 30 40 50 60 70 Time for a 200 F HGL SF Pns HRR [kW]

Duration [Min] Required HRR [kW] SF Pns SF*Pns 5 794 3.2E-02 1.9E-01 6.1E-03 10 668 4.8E-02 3.7E-02 1.8E-03 15 603 6.0E-02 7.1E-03 4.2E-04 20 561 6.9E-02 1.4E-03 9.4E-05 25 531 7.7E-02 2.6E-04 2.0E-05 17

One related current activity MCR abandonment analysis methods MCR abandonment is a complex issue

- Abandonment may arise from loss of habitability or control

- How is abandonment decision made for loss of control?

- When will abandonment occur - will it be timely?

- Partial abandonment - Operators stay in the MCR to supplement alternate shutdown operations (control maintained from the MCR)

- Quantifying transfer and alternate shutdown actions A FAQ was proposed by industry to address operations from alternate shutdown including abandonment

- FAQ 13-0002 - only available in draft form

- Proposed a framework for abandonment, containing relatively simple set of alternate shutdown conditions and related discussions 18

One related current activity MCR abandonment analysis methods Continued NRC Staff agrees that questions are important and need resolution

- Staff put out interim staff guidance as could not come to agreement on FAQ approach ML14080A028 Ongoing EPRI/RES research effort

- First phase addressing qualitative considerations Published in 2017 by EPRI, NRC publication in progress

- Publicly available at epri.com, 3002009215 (NUREG-1921 Supplement 1)

Second phase to address quantification

- Completed peer review

- Comment resolution process 19

Main Control Room Fire Analysis Concluding Remarks The main control room has unique characteristics that are addressed in detail in Task 11b.

Recommended fire scenarios for the MCR Evaluation of MCR abandonment due to fire generated conditions

- Loss of MCR habitability

- Loss of control functions due to fire in MCR or elsewhere 20