Use of Computational Fluid Dynamic Fire Models to Evaluate Operator Habitability for Manual Actions in Fire Compartments

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1 ANS PSA 2011 International Topical Meeting on Probabilistic Safety Assessment and Analysis Wilmington, NC, March 13-17, 2011, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2008) Use of Computational Fluid Dynamic Fire Models to Evaluate Operator Habitability for Manual Actions in Fire Compartments Robert L. Ladd Engineering Planning and Management, Inc Maxwell Drive, Suite 140 Hudson, WI ABSTRACT Conduct of a Fire PRA may identify situations that require the performance of operator manual actions (OMA) to mitigate the consequences of a fire. In cases where OMAs are required within the affected fire compartment or the action requires transit through the compartment to access components, human reliability analysis has traditionally assigned little to no credit for their performance. These situations typically require the performance of additional analysis to credit additional system options or the performance of modifications to relocate/protect affected circuits and/or equipment. However with the advent of advanced computational fluid dynamic (CFD) fire modeling tools such as Fire Dynamics Simulator (FDS), such cases can be evaluated to estimate feasibility and demonstrate the ability to perform necessary actions or transit through the fire environment. FDS fire models used to show feasibility of manual actions in a fire environment are designed much like those used to evaluate Fire PRA target damage. Feasibility of OMAs is demonstrated by establishing reasonable acceptance criteria and a means to measure the fire environment against those criteria. The acceptance criteria must ensure that the fire environment to which the operator is exposed, is acceptable for the performance of the required action and that it poses no immediate danger to the operator. In addition the model is designed to measure the time when equipment damage would precipitate performance of the action as well as the time when the required action must take place for successful mitigation of undesirable affects. This allows measurement of the expected environmental conditions when the operator would be required to be in the affected fire compartment to perform the required actions. Key Words: Fire Modeling, HRA, Feasibility, Manual Action, OMA, CFD

2 Robert L. Ladd 1 INTRODUCTION The performance of a Fire PRA can result in the identification of situations that require the performance of operator manual actions (OMA) to mitigate the consequences of fire. These actions are usually performed to recover equipment that is needed to achieve safe shutdown of the plant. The need to perform such actions is driven by fire damage to redundant trains of credited PRA equipment. Usually these actions are performed in areas of the plant that are not directly affected by the fire. In cases where the fire does not directly affect the ability to perform the action, the probability of its failure is based on typical Human Reliability Analysis parameters such as complexity of the action, timing, procedural guidance, diagnostic instrumentation, etc. However in cases where the required action requires transit through fire affected areas or actual performance of the action in the fire affected compartment, the failure probability that must be attributed to the action approaches 1.0. In cases where the HRA analyst concludes that the failure probability of a required manual operator action is unacceptably high the inability to perform the required action can become a dominant risk factor for the compartment. This can result in the need to perform plant modifications that will eliminate the need to perform the action. Alternatives to the conduct of expensive plant modifications are desirable, however such options are not always available using typical PRA approaches. In the performance of Fire PRAs there is an alternate method that is available for the evaluation of manual operator action feasibility. Evaluation of the fire environment in which the operator will conduct the action can be performed using fire modeling techniques for the purpose of demonstrating feasibility. Analysts can use these techniques to demonstrate that the products of combustion and heating from plant fires will be maintained within acceptable limits to allow the operator to access the plant areas to perform required actions. 1.1 Acceptance Criteria The performance of a feasibility analysis using fire modeling requires determination of acceptance criteria that will be used to judge the acceptability of the operator environment. Due to the fire the operator will be subjected to a number of challenges that must be overcome to ensure successful performance of the manual operation. At a minimum the operator must be able to access the equipment to be operated, identify the equipment, perform the operation and validate results. These requirements involve assurance that the environment is not only survivable but amenable to the performance of activities that may range from simple to relatively complex. To ensure that the operator will be able to perform the required action, all aspects of the action must be broken down to identify the critical parameters that will be measured to allow estimation of success. Fires affect the fire compartment in a number of ways which could prevent successful operation of manual operator actions. Parameters that may be considered include: Fire compartment temperature Opacity of smoke layer Smoke layer heat flux Timing required to perform action Page 2 of 11

3 Use of CFD Fire Models to Evaluate Habitability for Performance of OMAs Expected time of fire environment entry Operator access/egress paths Emergency lighting Ventilation Each parameter plays a role in designing the fire model used to evaluate manual action feasibility and in evaluating the results. The first step in using fire modeling to evaluate manual operator action feasibility is to identify the acceptance criteria to be applied. As discussed above a number of factors are considered in the design of the fire model. The performance of this type of evaluation assumes that a manual action will be performed in a fire environment therefore the use of staged protective equipment may also be assumed. Accordingly the use of equipment such as hand-held lights, self-contained breathing apparatus (SCBA), etc. may be assumed and factored into the selection of acceptance criteria. This being the case preservation of a breathable environment and the ability to perform actions without the use of supplemental lighting need not be considered in the selection of acceptance criteria. The most obvious, and likely most important, acceptance criteria that must be established involve acceptable temperature and smoke density requirements. The temperatures to which the operator will be exposed must be sufficiently low to prevent excessive stress or injury and should be low enough to ensure the operator can perform the required actions. Even assuming the use of SCBA to provide breathable air, smoke density must be sufficiently low to allow free movement of the operator, identification of equipment, and performance of the required actions given the use of normal and supplemental lighting. Acceptance criteria for these parameters are not well established. However industry guidance for other activities can be used to develop acceptance criteria for this type of evaluation. Reasonable acceptance criteria for the performance of manual actions can be found in NUREG/CR This document provides criteria that are used to estimate conditions created in a fire environment that would lead to the abandonment of the main control room. According to NUREG/CR control room abandonment is assumed to occur when a fire creates a smoke layer in the ceiling of the room which raises temperatures to untenable levels or when smoke obscures visibility to the point that identification and operation of equipment becomes unfeasible. Heat flux radiated from the smoke layer in the ceiling area of a main control room is assumed to lead to abandonment when it exceeds 1kW/m 2 at 6 above the floor. This heat flux criterion is considered to be the minimum level of heat flux that would result in pain to the skin. According to NUREG/CR a smoke layer with a temperature of approximately 95 C (200 F) could generate sufficient thermal radiation to result in a heat flux of 1kW/m 2. The smoke layer in the room may also create difficulty in performing an action simply because it obscures the equipment, labels and indications necessary to conduct the action. Smoke density which reduces visibility must be considered when determining the feasibility of the OMA to be performed. According to NUREG/CR an optical density level of 3.0 m -1 at 6 above the floor is sufficient to force abandonment of a main control room. At this level of obscuration, visibility is reduced to a point where light-reflecting objects cannot be seen at a Page 3 of 11

4 Robert L. Ladd distance of greater than 0.4 m (approximately 1-3 ) and a light emitting objects cannot be seen at a distance of greater than 1 m (approximately 3-3 ) away. From the stand point of measuring the acceptability of performing manual operator actions within a fire affected environment, measurement of the heat flux and obscuration of visibility to which an operator will be exposed become the primary parameters measured by the fire model. Acceptance criteria other than those discussed above may be selected as long as a reasonable justification is provided that establishes that the criteria selected are predictive of successful performance of the required actions. However use of the NUREG/CR prescribed main control room abandonment criteria is representative of a reasonable measure of success since the purpose of these criteria is to predict operator behavior related to plant shutdown under adverse conditions in a fire environment. With respect to operations in a fire environment, the performance of a safe shutdown action in general plant areas presents essentially the same physical challenges as performance of safe shut down actions in a fire affected control room. In both cases the fire environment adds heat and smoke which must be overcome to successfully perform the action. Accordingly it can be assumed that if the fire conditions in a general plant area are bounded by the main control room abandonment criteria, then the required OMAs can be successfully performed. 1.2 Manual Actions Reviewed The evaluation of the feasibility of performing a manual operator action in a fire affected compartment must include an assessment of several parameters related directly to the action that is required to be performed. This assessment should be performed to determine if fire modeling is likely to produce useful results. Consideration of several factors should be included in the decision to use fire modeling to establish OMA feasibility. These typically include examination of why the action is being performed, i.e., what is driving the performance of the action. Also consideration should be given to the physical parameters affecting the fire environment that can affect the likelihood of success. For example, is the action sufficiently removed from the postulated fire location. Typically post-fire manual operator actions are required due to fire related damage. Accordingly the manual action being evaluated is performed to recover functionality that was lost due to fire damaged targets. This determination provides insight concerning where the fire must be located to require the performance of the action. Establishment of this fact is necessary to properly design the fire model that will eventually be used to evaluate feasibility. Initial investigation of the physical layout of the affected area, postulated fire location, necessary fire strength, and available ventilation is critical to identifying good candidates for detailed evaluation using CFD fire modeling tools. Good candidates for evaluation should place the postulated fire a reasonable distance from the required action and ventilation of the products of combustion should be available either from mechanical ventilation systems that can be credited as surviving the fire or through openings to adjacent compartments. Essentially the postulated fire will be relatively small as compared to the volume that will contain the products of combustion and the required action will be located a reasonable distance away from the fire or located in areas that would be physically protected from direct fire effects. Consideration should also be given to the operators access and egress to the location of the manual action and the time within which the action will be performed. Conduct of the fire Page 4 of 11

5 Use of CFD Fire Models to Evaluate Habitability for Performance of OMAs modeling will demonstrate the importance of these attributes. However early assessment of these factors should be conducted when deciding whether fire modeling is a good approach. If access and egress to the manual action requires transit near the postulated fire location, then fire modeling may not predict success. Also success may be questionable if the area in which the fire and action is located is relatively small in comparison to the fire size and ventilation of the products of combustion to adjacent compartments is not available. Finally if performance of the OMA occurs late in the development of a relatively large fire, modeling may only demonstrate that performance of the action at the required time will not be possible due to advanced environmental degradation. Good candidates for evaluation of OMA feasibility include actions performed in rooms/compartments that are relatively large or open to adjacent areas such that a large volume is available for deposition of the products of combustion. Also OMAs evaluated using fire modeling should be located far enough away from the postulated fire such that access, egress or performance of the required action does not unduly expose the operator to direct fire affects. The analyst should judge the merits of the scenario presented to determine if fire modeling will likely provide useful insight prior to committing to the conduct of detailed evaluation. 1.3 Fire Models Selection of the fire modeling tool that will be used for the evaluation of the OMA feasibility is very important. Selection of a computational fluid dynamic model is advantageous for evaluation of these scenarios because these models allow investigation of non-averaged values within the modeling domain and establishment of gradients across the modeled environment. Fire Dynamics Simulator (FDS) 4,5 is specifically addressed in this paper as the preferred fire modeling tool for evaluation of fire environments with regard to the performance of manual actions. FDS is a computational fluid dynamics model of fire-driven fluid flow. FDS solves numerically a form of the Navier-Stokes equations appropriate for low-speed, thermally-driven flow with an emphasis on smoke and heat transport from fires. It provides the capabilities to evaluate the parameters necessary to analyze the environment within which the actions will be performed. FDS has the capability to model and observe (using the Smokeview visualization program to display results) heat flux, temperature (point and gradient), smoke distribution, optical density, ventilation, and hot gas layer height and thickness. This modeling software has been evaluated by the Nuclear Regulatory Commission as documented in NUREG ,2 to establish its predictive capabilities. This evaluation was performed to verify and validate FDS calculations to ensure correctness and suitability of the methods employed. The evaluations performed were used to establish that the model is capable of reproducing phenomena of interest. NUREG 1824 Volume 7 2 concluded that for known heat release rate, FDS can reliably predict gas temperatures, major gas species concentrations, and compartment pressures to within about 15%, and heat fluxes and surface temperatures to within about 25%. Accordingly FDS is capable of reliably predicting the parameters necessary to analyze the environment within which the OMAs may be performed. Page 5 of 11

6 Robert L. Ladd 1.4 Modeling Approach Several factors are considered when designing the fire model. To accurately predict successful performance of the action the model must be designed to simulate conditions that will exist for the operator from the time the environment is entered to the time it is exited. This requires that the conditions governing performance of the action can be anticipated and reasonably replicated in the fire model domain. The first challenge is to design the fire model to accurately reflect the physical environment of the compartments/rooms within which the fire is located and the OMAs are performed. Design of the compartment is critical to accurately predicting the resulting gas flows, heat flux, and temperatures. Openings to adjacent compartments via doorways, equipment hatches, stairways, etc. should be accurately depicted to ensure that products of combustion are vented away from the operator s environment. The size of adjacent compartments should be accurately represented to ensure the flow of combustion gases is accurately represented. In cases where deposition of gases is to the building exterior or extremely large spaces, restriction of the vented volume may not be necessary however the treatment of such variables must be carefully considered and justified by the analyst. Properly designing the compartment openings and the region of gas deposition is important to ensure that the hot gas layer created by the fire is reasonably approximated. Stairway opening to elevation above Equipment Hatch opening to elevation above Figure 1. Fire Model Domain with Equipment Hatches and Stairway Openings Accurate depiction of the hot gas layer is important because this feature defines the heat flux to which the operator will be subjected and the depth of the smoke layer that obscures visibility in the area. Other factors that can affect the hot gas layer, is the presence of fire dampers, held-open fire doors, ventilation systems, whose operation or automatic actuation in Page 6 of 11

7 Use of CFD Fire Models to Evaluate Habitability for Performance of OMAs response to fire conditions may change the character of the hot gas layer. When such features are present the model should be designed to replicate ventilation flows and/or compartment ventilation openings that open or close as conditions change in the compartment. Design of the overall fire compartment is also important to ensure that the gas flows are correctly portrayed. Typically small obstructions such as cable trays, conduits, ventilation ducts, etc. do not form a significant impediment to fire gas flows in the compartment. As a result small insignificant obstructions need not be modeled. However large obstructions or construction features that may alter gas flow paths or significantly reduce the compartments internal volume should be included. Inclusion of these details will ensure that the model realistically replicates the formation and flow characteristics of the hot gas layer in the compartment. Walls, partitions, equipment cubicles, beam pockets, etc. should be included in the design of the model such that gas flows from the fire will be channeled and diverted in a realistic manner. Possibly the most important attribute of the fire model is inclusion of a reasonable approximation of the anticipated fire which will provide a meaningful simulation of the fire environment. FDS allows fires to be designed in a number of ways. Fires can be designed as a material burns in a pre-described manner or a more controlled approach can be selected a where a specific heat-release-rate (HRR) is and fire location is defined. Use of the prescribed HRR method allows the analyst to strictly control the amount of heat input into the environment and the rate at which it is released. This also allows the fire to be modeled using inputs from accepted references. Cable Trays Cabinet Fire Vents Initial Fire Source Figure 2. Fire Vents with Fire Initiating in the Cabinet and Spreading to Cable Trays Above Page 7 of 11

8 Robert L. Ladd The HRR of the fire should be representative of the maximum expected fire so that the resultant fire environment simulates the worst case condition. NUREG/CR provides information that can be used to design the fire. The use of a 98% HRR for the selected initiator is suggested such that the postulated fire is conservative for most scenarios. Assuming the evaluation predicts an environment that is amenable to performance of the required action at the 98% fire strength, the analyst will be able to conclude that performance of the action is feasible from the standpoint of fire exposure. In addition to making a reasonable assumption for fire strength, the analyst should design the fire to include a growth, steady burn, and decay curve. As discussed above for HRR, NUREG/CR provides guidance on the design of fire growth and decay profiles. FDS provides the ability to ramp fire strength to predetermined levels over time and to provide a decay time. The analyst should make use of available information to approximate fire growth and decay to more realistically define how the postulated fire will grow and consequently how the fire environment will degrade over time to better describe the challenges that will be faced by an operator at any given time. Where fire spread to secondary combustibles is anticipated or possible, FDS device elements should be included to predict ignition times. Where ignition is indicated the HRR of the combustible is added to the model at the time indicated. Depiction of fire spread to secondary combustibles and spread along their length, as in the case of an ignited cable tray, is necessary to realistically predict the conditions in the fire compartment. Fire spread/growth along the cable tray and eventual decay should be postulated in a manner similar to that employed for the fire initiator and the values used taken from accepted references. As discussed earlier the need to perform the OMA is driven by fire damage caused by the postulated fire. The fire model is therefore additionally designed to include the targets whose damage leads to the performance of the OMA in question. These targets are included in the model for several reasons. Damage to these targets indicates the need to perform the actions; if the model shows that the required damage does not occur then the action is not necessary or, at the very least, the selected fire initiator will not result in performance of the action. However where the required damage is shown to occur, the length of time required to damage the targets establishes the time when the need to conduct the required manual action may first be recognized and recovery actions are initiated. Accordingly target damage that results in the performance of the OMA also sets time zero for evaluation of the feasibility of the action. Following development of a representative fire model the analyst must select and place the data gathering points within the computational domain that will describe performance of the OMA. FDS provides numerous methods to gather data from within the domain. Collection of data at individual points is conducted using a device element (thermocouple) that allows data collection at single computational cells within the domain at predetermined time steps. Other methods which present calculated values over two dimensional planes, along isometric surfaces, at obstruction surfaces, etc., are also available and are useful for analyzing model results however collecting data using thermocouples is the best approach to determine potential fire affects on the operator. As discussed earlier there are several factors to be considered when determining success of an OMA that is performed within the fire affected area. First the operator s access and egress path to the action must be considered. Emergency lighting in the area will typically define the Page 8 of 11

9 Use of CFD Fire Models to Evaluate Habitability for Performance of OMAs credited access and egress routes. Given this information and assuming an operator height of 6 feet, as suggested by the NUREG/CR main control room abandonment criteria, heat flux information may be obtained along the lighted access and egress routes in the room using FDS thermocouples which are set up to measure optical density and radiative heat flux. To ensure adequate information is collected the model should include thermocouples every few feet along the access and egress routes. This data is collected for the entire duration of the model run. Model run time is estimated based on a conservative estimate of the length of time required to damage the initiating targets, plus time for the operator to discover and perform the required OMA, including diagnosis, performance of the action, and egress from the area. Figure 3. Fire Scenario Arrangement The time required to damage the initiating targets, diagnose the problem, and perform the required action describes the pertinent timeframe for the fire model results. Assuming the fire model demonstrates a burn time of 10 minutes to damage the Fire PRA targets and it takes an additional 5 minutes for the operator to diagnose the problem and to travel to the compartment access point, then the fire model results recorded at 15 minutes describe the conditions to which Page 9 of 11

10 Robert L. Ladd the operator will be exposed upon arrival. Assuming the required action and egress from the area requires an additional 10 minutes, then the fire model results between 15 and 25 minutes describe the environment to which the operator will be exposed. The analyst can use this information to determine if the fire environment is tenable for the duration of the operators visit for the purpose of determining feasibility. 1.5 Fire Model Uncertainty The approach and discussion above suggests that building and running the fire model will produce results that that accurately describe fire environment conditions and timing for operator entry into and exit from the fire affected area. However due to uncertainty in the fire model calculations and the inputs/assumptions used to develop the fire model domain the analyst must consider the consider uncertainty of the fire mode results when determining feasibility of the OMA. As discussed in FDS Technical Reference Guide, Volume 2: Verification 4, uncertainty in CFD models can be introduced from a number of sources because of the complication inherent to such models. Sources of uncertainty may include the calculations employed in the model, sensitivity to the computational grid size, assumed heat release rates, ignition of additional sources, and model inputs that create changes over time, etc. Because of the numerous sources of uncertainty and the inherent complexity of these models it s unreasonable to attempt to determine uncertainty based on the cumulative effect of the component parts of the model. Rather comparison of model predictions to fire test results for typical output parameters provides a benchmark that can be used to estimate uncertainty for a predictive parameter thus providing assurance that the model predicted results provide a best estimate of realistic fire conditions. A simplified method for determining uncertainty associated with FDS is included in the FDS Technical Reference Guide, Volume 2: Verification 4, this guidance used with the experimental data available from NUREG 1824, Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications 1,2, for the desired model can be used to estimate uncertainty for the results achieved. Establishing the uncertainty associated with the fire model for a particular scenario allows the analyst to determine the range of probable results and potential deviation from that predicted. Through examination of the predicted results and potential deviation due to uncertainty the analyst can provide a prediction of OMA feasibility with an acceptably high level of confidence. 2 CONCLUSIONS Use of computational fluid dynamic fire models such as FDS allows the analyst to realistically evaluate the conditions expected to exist in a fire involved compartment for the purpose of estimating OMA feasibility. The current state of the art is such that these evaluations can provide a best estimate prediction of conditions provided sufficient information is available to adequately describe the modeled conditions. Accurate modeling requires that the analyst identify the fire targets that will result in the need to perform the OMA and information concerning conduct of the required action. The analyst must identify the access and egress paths to be taken to the location of the OMA and the required transit and action performance times. Additionally bounding fire scenarios must be identified and accurately modeled to ensure realistic representation of the environmental challenge. Using the approach described herein the Page 10 of 11

11 Use of CFD Fire Models to Evaluate Habitability for Performance of OMAs analyst can predict with reasonable confidence the feasibility of performing an OMA inside of a fire affected compartment. 3 ACKNOWLEDGMENTS I would like to acknowledge Eric Megow of Engineering Planning & Management for assistance in the development of the analytical approach discussed in this paper. 4 REFERENCES 1. Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications, Volume 1: Main Report, U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research (RES), Rockville, MD, 2007, and Electric Power Research Institute (EPRI), Palo Alto, CA, NUREG-1824 and EPRI Verification and Validation of Selected Fire Models for Nuclear Power Plant Applications, Volume 7: Fire Dynamics Simulator (FDS), U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research (RES), Rockville, MD, 2007, and Electric Power Research Institute (EPRI), Palo Alto, CA, NUREG-1824 and EPRI EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities: Volume 2: Detailed Methodology. Electric Power Research Institute (EPRI), Palo Alto, CA, and U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research (RES), Rockville, MD: 2005, EPRI TR and NUREG/CR National Institute of Standards and Technology, Gaithersburg, Maryland, USA, and VTT Technical Research Centre of Finland, Espoo, Finland. Fire Dynamics Simulator, Technical Reference Guide, 5th edition, October NIST Special Publication (Four volume set). 5. K.B. McGrattan, S. Hostikka, and J.E. Floyd. Fire Dynamics Simulator (Version 5), User s Guide. NIST Special Publication , National Institute of Standards and Technology, Gaithersburg, Maryland, October Page 11 of 11