Fire Modeling Parameters Outside the Validation Range

The development of the sample problems documented in the appendices to this report suggests that many commercial NPP fire modeling applications can fall outside the range of applicability of the validation study documented in NUREG-1824 (EPRI 1011999). The primary reason for this is that the range of applicability, as defined by the dimensionless parameters, is governed by the experiments selected for the validation study. The selected experiments are

representative of various types of spaces in commercial NPPs, but do not encompass all possible geometries or applications. Accordingly, the analyst will encounter many areas or

THE FIRE MODELING PROCESS applications that will fall outside this application range. The predictive capabilities of the fire models in specific scenarios can extend beyond the range of applicability defined in NUREG-1824 (EPRI 1011999). Additional analysis and justification is required by the analyst to address situations where some or all of the analysis parameters fall outside the range of applicability defined in NUREG-1824 (EPRI 1011999). The additional analysis and justification should address the applicability of fire modeling results generated by input parameters outside the validation range to support the conclusions of the study. This section describes the

recommended strategies for addressing this situation.

2.3.7.1 Sensitivity Analysis

In the context of applicability of validation results, sensitivity analysis refers to varying selected input parameters in the “conservative” direction so that they fall within the applicability range. If the fire modeling conclusions are not affected by the variations in the parameters, the analyst may use the sensitivity analysis results to further justify the conclusions. Based on the dimensionless terms listed above, the following sensitivities could be evaluated:

 Froude number: The two parameters that can be practically varied are the fire diameter and the HRR. For fire sizes (i.e., HRR) that are small for the postulated diameter, the resulting Froude number can fall under the low end of the applicability range. Similarly, for fires that are relatively large for the postulated diameter, the Froude number can fall above the applicability range. In the former situation, the analysts may consider

reducing the fire diameter and keeping the HRR profile unchanged. In most fire modeling tools, the fire diameter is simply used to determine HRRs or to calculate the fire plume conditions, such as the flame height or plume temperature. Considering that the HRR is “fixed” in this sensitivity study, the fire diameter may not be a relevant

parameter in the analysis, with the important exception of scenarios where the fire plume conditions are relevant. A similar approach could be used for the latter situation.

Increasing the fire diameter can “force” the dimensionless term into range. It should be stressed that fire diameter is often a parameter that influences predicted flame height and fire plume conditions, and that the effects of diameter variations should be explicitly addressed in the analysis. This includes other dimensionless terms where the fire diameter is a key input (e.g., target distance to diameter (r/D), etc.).

 Flame length relative to ceiling height: This is a convenient parameter for expressing the

“size” of the fire relative to the height of the compartment. A value of 1 means that the flames reach the ceiling. The validation range extends up to a value of 1.0, which should cover most of the scenarios of interest in commercial NPPs. Scenarios that are expected to fall out of the range are:

o Those associated with relatively short flames. Typical ceiling heights in NPP scenarios range from about 3 to 6.1 m (10 to 20 ft), excluding the containment and turbine buildings, which have relatively large openings between elevations.

Consequently, flame lengths shorter than 0.6 to 1.2 m (2 to 4 ft) will be considered outside of validation range. A sensitivity analysis increasing the HRR values should provide a conservative estimate of fire conditions within the validation range. In cases where the conclusion of the analysis does not change given the increased fire intensity (e.g., no damage within the flame length of fire plume), the suggested sensitivity analysis can be used as the justification for the evaluation of a compartment that falls outside the validation range.

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o Flame extensions under ceilings. In this particular case, not only are such flame lengths out of the range of validation, but also the models for predicting this phenomenon have not been verified or validated with a process similar to the one documented in NUREG-1824 (EPRI 1011999).

 Ceiling jet radial distance relative to the ceiling height: Ceiling jet temperature and velocity correlations use this ratio to express the horizontal distance from target to plume. Ceiling jet applications in commercial NPPs should be carefully evaluated due to the numerous obstructions near the ceiling (e.g., cable trays, HVAC ducts, piping, etc.).

Most of its applications include determination of time to detection and sprinkler

activation, in which the ceiling jet velocity is a sub-model in the analysis. An alternative option is a sensitivity analysis consisting of moving the fire location to distances that would fall within the validation range; it is recognized, however, that in many situations the fire location cannot be altered, particularly in the case of fixed ignition sources or transient fires postulated near areas where redundant targets are in close proximity (pinch-points). In general, longer horizontal distances will result in longer activation time results; by contrast, shorter horizontal distances would result in “conservative” time-to-damage results. In situations where the ceiling jet geometry deviates significantly from the idealized flat horizontal surface, as may be the case when there are large numbers of obstructions or bays, a CFD model may be the better choice for calculating detector response times.

 Equivalence ratio as an indicator of the ventilation rate: The validation available is for well-ventilated fires: that is, no model validation information is available for under-ventilated compartment fires, including fire extinction due to lack of oxygen. In general, fires that are considered well ventilated in the enclosure should result in bounding conditions as long as the HRR profile is appropriate. Conditions in the enclosure are not expected to be worse in a fire where the combustion process is affected by lack of oxygen than they would be under fire conditions where the combustion process is unaffected. However, under-ventilated fire conditions should be considered carefully as sudden air inflows into compartments with under-ventilated fire conditions could produce relatively severe fire conditions.

 Compartment aspect ratio: It is expected that some compartments in commercial NPPs would have geometric characteristics outside the validation range (e.g., relatively long, narrow corridors with high ceilings, etc.). These parameters are important in fire scenarios involving HGL calculations, as the size and configuration of the compartment are important input parameters. Clearly, these parameters should not be applicable in scenarios where the enclosure conditions are not considered, such as flame radiation calculations using the point source model and plume temperature calculations using algebraic models where it has been determined that enclosure conditions are not a factor. As part of the sensitivity analysis, the analyst may consider “shortening” the length, width, or height of the compartment to values that fall within the validation range, with the expectation that this will result in an elevated level of hazardous fire-generated conditions, as predicted by the model (i.e., a conservative calculation). In cases where the conclusion of the analysis does not change given the “smaller” compartment (e.g., the HGL temperature does not exceed damage threshold of cables in either case), the

THE FIRE MODELING PROCESS suggested sensitivity analysis can be used as the justification for the evaluation of a compartment that falls outside the validation range.

 Radial distance relative to the fire diameter: This ratio is the relative distance from a target to the fire, and is important when calculating the radiative heat flux. Note that the validation range starts at a distance approximately twice the fire diameter. In practice, targets at a very close distance to the fire (approximately two fire diameters or less) should be expected to fail, given the relatively low damage threshold levels for cables.

An alternative option is a sensitivity analysis, which consists of moving the fire location to distances that would fall within the validation range; it is recognized, however, that in many situations the fire location cannot be altered, particularly in the case of fixed ignition sources or transients fires postulated near “pinch-points.” In general, shorter horizontal distances will result in higher heat flux levels.

2.3.7.2 Additional Validation Studies

There are other fire model validation studies besides NUREG-1824 (EPRI 1011999) that can serve as a basis for establishing the applicability of fire modeling results. In developing the examples documented in the appendices of this report, the research team identified relevant validation studies outside of NUREG-1824 (EPRI 1011999), as summarized below:

 Scenarios involving targets within the fire plumes: A useful discussion of fire plumes is contained in Gunnar Heskestad’s chapter in the SFPE Handbook of Fire Protection Engineering, 4^th ed., “Fire Plumes, Flame Height, and Air Entrainment.” The plume correlations used in the empirical and zone models are described, as well as their range of applicability. NUREG-1824 (EPRI 1011999) contains experimental measurements of fire plumes, but the range is somewhat limited. The plume correlations used by the models have a much wider range of applicability than that exercised in NUREG-1824 (EPRI 1011999).

 Scenarios involving targets within the ceiling jet: Similarly, Ronald Alpert’s chapter

“Ceiling Jet Flows” in the SFPE Handbook contains a description of the various

correlations used to estimate the temperature and gas velocity of ceiling jets. There are extensive references to the original experimental test reports from which the correlations were derived.

 Scenarios involving targets exposed to flame radiation: A useful collection of techniques and validation data for thermal radiation calculations is found in the SFPE Engineering Guide for Assessing Flame Radiation to External Targets from Pool Fires, written by the SFPE Task Group on Engineering Practices, 1999.

 Scenarios involving flashover/post-flashover conditions: A series of experiments was conducted at NIST as part of an investigation of the collapse of the World Trade Center towers. Validation calculations with FDS are described in the report NIST NCSTAR 1-5F, Federal Building and Fire Safety Investigation of the World Trade Center Disaster:

Computer Simulation of the Fires in the WTC Towers, September 2005.

 Scenarios involving electrical failure of cables: The Cable Response to Live FIRE (CAROLFIRE) program led to the development and validation of the Thermally-Induced

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Electrical Failure (THIEF) model (NUREG/CR-6931, Volume 3). This model can be used to estimate the temperature within an electrical cable that is exposed to an elevated temperature or heat flux.

 Scenarios involving cable burning: The Cable Heat Release, Ignition, and Spread in Tray Installations in Fire (CHRISTIFIRE) program led to the development and validation of the Flame Spread in Horizontal Cable Trays (FLASH-CAT) model (NUREG/CR-7010, Volume 1). This model addresses the growth and spread of fire within vertical stacks of horizontal, open-back cable trays.

In addition to NUREG-1824 (EPRI 1011999) and the various documents cited above, the individual model developers typically maintain a collection of validation cases that are included as part of the model documentation. The algebraic spreadsheet models, FDT^s and FIVE-Rev1, are based directly on experimental correlations. Validation of these models is typically not part of the model documentation; rather, there are references to source material like the SFPE Handbook or the original test reports. Validation studies by the CFAST and FDS developers are contained within:

NIST Special Publication 1086, CFAST – Consolidated Model of Fire Growth and Smoke Transport, Software Development and Model Evaluation Guide, 2008.

NIST Special Publication 1018, Fire Dynamics Simulator, Technical Reference Guide, Volume 3, Validation, 2007.

In summary, the purpose of the sensitivity analysis is to “re-shape” the scenario with parameters that fall within the V&V range and result in more severe fire generated conditions (e.g., higher HGL or plume temperature, higher incident heat flux, etc.). Depending on the application, one or more parameters may need to be varied affecting multiple dimensionless parameters. It is recommended that the results from the sensitivity calculations always be compared to those resulting from the base case to ensure that the input parameter manipulation produces more severe fire generated conditions.