When determining how best to prevent, mitigate, and suppress a fire, engineers must first understand how a fire would behave in a particular space. As part of the fire protection services provided, ARS uses a variety of programs to determine every potential behavior of any fire and how each variable within a space would impact that behavior.
There is a movement in the nuclear power plant fire protection community to use risk-informed and performance-based analyses. Since 2004, the NRC has permitted plants to take advantage of the performance-based requirements contained in NFPA 805 as an alternative to the existing deterministic or prescriptive fire protection requirements. Fire modeling is a key tool in furthering risk-informed and performance-based fire protection for nuclear power plants.
ARS uses fire models for the following purposes:
- To demonstrate code compliance
- To demonstrate ability to meet performance-based criteria
- To analyze configurations not addressed by codes
- To support fire probabilistic risk analysis using NUREG/CR-6850
Types of Fire Models
When performing fire modeling, ARS uses a variety of programs that have been verified and validated to demonstrate code compliance. The fire modeling engineers at ARS employ three different types of fire modeling programs: algebraic, zone, and the fluid dynamic or computational fluid dynamic model.
FDTs – Algebraic Model
This model is used by ARS fire protection engineers to solve for correlation based on experiments, and some algebraic models go straight into the zone model. In these applications, the engineer is evaluating one data point. This model usually contains closed-form algebraic expressions. In some cases, algebraic models, when used properly, can provide an estimate of fire variables such as hot gas layer (HGL) temperature, heat flux from flames or the HGL, smoke production rate, depth of the HGL, and the actuation time for detectors. Algebraic models are helpful because they require a minimum of computational time. These models are useful primarily as screening tools.
CFAST – Zone Model
ARS fire protection engineers use this model when performing a two-zone model analysis. This model is a classic two-zone fire model that predicts the compartment conditions for the upper and lower gas layers. CFAST subdivides each compartment into two control volumes with a relatively hot upper layer (hot gas layer) and a relatively cool lower layer (fresh air). Each layer has its own energy and mass balances, and the model assumes that the temperature and gas concentrations are constant throughout the zone. The CFAST model accounts for the size and geometry of the room when simulating the effects of a fire and can model the flame and plume regions around a fuel source.
FDS – Computational Fluid Dynamic Model
When determining compartment variables at a specific location, or when geometric features are expected to play a significant role in the results, ARS fire protection engineers use the FDS computational fluid dynamic model. Some engineers prefer the FDS model because it can solve for a temperature profile, and the engineer receives a complete set of data containing all the information needed for a comprehensive fire risk analysis.