Detailed Fire Modelling Approach Overview
Over the last decade Jacobsen Analytics has gained extensive experience in performing and reviewing detailed fire models. The detailed fire modelling methodology was initially created on the basis of NUREG/CR-6850 and was gradually enhanced in order to reflect the ongoing development of the fire analysis techniques.
Generally, a detailed fire analysis is conducted for compartments which have been found to be potentially risk significant. It considers the growth and propagation of a fire and the possibility of it being detected and suppressed before a specific target set is damaged. Three categories of fire scenarios are normally analysed, general single compartment, main Control Room (MCR) and multicompartment scenarios. The ultimate output of the detailed fire analysis is a set of fire scenarios, frequency of occurrence of those scenarios, and a list of target sets (in terms of fire PRA components) associated with the scenarios. For scenarios involving the MCR, the possibility of forced abandonment is also evaluated.
Overview of the Single Compartment Detailed Fire Modelling Process
The detailed fire modelling process relies on a set of tools for simulating the fire effects. The most often applied tools are the empirical correlations (Fire Dynamics Tools) and the zone models.
Fire Dynamics Tools
A series of tools published with NUREG 1805 to provide first-order quantitative methods for performing fire dynamics calculations. Utilising these tools, with respect to performing detailed fire modelling, enables the analyst to determine relevant fire growth parameters, such as:-
- Hot gas layer temperature and smoke layer height
- Radiant heat flux from fire to target fuel
- Ignition time of a target fuel exposed to a constant radiative heat flux
- Full scale heat release rate of a cable tray fire
- Burning durations
- Detector and suppression response times
The above calculations are mainly used in the development of the fire heat release rate profile and establishing the detector and suppression response times. The stages for which the above tools are used in the process are outlined in the process flow chart above.
Consolidated Model of Fire and Smoke Transport (CFAST).
The Consolidated Model of Fire and Smoke Transport, CFAST, is a zone model code that is typically used to simulate the impact of postulated fires and smoke in a specific building environment. CFAST is a two-zone fire model used to calculate the evolving distribution of smoke, fire gases and temperature throughout compartments of a building during a fire. The software is typically utilised in step 5.0 of the process where the postulated fire is simulated in a user-defined room-geometry model. The typical output of the simulation are a series of variables (but not limited to):-
- environmental conditions in the room (such as hot gas layer temperature; plume centerline temperature; oxygen and smoke concentration; and ceiling, wall, and floor temperatures)
- heat transfer-related outputs to walls and targets (such as incident convective, radiative, and total heat fluxes)
- fire intensity and flame height
Jacobsen’s fire modelling methodology also covers the applicability analysis of fire modelling parameters, also known as normalized parameters check. This process is outlined in NUREG-1824 along with the verification and validation of the fire modelling tools. It consists of a series of checks applied in order to confirm that the fire modelling tools are used within the range of their proven applicability.
Based on the results of the fire simulations, the detailed fire modelling process continues with the construction of a fire event tree (FET). The purpose of the FET is to give a graphical representation of the potential pathways of fire development as a function of the availability of the compartment specific fire protection features and also to provide a basis for the calculation of the fire damage state probabilities. Each one of the end state (fire damage state) probabilities calculated from FET is associated with the damage that the fire can cause to a subset of the PSA related equipment. These probabilities are then used for scenario frequency calculation. The detailed fire model of each ignition source results in one or more fire scenarios, consisting of frequency of occurrence and related set of damaged equipment. For the purposes of Fire PSA, the defined fire scenarios are fed into the PSA software of choice (e.g. CAFTA/FRANX, RiskSpectrum, etc.) for further risk quantification.