Simulation Models and Codes Used for Analyzing the Production and Spreading of

Heat

Up to the time being, only in some countries the production and spreading of smoke is simulated. Most of the fire PSA studies do not include modeling of smoke.

In Germany, smoke generation is included in analytical fire simulation modeling but was not yet considered in the fire PSAs carried out.

Smoke production was experimentally analyzed in all HDR fire experiments as well as in actual cable fire experiments by iBMB of TU Braunschweig, the results were implemented in codes participating in the HDR studies and in codes used by iBMB of TU Braunschweig. At the time being, the models used are still under development.

For the time being, the French FLAMME-S code /CAS 96/ models the spreading of smoke in the following way:

• for each fire, a quantity of soot, depending of the burnt material, is produced in a room. This

quantity of soot is used to estimate the smoke concentration in the hot zone which is considered to be homogeneous,

• due to air exchange between hot and cold zones, the smoke concentration in the cold zone,

considered homogeneous, is evaluated,

• in the adjacent rooms the smoke concentration is also calculated taking into account the flow

rate exchange between the different rooms by the openings.

The smoke impact is taken into account for its impact on the radiation heat exchange.

To date, no U.S. nuclear power plant fire PSAs have explicitly modeled the production, transport, and deposition of smoke, and resulting equipment damage from this process. Experiments on some of the basic physical processes have been (e.g., see /JAC 86/) or are being performed (e.g., see /TAN 96/), and simple transport codes (e.g. CFAST) are available, so a credible analysis may be possible in the near future. At present, analyzes use experimental information (e.g., from /CHA 88/) directly in simplistic models.

In Finland, experimental case studies and related simulation codes are used for analyzing the production and spreading of smoke:

• PHOENICS code /CHM 91/ (numerical field code) and developed heat transfer and fire and

smoke product models (CO2, H2O, N2, HCl),

• Smoke spreading and content of smoke (fire gas) product components can be calculated as a

part of the fire simulation /HST 98/.

Some cases are simulated at NPPs, e.g., a main control room, an electrical room, two different cable spreading rooms. The results of the simulations can be used for evaluating the following issues:

− habitability of a main control room,

− toxic products,

− fire fighting environment in electrical and cable rooms,

Examples of the application of recent CFD codes are:

• Simulation of a cabinet fire in a control room of a nuclear power plant (4 cases): The simulation

provided estimates for the smoke and hydrochloride concentrations in the control room and the impact of ventilation and flow obstacles to the local conditions near the control desk.

• Simulation of smoke movement in a room with no ventilation system: The initial temperature

stratification (which is enhanced due to structural objects) may restrict smoke movement so that smoke does not rise to the detectors at the ceiling if the fire source is weak. It may be beneficial

to move the detectors to a lower position (depends on the obstacles

).

For the time being, in Japan the effects of smoke production and spreading have not been taken into account implicitly in the level 1 fire risk analysis by NUPEC. However, a detailed sub-scenario analysis for the main control room fire will be carried out in the near future. The respective code (/TAK 92/), an advanced fluid-dynamic analysis code, the alpha-FLOW code, has the capability of analyzing the production and spreading of smoke. Namely, the time dependent flow and temperature fields induced by fire can be simulated to solve mass conservation equations, mass conservation equations for chemical species, momentum conservation equations, energy conservation equations and equations of states for compressive fluid. The mass conservation equations for chemical species consist of transport equations, which dominate production and consumption, diffusion and convection of chemical species. The alpha- FLOW code has no generation models of smoke but if generation rates are given, it can analyze smoke spreading using the above transport equations. This code will be used on occasional demands.

In the United Kingdom, no modeling of smoke production and spread has been carried out in the PSA. In the reactor safety cases, the spread of smoke has been considered deterministically as a hazard. Barriers and the shutdown of ventilation systems are provided where appropriate to minimize the potential for damage.

In addition, within the HSE mainstream research program on major hazards and risk assessment, work has been undertaken on fire and smoke spread modeling originally to explain the rapid fire spread in the London King’s Cross Underground Station fire. This work used a Computational Fluid Dynamics (CFD) software code called CFX /HAR 96/ developed by AEA Technology. The CFX code has now been widely used by AEA Technology and its clients and is considered effective for a range of applications including

• low-momentum compartment fires,

• high momentum compartment fires,

• fire and smoke movements in tunnels,

• interactions of fire and water spray,

• gas dispersion at nuclear plants,

• deflagrations.

Other general purpose CFD codes available in the UK have been used for fire modeling, including FIREDASS /GRA 98/, FLUENT /AND 88/, and PHOENICS /SPA 87/, /CHM 91/, /GRE 96/. JASMINE /KUM 83/, /KUM 95/ is a CFD code specifically developed in the UK at the UK Fire Research Station for fire modeling. JASMINE has a wide choice of fire source details, including corrections to the fire source for radiative heat loss and in built abilities to model fire spread, flash-over and the effects of sprinkler systems. Future code developments include implementation of the more advanced DTM radiation model (discrete transfer model) and a two-step combustion model. A limitation of the code is that it is currently restricted to rectangular shaped rooms.

in the control room, smoke will fill the room after 8 to 15 minutes. If this situation is reached, the plant should be shutdown from the remote shutdown panel. Some time before, however, self breathing equip- ment (breathing apparatus) would be necessary to stay in the control room.

This time period has been obtained from /NOW 88/. It is dependent of several factors, such as room size, ventilation rate, etc.