Horizontal cable trays fire in a well‐confined and mechanically ventilated enclosure using a two‐zone model

International audienceElectrical cable trays are used in large quantities in nuclear power plants (NPPs) and are one of the main potential sources of fire. A malfunction of electrical equipment due to thermal stress for instance may lead to the loss of important safety functions of the NPPs. The investigation of such fires in a confined and mechanically ventilated enclosure has been scarce up to now and has been investigated in the nuclear industry. In the scope of the OECD PRISME-2 project, the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) conducted more than a dozen of fire tests involving horizontal electrical cable trays burning either in open atmosphere under a calorimetric hood or inside mechanically ventilated compartments to investigate this topic. Calorimetric hood experiments in open atmosphere highlighted that the halogenated flame retardant cable tests had shorter ignition time, faster fire growth rate and higher peak of Heat Release Rate (HRR), compared with the mineral flame retardant cables tested. The influence of the enclosure on the fire behavior depends on the temperature of the surrounding gas of the cables, as well as on the oxygen content at the level of cables. The enclosure strongly impacts the pyrolysis of the fuel, decreasing the mass loss rate and the HRR of the fuel, affecting the fire duration. For tests performed at low ventilation level, combustion of unburned gases occurred due to a high production of pyrolysed gas in excess. A semi-empirical model of horizontal cable trays fires in a well-confined enclosure was developed. This model is partly based on the approach used in FLASH-CAT and on experimental findings from the IRSN cables fire tests. It was implemented in the two-zone model SYLVIA. The major features of the compartment fire experiments, such as characteristic HRR and fire duration, could then be reproduced with acceptable error, except for combustion of unburned gases, occurring in the upper part of the fire compartment. The development of such a semi-empirical model is a common practice in fire safety engineering concerned with complex solid fuels