Experimental study at medium-scale of the pyrolysis and flame spread on horizontal panels of electrical cable sheath model materials (Ref PsD RES 23-01)

IRSN, a Public Industrial and Commercial Establishment (EPIC), is the national public expert on nuclear and radiological risks. Its missions are now defined by Law 2015-992 of 17 August 2015 on the energy transition for green growth (TECV). IRSN contributes to public policies on nuclear safety and the protection of health and the environment from ionising radiation. As a research and expert assessment body, it acts in consultation with all the players involved in these policies, while maintaining its independence of judgement.

The Fire Test Laboratory of the French Institute for Radiation Protection and Nuclear Safety (IRSN), in partnership with the Polymers Composites and Hybrids Department of IMT Mines Ales, has started an experimental research for more than 5 years on the study of cable ignition and cable flame propagation. These experimental studies were conducted with the CISCCO test device that allows to conduct controlled flame spread experiments at medium-scale.

Electrical cables are one of the main fire hazards in many industrial sectors such as buildings, aircraft, spacecraft, and nuclear power plants (NPPs). Several hundred kilometers of electrical cables can be found in most industrial plants that contain numerous electrical devices (electrical cabinets, digital switch racks, …) and multiple cable trays connecting these appliances. The electrical equipment and cables pose a potential source of fire since they contain both combustible materials and live electrical circuits. This kind of equipment can undergo electrical failures, such as a short circuit, overheating, or electrical arcing, which first ignite plastic materials of both electric wires and cables. To assess the potential damages of the cable fires on safety-related equipment, fire safety analyses rely on tools that intend to determine the fire spread over multiple cable tray installations and the resulting heat release rate (HRR). The cable fire hazard may be assessed in NPPs with simple and advanced cable fire models. The latter models may involve detailed pyrolysis models that are implemented in computational fluid dynamics (CFD) tools. In parallel, more simple modeling was developed based on international experimental programs and were used for safety analyses. 

The Fire Test Laboratory of the French Institute for Radiation Protection and Nuclear Safety (IRSN), in partnership with the Polymers Composites and Hybrids Department of IMT Mines Ales, has started an experimental research for more than 5 years on the study of cable ignition and cable flame propagation. These experimental studies were conducted with the CISCCO test device that allows to conduct controlled flame spread experiments at medium-scale. This original device is equipped with radiant panels that can impose controlled heat flux on the combustible surfaces to simulate the preheating of the fuel material, the ignition and flame propagation. The measurements implemented in the CISCCO device such as fuel temperature, flame heat flux, flame spread velocity and heat release rate, are suitable for the validation of flame spread models. From these experimental findings, simple models were developed, and a database was built to support the development and the validation of these models. These studies used polyvinyl chloride (PVC)-based and halogen free flame retardant (HFFR) cables that are two common electrical cable types used in NPPs.

To go further on the comprehension and the validation of pyrolysis models, the fire community agrees on the need for perfectly instrumented analytical experiments. To the best of our knowledge, the potential of such experimental tests in the field of pyrolysis and flame propagation has remained largely unexplored to date, and the project aims at filling this science knowledge gap. For this purpose a simple combustible material in terms of composition and geometry will be defined and used instead of the complex geometry presented by the cable layer and used in previous studies.

The study will focus on panels of homogeneous composition but representative of the outer sheaths of electrical PVC or HFFR cables. An initial theoretical approach will be carried out on a well-known academic fuel such as black PMMA, to conduct an exhaustive assessment of the metrology and of the experimental protocol. The second major objective of this research project is to develop and implement a metrology system that will ultimately evaluate or determine the main parameters used in current models of pyrolysis or flame propagation on surfaces. For example, the mass loss rate will be measured, and fine thermocouples will be inserted into the material to accurately estimate thermal wave propagation within the material. Infrared imaging will also be used to quantify in detail the temperature of preheating zones, a key parameter in propagation models. Finally, analytical pyrolysis models will be compared with experimental results, and model parameters will be clearly discussed and analyzed. A correlative approach may also be carried out in parallel, since it is of considerable interest for the validation of simplified pyrolysis models. In the longer term, the database will serve the fire research community both analytically and in terms of numerical simulations. These experiments may be used as reference experiments for international benchmarks.

PhD in fire science, significant experience in the conduct of fire experiments and solid knowledge of the related metrology, self-motivation, scientific rigor, work autonomy, ability to work as part of a team, English language proficiency, good writing skills and experiences with writing academic articles are highly beneficial