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Simulation of the pyrolysis of electrical cable sheaths exposed to fire: Characterization and modelling of the morphology and thermal conductivity evolution during the degradation

Jianwei Shi has defended his thesis​ on 9th December 2019 in Chasseneuil-du-Poitou, France.

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Research Unit > IRSN/PSN-RES/SA2I/LIE

Authors > SHI Jianwei

Publication Date > 09/12/2019

Summary

The study of fire spread along cable trays in Nuclear Power Plants (NPP) has shown that the type of cable and sheath plays a key role in the fire growth. In the purpose of simulating the pyrolysis and the fire spread along these cables, the characterization of the thermal and thermokinetic properties of these materials all along their degradation process is required. Besides, several properties, in particular the conductivity, cannot be easily characterized insofar as the volume and the morphology of these materials can evolve and their robustness can be altered during pyrolysis. That is why this work aims at estimating the conductivity of these materials, accounting for their morphology characterized by micro-tomography and the conductivity of each of their components.

This approach involves four steps:
  • Build 3D representations of the degraded polymers at the most significant steps of their pyrolysis, using X-ray tomography to characterize the macro-structure and Scanning Electron Microscopy (SEM) for the micro-structure.
  • Evaluate the effective thermal conductivities of the degraded polymers at these various stages using a numerical homogenization technique.
  • Propose a conceptual model for the morphology evolution during the material degradation, from which a thermal conductivity model can be inferred.
  • Use these effective conductivities in the complete simulation of the material degradation, accounting for the coupled heat and mass transfers and chemical reactions.

In this work, EVA-ATH (i.e. Ethylvinyl Acetate and alumina trihydrate as fire retardant) compounds will be more specifically considered. These materials contain a dispersion of mineral grains of approximately 2 µm corresponding to the ATH load, and their degraded states show pores whose size does not exceed a few hundred microns. Heat transfer is therefore expected to be dominated by conduction and radiation to play a minor role. In addition, there is a large conductivity contrast between the various components, namely the gas within the pores, the polymer matrix, ATH and alumina produced by its dehydration. Such contrasts induce large uncertainties on the material effective conductivity. In this context, the proposed approach allows estimating the value of the effective conductivity but also the uncertainties associated to various material characteristics (porosity scales, anisotropy, conductivity of the components). The particular degradated states observed by tomography and SEM imaging are addressed in the first place. But the formulation of a conceptual model gives also access to the conductivity all along the material degradation. This model is eventually used to carry out pyrolysis simulations with the IRSN CALIF3S-ISIS fire simulation software. On the one hand, such simulations aim at reproducing cone calorimeter degradation experiments of EVA-ATH samples from the literature, for validation purposes; on the other hand, a sensitivity analysis to the uncertainties of the various parameters, including those of the effective conductivity model, is carried out in order to determine the most influential parameters for the pyrolysis process of this kind of material.


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