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.
