Self ignition of layers of powder mixtures: Effect of solid inertants

Within the scope of risk assessment in the nuclear industries, the influence of alumina insertion on the self-ignition behavior of zirconium powder layers has been studied. A model, based on a set of partial differential equation describing the distribution of temperature and species concentrations and their evolution with time has been developed. Numerous experiments, both in static and dynamic modes, have been carried out in order to validate the model. They have notably allowed the identification of the oxidation kinetics, which is governed by the diffusion into the oxide layer and represented by Jander's model. The activation energy has been set at 97.5 kJ mol-1. The experimental minimum ignition temperature of pure zirconium has been determined to be 213 degrees C, whereas the model leads to a value of 210 degrees C. It has also shown a good agreement with the tests performed both in static and dynamic modes at various alumina concentrations ranging from 0 to 90 wt.%. The average deviation obtained is 3.5%, which confirms the adequacy of the model. In static mode, a threshold-like behavior has been observed with a threshold level as high as 85 wt.%, whereas a more progressive increase has been obtained for the dynamic heating mode. A parametric study including especially the influences of layer porosity, oxygen partial pressure and layer dimensions has notably demonstrated the importance of the thermal effusivity and of the oxidation kinetics. The limitation due to the oxygen diffusion being predominant, the chemical reactions between nitrogen and zirconium could not be neglected in the lower part of the dust layers. This model allows a good visualization of the ignition process. Thus, it enables the estimation of the ignition delay and temperature, parameters which are essential to evaluate quantitatively the ignition risk of such powders and to propose adequate prevention and protection means. (authors)