Pyrophoric properties of zircaloy particles from irradiated fuel element claddings

In order to assess the fire and explosion hazards of zircaloy fines arising during chopping of fuel elements in a reprocessing plant, artificially produced particles from irradiated and non-irradiated LWR-fuel rod hulls were examined with regard to the pyrophoric properties, using standardized test procedures and devices. Explosion characteristics indicate that the irradiated fines must be classified in category 2 and the non-irradiated particles in a less dangerous category 1 of a scale of explosion hazards of dusts. Original fines from operational tests with a commercial shear proved to be far less pyrophoric than the artificial material. Highly active fission products, when mixed with zircaloy fines, merely lowered the ignition temperature of the latter material, if the mixtures were heated up at low rates in a thermoanalytical device (DTA/TG). At higher heating rates (300C/min), these mixtures produced a vigorous reaction supposedly of the thermite type. Inertisation with nitrogen proved to be an efficient means for suppressing ignition and explosion of the zircaloy particles. (orig./RB)