Pyrophoric properties of radioactive zircaloy fines

Fragmentation of light water reactor fuel rods by chopping with a bundle shear in the head-end of a reprocessing plant is always accompanied by the occurrence of fine zircaloy particles or even dust, which had given rise to concern over safety because of the known pyrophoric behaviour of this alloy. In order to assess the dangers posed by this material to safe operation of the plant, an experimental program was carried out covering standard methods for obtaining safety relevant data for metallic dusts as well as special investigations for determining the oxidation kinetics of zircaloy and the effect of other material, i. e. metallic fission products, on the ignitability. Hazards could arise from zircaloy particles through self-ignition of dust layers accumulating in particular areas of the plant or from dust explosions when particles are raised by air flows. The experiments covered both possibilities. From the operational point of view, three plant areas (shear housing, dissolver and centrifuge or filter) for a potential occurrence of zircaloy fines were taken into consideration and will be discussed below