In-Pile Criteria for the Initiation of Massive Hydriding of Zr in Steam-Hydrogen Environment

摘要

It is shown that irradiation of oxide film on the surface of zirconium alloy by fission fragments may promote the breakdown of oxide protective properties and lead to massive hydriding of zirconium in a steam-hydrogen environment far beyond the condition of "oxygen starvation." Massive hydriding of zirconium claddings may cause severe secondary degradation of failed fuel rods in commercial nuclear reactors. The effect of irradiation is concerned with two factors. The first one is radiolysis of steam-hydrogen mixture and chemical interaction of highly reactive radicals with the oxide surface. The second factor is intense generation of radiation-induced defects in crystal lattice of ZrO_2 which enhance the rate of oxide reduction. A model is proposed for assessment of the threshold conditions for the onset of massive hydriding depending on composition of steam-hydrogen mixture and irradiation dose rate.