Papers by Author: Hiroshi Abe

Abstract: During power transient conditions in nuclear reactors, uranium oxide pellets expand and crack due to the increase in temperature and their poor thermal conductivity. Moreover, the cladding undergoes creep because of the external pressure, and its diameter shortens. These antagonistic phenomena lead to the establishment of a contact between the pellet and the cladding, called the pellet-cladding interaction. The synergistic effect of the hoop tensile stress and strain imposed on the cladding by fuel thermal expansion and corrosion by iodine released from the UO₂ fuel as a fission product at the same time can lead to Iodine-induced Stress Corrosion Cracking (I-SCC) of the Zircaloy-4 cladding. I-SCC failures of zirconium alloys are usually described in three steps: initiation of cracks, intergranular subcritical propagation, and critical propagation with a brittle transgranular propagation mode [1]. Transgranular propagation occurs as soon as the stress intensity factor overshoots a threshold value K_I,SCC. It is the critical step and leads to the final ductile failure of the cladding. Transgranular cracks propagate by cleavage-like fracture on basal planes of the hexagonal lattice and fluting; it is the result of a competition between a plastic accommodation of the applied strain and the brittle fracture of basal planes by iodine assisted cleavage.

Abstract: A number of mechanisms have been proposed to understand stress corrosion cracking (SCC) of metals, e.g. (1) slip dissolution and active pass corrosion based on anodic dissolution of metals, (2) tarnish rupture and internal oxidation based on oxidation ahead the crack tip followed by cracking of the oxides, and (3) hydrogen cracking, etc. If dissolution of metals takes the essential role in the stress corrosion cracking concerned, cracking susceptibility is expected to be significantly affected by dielectric constant of water. Because dielectric constant represents a character of water as a solvent, which determines solubility of metal oxides, and therefore corrosion rate of metals is strongly dependent on dielectric constant of water. K-constant type SCC growth rate tests have been done as a function of physical property (dielectric constant) of water by either manipulating temperature under iso-pressure condition (15MPa) or manipulating pressure under iso-thermal condition (330oC). Intergranular cracking was more enhanced and the crack growth was significantly accelerated under the condition of higher dielectric constant, indicating that dissolution of metal plays important role in the cracking mechanism of 316L stainless steels under the present testing conditions.