Crack initiation and propagation behavior of zirconium cladding under an environment of iodine-induced stress corrosion

Abstract

Tests of iodine-induced stress corrosion cracking (ISCC) were carried out to elucidate the initiation and propagation of cracks in the claddings of zirconium alloys. Zircaloy-4 cladding and Nb-contained zirconium cladding were pressurized with and without a pre-cracked state at 350 o C in an iodine environment. The results show that pitting nucleation and growth play an important role in initiating ISCC. Pits preferentially grow and agglomerate around the grain boundary, where the number of pits increases with the iodine concentration and the hoop stress of the claddings. A model of grain-boundary pitting coalescence and a model of pitting-assisted slip cleavage, which were proposed to clearly elucidate the crack initiation and propagation process under ISCC, produce reasonable results. The Nb-contained zirconium cladding exhibits higher ISCC resistance than Zircaloy-4 from the standpoint of a higher threshold stress-intensity factor and a lower crack propagation rate.

Effects of the microstructure and alloying elements on the iodine-induced stress-corrosion cracking behavior of nuclear fuel claddings

“…The K I value was adjusted so that the stress state around the crack tip was in a plane strain condition. The detailed experimental procedure regarding a fatigue pre-cracking and the ISCC procedure can be found in previous papers [1,2]. Fig.…”

“…Recently, it has been proposed that grain-boundary pitting coalescence (GBPC) and pitting-assisted slip cleavage (PASC) models are appropriate for modeling crack nucleation and growth in fuel claddings, such as a commercial grade of nuclear fuel cladding [1,2]. The main factors affecting the crack initiation step are pitting nucleation, growth and agglomeration in that the pits are preferentially formed around a grainboundary (GB) as a result of a local corrosion reaction by iodine.…”

Atomistic Simulations of Formation of Elementary Zr-I Systems

“…(2) with the combination of the impure Zr and I 2 gas forming volatile ZrI 4 , which then deposits on a filament, and can decompose under the equilibrium of (1), resulting in pure zirconium. This same mechanism is suspected to be a means of I-SCC in Zr cladding [11]. Formation of zirconium iodides, however, can occur in multiple stoichiometries.…”

“…The formation of zirconium iodides are important due to observations seen in previous research [15][16][17], which can lead to pit formation and nucleation [5,11], followed by material failures. A suspected mechanism of pit formation is due to weakening of Zr-Zr bonds [7] at and around the site of iodine adsorption.…”

Reduced yield stress for zirconium exposed to iodine: reactive force field simulation

“…It is possible that synergistic interactions with the electropositive fission products (Cs, Cd, Sr, among others) can chemically modify and/or mechanically weaken the oxide, undermining its passivity. The significance of grain-boundaries and triple-points was made apparent in recent investigations by Park [15]. A fundamental approach to modeling ISCC, therefore, should involve a deconstruction of these effects, to assess their individual contributions, prior to a final integration.…”