Iodine-induced Stress Corrosion Cracking Research Articles

Influencing factors and mechanism of iodine-induced stress corrosion cracking of zirconium alloy cladding: A review

Influencing factors and mechanism of iodine-induced stress corrosion cracking of zirconium alloy cladding: A review

Effects of duty cycle on microstructure and iodine-induced stress corrosion cracking behavior of pulse electro-deposited zirconium coating

ABSTRACT Compact and dense zirconium coatings with silvery-white metallic luster were deposited on the surfaces of alloy ZIRLO under different duty cycles of 20%, 50% and 80% by pulse electro-deposition (PED) method in FLiNaK molten salt at 1023 K. The characteristics of the Zr coatings were investigated by SEM, EDS and XRD. The iodine-induced stress corrosion cracking(I-SCC) behavior of the Zr-coated alloy ZIRLO was investigated using slow strain rate tensile (SSRT) tests. The results showed that as the duty cycle increased from 20% to 80%, the average grain size of zirconium deposits increased but current efficiency decreased progressively. All of the Zr-coated samples improve the ultimate tensile strength and ductility in iodized butanol solution with respect to the uncoated Zr. The sample with Zr coating of 50% duty cycle has the highest ultimate tensile strength and ductility during the SSRT test in iodized butanol solution.

Iodine-induced stress corrosion cracking behavior of alloy ZIRLO with Zr coatings by electrodepositing with different pulse current densities

To improve the iodine-induced stress corrosion cracking(I-SCC) resistance of alloy ZIRLO used as nuclear fuel cladding in light water reactor, Zr coatings were prepared on ZIRLO with different current densities by pulse electrodeposition (PED) method. I-SCC behavior of the Zr-coated alloy ZIRLO was tested by slow strain rate tensile (SSRT). The results revealed that the I-SCC resistance of alloy ZIRLO was enhanced by Zr coatings. I-SCC susceptibility indices of ultimate tensile strength and failure time were reduced from 0.23 to 0.12 and 0.55 to 0.40, respectively. Zr coatings with columnar crystals could suppress pitting generation and reduce the diffusion rate of iodine perpendicular to the surface from 3.258 × 10−12 m2/s to 0.175 × 10−12 m2/s. The sample with Zr coating obtained under the current density of 90 mA/cm2 had the highest ultimate tensile strength and ductility, which is attributed to the high interfacial adhesion strength and the structure of continuous columnar crystal.

Toward a Mechanistic Model of Stress Corrosion Cracking in PHWR Fuel Sheaths

Abstract This work builds on the iodine-induced stress corrosion cracking (ISCC) model of Lewis and Kleczek by integrating the fuel performance model fuel and sheath modeling tool (FAST) to provide thermal and mechanical analysis of the fuel sheath, which was previously required as input parameters. The iodine transport methodology of the Lewis–Kleczek model has been modified to utilize the more mechanistic diffusion model in FAST, and an empirical surface multiplier term has been derived to predict iodine release rates under normal operating conditions based on measured release rates from in-reactor sweep gas tests. A fracture mechanics analysis is implemented using threshold stress intensity values and crack growth rates reported in literature. A correlation to predict crack initiation has been derived by analysis of a database of power histories with known ISCC defects. This correlation is based on the change in sheath hoop strain during a power ramp and is shown to be more accurate at discerning failure versus nonfailure than the correlations used in the previous Lewis–Kleczek model and FUELOGRAMS. Failure time prediction of the model is compared against power ramp test FFO-104 performed at the National Research Experimental (NRX) reactor. Fuel failure is predicted to occur 27% faster than experimentally measured; failure time is explored in a series of sensitivity studies to suggest areas for further development. The model improvements represent a step forward in the mechanistic modeling of stress corrosion cracking (SCC) in pressurized heavy water reactor (PHWR) nuclear fuel.

Investigating iodine-induced stress corrosion cracking of zirconium alloys using quantitative fractography

Pellet Cladding Interaction (PCI) is a fuel failure phenomenon that limits power manoeuvrability of light water nuclear reactors with Iodine-Stress Corrosion Cracking (I-SCC) the most commonly hypothesised cause. To increase mechanistic understanding, I-SCC tests have been performed by compressing zirconium alloy c-rings at fixed load while submerged in iodised ethanol. Samples left until failure produced fracture surfaces subsequently analysed using quantitative Scanning Electron Microscopy (SEM) fractography, and samples with cracks arrested part way produced incipient cracks assessed by optical imaging. The influence of microstructure, stress and iodine concentration on I-SCC performances has been assessed by comparison between cold-worked and recrystallised material.Increasing the stress resulted in faster time to failure and produced smaller regions of intergranular cracking; explained using the stress intensity factor and its control over the prevalent mechanism, further evidenced by tests that removed either iodine or stress part way through cracking. Increasing iodine concentrations resulted in faster times to failure and more intergranular cracking indicating the importance of an incubation period and a possible crack blunting effect.Cracks in cold-worked material progress at a non-radial angle, whereas cracks within recrystallised and stress-relieved material cracks progress radially. Analysis suggests that the mechanisms that lead to this difference may be related to grain morphology rather than residual stress as is typically suggested in the literature, with fractography and incipient cracks used as evidence.

Advanced 3D characterisation of iodine induced stress corrosion cracks in zirconium alloys

In this study, a range of advanced techniques have been used to characterise iodine-induced stress corrosion cracks, and investigate their interaction with microstructure in zirconium alloys used as cladding material in nuclear power reactors. C-ring samples, machined from cladding tubes in a recrystallised and cold worked condition, were subjected to compressive stress while submerged in an iodine-ethanol solution to simulate stress corrosion cracks arising from pellet cladding interaction. The complex morphology of these stress corrosion cracks was imaged in 3D using X-ray computed tomography (XCT), and subsequent analysis using both 2D and 3D electron backscatter diffraction (EBSD) allowed for the interaction of these cracks with microstructure to be investigated. Intergranular cracking modes were observed to dominate in the recrystallised microstructure, resulting in cracking in the radial direction with variations on the order of the grain size. Crack propagation in the cold worked microstructure was observed to be correlated with the direction of the elongated grains, resulting in a more tangential crack path with less local variation and a microstructure potentially more susceptible to iodine-induced stress corrosion cracking. 3D-EBSD using serial sectioning allows for the determination of the full five-parameter grain boundary description, and opens up the possibility of a statistical analysis of susceptible boundaries to SCC. Nanoscale secondary ion mass spectrometry (NanoSIMS) was also used to map the chemical distribution in the crack tip region and provides possible evidence of iodine segregation ahead of the crack tip. The results provide new insight into crack propagation mechanisms in recrystallised and cold worked microstructures, and pave the way for a more mechanistic understanding of crack propagation through advanced characterisation of iodine-induced stress corrosion cracks in zirconium alloys.

Stress corrosion crack initiation of Zircaloy-4 cladding tubes in an iodine vapor environment during creep, relaxation, and constant strain rate tests

During accidental power transient conditions with Pellet Cladding Interaction (PCI), the synergistic effect of the stress and strain imposed on the cladding by thermal expansion of the fuel, and corrosion by iodine released as a fission product, may lead to cladding failure by Stress Corrosion Cracking (SCC). In this study, internal pressure tests were conducted on unirradiated cold-worked stress-relieved Zircaloy-4 cladding tubes in an iodine vapor environment. The goal was to investigate the influence of loading type (constant pressure tests, constant circumferential strain rate tests, or constant circumferential strain tests) and test temperature (320, 350, or 380 °C) on iodine-induced stress corrosion cracking (I-SCC). The experimental results obtained with different loading types were consistent with each other. The apparent threshold hoop stress for I-SCC was found to be independent of the test temperature. SEM micrographs of the tested samples showed many pits distributed over the inner surface, which tended to coalesce into large pits in which a microcrack could initiate. A model for the time-to-failure of a cladding tube was developed using finite element simulations of the viscoplastic mechanical behavior of the material and a modified Kachanov's damage growth model. The times-to-failure predicted by this model are consistent with the experimental data.

Multiphysics Modeling of the Role of Iodine in Environmentally Assisted Cracking of Zirconium via Pellet-Clad Interaction

This paper reviews the development of a multiphysics approach to studying the environmentally assisted cracking of zirconium as induced by iodine, a failure mechanism pertinent to the degradation of clad materials encasing nuclear fuel rod assemblies. The phenomenological model of Lewis is reviewed, along with atomistic modeling of surface and grain boundary effects. The paper then surveys the modeling work performed in the authors’ own research group to model the physical and chemical effects surrounding Lewis’ phenomenological model, culminating in a molecular dynamics simulation of intergranular failure. Then, recent trends in modeling are surveyed that will lead to the next steps in the multiphysics simulation of iodine-induced stress corrosion cracking of zirconium and Zircaloy materials, and potentially other systems in which intergranular corrosion cracking is a key mechanism.

I-SCC Behavior Evaluation of N36 Zirconium Alloy Cladding Using Ring Tensile Test

Abstract A ring tensile test was performed to study the iodine-induced stress corrosion cracking (I-SCC) behavior of N36 zirconium alloy cladding at 350°C and in 10 2 , 10 3 , 10 4 Pa iodine vapor pressures. Morphologies of ring samples were observed by optical microscope, ring tensile load-displacement curves and parameters were analyzed to evaluate I-SCC susceptibility. The fracture surfaces of samples after test show the brittle fracture in iodine vapor which is different from cup-cup ductile fracture with non-iodine. Load-displacement curves and parameters show that the mechanical properties are degraded in iodine vapor environment compared to that in non-iodine environment, and more significantly decrease when increasing iodine content. Cladding tube toughness is considered to be an appropriate parameter to assess the I-SCC susceptibility; as a result, cladding toughness decreases gradually with the increasing of iodine content.

First-principles insights into the nature of zirconium–iodine interactions and the initiation of iodine-induced stress–corrosion cracking

We present research relating to iodine-induced stress corrosion cracking (ISCC) and draw insights relevant to the initiation process. The means by which this corrosion initiates is currently unknown. Our previous work has highlighted some of the chemical processes and properties that must be considered in ISCC, and enable us to make some possible connections in the overall corrosion mechanism. A series of calculations has been performed to better characterize the iodine interaction with zirconium, following a hybrid approach that integrates both molecular and solid-state calculations for incorporation into large scale simulations.

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

Iodine-induced stress-corrosion cracking (ISCC), a known failure mode for nuclear fuel cladding, occurs when iodine generated during the irradiation of a nuclear fuel pellet escapes the pellet through diffusion or thermal cracking and chemically interacts with the inner surface of the clad material, inducing a subsequent effect on the cladding’s resistance to mechanical stress. To complement experimental investigations of ISCC, a reactive force field (ReaxFF) compatible with the Zr-I chemical and materials systems has been developed and applied to simulate the impact of iodine exposure on the mechanical strength of the material. We show that the material’s resistance to stress (as captured by the yield stress of a high-energy grain boundary) is related to the surface coverage of iodine, with the implication that ISCC is the result of adsorption-enhanced decohesion.

Iodine-Induced Stress Corrosion Cracking of Zircaloy-4: Identification of Critical Parameters Involved in Intergranular to Transgranular Crack Propagation

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 KI,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.

Modelling of iodine-induced stress corrosion cracking in CANDU fuel

Iodine-induced stress corrosion cracking (I-SCC) is a recognized factor for fuel-element failure in the operation of nuclear reactors requiring the implementation of mitigation measures. I-SCC is believed to depend on certain factors such as iodine concentration, oxide layer type and thickness on the fuel sheath, irradiation history, metallurgical parameters related to sheath like texture and microstructure, and the mechanical properties of zirconium alloys. This work details the development of a thermodynamics and mechanistic treatment accounting for the iodine chemistry and kinetics in the fuel-to-sheath gap and its influence on I-SCC phenomena. The governing transport equations for the model are solved with a finite-element technique using the COMSOL Multiphysics® commercial software platform. Based on this analysis, this study also proposes potential remedies for I-SCC.

An Out-of-Pile Method to Investigate Iodine-induced SCC of Irradiated Cladding

Pellet Cladding Interaction (PCI) during power changes can be a limiting factor for nuclear reactors operation as it can cause fuel cladding failure by Iodine-induced Stress Corrosion Cracking (ISCC) [1]. In the as-fabricated condition, there is a gap between the fuel cladding and the pellets. During reactor operation when fast power transient changes occur, the fuel experiences power ramp and the pellets expand more than the cladding, such that the gap is reduced and eventually closed. When the fuel and the cladding are in direct contact, further fuel swelling and thermal expansion produces tensile stresses in the cladding. Moreover, pellet cracking occurs causing the stress to increase locally in the cladding at the location of the cracks. ISCC initiates at locations where both the stress and the iodine content locally reach critical levels. The cracking initiates at the inner surface of the cladding and propagates towards the outer surface [2, 3].ISCC can be investigated out of pile by means of an expanding mandrel test technique [4, 5]. In this design of the mandrel test equipment, the fuel pellet is simulated using a ceramic sheath with 4 longitudinal slits to facilitate the transport of iodine inside the cladding, which contains a Zr slug (mandrel) inserted in the center. The Zr slug is axially compressed and expands radially forcing the surrounding ceramic sheath to crack. The ceramic fragments can strain the cladding with further compression of the Zr slug.Different versions of the mandrel technique have previously been used to test unirradiated cladding. However, previous studies show that ISCC is sensitive to the condition of the cladding inner surface as well as the material strength, thus the irradiated cladding is expected to behave differently with respect to ISCC than unirradiated cladding.To evaluate the susceptibility to PCI failure of a given fuel rod, power ramp tests are performed in experimental reactors [2]. However, a good out of pile test to complement single ramp tests is crucial to improve the understanding of the mechanisms behind the PCI failures as well as the effect of different parameters.Within the SCIP program<sup>3<sup/>, in addition to ramp testing, a mandrel testing technique that enables out-of-pile testing of irradiated cladding has therefore been developed. The equipment is designed to investigate critical parameters that are known to affect PCI failures such as: irradiation, the strain rate, pellet performance (expansion and cracking), manufacturing (missing pellet surface) and cladding properties (texture, hardness, liner composition etc) [2].The tests are conducted under carefully controlled environmental conditions. Ar gas (99,99%) is passed through a temperature-controlled bath with iodine crystals (reagent grade 99,8%).The effect of the following parameters can be investigated:- Strain rate,- Iodine concentration,- Oxygen potential,- Temperature,- Different crack pattern of the pellet or missing chip,- Cladding properties.The mandrel equipment at Studsvik is specially designed and installed in a hot cell to enable testing on irradiated cladding. The mandrel test technique has been qualified within SCIP using unirradiated cladding. The effects of stress/strain rate, iodine concentration and oxygen potential, on the ISCC behavior were investigated by mandrel testing coldworked non-liner Zircaloy-2 cladding materials.After qualification of the test equipment, several tests have been performed on irradiated non-liner recrystallized Zircaloy-2 claddings. The tests were performed at a temperature of 320°C and iodine partial pressure in the range of 0 to 200 Pa. The tests were run by controlling the strain rate with an initial fast strain rate (21.6%/h) up to 0.6% strain from the initial diameter followed by a slow strain rate (0.36%/h) up to a strain of 2% and with a holding time of 20 hours or until failure. After the mandrel tests, the samples have been characterized by visual inspection, profilometry, LOM and SEM<sup>4<sup/>.The results of the irradiated recrystallized Zircaloy-2 claddings showed that in the presence of iodine partial pressures in the range of 60 to 200 Pa, ISCC occurred at 0.7-0.78% total (elastic + plastic) diametral strain. The low strain to failure as well as typical ISCC features like fluting, observed at the fractured surfaces, are consistent with previous hot cell examinations of PCI failures, and thus the new expanding mandrel technique seems promising.

Proton irradiation effect on microstructure, strain localization and iodine-induced stress corrosion cracking in Zircaloy-4

The radiation-induced microstructure, strain localization, and iodine-induced stress corrosion cracking (I-SCC) behaviour of recrystallized Zircaloy-4 proton-irradiated to 2 dpa at 305 °C was examined. <a> type dislocation loops having 1/3〈1 1 2 ¯ 0〉 Burgers vector and a mean diameter and density of, respectively, 10 nm and 17 × 10 21 m −3 were observed while no Zr(Fe,Cr) 2 precipitates amorphization or Fe redistribution were detected after irradiation. After transverse tensile testing to 0.5% macroscopic plastic strain at room temperature, almost exclusively basal channels were imaged. Statistical Schmid factor analysis shows that irradiation leads to a change in slip system activation from prismatic to basal due to a higher increase of critical resolved shear stresses for prismatic slip systems than for basal slip system. Finite element calculations suggest that dislocation channeling occurs in the irradiated proton layer at an equivalent stress close to 70% of the yield stress of the irradiated material, i.e. while the irradiated layer is still in the elastic regime for a 0.5% applied macroscopic plastic strain. Comparative constant elongation rate tensile tests performed at a strain rate of 10 −5 s −1 in iodized methanol solutions at room temperature on specimens both unirradiated and proton-irradiated to 2 dpa demonstrated a detrimental effect of irradiation on I-SCC.

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

A quantitative analysis of the iodine-induced stress-corrosion cracking (ISCC) process for a Zircaloy-4 cladding and a ZIRLO cladding was performed to support grain-boundary pitting coalescence (GBPC) and pitting-assisted slip cleavage (PASC) models for an ISCC behavior. It was focused on the effects of the microstructure on a grain-boundary pitting and the transgranular cracking phenomenon during a crack propagation step. Also, a microscopic analysis of the stress intensity applied to pits on a grain surface was performed to evaluate the cleavage crack propagation rate of the stress-relieved (SR) and re-crystallized (RX) grains. During the ISCC cracking, it was revealed that the grain shape and cleavage habit plane played important roles in a grain cracking, which resulted in an IG–TG c (TG cracking by cleavage) or a TG f (TG cracking by fluting)–TG c cracking mode. An IG–TG c cracking took place for the RX microstructure through a GB pitting, however, a TG f–TG c cracking did occur for the SR one which in turn increased its propagation rate. The increase of the pitting resistance at the grain-boundary played a critical role in the crack propagation rate of the ZIRLO cladding.

Strain-hardening influence on iodine induced stress corrosion cracking of Zircaloy-4

The iodine induced stress corrosion cracking (SCC) of recrystallized Zircaloy-4 is first intergranular, and then turns to transgranular propagation, before leading to final ductile rupture. In order to progress in the identification of the factors enhancing transgranular propagation of SCC cracks, the effect of strain-hardening on SCC propagation modes is studied. Fractographic observations by SEM of as-received or pre-strained specimens, submitted to iodine methanol under constant load until rupture, show that strain-hardening enhances transgranular propagation of SCC cracks. This propagation mode is associated with a significant acoustic emission. On the base of these results and metallographic observations and texture measurements, the mechanisms associated with strain-hardening at grain scale and their potential consequences on transgranular initiation and propagation of SCC cracks are discussed.

Stress-corrosion crack initiation and propagation behavior of Zircaloy-4 cladding under an iodine environment

Iodine-induced stress-corrosion cracking (ISCC) properties and the associated ISCC process of Zircaloy-4 fuel cladding were evaluated. Cladding was heat-treated to have either stress-relieved (SR) or recrystallized (RX) microstructures, and then an internal pressurization with a smooth and pre-cracked specimen was performed at 350 °C, in an iodine environment. The results showed that the threshold stress-intensity factor ( K ISCC) of the SR and RX Zircaloy-4 claddings were 3.3 and 4.8 MPa m 0.5, respectively. The crack propagation rate of the RX Zircaloy-4 was 10 times lower than that of the SR one. Crack initiation and propagation mechanisms of Zircaloy-4 claddings, which had different microstructures, were proposed by a grain-boundary pitting model and a pitting-assisted slip cleavage model; they showed reasonable results.

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

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°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.

The development of the crack growth model in zirconium claddings in iodine environment

The theoretical model of iodine induced stress corrosion cracking of zirconium claddings which takes into account the cladding texture has been developed. The process of quasistationary crack growth is considered. In the model the crack is described with the set of brittle and ductile regions which are alternating along the crack front. The cladding material has polycrystalline structure and mechanical behavior of the grains depends on their orientation relative to the applied stresses. Chemical potential of dissolved iodine has minimum in the places with maximum tensile stresses which occur in the brittle region of the crack tip. Iodine weakens interatomic bonds in the crack tip and leads to growth of the crack. The stresses in the brittle region relax and the redistribution of the stresses causes the crack growth in the ductile region. In the model the stresses in both regions are calculated self-consistently. Critical iodine concentration in the gaseous phase near the tip determines the condition of crack growth This criterion can be presented in the form that the stress intensity factor exceeds some threshold value. The value of iodine flux to the crack tip has been calculated. The comparison of calculated rate of crack growth with the experimental results has demonstrated a satisfactory agreement. The model predicts the decrease of the threshold SIF for the increase of the tangential component of texture factor that is in a good agreement with the experimental observations.