Oxidation-induced creep in Zircaloy-2: II. Stress distribution in the oxygen-stabilised α-phase layer

Steam oxidation of Zircaloy-2 produces tensile extension; at a given temperature the strain increases parabolically with time. Oxygen dissolution in Zircaloy-2 during oxidation stabilises the α-phase and causes lattice dilatation. These two observations are used to develop a model of the oxidation-induced deformation behaviour of the stabilised α-layer and hence to calculate the stress distribution across the layer.     

Air oxidation of Zircaloy-4 in the 600-1000 °C temperature range: Modeling for ASTEC code application

Progress in the treatment of air oxidation of zirconium in severe accident (SA) codes are required for a reliable analysis of severe accidents involving air ingress. Air oxidation of zirconium can actually lead to accelerated core degradation and increased fission product release, especially for the highly-radiotoxic ruthenium. This paper presents a model to simulate air oxidation kinetics of Zircaloy-4 in the 600-1000 °C temperature range. It is based on available experimental data, including separate-effect experiments performed at IRSN and at Forschungszentrum Karlsruhe. The kinetic transition, named “breakaway”, from a diffusion-controlled regime to an accelerated oxidation is taken into account in the modeling via a critical mass gain parameter. The progressive propagation of the locally initiated breakaway is modeled by a linear increase in oxidation rate with time. Finally, when breakaway propagation is completed, the oxidation rate stabilizes and the kinetics is modeled by a linear law. This new modeling is integrated in the severe accident code ASTEC, jointly developed by IRSN and GRS. Model predictions and experimental data from thermogravimetric results show good agreement for different air flow rates and for slow temperature transient conditions.     

Model to simulate oxidation kinetics of zircaloy-4 during temperature transients between 600 and 800°C

Numerical methods have been used extensively to solve differential equations in diffusion problems, particularly the finite difference method in its explicit version. A substantial modification is introduced here in the implicit finite difference method (parabolic interpolation in the interface) to allow for accurate tracing on the interface movement even in points non-coincident with the mesh. This procedure results in a powerful code, adequate to simulate any interface movement with good accuracy and low expense in calculation time.The code was employed to simulate the linear oxidation kinetics of Zircaloy-4 (Zry-4) in water vapor during various temperature transients: upward ramp, downward ramp and sawtooth.The fundamental assumption of this model is that the parabolic-to-linear transition is due to the interconnection of pores generated in the oxide together with the oxide itself. Moreover, the model assumes that on account of pore interconnection, two zones may be recognized in the oxide: a channeled region formed by stoichiometric oxide and a diffusion zone, separated by another moving interface. The hypothesis is formulated that the pore interconnection speed is the variable which controls the overall linear process.Experimental evidence suggests that under variable temperature conditions, Zr or Zry oxidation is not only temperature-dependent but also shows some memory effects. These are reflected in the model through the speed of pore interconnection which is allowed to be delayed with reference to the temperature variations.Although the results obtained with the model seem to be satisfactory, further experimental evidence is required for comparison with the predictions of the model.     

Development of Computer Code PRECIP-II for Calculation of Zr-Steam Reaction

A code PRECIP-II has been developed for the prediction of Zircaloy cladding oxidation under loss-of-coolant accident conditions of a light water reactor. This code is an extended, improved version of SIMTRAN-I produced by S. Malang. Main improvements have been made on the treatment of boundary conditions in a cooling period, which enabled the calculation to take into account the structure change of β-phase, i.e. α-phase precipitation. Modifications were made on the values of oxygen solubilities and diffusivities so that the values of isothermal reaction rate constants derived from calculations may match to the experimental data. Then, calculations and experiments of oxidation were carried out along various types of temperature histories, making comparison between the results of calculation and measurement on weight gain, thicknesses of oxide and α-phase layers. The results were that the differences between the calculated and experimental data lie for the most part within ±10%.     

Examination of the chemical composition of irradiated zirconium based fuel claddings at the metal/oxide interface by TEM

Detailed post-irradiation examinations have been performed at PSI on three fuel rods with differing cladding materials revealing different corrosion behaviour. The rods had been irradiated for 3-5 cycles at Gösgen nuclear power plant (pressurised water reactor), Switzerland. As zirconium corrosion is proceeding at the metal/oxide interface, extended micro-structural analyses were performed by transmission electron microscopy (TEM), expecting to possibly reveal phenomena explaining the varying corrosion resistance. This paper reports on the distribution of oxygen at the metal/oxide interface examined by energy dispersive X-ray spectroscopy (EDS) in TEM, while other micro-structural investigations have been published earlier [1]. In order to get some statistical confidence in the analyses, three neighbouring TEM samples of each cladding variant were studied. The oxygen concentration profiles of the three alloys (i.e. low-tin Zircaloy-4, Zr2.5%Nb and extra low-tin (Sn 0.56%)) both in the oxide and metal close to the metal/oxide interface are compared. The results of the examinations show the composition of the oxide in the vicinity of the interface to be sub-stoichiometric for all three materials, indicating an oxide layer adjacent to the interface, with diffusion-controlled access of oxygen to the metal/oxide interface. The metallic parts show highest oxygen concentrations at the metal/oxide interface which are reduced towards the bulk metal, pointing towards the expected second diffusion-controlled process leading to α-Zr (O). Based on the experimental results values for the diffusion coefficients in the range of 0.8-6.0 × 10⁻²⁰ m² s⁻¹ are estimated for the oxygen dissolution process, the diffusion coefficient in Zircaloy-4 being six times higher than for the other two less corroding alloys. This finding is in contradiction with the present assumptions about the corrosion mechanism, and confirms the expected but not so far reported diffusion controlled oxidation of different zirconium alloys. It also points towards a corrosion rate that is at least partly governed by the diffusion coefficient of oxygen in metal that is different for different alloys, unlike what has been assumed till present.     

Oxidation-induced creep in Zircaloy-2: III. The average stress in the oxide layer

Oxidation of Zircaloy-2 at high temperatures results in the formation of a surface oxide, growth of an underlying layer of oxygen-stabilised α-phase and tensile deformation of the metal substrate core. A model, developed to account for this oxidation-induced deformation, is used to calculate the mean stress generated within the oxide during the course of oxidation.     

Effect of Fuel Pin Pitch on Core Characteristics of Large LMFBR

A simple model of hydrogen diffusion and hydride precipitation was developed to estimate the radial re-distribution of hydrogen in zirconium-lined Zircaloy-2 claddings for light water reactors. Numerical calculation based on the model agreed well with experimental data and clearly showed that a slight difference of the terminal solid solubility of hydrogen in the two materials caused a significant flow of hydrogen from Zircaloy-2 to zirconium during slow cooling. The model would also evaluate the hydrogen distribution in the claddings of which the terminal solid solubility is not uniform.     

Development of the Ultra-microhardness Technique for Post Irradiation Examination of Fuel Cladding Tubes

Hardness measurements are potentially valuable for a quantitative discussion of embrittlement in the inner portions of fuel cladding tubes. The size of the indentation, however, is not negligible compared to the measuring region, even when a micro Vickers hardness tester is employed. This limits the measuring technique, and very little has been studied about degradation phenomena in the inner portion of the tubes.

A hardness measurement system, equipped with a depth-sensing indentation instrument, and the necessary post irradiation examination technique for specimens with high radioactivity were successfully developed and the following observations were obtained from the system's application example. The diffusion coefficient of oxygen obtained from the hardness of an unirradiated zirconium lined cladding with simulated oxidation in the fuel rod showed good agreement with literature data. The calculated diffusion coefficient from hardness in the inner portion of irradiated Zircaloy-2 fuel rods was almost the same value as that of unirradiated zirconium, which implied that neither neutron irradiation nor fission fragment bombardment enhanced the oxygen diffusion in the inner portion of cladding tube.     

High-temperature oxidation of zircaloy-2 and zircaloy-4 in steam

At temperatures above the $\text{(α + β)}\text{β}$ transformation temperature for zirconium alloys, steam reacts with β-Zr to form a superficial layer of zirconium oxide (ZrO₂) and an intermediate layer of oxygen-stabilized α-Zr. Reaction kinetics and the rate of growth of the combined (ZrO₂ + α-Zr) layer for Zircaloy-2 and Zircaloy-4 oxidation in steam were measured over the temperature range 1050–1850°C. The reaction rates for both alloys were similar, obeyed parabolic kinetics and were not limited by gas phase diffusion. The parabolic rate constants were consistently less than those given by the Baker and Just correlation for zirconium oxidation in steam. A discontinuity was found in the temperature dependence of both the reaction rate and the rate of growth of the combined (ZrO₂ + α-Zr) layer. The discontinuity is attributed to a change in the oxide microstructure at the discontinuity temperature, an observation which is consistent with the zirconium-oxygen phase diagram.     

Application of Zeolites to Remove Iodine from Dissolver Off-Gas, (I)

The oxidation behavior of Zr(Fe,Cr)₂ precipitates being present at the surface of Zircaloy-4 was examined with microprobe Auger electron analysis, focussing attention on the oxidation behavior of chromium and iron in the early stage of oxidation where the oxide film was coherent. Chromium formed a thin oxide layer at the top surface of the zirconium oxide film of precipitate and remained in a metallic state inside the oxide film. Iron was oxidized via dissolution in the matrix zirconium oxide near the top surface and remained in a metallic state inside the oxide film. Such variety of chemical state of chromium and iron with depth in the oxide film was attributed to the existence of oxygen potential gradient in the oxide film.     

Hydrogen absorption by zirconium alloys at high temperatures

At high temperatures, e.g. during a hypothetical severe accident, zirconium and its alloys are not stable to other materials and to oxidising atmospheres. Exothermic reactions with steam cause the production of hydrogen which will be released to the atmosphere and, thus, endanger the containment or may be absorbed by the remaining metal. The hydrogen solubility in Zircaloy-4 and Zr-1Nb was measured in the temperature range of 1230-1730 K and at hydrogen partial pressures between 10 and 100 kPa. The parameters of the Sieverts constant were determined. No significant differences between the two alloys were observed. The hydrogen solubility of oxygen containing Zircaloy-4 decreases with increasing oxygen content.     

Air oxidation of Zircaloy-4, M5® and ZIRLO™ cladding alloys at high temperatures

The paper presents the results of isothermal and transient oxidation experiments of the advanced cladding alloys M5® and ZIRLO™ in comparison to Zircaloy-4 in air at temperatures from 973 to 1853 K.Generally, oxidation in air leads to a strong degradation of the cladding material. The main mechanism of this process is the formation of zirconium nitride and its re-oxidation. From the point of view of safety, the barrier effect of the fuel cladding is lost much earlier than during accident transients with a steam atmosphere only.Comparison of the three alloys investigated reveals a qualitatively similar, but quantitatively varying oxidation behavior in air. The mainly parabolic oxidation kinetics, where applicable, is comparable for the three alloys. Strong differences of up to 500% in oxidation rates were observed after transition to linear kinetics at temperatures below 1300 K.The paper presents kinetic rate constants as well as critical times and oxide scale thicknesses at the point of transition from parabolic to linear kinetics.     

High-temperature internal friction in polycrystalline Zirconium and Zircaloy-4

The data on the high-temperature internal friction of zirconium and zirconium alloys are reviewed and new results on zirconium and Zircaloy-4, measured at low and at intermediate frequencies, are presented. It is shown that the damping spectrum of pure zirconium, for annealed polycrystals, shows a peak probably related to relaxation of grain or subgrain boundaries. The data on Zircaloy-4 show two peaks: one near the grain-boundary peak in the pure metal and another one at a higher temperature. Possible mechanisms for these peaks are discussed. Finally, the high-temperature internal friction background of zirconium and zirconium alloys is analyzed and, for Zircaloy-4, the apparent activation enthalpy is found to be related to the grain size.     

Comparative study and imaging by PhotoElectroChemical techniques of oxide films thermally grown on zirconium and Zircaloy-4

The oxidation of iron and chromium that are present as impurities in zirconium metal or as alloying elements in Zircaloy-4 was investigated with PhotoElectroChemical techniques (PEC), highlighting the chemical nature, the size and the lateral distribution of Fe and Cr-containing phases in thin zirconia scales formed during the oxidation of pure zirconium and Zircaloy-4 at 470 °C in oxygen. In the case of zirconium, iron and chromium impurities led to the formation of oxides distributed in a homogeneous way in the zirconia scale, while in the case of Zircaloy-4 these elements, present in the form of intermetallic particles in the substrate, led to the formation of localised haematite Fe₂O₃, rhomboedric solid solution (Fe_xCr_1−x)₂O₃ and chromia Cr₂O₃ phases. These phases were accurately studied via the measurement of their respective band-gap (Fe₂O₃: 2.2 eV, (Fe_xCr_1−x)₂O₃: 2.6 eV and Cr₂O₃: 3.0 eV). It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale at a micron scale.     

Prototypical experiments relating to air oxidation of Zircaloy-4 at high temperatures

The mechanism of the reaction between Zircaloy-4 and air at temperatures from 800 to 1500 °C was studied. Air attack under prototypical conditions with air ingress during a hypothetic severe nuclear reactor accident was investigated. Oxidation in air and in air and nitrogen-containing atmospheres leads to a major degradation of the cladding material. The main mechanism is the formation of zirconium nitride and its re-oxidation. Pre-oxidation in steam prevents air attack as long as the oxide scale is intact. Under steam/oxygen starvation conditions, the oxide scale is reduced and significant external nitride formation takes place. When modeling air ingress in severe accident computer codes, parabolic correlations for oxidation in air may be applied only for high temperatures (>1400 °C) and for pre-oxidized cladding (?1100 °C). Under all other conditions, faster, rather linear reaction kinetics should be applied.     

The Effect of Oxide Microstructure on Kinetic Transition in Out-of-pile Steam Corrosion Test for Zircaloy-2 and Nb-added Zircaloy-2

In order to study the mechanism of kinetic transition of corrosion rate for zirconium alloys, oxide films formed on Zircaloy-2 (Zry-2) and Nb-added Zircaloy-2 (0.5Nb/Zry-2) in steam at 673 K and 10.3 MPa were examined with TEM and SIMS.

Kinetic transition occurred at almost the same oxide thicknesses for both Zry-2 and 0.5Nb/Zry-2, but the corrosion rate after the transitions were quite different for the two alloys. Zircaloy-2 showed cyclical oxidation, while the weight gain of 0.5Nb/Zry-2 increased linearly.

The morphology and crystal structure were similar for the oxides of the two alloys and both the oxide films still mainly consisted of columnar grains even after the transition. Interface layers which mainly consisted of a-Zr crystallites were observed for both alloys and the oxygen content in the interface layers increased after the transition.

The solute concentrations of Fe, Cr and Ni became higher, accompanying the increase of oxygen concentrations at columnar grain boundaries in the oxide films after the transition for 0.5Nb/Zry-2. It was thought that the properties of grain boundaries of the 0.5Nb/Zry-2 oxide films changed after the transition, and the increase in oxygen diffusivity at grain boundaries caused the linear increase in weight gain.     

Crystal structure and grain size of Zr oxide characterized by synchrotron radiation microdiffraction

Zirconium oxides formed on Zirclaoy-4 and Zr-1.5Nb (in wt%) were characterized by the microbeam X-ray diffraction using a synchrotron radiation. The phase fraction and the grain size were determined as a function of the position in the oxide. It was found that Zr-1.5Nb showed the better corrosion resistance than Zircaloy-4 in 360 °C pure water although the tetragonal phase was more stabilized to a further distance from the metal/oxide interface in the oxide of Zircaloy-4. The calculation of the grain size revealed that the oxide of the Zr-1.5Nb had larger grains than that of Zircaloy-4 with the tetragonal phase being smaller than the monoclinic one. It seems reasonable to suppose that the superior corrosion resistance of Zr-1.5Nb was attributed to the lager grain size of the oxide in which the oxygen diffusion is expected to be lowered when compared to the smaller grain size of the oxide on Zircaloy-4.     

Localised electron transport in corroding zirconium alloys

Zircaloy-2, in the as-received and heat-treated conditions, and the binaries of Zr with Sn, Fe, Cr and Ni, were oxidised in a fused salt medium at 300 and 400° C. The electrochemical polarisation behaviour was studied at various oxide film thicknesses. Metal electrodes were evaporated on to the oxide films and the dc conduction of the alloy-oxide-metal diodes was investigated. Analysis of the data shows that the presence of the Zr-Fe intermetallic phase is associated with an enhanced localised electron transport which leads to low negative corrosion potentials and fast oxidation rates; the high temperature oxidation behaviour of zirconium alloys falls into two distinct groups depending on the presence or absence of this intermetallic phase. In proposing a model for the oxidation of zirconium alloys, information regarding the processes controlling ion and electron transport is essential. Because of the possibility of electron transport occurring through the bulk oxide and at localised second-phase precipitates by different mechanisms, it is unlikely that there is a simple relation between the electron current and potential drop across the oxide.     

Raman investigation of pre- and post-breakaway oxide scales formed on Zircaloy-4 and M5® in air at high temperature

For nuclear facility safety analysis purposes, the Zircaloy-4 and M5® air oxidation scales produced in the 800–1000 °C temperature range have been investigated with micro-Raman spectroscopy. The oxides were characterized to determine the microstructures associated with protective and non-protective behaviors. Systematic scans, from the oxide/metal interface to the scale surface, were performed to study the variations in crystal structure and composition as a function of position in the oxide. The results presented here tend to indicate that the crystallographic phases that are present in the scales are strongly dependent on alloy composition. In particular, looking at scales grown at 800 °C, the crystallographic phases observed for the M5® alloy, t-ZrO₂ and m-ZrO₂, are similar in many respects to those already identified in scales grown in pure oxygen or steam. This is not the case for Zircaloy-4, since we clearly observed additional Raman signatures which most probably track the presence of nitrogen in the layers well before the occurrence of the kinetic transition. Indeed, the Raman measurements first show the presence of cubic zirconia in the layers, and strongly suggest the presence of zirconium nitride and oxynitride. Increasing the oxidation temperature, zirconium nitride is clearly detected in the scales well before the occurrence of the kinetic transition. Thus, any mechanism proposed to explain this oxidation behavior must consider the fact that both nitrogen and oxygen participate in the scaling reaction.     

Effect of dissolved oxygen on oxidation and hydrogen pick up behaviour—Zircaloy vs Zr-Nb alloys

Upcoming advanced nuclear power reactors contemplate partial boiling inside the core and it is important to establish the effects of partial boiling on oxidation and hydrogen pick up for sufficiently long periods of time. The effect of partial boiling on oxidation behaviour was assessed in this study in terms of change in the level of dissolved oxygen (DO) in static autoclaves. The results clearly demonstrated that the oxidation behaviour of Zircaloy-2 is quite immune to the level of dissolved oxygen, while Zr-Nb alloys showed very different oxidation behaviour with change in the dissolved oxygen level. The weight gain due to oxidation in Zr-Nb alloys during long term exposure was almost doubled with increase in the DO level. Nevertheless, in case of all the three alloys, the corresponding hydrogen pick up was found to be lower for materials oxidized at higher DO levels irrespective of the effect of DO level changes on weight gain. This is attributed to the preferential reaction of the metal with free oxygen rather than water to produce the oxide when the DO levels are high. Further, the oxidation behaviour of Zr-1 wt%Nb alloy, as a function of dissolved oxygen content in lithiated water, was monitored in situ by electrochemical impedance spectroscopic (EIS) measurements using controlled displacement electrochemistry (CDE) technique in a recirculation loop at reactor operation temperature. The oxidation behaviour (for upto 90 h duration at 310 °C) in high DO content of water was in good correlation with the results obtained from accelerated oxidation experiments during short term exposure (up to 24 h at 400 °C) in static autoclaves.