A model for uniform Zircaloy clad corrosion in pressurized water reactors

A phenomenological water-side corrosion model for Zircaloy fuel cladding for pressurized water reactors (PWRs) is considered. The model acounts for the breakaway transition in the Zircaloy oxidation rate that takes place in an isothermal condition and the changes that occur during reactor operation, i.e. the dependence of oxide growth on fast neutron flux and cladding oxide layer thickness. Closed-form analytical solutions of the oxidation kinetics equations are obtained. The corrosion kinetics model is coupled to PWR thermal and hydraulic models which assume a subchannel that is either a closed single channel or a multichannel which accounts for coolant cross-flow and coolant enthalpy mixing. Both single-phase forced convection and subcooled nucleate boiling are accounted for in the thermal-hydraulic models. The model calculates the coolant temperature at the axial midplane of each axial segment of the fuel rod. When an oxide layer is present, the temperature at the metal-oxide interface is determined. This temperature in turn is used to determine the oxide growth via the Arrhenius temperature dependence of the Zircaloy oxidation rate. The predictions of the model have been compared with the measured cladding oxide data obtained in PWRs. The data for a given rod were obtained at various burn-ups (at the end of reactor cycles) and various axial positions of the rod. Our evaluations show that the model predicts the measured data satisfactorily; however, the deviations are discussed. The model has been used to study the effect of core loading patterns on cladding oxide growth. Our analyses show that core nuclear design is an important factor for water-side corrosion of fuel rods.     

CEMS AND COUPLING EFFECT OF INTERLAYERS IN MULTILAYERED FeSi/Si AMORPHOUS FILMS

Multilayered FeSi/Si amorphous films with fixed FeSi layer thickness and different Si layer thicknesses have been studied by conversion electron Mossbauer spectroscopy at room temperature. The results showed that with decreasing the Si layer thickness, the hyperfine field of samples increased and the thickness of interface dead layers arisen from the atomic interdiffusion effect decreased. These are due to the coupling effect between the magnetic layers. When the Si layers are thinner than 0.88 nm, the direction of the magnetization is out of the film plane.     

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.     

模拟高放玻璃体在高温湿气中的蚀变行为

模拟地下水穿透地质处置多重屏障情形下，地下水会以高温蒸气的形式与高放玻璃体发生反应，加速玻璃体的蚀变。采用纯水和特定的盐溶液作为浸泡剂以控制体系的相对湿度分别在100%和90%左右。模拟高放玻璃体样片用聚四氟乙烯线悬挂在浸泡剂上方并在低氧箱中平衡，分别在120 ℃和150 ℃的干燥箱内反应。通过测试浸泡液中各元素的浓度和分析玻璃体表面的腐蚀产物，以表征玻璃的蚀变速率。实验发现，温度对玻璃腐蚀的影响最为明显，湿度的影响较弱。玻璃体中大多数元素在150 ℃下的浸出速率高于120 ℃下的浸出速率2～3个数量级。90 d时，玻璃体在150 ℃下已观察到显著的腐蚀。900 d后，150 ℃下玻璃体已基本完全腐蚀；而在120 ℃下的玻璃腐蚀程度很小，900 d时腐蚀厚度仍小于50 μm。150 ℃时随着腐蚀的发生，玻璃表面的SiO2腐蚀层逐渐形成以钙的硅酸盐为主的二次矿相。     

模拟高放玻璃体在高温湿气中的蚀变行为

模拟地下水穿透地质处置多重屏障情形下，地下水会以高温蒸气的形式与高放玻璃体发生反应，加速玻璃体的蚀变。采用纯水和特定的盐溶液作为浸泡剂以控制体系的相对湿度分别在100%和90%左右。模拟高放玻璃体样片用聚四氟乙烯线悬挂在浸泡剂上方并在低氧箱中平衡，分别在120 ℃和150 ℃的干燥箱内反应。通过测试浸泡液中各元素的浓度和分析玻璃体表面的腐蚀产物，以表征玻璃的蚀变速率。实验发现，温度对玻璃腐蚀的影响最为明显，湿度的影响较弱。玻璃体中大多数元素在150 ℃下的浸出速率高于120 ℃下的浸出速率2～3个数量级。90 d时，玻璃体在150 ℃下已观察到显著的腐蚀。900 d后，150 ℃下玻璃体已基本完全腐蚀；而在120 ℃下的玻璃腐蚀程度很小，900 d时腐蚀厚度仍小于50 μm。150 ℃时随着腐蚀的发生，玻璃表面的SiO2腐蚀层逐渐形成以钙的硅酸盐为主的二次矿相。     

Structure of zirconium alloy oxides formed in pure water studied with synchrotron radiation and optical microscopy: relation to corrosion rate

A detailed study was undertaken of oxides formed in 360 °C water on four Zr-based alloys (Zircaloy-4, ZIRLO™,¹ Zr-2.5%Nb and Zr-2.5%Nb-0.5%Cu) in an effort to relate oxide structure to corrosion performance. Micro-beam X-ray diffraction was used along with transmitted light optical microscopy to obtain information about the structure of these oxides as a function of distance from the oxide-metal interface. Optical microscopy revealed a layered oxide structure in which the average layer thickness was inversely proportional to the post-transition corrosion rate. The detailed diffraction studies showed an oxide that contained both tetragonal and monoclinic ZrO₂, with a higher fraction of tetragonal oxide near the oxide-metal interface, in a region roughly corresponding to one oxide layer. Evidence was seen also of a cyclic variation of the tetragonal and monoclinic oxide across the oxide thickness with a period of the layer thickness. The results also indicate that the final grain size of the tetragonal phase is smaller than that of the monoclinic phase and the monoclinic grain size is smaller in Zircaloy-4 and ZIRLO than in the other two alloys. These results are discussed in terms of a model of oxide growth based on the periodic breakdown and reconstitution of a protective layer.     

Residual stresses and tetragonal phase fraction characterisation of corrosion tested Zircaloy-4 using energy dispersive synchrotron X-ray diffraction

To improve the understanding of the oxidation mechanism in zirconium alloys for fuel clad applications, detailed residual stress and phase fraction analysis was carried out for the oxides formed on Zircaloy-4 after autoclave exposure at 360 °C for various times by means of synchrotron X-ray diffraction. In a post-transition sample (220 days), significant stress variation through the oxide thickness was found for the monoclinic phase in individual oxide layers, with maximum in-plane compressive stresses located towards the metal–oxide interface and a discontinuity in the residual stress profile. The depth of this discontinuity matched well with the depth at which electron microscopy analysis showed an interface between two distinct oxide layers. Analysis of the tetragonal phase with exposure time demonstrated changes of the total volume of tetragonal phase before and during transition. These observations are put into the context of residual stress evolution presented previously, to provide further insight into the importance of phase transformations and residual stresses in determining the corrosion kinetics of Zr alloys.     

T91钢在氧浓度为0.01ppm静态铅铋合金中的界面腐蚀特征

开展了铅基反应堆候选结构材料T91钢在500℃、0.01ppm氧浓度、静态铅铋共晶合金(LBE)中的腐蚀行为研究,腐蚀时间依次为500、1 000、2 000h。采用SEM观察腐蚀界面组织形貌,并结合EDX分析界面产物成分及元素扩散行为。结果显示:T91钢发生了氧化腐蚀,表面生成了具有3层结构的氧化膜。最外层为疏松且有LBE渗透的Fe3O4层,中间层为致密且具有保护性的(Fe,Cr)3O4层,最内层为富含铬元素的内氧化层(IOZ)。随着腐蚀时间的增加,Fe3O4层和(Fe,Cr)3O4层的厚度先快速增加,在1 000h时分别达到6.5μm和7.4μm;随着腐蚀时间进一步增加,Fe3O4层的厚度略有减小而(Fe,Cr)3O4层的厚度略有增加,而IOZ的厚度却一直近似以线性规律缓慢增加。     

Transmission electron microscopy of oxide development on 9Cr ODS steel in supercritical water

Oxide layers formed on 9Cr oxide dispersion strengthened ferritic steel alloys during exposure to 600 °C supercritical water for 2- and 4-weeks were examined using cross-sectional transmission electron microscopy. A focused ion beam insitu lift-out technique was used to produce site-specific samples with electron transparent areas up to 8 μm by 10 μm. The oxide layers consist of several sub-layers: an Fe-rich outer oxide, a Cr-rich inner oxide, and a diffusion layer, extending beyond the oxide front into the metal. An evolution of the oxide layer structure is seen between 2 and 4 weeks, resulting in the development of a band of Cr₂O₃ at the diffusion layer/metal interface from the previously existing continuous mixture of FeCr₂O₄ ‘fingers’ and bcc metal. It is believed that transport in this Cr₂O₃ layer at the diffusion layer/metal interface becomes the rate-limiting step for oxide advancement, since this change in oxide structure also corresponds to a decrease in corrosion rate.     

Nano-indentation measurement of oxide layers formed in LBE on F/M steels

Ferritic/martensitic (F/M) steels (T91, HT-9, EP 823) are candidate materials for future liquid lead or lead bismuth eutectic (LBE) cooled nuclear reactors. To understand the corrosion of these materials in LBE, samples of each material were exposed at 535 °C for 600 h and 200 h at an oxygen content of 10⁻⁶ wt%. After the corrosion tests, the samples were analyzed using SEM, WDX and nano-indentation in cross section. Multi-layered oxide scales were found on the sample surfaces. The compositions of these oxide layers are not entirely in agreement with the literature. The nano-indentation results showed that the E-modulus and hardness of the oxide layers are significantly lower than the values for dense bulk oxide materials. It is assumed that the low values stem from high porosity in the oxide layers. Comparison with in-air oxidized steels show that the E-modulus decreases with increasing oxide layer thickness.     

Corrosion Resistance of Fe-Al-Alloy-Coated Ferritic/Martensitic Steel under Bending Stress in High-Temperature Lead-Bismuth Eutectic

The corrosion test was performed for ferritic/martensitic steel HCM12A with and without Fe-Al alloy coating in LBE at temperatures of 550 and 650°C under loading for an immersion time up to 500 h. After the corrosion test at 550°C for 500 h, both of the uncoated and Fe-Al-coated HCM12A showed a good corrosion resistance without the influence of the tensile stress on the LBE corrosion. On the other hand, after the corrosion test at 650°C, the Fe-Al coating layer on the specimen surface exhibited no LBE dissolution corrosion because of the formation of a stable oxide protection film on the coating layer surface, although the coating layer cracked. The LBE penetrated into the cracks and corroded the base metal and the precoating layer. The uncoated HCM12A exhibited the double oxide layers of FeO and Fe- Cr spinel. The FeO was damaged in the bent zone by the stress, and the Fe-Cr spinel layer was not destroyed by the influence of cracking and the tensile stress. The cracking and stress did not have a large influence on the overall oxidation corrosion rate.     

模拟高放玻璃固化体在多重介质下的腐蚀行为

考察了模拟高放玻璃固化体在多重介质下关键元素的浸出和玻璃表面的腐蚀行为。在90 ℃下，北山二长花岗岩、高庙子膨润土和北山地下水按一定质量比加入微型反应釜中，将玻璃片放入膨润土中，反应从7 d持续至720 d。利用电感耦合等离子体质谱（ICP-MS）/电感耦合等离子体原子发射光谱（ICP-AES）分析溶液中元素的浸出浓度。采用扫描电子显微镜和电子能谱分析玻璃断面的形貌和腐蚀界面的元素分布。结果表明，玻璃体腐蚀实验180 d后，各元素的浸出浓度趋近饱和。玻璃的腐蚀在初期较为缓慢，90 d后在玻璃表面有明显的腐蚀层形成。实验360 d后，玻璃表面发生非均匀腐蚀，在断面形成楔形的腐蚀区域。在多重介质下，玻璃体的平均腐蚀速率为0.4 μm/d。     

The growth of oxide and oxygen-stabilized alpha layers in steam-oxidized zircaloy

Experimental investigations of the oxidation of Zircaloy in steam at high temperatures suggest temperature gradients exist across the oxide and oxygen-stabilized α layers even when specimens are exposed under nominally isothermal conditions. This paper presents a simple model that permits one to calculate the ratio of the thickness, of the oxide to oxygen-stabilized α layers in the presence of temperature gradients as well as under truly isothermal exposure conditions. The shape of the oxide to oxygen-stabilized α thickness ratio curve as a function of temperature was found to be in excellent agreement with oxidation kinetics data that were used to derive a scaling factor for the model. Variations in the temperature dependence of this ratio from independent measurements can be reproduced if it is assumed that temperature differences on the order of 10°C exist between the oxide layer and the oxygen-stabilized α layer. Metallographic evidence is presented that suggests the rate-controlling oxidation step occurs in the vicinity of the interface between the oxide and oxygen-stabilized a layers     

Growth of the interaction layer around fuel particles in dispersion fuel

Corrosion of uranium particles in dispersion fuel by the aluminum matrix produces interaction layers (an intermetallic-compound corrosion product) around the shrinking fuel spheres. The rate of this process was modeled as series resistances due to Al diffusion through the interaction layer and reaction of aluminum with uranium in the fuel particle to produce UAl_x. The overall kinetics are governed by the relative rates of these two steps, the slowest of which is reaction at the interface between Al in the interaction layer and U in the fuel particle. The substantial volume change as uranium is transferred from the fuel to the interaction layer was accounted for. The model was compared to literature data on in-reactor growth of the interaction layer and the Al/U gradient in this layer, the latter measured in ex-reactor experiments. The rate constant of the Al-U interface reaction and the diffusivity of Al in the interaction layer were obtained from this fitting procedure. The second feature of the corrosion process is the transfer of fission products from the fuel particle to the interaction layer due to the reaction. It is commonly assumed that the observed swelling of irradiated fuel elements of this type is due to release of fission gas in the interaction layer to form large bubbles. This hypothesis was tested by using the model to compute the quantity of fission gas available from this source and comparing the pressure of the resulting gas with the observed swelling of fuel plates. It was determined that the gas pressure so generated is too small to account for the observed delamination of the fuel.     

Models of liquid metal corrosion

In the present study, models for liquid metal corrosion are reviewed and their applications in nuclear reactor engineering are discussed. The paper presents mathematical analysis of liquid metal corrosion, including species transport in solid steels, in flowing liquid metals, and mass exchange at liquid/solid interface. The survey illustrates the mechanisms of the liquid metal corrosion and sets up a system to calculate the corrosion rate and to study the corrosion species distributions in the solid and liquid metal/alloys. Both light liquid metal/alloy (sodium and sodium-potassium) and heavy liquid metal/alloy (liquid lead and lead-bismuth) are considered. Oxygen effects on liquid metal corrosion are also discussed. For liquid sodium and sodium-potassium the corrosion rate increases with increasing oxygen concentration, while for liquid lead and lead-bismuth it is reasonable to produce a protective oxide layer using an oxygen control technique which can mitigate the corrosion rate significantly. Finally, the corrosion-oxidation interaction in liquid lead and lead-bismuth are discussed.     

FOCUS-type fuel assembly for PWRs

In recent years the development efforts for Siemens PWR fuel assemblies were mainly concentrated on reducing the fuel cycle costs and increasing the operational reliability of the fuel assemblies.The first objective was aimed at increasing the average discharge burnup to > 50 MWd/kgU and increasing the critical heat flux. The high envisaged burnup required to develop a corrosion resistant cladding tube outside the Zry-4 range. The decision was made to use a Duplex cladding tube consisting of a corrosion optimized outer layer on a Zry-4 base material. A ZrSnFeCr alloy with reduced tin content was chosen for the outer layer. The critical heat flux could be increased by introducing mixing vanes on the spacer grids within the active length.To reach the second objective, reliable avoidance of spacer grid damage during core loading and unloading and reduction of fuel rod defects by debris fretting, the spacer grid corners were improved and a debris separation grid was developed.These design improvements were introduced into the new FOCUS-type fuel assembly. The name FOCUS stands for “Fuel assembly with Optimized Cladding and Upgraded Structure”.     

Nb-1Zr与304不锈钢电子束熔钎焊界面微观组织研究

利用光学显微镜、扫描电镜(SEM)和电子衍射能谱仪(EDS)分析了Nb-1Zr合金与304不锈钢电子束熔钎焊接头的反应相种类和界面结构。研究结果表明,接头界面生成了ε-(Fe₂Nb)和σ-(FeCr)两种反应相,形成了两个反应层。反应层Ⅰ是Fe₂Nb的疏松组织,反应层Ⅱ是Fe₂Nb与FeCr的双相片层柱状晶组织。反应层Ⅰ是接头脆化、产生裂纹的主要原因。接头的强度与扩散层厚度有关,当扩散层厚度为22μm时,接头试样的强度达到355MPa的最大值。     

Evaluation of flow accelerated corrosion by coupled analysis of corrosion and flow dynamics. Relationship of oxide film thickness, hematite/magnetite ratio, ECP and wall thinning rate

Systematic approaches to evaluate flow accelerated corrosion (FAC) are desired before discussing application of countermeasures for FAC. First, future FAC occurrence should be evaluated to identify locations where a higher possibility of FAC occurrence exists, and then, wall thinning rate at the identified FAC occurrence zone is evaluated to obtain the preparation time for applying countermeasures.Wall thinning rates were calculated with two coupled models:

1.

static electrochemical analysis and

2.

dynamic oxide layer growth analysis.

The anodic current density and the electrochemical corrosion potential (ECP) were calculated with the static electrochemistry model based on an Evans diagram. The ferrous ion release rate, determined by the anodic current density, was applied as input for the dynamic double oxide layer model. Some of the dissolved ferrous ion was removed to the bulk water and others precipitated on the surface as magnetite particles.The thickness of oxide layer was calculated with the dynamic oxide layer growth model and then its value was used as input in the electrochemistry model. It was confirmed that the calculated results (corrosion rate and ECP) based on the coupled models were in good agreement with the measured ones.Higher ECP was essential for preventing FAC rate. Moderated conditions due to lower mass transfer coefficients resulted in thicker oxide layer thickness and then higher ECP, while moderated corrosion conditions due to higher oxidant concentrations resulted in larger hematite/magnetite rate and then higher ECP.     

涂硼正比计数管涂硼厚度对探测效率影响的研究

涂硼正比计数管是可用于反应堆压力容器外对中子注量率进行监测的关键设备,涂硼正比计数管的涂硼厚度对自身的本征探测效率有影响。仿真不同厚度硼层中¹⁰B(n,α)⁷Li生成的⁷Li和α粒子的输运过程,计算硼层界面位置离子整体射出率并给出涂硼正比计数管本征探测效率的计算方法。仿真与计算结果表明：单位核反应率下硼层界面处离子整体射出率在涂硼厚度小于1.5μm时与硼层厚度近似呈正线性相关,在涂硼厚度大于1.5μm后随涂硼厚度增大而增速变缓,在涂硼厚度为3.6μm时达最大为1.3×10^-4 (cm²·s)^-1。基于离子整体射出率的结果进一步计算得到涂硼正比计数管本征探测效率与涂硼厚度之间关系曲线,该关系曲线可以为涂硼正比计数管研制中选择合适涂硼厚度、确定最佳的探测效率提供参考。     

奥氏体304NG不锈钢在550℃/25MPa超临界水中的腐蚀行为

研究了304NG不锈钢在550℃/25MPa超临界水中的腐蚀特性。采用扫描电镜、X射线能谱仪和X射线衍射分析了氧化膜的腐蚀形貌、组织结构和元素成分分布。实验结果表明,在550℃/25MPa的超临界水中腐蚀1000h后,304NG不锈钢显示出优越的耐腐蚀性能,其均匀腐蚀增重速率仅为0.01299mg•dm^-2•h^-1。304NG不锈钢在超临界水中形成均匀致密、但带有疖状腐蚀的双层氧化膜,厚度约为2.0μm,内层氧化膜致密而富Cr和Ni,外层氧化膜疏松而富Fe。