Internal cladding corrosion effects

The structure of a fuel element has the primary function of holding fuel pins in a regular array throughout their irradiation life, and secondary functions of permitting loading and unloading operations and transportation to be carried out without damage. Differential thermal expansions, irradiation-induced dimensional changes and flow-induced phenomena have to be accommodated. These factors can lead to fuel pin damage and ultimate loss of cladding integrity, or to distortions which may affect the thermal performance either in normal operation or in a loss-of-coolant accident. This paper discusses the various types of interaction that have been experienced and their consequences, as well as the design principles that should be followed to avoid them.     

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.     

金属腐蚀探测技术及其应用

地下金属管及建筑物中的钢筋发生腐蚀不仅导致重大经济损失，而且还可引发环境污染，并孕育安全事故隐患。为此，必须要对金属物体发生腐蚀的区段实施防腐工程。为做到准确施工，必须在施工前查明腐蚀区段的分布，并确定其位置。物探中的许多方法，如电化学方法、电阻率法和电磁法等均可在不破坏介质的情况下，快速探测出金属物体的腐蚀区段，为防腐工程提供腐蚀定位资料。此外。还介绍了由上述物探方法演变而成的金属腐蚀探测技术及其在国外的应用实例。     

The aluminum chemistry and corrosion in alkaline solutions

Aluminum-alkaline solution systems are very common in engineering applications including nuclear engineering. Consequently, a thorough knowledge of the chemistry of aluminum and susceptibility to corrosion in alkaline solutions is reviewed. The aluminum corrosion mechanism and corrosion rate are examined based on current experimental data. A review of the phase transitions with aging time and change of environment is also performed. Particular attention is given to effect of organic and inorganic ions. As an example, the effect of boron is examined in detail because of the application in nuclear reactor power systems. Methods on how to reduce the corrosion rate of aluminum in alkaline solutions are also highlighted.     

Developments in PWR cladding corrosion: materials and models

In pressurized water reactors Zircaloy-4 is a standard fuel cladding material. The aim of this paper is to present and evaluate corrosion data generated both in-reactor, and out-of-reactor on PWR claddings made of both Zircaloy-2 and Zircaloy-4 materials. The oxide thickness measurements of cladding tubes irradiated in the Ringhals 3 reactor, and oxide weight gain measurements carried out in Sandvik autoclaves at 400°C, 10.3 MPa clearly show that the stress relief annealed Zircaloy-2 is more corrosion resistant than Zircaloy-4 produced with an identical fabrication route. Furthermore, autoclave tests indicate that the hydrogen pickup fraction of the two alloys is very similar. The obtained data have been evaluated in regard to chemical composition and heat treatment. In addition, computer models, which simulate thermal and hydraulic reactor conditions and corrosion kinetic processes simultaneously, have been used to predict the in-reactor corrosion behaviour of the claddings.     

External corrosion of cladding in PWRs

In-reactor corrosion of Zircaloy is strongly influenced by the fast neutron flux and water chemistry of the primary coolant. Under typical PWR coolant conditions with low oxygen content the fast neutron flux increases the corrosion rate only slightly. On the other hand, under fast neutron irradiation at a high oxygen content in the primary coolant the corrosion is accelerated 5- to 10-fold. In addition localized oxide lenses (nodular corrosion) have been observed. However, hydrogen pick-up rates were generally low. The results are discussed in view of life-limiting aspects; under normal operating conditions of a PWR the external corrosion is not life limiting.     

Three-dimensional characterization of stress corrosion cracks

Understanding crack propagation and initiation is fundamental if stress corrosion cracking (SCC) mechanisms are to be understood. However, cracking is a three-dimensional (3D) phenomenon and most characterization techniques are restricted to two-dimensional (2D) observations. In order to overcome this problem, different approaches to extract 3D information have been used in the recent years. In this work we will present the benefits of using 3D focused ion beam (FIB) slicing and electron tomography. 3D FIB slicing offers a fast and high throughput characterization while electron tomography offers the best spatial resolution. To illustrate the power of these techniques, different parts of dominant stress corrosion cracks in Ni-alloys and stainless steels have been reconstructed in 3D. All relevant microstructural features can now be studied in detail and its relative orientation respect to the strain direction and grain boundary plane accurately measured.     

Long term corrosion on T91 and AISI1 316L steel in flowing lead alloy and corrosion protection barrier development: Experiments and models

Considering the status of knowledge on corrosion and corrosion protection and especially the need for long term compatibility data of structural materials in HLM a set of experiments to generate reliable long term data was defined and performed. The long term corrosion behaviour of the two structural materials foreseen in ADS, 316L and T91, was investigated in the design relevant temperature field, i.e. from 300 to 550 °C. The operational window of the two steels in this temperature range was identified and all oxidation data were used to develop and validate the models of oxide scale growth in PbBi. A mechanistic model capable to predict the oxidation rate applying some experimentally fitted parameters has been developed. This model assumes parabolic oxidation and might be used for design and safety relevant investigations in future. Studies on corrosion barrier development allowed to define the required Al content for the formation of thin alumina scales in LBE. These results as well as future steps and required improvements are discussed. Variation of experimental conditions clearly showed that specific care has to be taken with respect to local flow conditions and oxygen concentrations.     

Effect of corrosion potential on the corrosion fatigue crack growth behaviour of low-alloy steels in high-temperature water

The low-frequency corrosion fatigue (CF) crack growth behaviour of different low-alloy reactor pressure vessel steels was characterized under simulated boiling water reactor conditions by cyclic fatigue tests with pre-cracked fracture mechanics specimens. The experiments were performed in the temperature range of 240-288 °C with different loading parameters at different electrochemical corrosion potentials (ECPs). Modern high-temperature water loops, on-line crack growth monitoring (DCPD) and fractographical analysis by SEM were used to quantify the cracking response. In this paper the effect of ECP on the CF crack growth behaviour is discussed and compared with the crack growth model of General Electric (GE). The ECP mainly affected the transition from fast (‘high-sulphur’) to slow (‘low-sulphur’) CF crack growth, which appeared as critical frequencies ν_crit = f(ΔK, R, ECP) and ΔK-thresholds ΔK_EAC = f(ν, R, ECP) in the cycle-based form and as a critical air fatigue crack growth rate da/dt_Air,crit in the time-domain form. The critical crack growth rates, frequencies, and ΔK_EAC-thresholds were shifted to lower values with increasing ECP. The CF crack growth rates of all materials were conservatively covered by the ‘high-sulphur’ CF line of the GE-model for all investigated temperatures and frequencies. Under most system conditions, the model seems to reasonably well predict the experimentally observed parameter trends. Only under highly oxidizing conditions (ECP ? 0 mV_SHE) and slow strain rates/low loading frequencies the GE-model does not conservatively cover the experimentally gathered crack growth rate data. Based on the GE-model and the observed cracking behaviour a simple time-domain superposition-model could be used to develop improved reference CF crack growth curves for codes.     

Degradation of aged plants by corrosion: “Long cell action” in unresolved corrosion issues

In a series of previously published papers the author has identified that ‘long cell action’ corrosion plays a pivotal role in practically all unresolved corrosion issues for all types of nuclear power plants (e.g. PWR/VVER, BWR/RBMK and CANDU). Some of these unresolved issues are IGSCC, PWSCC, AOA and FAC (erosion-corrosion).In conventional corrosion science it is well established that ‘long cell action’ can seriously accelerate or suppress the local cell corrosion activities. Although long cell action is another fundamental mechanism of corrosion, especially in a ‘soil corrosion’ arena, potential involvement of this corrosion process has never been studied in nuclear and fossil power plants as far as the author has been able to establish. The author believes that the omission of this basic corrosion mechanism is the root cause of practically all un-resolved corrosion issues.In this paper, the author further elaborated on his assessment to other key corrosion issues, e.g. steam generator and turbine corrosion issues, while briefly summarizing previous discussions for completeness purposes, as well as introducing additional experimental and theoretical evidence of this basic corrosion mechanism. Due to the importance of this potential mechanism the author is calling for institutional review activities and further verification experiments in the form of a joint international project.     

Effect of addition of corrosion inhibitors on corrosion behaviors of PCV material under diluted artificial seawater environment

ABSTRACT

Fukushima Daiichi nuclear power plants (1F) were damaged by unprecedented severe accident in the great east Japan earthquake, and seawater and freshwater had been injected as an emergency countermeasure for the core cooling. Although, the primary containment vessel (PCV) was not supposed to be exposed to diluted seawater, the PCV will be exposed to diluted seawater environment until fuel debris removal is completed. Therefore, it is necessary to establish a countermeasure of corrosion for the PCVs made of carbon steel. In this study, the effect of the addition of corrosion inhibitor, which is one of the corrosion countermeasures, was investigated by two types of corrosion tests. As a result of the immersion corrosion test, it was found that any of the three kinds of corrosion inhibitor could suppress corrosion of carbon steel. In addition, as a result of the inhibitor interim addition test, it was found that corrosion of carbon steel covered with corrosion products could be suppressed by optimizing the additive amount of corrosion inhibitor in the cooling water.     

Correlation between microstructure and corrosion behavior of Zr-Nb binary alloy

To investigate the correlation between microstructure and corrosion characteristics of Zr-Nb alloy, the microstructural observation and corrosion test with the change of cooling rate from beta temperature and the variation of Nb content were performed. The oxide characterization was also carried out by synchrotron XRD and TEM. When the Nb is contained less than solid solution limit (0.6 wt%) in Zr matrix, the difference of corrosion rate was not observed in spite of showing the significant changes of microstructures with cooling rate. While, when the Nb content in the alloy is more than 0.6 wt%, the corrosion properties were deteriorated with increasing the supersaturated Nb concentration in matrix and increasing the area fraction of β_Zr. Also it was observed that the supersaturated Nb in matrix was more effective to decrease the corrosion resistance than the β_Zr phase in the same Nb containing alloy, while the equilibrium Nb concentration below solubility limit in the matrix played an important role to enhance the corrosion resistance. During the corrosion testing in steam at 400 °C, the formation of β_Nb phase in water-quenched specimen would result in the reduction of Nb concentration in matrix. Thus, the corrosion resistance is enhanced with the formation of β_Nb phase. It is suggested from this study that the equilibrium Nb concentration below solubility limit in α matrix would be a more dominant factor in the enhancement of corrosion resistance than β phase (β_Nb or β_Zr), supersaturated Nb, precipitate, and internal microstructure such as twin, dislocation and plate.     

强流脉冲电子束处理对过共晶铝硅合金Al-25Si微观组织和耐蚀性的影响

用强流脉冲电子束处理过共晶铝硅合金表面,并对Al-25Si合金微观组织形貌、物相及耐蚀性进行分析和测试。结果表明,强流脉冲电子束处理在合金表面诱发的快速熔凝过程实现了铝硅合金表层晶粒细化,粗大初生硅颗粒熔化,硅元素固溶到铝基体中,合金表面成分趋于均匀化分布。截面组织分析指出重熔层(8-9μm)与基体之间有明显界限,致密度提高。24 h盐浸失重法测试中,A1-25Si合金经强流脉冲电子束处理后腐蚀失重减小0.008 g,腐蚀速度降低12.1%,耐蚀性能提高。     

Magnetite dissolution and corrosion behavior in high temperature EDTA solvents

The effect of temperature on magnetite dissolution and corrosion behavior was evaluated in ethylenediaminetetraacetic acid (EDTA)-based solvents for iron oxide chemical cleaning of nuclear steam generators at temperatures ranging from 93 to 150 °C. Magnetite dissolution was drastically accelerated as the temperature was raised to 150 °C, while the amount of corrosion was controlled well within an allowable limit. Periodic venting and excessive EDTA concentration in the solvents were expected to be very effective in dissolving hard tube sheet sludge piles. The linear polarization (LP) method was also suitable to monitor the changes of corrosion rates and process conditions such as venting.