Boiling Water Reactor Environment Research Articles

Oxidation of Alloy X-750 with Low Iron Content in Simulated BWR Environment

This paper presents an investigation of the oxidation of Alloy X-750 containing 5 wt% iron in a simulated boiling water reactor (BWR) environment. The specimens were exposed by a water jet (10 m/s) at 286 °C for durations ranging from 2 to 840 h, and the development of the oxide microstructure was mainly studied using electron microscopy. The results showed that the oxide scale consists of blocky crystals of trevorite on top of a porous inner layer rich in Ni and Cr. After the longest exposure time, the trevorite crystals completely covered the specimen surface. The study further revealed that the rate at which the oxide grew and the metal dissolved both decreased with time, and the metal thinning process appeared to be sub-parabolic. Given the significant variation in iron content in the X-750 specification, the influence of this element on the material’s corrosion performance in BWR was examined by comparing the results from this investigation with those from previous work on material containing 8 wt% Fe. The study demonstrates that the oxide growth, metal dissolution and metal thinning were slower in the material with a higher iron content, indicating the importance of this element in limiting the degradation of Alloy X-750 in BWR environments.

High temperature electrochemical reaction parameters affecting electrochemical corrosion potential of stainless steels in hydrogen peroxide environment

ABSTRACT High-temperature electrochemical reaction parameters for stainless steels have been determined based on data measured in pure water at high temperatures to realize an analytical model that can predict electrochemical corrosion potential (ECP) of stainless steels in a hydrogen peroxide (H2O2) environment without using any adjustment parameters based on ECP data. Behaviors of measured cathodic polarization curves for H2O2 were explained well with the determined parameters. Calculated ECPs of stainless steels agreed with measured values in the cases that it was acceptable for the model to have an error range of ±0.1 V in a range from the deaerated condition to 1000 ppb (= μ g kg−1) H2O2 concentration. It is expected that the ECP model in the H2O2 environment with the determined electrochemical reaction parameters will have greater validity for, and better interpretability of, the ECPs of stainless steels determined in a boiling water reactor environment than previous models.

Corrosion behavior of a series of combinatorial physical vapor deposition coatings on SiC in a simulated boiling water reactor environment

The simulated boiling water condition corrosion properties of multilayer and combinatorial physical vapor deposition metal coatings (Zr, Cr, and/or Ti) deposited on a SiCf-SiC composite were investigated. Various compositions within the ternary system were corroded to the most suitable composition preventing underlying SiCf-SiC from further attack. Mass measurements and visual observations were conducted to identify which coatings formed protective oxides and which coatings spalled. Transmission electron microscopy was conducted to observe the microstructure of the physical vapor deposition metal layer before and after corrosion as well as identify the oxide which were forming.

High temperature electrochemical reaction parameters affecting electrochemical corrosion potential of Ni-base Alloy 182

ABSTRACT High-temperature electrochemical reaction parameters for nickel-base Alloy 182 weld metal (Alloy 182) have been determined based on data measured in pure water at high temperatures to realize an analytical model that can predict electrochemical corrosion potential (ECP) of Alloy 182 without using any adjustment parameters based on ECP data. Behaviors of measured polarization curves for Alloy 182 could be explained well with the determined parameters. Calculated ECPs of Alloy 182 agreed with the measured values in the cases that it was acceptable for the model to have an error range of ±0.1 V in a range from the deaerated condition to 10,000 ppb (=μg kg−1) oxygen concentration. It is expected that the ECP model with the determined electrochemical reaction parameters will have greater validity for, and better interpretability of, the ECPs of Alloy 182 determined in a boiling water reactor environment than previous models.

Investigation of surface treatment effects on the environmentally-assisted cracking behaviour of Alloy 182 in boiling water reactor environment

ABSTRACT Surface treatments of primary circuit components in light water reactors are regarded as possible ways to mitigate environmentally-assisted cracking (EAC). To date, it is not fully conclusive which surface condition is suitable to reduce the EAC initiation susceptibility. Constant extension rate tensile (CERT) tests were performed by several labs using flat tapered tensile specimens with different surface conditions (ground, industrial face milled, advanced face milled and shot peened), exposed to a boiling water reactor normal water chemistry environment at 288°C. Despite some scatter in the results, the CERT tests revealed that the EAC initiation susceptibility seems lowest for the advanced face milled surface and highest for the shot peened surface. However, it must be emphasised that the differences were moderate and that the surprising behaviour of the shot peened surface can be explained. The mechanical grinding of the surface did not significantly retard EAC initiation compared to industrial face milling.

Stress corrosion cracking initiation susceptibility of Alloy 182 with different surface treatments

The effects of machining surface preparations on the stress corrosion cracking (SCC) susceptibility in boiling water reactor environment were evaluated for Alloy 182 weld metal. Constant extension rate tensile test, constant load test and electrochemical tests (passivation, current-voltage curves and electrochemical impedance spectroscopy) showed that the surface treatments strongly influence the resistance to SCC initiation. The machining-induced deformation and surface oxide layer from the surface treatments affect the re-passivation behaviour and SCC initiation susceptibility. The SCC susceptibility of the studied surface treatments was found following the sequence of shot peened > industrial surface milled > polished > advanced manufactured surface.

High temperature electrochemical reaction parameters affecting elecrochemical corrosion potential of type 316L stainless steel

ABSTRACT High temperature electrochemical reaction parameters for type 316 L stainless steel (316 L SS) have been determined based on data measured in pure water at high temperatures to realize an analytical model that can predict electrochemical corrosion potential (ECP) of 316 L SS without using any adjustment parameters based on ECP data. Behaviors of measured polarization curves for 316 L SS could be explained well with the determined parameters. Calculated ECPs of 316 L SS agreed with measured values in the case that it was acceptable for the model to have an error range of ±0.1 V in a wide oxygen concentration range. However, the error was relatively larger in the range from 10 to 100 ppb oxygen concentration because of the shape of the anodic polarization curve of 316 L SS; in other words, ECP could not be uniquely determined in such an oxygen concentration range. It is expected that the ECP model with the determined electrochemical parameters will have greater validity for, and better interpretability of, the ECPs of SS determined in a boiling water reactor environment.

Effects of surface treatments on environmentally-assisted cracking susceptibility of Alloy 182 in BWR environment

The effects of four machining surface preparations on the environmentally-assisted cracking (EAC) initiation and early crack growth were evaluated for Alloy 182 flat tapered specimens exposed to a simulated boiling water reactor (BWR) environment under constant extension rate tensile test conditions. Results showed that the apparent EAC critical threshold stress for initiation was highly dependent on the surface treatment. Detailed microstructural characterization revealed the nature of the machining-induced deformation associated with the four surface treatments. Surface treatments affected the surface oxidation during exposure in the simulated BWR environment. The machining-induced deformation structure affected the EAC initiation susceptibility.

Role of mean stress on fatigue behavior of a 316L austenitic stainless steel in LWR and air environments

Mean stress effects on fatigue life of a stainless steel in air and boiling water reactor (BWR) environments were evaluated by load-controlled tests. In stress-life, non-zero mean stresses increase life in low cycle fatigue regime in both environments and in high cycle regime in air. In high cycle fatigue regime in BWR environment, higher fatigue life was observed with negative mean stress while a decrease of the fatigue limit was found for positive one. In strain-life, the mean stress effects are insignificant. All data are well correlated with a Smith-Watson-Topper parameter showing no interaction between mean stress and BWR environment.

Investigation of the irradiation effects in additively manufactured 316L steel resulting in decreased irradiation assisted stress corrosion cracking susceptibility

While additive manufacturing (AM) is an area of increasing interest to the nuclear industry, there are significant research and development needs prior to successful implementation of additively manufactured components into reactors. AM steel behaves differently than traditional wrought material, as has been observed in numerous mechanical testing studies. The nuclear industry must also understand the material response to irradiation. This work examines the irradiation damage and the irradiation assisted stress corrosion cracking behavior (IASCC) of both proton irradiated AM and wrought 316L steel. For IASCC testing, specimens were strained to 4% plastic strain in a simulated boiling water reactor environment (288°C, 0.2 µS/cm). Transmission and scanning electron microscopy, confocal microscopy, and X-ray computed tomography were used to examine coupons of the material as well as tensile bars used in the IASCC testing. This work found that the wrought 316L was more susceptible to IASCC than the AM steel, however, some IASCC was observed in the AM steel when tested with the tensile axis parallel to the build direction. Electron microscopy has shown significantly fewer radiation induced voids formed in the AM steel. While the pre-existing pore network in the AM steel may act as a sink to point defects, this is not expected to be the major factor in the reduced swelling observed in the AM steel. The lower density of radiation induced voids was expected to contribute to the IASCC resistance by decreasing the localization of deformation in the irradiated specimens, however, severity of localized deformation in AM steel, measured in dislocation channel spacing and height, was similar or more severe to that found in wrought. The presence of severe cracking in the wrought specimens did not allow for a direct comparison of localized deformation severities between the AM and wrought steel.

The effect of post-irradiation annealing on the stress corrosion crack growth rate of neutron-irradiated 304L stainless steel in boiling water reactor environment

The effect of post-irradiation annealing (PIA) on the irradiation-assisted stress corrosion cracking (IASCC) of 304L stainless steel in boiling water reactor environments was studied. PIA has little effect on the crack growth rate (CGR) in normal water chemistry even it restored most of the irradiation-induced microstructure change. Nevertheless, PIA can significantly suppress the CGR in hydrogen water chemistry (HWC). The mitigation factor of HWC increases with the degree of PIA up to 5 h at 550 °C and correlates with the recovery of Si segregation at grain boundary. PIA is an effective strategy for decreasing IASCC susceptibility in reducing water environment.

Effects of dendrite axis and fusion boundary on stress corrosion cracking of ER 308 L/SS 304L welds in a high-temperature water environment

This study investigated the effects of dendrite axis and fusion boundary (FB) on the stress corrosion cracking (SCC) of 308 L/304 L welds by using an alternating current potential drop (ACPD) technique to measure the SCC growth rates in a simulated boiling water reactor environment. The results show that the ferrite/austenite interfaces are more susceptible to SCC in the SS 308 L welds. Furthermore, the angle between the applied load and the dendrite axis affects the SCC growth rate, and as this angle approached 90°, the SCC crack growth rate increases. The SCC cracks in the heat affected zone (HAZ) are mainly intergranular and some cracks propagate transgranularly. Furthermore, the primary crack in the HAZ propagates along the FB; when reaching FB, some secondary cracks are arrested at the FB. The residual strain caused by weld shrinkage is the crucial factor for the increase in SCC susceptibility in HAZ.

Electrochemical and spectroscopic characterization of oxide films formed on Alloy 182 in simulated boiling water reactor environment: Effect of dissolved hydrogen

In boiling water reactor (BWR) environments, the content of dissolved hydrogen (DH) in the high-temperature water has an influence on the stress corrosion cracking (SCC) susceptibility of Ni-based alloys. In order to understand the possible role of different types of surface oxide films in the SCC initiation process, oxide films formed on Alloy 182 weld metal under simulated BWR conditions at 274 °C with varying contents of DH from 10 to 2200 ppb, were investigated by electrochemical and spectroscopic methods. The influence of the DH content on the nature of the oxide films is discussed in the context of SCC mechanisms.

TEM/STEM study of Zircaloy-2 with protective FeAl(Cr) layers under simulated BWR environment and high-temperature steam exposure

FeAl(Cr) thin-film depositions on Zircaloy-2 were studied using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with respect to oxidation behavior under simulated boiling water reactor (BWR) conditions and high-temperature steam. Columnar grains of FeAl with Cr in solid solution were formed on Zircaloy-2 coupons using magnetron sputtering. NiFe2O4 precipitates on the surface of the FeAl(Cr) coatings were observed after the sample was exposed to the simulated BWR environment. High-temperature steam exposure resulted in grain growth and consumption of the FeAl(Cr) layer, but no delamination at the interface. Outward Al diffusion from the FeAl(Cr) layer occurred during high-temperature steam exposure (700 °C for 3.6 h) to form a 100-nm-thick alumina oxide layer, which was effective in mitigating oxidation of the Zircaloy-2 coupons. Zr intermetallic precipitates formed near the FeAl(Cr) layer due to the inward diffusion of Fe and Al. The counterflow of vacancies in response to the Al and Fe diffusion led to porosity within the FeAl(Cr) layer.

Stress–Corrosion Cracking of Surface-Engineered Alloys in a Simulated Boiling-Water Reactor Environment

The potential of surface engineering by concentrated interstitial solute (SECIS) for improved resistance to stress–corrosion cracking (SCC) in a simulated boiling-water reactor (BWR) environment was studied for an austenitic stainless steel (AISI-316L) and a Ni-base superalloy (IN-718) via slow-strain-rate tests. For these tests, tensile rods of both alloys were carburized at a low temperature, which generates a concentrated solution (supersaturation: 105) of interstitially dissolved carbon within a ≈25 μm deep zone below the surface. In BWR-NWC (normal water composition), SECIS AISI-316L exhibits high susceptibility to SCC with a reduction in elongation and a fracture surface consistent with transgranular SCC (TGSCC). We found that the crack-tip strain rate plays an important role for crack initiation and propagation. To understand the role of concentrated interstitial carbon on the failure mode, we also studied specimens treated by ultrasonic nanocrystal surface modification (UNSM), a carbon-free method for surface hardening. We conclude that the carbon-rich zone enables nucleation of sharp cracks, which normally do not form in AISI-316L. Crack propagation in the underlying carbon-free alloy core and, ultimately, TGSCC failure, depends on the environment (solution and potential), as demonstrated by tests in BWR-HWC (hydrogen water composition). The stress–strain curve of SECIS IN-718, in contrast, is similar to that of non-treated material. In this case, the fracture surface was consistent with ductile failure.

Formation and deposition of platinum nanoparticles under boiling water reactor conditions

Stress corrosion cracking (SCC) is a well-known degradation mechanism for components of boiling water reactors (BWRs). Therefore the mitigation of SCC is important for ensuring the integrity of the reactor system. Noble metal chemical application (NMCA) has been developed by General Electric to mitigate SCC and reduce the negative side-effects of hydrogen water chemistry used initially for SCC mitigation. NMCA is now widely applied as an online process (OLNC) during power operation. However, the understanding of the parameters that control the formation and deposition of the noble metal (Pt) particles in a BWR was still incomplete. To fill this knowledge gap, systematic studies on the formation and deposition behaviour of Pt particles in simulated and real BWR environment were performed in the framework of a research project at PSI. The present paper summarizes the most important findings.Experiments in a sophisticated high-temperature water loop revealed that the flow conditions, water chemistry, the Pt injection rate, and the pre-conditioning of the stainless steel surfaces have an impact on the Pt deposition behaviour. Slower Pt injection rates and stoichiometric excess of H2 over O2 produce smaller particles, which may increase the efficiency of the OLNC technique in mitigating SCC. Surfaces with a well-developed oxide layer retain more Pt particles. Furthermore, the pre- and post-OLNC exposure times play an important role for the Pt deposition on specimens exposed at the KKL power plant. Redistribution of Pt in the plant takes place, but most of the Pt apparently does not redeposit on the steel surfaces in the reactor system. Comparison of lab and plant results also demonstrated that plant OLNC applications can be simulated reasonably well on the lab scale.

Application of the FRI crack growth model for neutron-irradiated stainless steels in high-temperature water of a boiling water reactor environment

This study considered a methodology to reflect the effect on material properties by neutron irradiation onto the FRI model of the crack growth rate (CGR) of stress corrosion cracking developed at Tohoku University. Yield strength and strain hardening exponent were evaluated by a tensile test for irradiated stainless steels, and the relationship between mechanical property and strain to fracture of the oxide film was derived. Effect of the radiation-induced segregation on CGR was also discussed. The experimental tendencies for the CGR to increase with dose and almost saturate above 3dpa were well replicated by the model calculations.

The Effects of Oxidizing Species on Cobalt Buildup in a BWR Environment

We have studied the effects of the oxidizing species on the cobalt radioactivity buildup behavior in boiling water reactors (BWRs) using both experimental results and existing literature data. The oxidizing species used to simulate the normal water chemistry (NWC) condition of BWRs were 200 ppb dissolved oxygen or 200 ppb hydrogen peroxide accompanied by 100 ppb dissolved oxygen. We found that the amount of cobalt deposited on stainless steel specimens in the oxygen-based water chemistry (200 ppb dissolved oxygen) was larger than that in the hydrogen peroxide–based water chemistry (200 ppb hydrogen peroxide and 100 ppb dissolved oxygen). The rate of cobalt deposition in the former chemistry was more than four times larger than that in the latter chemistry. This difference in cobalt deposition behavior can be attributed to two properties of oxides: surface morphology and composition. The film formed in the oxygen-based environment was less dense than the film formed in the hydrogen peroxide–based environment. Regarding the chemical constituents of the oxides, iron chromite is considered to be a major spinel-type oxide formed in oxygen-based environments. Furthermore, some literature data suggest that in hydrogen peroxide–based conditions, hematite-rich oxides are formed instead of magnetite-rich films, which are observed in oxygen-based conditions. These are likely reasons why the stainless steel specimens incorporate more cobalt radioactivity in the oxygen-based environment than in the hydrogen peroxide–based environment. The cobalt buildup behavior after switching from NWC to hydrogen water chemistry (HWC) is also affected by the oxidizing species used to simulate NWC; exposure to hydrogen peroxide–based NWC conditions tends to suppress the cobalt radioactivity buildup after switching from NWC to HWC compared to exposure to oxygen-based NWC.

Corrosion of pre-oxidized nickel alloy X-750 in simulated BWR environment

Samples of pre-oxidized Alloy X-750 were exposed to a simulated boiling water reactor environment in an autoclave at a temperature of 286 °C and a pressure of 80 bar for four weeks. The effect of alloy iron content on corrosion was investigated by comparing samples with 5 and 8 wt% Fe, respectively. In addition, the effect of two different surface pre-treatments was investigated. The microstructure of the formed oxide scales was studied using mainly electron microscopy. The results showed positive effects of an increased Fe content and of removing the deformed surface layer by pickling. After four weeks of exposure the oxide scale consists of oxides formed in three different ways. The oxide formed during pre-oxidization at 700 °C, mainly consisting of chromia, is partly still present. There is also an outer oxide consisting of NiFe2O4 crystals, reaching a maximum size of 3 μm, which has formed by precipitation of dissolved metal ions. Finally, there is an inner nanocrystalline and porous oxide, with a metallic content reflecting the alloy composition, which has formed by corrosion.

Oxide evolution on Alloy X-750 in simulated BWR environment

In order to simulate the environment experienced by spacer grids in a boiling water reactor (BWR), specimens of the Ni-based Alloy X-750 were exposed to a water jet in an autoclave at a temperature of 286 °C and a pressure of 80 bar. The oxide microstructure of specimens exposed for 2 h, 24 h, 168 h and 840 h has been investigated mainly using electron microscopy. The specimens suffer mass loss due to dissolution during exposure. At the same time a complex layered oxide develops. After the longest exposure the oxide consists of two outer spinel layers consisting of blocky crystals, one intermediate layer of nickel oxide interspersed with Ti-rich oxide needles, and an inner layer of oxidized base metal. The evolution of the oxide leading up to this structure is discussed and a model is presented.