PWR Primary Water Research Articles

The effect of nano-sized precipitates on stress corrosion cracking propagation of Alloy 718 under constant loads in PWR primary water

The effect of nano-sized precipitates on stress corrosion cracking propagation of Alloy 718 under constant loads in PWR primary water

Effects of irradiation and stress on crack initiation behavior of 316SS flux thimble tubes in PWR environment

Irradiation assisted stress corrosion cracking (IASCC) behavior and irradiated microstructural features of cold-worked 316 stainless steel flux thimble tube specimens were investigated in 340 °C simulated PWR primary water. It covers three dose levels (∼40, ∼60 and ∼100 dpa) and five stress levels (35%, 40%, 50%, 60% and 70% of irradiated yield strength at high temperature. Crack initiation in highly irradiated materials occurred rapidly in 33 failed specimens at high-stress level. A clear dose dependence for IASCC initiation was observed. The susceptibility to IASCC initiation reached saturation around 40–60 dpa. Fracture surface of different dose level O-ring specimens showed that intergranular cracking was the main fracture mode. Discrete slip steps were observed on intergranular fractured regions of each dose level specimens, which is associated as evidence of dislocation channeling. A threshold stress level which was close to 30% yield strength was determined on various testing results from literature and this study. Data scatter was observed for those results from different studies but all agreed on that the failure rate increases with stress level and neutron dose.

Ameliorating dilution of Cr and Ni and improving PWSCC resistance of 52 M overlay by hot wire plasma arc cladding

The multi-scale chemical composition distribution and microstructure of Alloy 52 M overlay on low alloy steel by hot wire plasma arc cladding (HWPC) were characterized. Stress corrosion cracking (SCC) growth rates were quantified in the dilution zone (DZ) near the fusion boundary and in the bulk of Alloy 52 M using compact tension samples with a thickened cladding overlay. The DZ of the Alloy 52 M side exhibited a single macroscopic dilution zone containing approximately 27–28 wt% Cr and 50 wt% Ni, in contrast to the multiple dilution zones observed in the DZ of Alloy 52 M overlays or joints prepared using other welding technologies such as submerged arc welding (SMAW). The DZ of the HWPC overlay exhibited less dilution of Cr and Ni compared to SMAW weldment. This resulted in a shorter average SCC band and fewer local intergranular cracks in the DZ of HWPC, as observed from SCC crack growth rate specimens tested in simulated PWR primary water at 325 °C and 350 °C. These findings highlight the advantages of utilizing HWPC to enhance SCC resistance in the DZ of Alloy 52 M overlays over conventional cladding technologies.

Fretting corrosion behavior of a Cr-coated Zr-1Nb alloy cladding in simulated PWR primary water environment

The fretting corrosion behaviors of Cr-coated Zr‐1Nb alloy claddings with Zr-4 dimple and Inconel 718 spring in simulated PWR primary water were investigated. The results indicated that by increasing the fretting duration from 192 to 427 h, the maximum wear depth and volume of Cr coating with dimple increased by 15% and 3%, respectively. In the case of spring as a grinding material, however, the maximum wear depth and volume significantly increased by about 8 and 300 times, respectively. At 427 h, the Cr coating was worn through, and oxidation of the underlying Zr‐1Nb substrate formed a dual-layered oxide film.

The time dependence of proton irradiation effect on the intergranular oxidation of 316 L stainless steel in high-temperature hydrogenated water

The time dependence of proton irradiation effect on the intergranular oxidation of 316 L stainless steel in simulated PWR primary water was clarified for the first time by ruling out the interference from grain boundary structure dependence. Interestingly, proton irradiation has an acceleration effect on the intergranular oxidation of 316 L stainless steel after shorter-term oxidation (less than 500 h) but a mitigation effect at later stage (more than 500 h). It was found that Cr depletion at the pristine grain boundary due to radiation-induced segregation (RIS) plays a dominant role at the early stage of oxidation, resulting in a deeper intergranular oxidation than in the non-irradiated region. Nevertheless, the Cr content at the intergranular oxide tip in the irradiated region builds up faster than in the non-irradiated region with time due to the enhanced solute (especially Cr) diffusivity in the irradiated region. The positive effect of enhanced Cr diffusivity on resistance to intergranular oxidation gradually dominates after longer-term immersion and eventually leads to a shallower intergranular oxide penetration in the irradiated region. The faster intergranular oxidation in irradiated region due to RIS at the early stage partly explains the accelerated crack propagation of irradiated material as the oxidation condition at the crack tip is similar to that at the early stage.

X-ray absorption study of oxide film formed on Alloy 600 in PWR primary water at 290°C with a decreased concentration of dissolved hydrogen

ABSTRACT Primary water stress corrosion cracking (PWSCC) of Alloy 600 remains an important degradation phenomenon in PWRs. In particular, the susceptibility to the PWSCC initiation at 290°C is of recent interest because that is one of the typical temperatures of the primary water in contact with Alloy 600 which remains in PWRs. In this paper, oxide film formed on Alloy 600 in simulated primary water at 290°C was analyzed by X-ray absorption fine structure (XAFS) measurements at the Fe K-edge. For the preparation of the oxide film, Alloy 600 was immersed in simulated primary water with dissolved hydrogen (DH) concentration of 5 or 30 ml/kg-H2O. The near edge region of the XAFS suggested that the oxide film for 5 ml/kg-H2O DH had a larger Fe3+/Fe2+ ratio than for 30 ml/kg-H2O DH. Additionally, the extended region of the XAFS revealed that Fe3+ mainly occupied the octahedral site of the spinel oxide. Previous reports indicate that an Fe3+-rich spinel oxide is less defective and protective against PWSCC initiation. Therefore, our results suggest that the lower DH concentration (5 ml/kg-H2O) could potentially mitigate the susceptibility of Alloy 600 to the PWSCC initiation at 290°C.

Clarifying the irradiation effect on the general oxidation of 316L stainless steel in high temperature hydrogenated water after 1000h immersion

The role of proton irradiation in the general oxidation of 316 L stainless steel in simulated PWR primary water after 1000 h immersion was clarified by comparing the microstructural and microchemical features of oxides formed on both the irradiated and un-irradiated regions of identical grains. Hence, the interference from differences in crystallographic orientation can be ruled out. Interestingly, the average inner oxide thickness on the irradiated region is significantly thinner than that on the un-irradiated region. The enhanced resistance to oxidation on the irradiated region originates from the more protective inner oxide layer and the formation of a continuous Ni-rich zone near the oxide/matrix interface. The inner oxide formed on irradiated region is less deficient in cation due to the enhanced diffusion of Cr from matrix by the irradiation-induced defects, thus making a better diffusion barrier. Meanwhile, the formation of continuous Ni-rich zone near the oxide/matrix interface, which is facilitated by the Ni-rich defects serving as nuclei, fast generation of vacancy at the interface and suppression of the outward diffusion of surplus Ni, can diminish the available space for the growth of inner oxide. The limited oxidation space and high Cr content at the oxide/matrix interface in the irradiated region result in the formation of rocksalt oxide while spinel oxide is formed near the interface in the un-irradiated region.

The correlation between the dislocation slip/grain boundary interaction mode and the resistance to SCC initiation of Alloy 690 in simulated PWR primary water

Recurring step straining was applied on Alloy 690 to establish a correlation between the dislocation slip/grain boundary (GB) interaction mode and the resistance to stress corrosion crack (SCC) initiation in high temperature hydrogenated water. The intergranular carbides can suppress the dislocation transmission across GB in thermally treated Alloy 690. GBs that block dislocation slip show higher resistance to SCC initiation than those that transmit slip. Such correlation reveals that the local normal strain near GB is the primary driving force for breaching the surface oxide film over GB which is a critical prerequisite in the SCC initiation of Alloy 690.

Effect of Water Jet Cavitation Peening on Short-Period Oxidation Behavior of Alloy 600 in PWR Primary Water

Water jet cavitation peening (WJCP) was used to strengthen the surface of nickel-based alloy 600. Residual stress, hardness, surface roughness, microstructure, and short-period oxidation behavior in untreated (UT) and WJCP-treated alloy 600 were characterized. A continuous oxide film was formed on the WJCP-treated surface after a short period of exposure, while it was discontinuous in the UT specimen. The change in oxidation behavior was attributed to the ultrafine grain and dislocation structure resulting from the WJCP treatment.

The effect of zinc addition on corrosion behavior of stainless steels with different phases in simulated PWR primary water

In this study, corrosion behaviors of commercial austenitic, ferritic stainless steels, and duplex stainless steel with B2-NiAl were investigated in simulated PWR primary water environments with and without zinc addition. In addition, electrochemical methods such as EIS and Mott-Schottky analyses were utilized to investigate oxide layers. Zinc was incorporated mainly into chromium-rich spinel. For austenitic stainless steel, improvement on corrosion was observed, however other stainless steels showed less effects by zinc addition. It was found that beneficial effect of zinc addition depends on chromium content of matrix and the presence of low chromium-containing phase reduced beneficial effect.

Clarifying the mitigation effect of proton irradiation on the intergranular oxidation of 316L stainless steel in high temperature water

The role of proton irradiation in the intergranular oxidation of 316L stainless steel after long-term immersion in simulated PWR primary water was clarified by comparing the microstructural and microchemical features of oxide between the irradiated grain boundary (GB) and its un-irradiated counterpart. As such, the interference from difference in GB structure can be ruled out. Surprisingly, the results reveal that the intergranular oxide penetration in the irradiated region is shallower than that in the un-irradiated region. It is found that Si segregated at the irradiated GB diffuses outwards preferentially and gets oxidized due to its high diffusivity and affinity to oxygen. The Si-enriched oxide in the intergranular oxide tip of irradiated GB can act as a temporary diffusion barrier for oxygen, although it tends to dissolve near the sample surface. Meanwhile, the efficiency of elements (especially Cr) transport along irradiated GB to the oxidation front is promoted mainly due to the vacancies created by preferential diffusion of Si, resulting in higher Cr content at the intergranular oxide tip. The combination of Si and Cr enrichments in the intergranular oxide tip can enhance the resistance to oxidation and eventually leads to a lower oxidation rate for the irradiated GB. For the first time, our study determines that irradiation can enhance the intergranular oxidation resistance of stainless steel after long-term immersion in simulated PWR primary water when the sample is not stressed.

Deformation and cracking behaviors of proton-irradiated 308L stainless steel weld metal strained in simulated PWR primary water

The proton-irradiated 308L stainless steel weld metal was strained by using constant extension rate tensile testing in simulated PWR primary water, and its deformation microstructures and irradiation assisted stress corrosion cracking (IASCC) behavior were investigated. The results suggest that the irradiation significantly increases the SCC susceptibility of 308L weld metal and causes various deformation microstructures including lathy faulted planes, dislocation channels and deformation twins in austenite and atomic plane rotation in δ-ferrite. The propagation of intergranular IASCC cracks is closely related to the location of the crack tip. For the crack tip in the specimen matrix interior, localized deformation is likely the key factor responsible for the crack growth. For the crack tip close to the specimen surface, however, localized corrosion along the grain boundary rather than the localized deformation appears to dominate the crack propagation. Unlike the intergranular cracks, the IASCC cracks along the δ-ferrite/austenite phase boundary can initiate either by crack initiation at the phase boundary or by crack propagation from the grain boundary. In both cases, the cracked phase boundaries contain a large number of carbides and are severely corroded, but no deformation microstructures are observed, which implies that the localized corrosion may play an important role in the IASCC along the phase boundary. In addition, δ-ferrite can retard the IASCC crack propagation along the grain boundary, which is probably related to the reduction of localized deformation by δ-ferrite.

Interpreting electrochemical noise signal arising from stress corrosion cracking of 304 stainless steel in simulated PWR primary water environment by coupling acoustic emission

Electrochemical noise (EN) signals arising from stress corrosion cracking (SCC) of 304 stainless steel (SS) in borated and lithiated high-temperature water were monitored. Acoustic emission (AE) signals were simultaneously measured to relate the EN signals with the SCC process. Through Weibull analysis of the EN signals, it was demonstrated that the EN technique can in-situ capture the SCC events in high-temperature water. With evolution of the SCC process, Hilbert time-frequency spectrum of the EN signals gradually migrated from high frequency to low frequency. The capacity of EN and AE techniques to monitor the SCC process in high-temperature water is compared.

Strain rate dependence of the effect of intergranular carbides on the resistance to stress corrosion cracking initiation of alloy 690 in simulated PWR primary water

The stress corrosion cracking (SCC) initiation of alloy 690 in simulated PWR primary water was evaluated in constant extension rate tensile tests with different strain rates. The SCC initiation was inhibited by intergranular carbide precipitation. However, the carbide effect weakened with increasing strain rate. The SCC mitigation effect of carbides mainly arises from the carbides’ ability to impede the transfer of dislocations across grain boundaries and delay the local break-down of surface oxide films. However, dislocations accumulated near the grain boundary can promote intergranular oxidation which induces cracking after the oxide film fails.

Effects of surface treatments and temperature on the oxidation behavior of 308L stainless steel cladding in hydrogenated high-temperature water

The properties of the oxide films grown on 308 L SS cladding with various surface treatments and temperatures in simulated PWR primary water are studied. The thickness of the inner oxide layer increases with the decrease of the surface roughness. The high oxidation resistance of the ground surface is due to the thicker fine-grained layer with high dislocations density and subgrain boundaries in the near-surface. The ferrite-affected oxidation zone (FAOZ) expands with the increase in temperature, resulting from the thermally activated self-diffusion of Cr in ferrite. The effects of surface states, ferrite and temperature on oxidation performance of 308 L SS cladding are discussed.

A high-resolution characterization of irradiation-assisted stress corrosion cracking of proton-irradiated 316L stainless steel in simulated pressurized water reactor primary water

Irradiation-assisted stress corrosion cracks (IASCCs) from a proton-irradiated 316 L stainless steel (2.5 dpa) after constant load test in simulated PWR primary water were analyzed. The irradiation damages are consistent with those from neutron irradiations at similar dose. The IASCC cracks show microstructural and microchemical features similar to those reported from neutron-irradiated samples, confirming that proton is effective in emulating neutron for IASCC study. Si segregated at grain boundary gets oxidized preferentially beyond the crack tip and then dissolves into solution, resulting in enhanced porosity within the intergranular oxide. The Si effect is an important factor behind IASCC in reducing environment.

Overview of Nuclear Piping Seismic Verification

Abstract Nuclear power has gone through several transitional phases worldwide with early development, design, and construction in 1960s,1970s, and 1980s, particularly in Japan and US. There were some major research programs conducted during those years to develop methodology and prove the technology. Much of this knowledge is still very relevant and applicable and would be useful to young engineers and new professional coming to the nuclear industry. One of such programs is seismic verification of major structures, systems, and components (SSCs) by tests in Japan during the period of 1981 through 2004 at a large shaking table facility. Opportunities for conducting such large tests are not likely in the current environment of the industry and, therefore, one of the objectives of this paper is to summarize some earlier tests to raise awareness for the newcomers to the industry. This paper discusses on Seismic verification tests of PWR and BWR Primary Loop, Main Steam and Feed Water line, Ultimate Strength of Piping and eroded piping , conducted in Japan. In particular, this paper focuses on: insights obtained from the test results; comparisons of test results with responses during actual earthquakes; collaboration of Japan and US in these tests; observations on robustness of Japanese seismic design; how the results relate to current issues; and use of these results from the future perspectives.

Correlating oxidation resistance to stress corrosion cracking of 309L and 308L stainless steel claddings in simulated PWR primary water

Stress corrosion cracking (SCC) growth behaviors of 309L and 308L stainless steel claddings in simulated pressurized water reactor primary water at 325 °C and 350 °C were investigated by compact tension specimens and correlated to the oxide film properties in terms of microstructure. SCC growth rates of 308L were much lower than those of 309L. 309L showed typical intergranular SCC characteristics. The high SCC resistance of 308L was related to the low oxidation rate and crack tip retardation due to the presence of highly networked ferrites. The lower Cr content, as well as less and sparsely distributed δ-ferrite phases in 309L than in 308L, led to more severe local oxidation penetration beneath the metal/inner oxide interface and more porous inner oxide, therefore, contributed to its enhanced SCC growth in 309L. The SCC growth rate results by compact tension specimens are consistent with those by slow strain rate tests.

Effect of thermal ageing and dissolved gas on corrosion of 308L stainless steel weld metal in simulated PWR primary water

• 7000-h thermal ageing had limited effect on the corrosion of 308L weld metal. • Inner oxide thickness of aged δ-ferrite was increased under deaerated condition. • Dissolved regions at O/M interface were observed under aerated condition. • A FeCr 2 O 4 layer presented on oxide particle surface under aerated condition. • Localized corrosion along phase boundary occurred under aerated condition. The corrosion of unaged and 7000-h thermally aged 308L stainless steel weld metals in simulated PWR primary water under aerated and deaerated conditions was investigated, involving the corrosion of austenite, δ-ferrite and δ-ferrite/austenite phase boundary. The results revealed that thermal ageing for 7000 h had a limited effect on the corrosion behavior of 308L weld metal as it only increased the inner oxide thickness of δ-ferrite slightly under the deaerated condition. No obvious corrosion enhancement of 308L weld metal under the aerated condition was found compared to the deaerated condition regardless of the thermal ageing. Nevertheless, Cr-enrichment on the surface of oxide particles, dissolved regions at the metal/oxide interface and localized corrosion along the δ-ferrite/austenite phase boundary occurred under the aerated condition.

Strain Rate Dependence of the Effect of Intergranular Carbides on the Resistance to Stress Corrosion Cracking Initiation of Alloy 690 in Simulated Pwr Primary Water

Strain Rate Dependence of the Effect of Intergranular Carbides on the Resistance to Stress Corrosion Cracking Initiation of Alloy 690 in Simulated Pwr Primary Water