Ulla Ehrnstén

Cyclic crack growth evaluation of a 20MnMoNi55 piping steel in high-oxygen reactor water

Abstract Samples of a low alloy steel piping material taken from the full scale corrosion fatigue test loop of the Heissdampfreaktor (HDR) plant have been tested at 240°C in high oxygen reactor water. The small-scale specimens (CT25) were exposed to a similar loading spectrum to that which has been used in the full-scale corrosion fatigue tests at the HDR-plant. During the autoclave tests cyclic crack growth rates were determined. Fracture surface investigations were performed not only for the laboratory test specimens but also for the fracture surface of a sample taken from the HDR test loop piping after the full scale test. In this paper the autoclave testing results and fracture surface observations are presented and compared to those obtained in the HDR piping tests.

Plastic strain and residual stress distributions in an AISI 304 stainless steel BWR pipe weld

In AISI 304 stainless steel pipe welds weld shrinkage causes large variations in residual plastic strain in different parts of the weld metal and heat-affected zone (HAZ). The amount of strain was analyzed by EBSD quantitatively by comparing the intra-grain misorientations to the calibration curve. Highest degrees of plastic strain (10...20%) were detected in the HAZ close to the root area of a prototypical BWR plant weld. Strain in the weld metal varies in the different directions of solidification, being high in the weld bead boundaries and near the fusion lines. Preliminary studies of the effects of mechanical and elastic anisotropy of the weld metal microstructure on the grain size level were performed by EBSD and nanoindentation. The residual stress distribution in the same weld cross-section was determined by a contour method. The residual strain and stress distributions are superimposed and EAC susceptibility of various areas of the pipe weld is evaluated and discussed.

Microstructural Characterization of Alloy 52 Narrow-Gap Dissimilar Metal Weld After Aging

The safe-end dissimilar metal weld (DMW) joining the reactor pressure vessel to the main coolant piping is one of the most critical DMWs in a nuclear power plant (NPP). DMWs have varying microstructures at a short distance across the ferritic-austenitic fusion boundary (FB) region. This microstructural variation affects the mechanical properties and fracture behavior and may evolve as a result of thermal aging during long-term operation of an NPP. This paper presents microstructural characterization performed for as-manufactured and 5000 h and 10,000 h thermally aged narrow-gap DMW representing a safe-end DMW of a modern pressurized water reactor (PWR) NPP. The most significant result of the study is that the thermal aging leads to a significant decrease in a hardness gradient observed across the ferritic-austenitic FB of the as-manufactured DMW.

Microbially Induced Corrosion in Firefighting Systems—Experience and Remedies

Firefighting water systems are important safety systems in all industries, including nuclear power plants (NPPs). However, they are susceptible to microbially induced corrosion, which is a degradation mode needing special attention. Leakages were observed in a fire fighting system made from stainless steel at a nuclear power plant shortly after maintenance and modernization work, which included replacement of part of the old carbon steel pipelines with stainless steel pipelines, as well as exchange of some Type 304 stainless steel pipes with Type 316 pipes due to relining parts of the system. The failure analysis revealed sub-surface corrosion cavities with pinholes at the inner surface and finally penetrating the whole pipe wall thickness. It was concluded that the reason for the leaks was due to microbially induced corrosion, (MIC). The paper will present the results from failure analyses, explain the remedial actions taken at the power plant, and discuss the implication of these findings on new similar systems, including the importance of avoiding iron deposits and optimization of water quality.

Development of Short-Range Order and Intergranular Carbide Precipitation in Alloy 690 TT upon Thermal Ageing

Thermal ageing promotes intergranular carbide precipitation and atomic ordering reaction in most commercial nickel-base alloys, and it affects the long-term primary water stress corrosion cracking (PWSCC) resistance of pressurized water reactor components. Alloy 690 with 9.8 wt% Fe was solution annealed and heat-treated at low temperature, then aged between 350 and 550 °C for 10,000 h. No direct observation of ordering was possible, but variations in hardness and lattice parameter suggested the formation of short-range ordering (SRO) with a peak level upon ageing at 420 °C, while a disordering reaction occurred at higher temperatures. Heat treatment induced ordering before thermal ageing was compared to the solution-annealed state. Thermal ageing resulted in the precipitation of Cr-rich M23C6 carbides at grain boundaries and twin boundaries. Although no link between SRO and an increase in strain localization was observed, the combination of intergranular carbide formation and SRO over longer ageing times was deemed detrimental to the PWSCC resistance of Alloy 690 TT.

Effect of thermal aging on fracture mechanical properties and crack propagation behavior of alloy 52 narrow-gap dissimilar metal weld

Determination of the fracture toughness properties and thermal aging behavior of dissimilar metal weld (DMW) joints is of utmost importance for successful structural integrity and lifetime analyses. This paper presents results from fracture resistance (J-R), fracture toughness (T0) and Charpy-V impact toughness tests as well as fractography performed for an industrially manufactured narrow-gap DMW mock-up (SA508-Alloy 52-AISI 316L). Tests were performed on post-weld heat treated, 5000 h aged and 10,000 h aged material. The results show that this DMW is tough at the SA 508-Alloy 52 interface, which typically is the weakest zone of a DMW. The DMW joint maintains its high fracture resistance also after thermal aging. Crack propagates for a large part in the carbon-depleted zone (CDZ) of SA 508 but deflects occasionally to the Alloy 52 side due to small weld defects in µm scale. Ductile-to-brittle transition temperature determined from Charpy-V impact toughness tests increases due to thermal aging, but only to a minor extent. No significant change is observed for the T0 transition temperature due to aging.

Post-Irradiation Examinations of Irradiation Creep Tested Zircaloy-2

This work stems from a test program studying the in-pile creep behaviour of Zircaloy-2 materials. The work involves the post-irradiation examinations (PIE) of the Zircaloy-2 specimens creep tested under neutron irradiation at the HALDEN Test Reactor. The multiple specimens provided to VTT for PIE were creep tested to various plastic strain levels with some failing during testing. In-pile creep testing in the HALDEN Test Reactor involved final fluence levels of ~3×10 n/cm to 5×10 n/cm (>1 MeV) and temperatures of ~550 to ~650oF (~288 to ~343oC). The PIE of the various specimens included mainly fractography of the failed specimens using scanning electron microscopy (SEM) and detailed transmission electron microscopy (TEM) characterization.

Investigations of a Type 316L Steam Dryer Plate Material Suffering from IGSCC After a Few Years in BWR Conditions.

Detailed microstructural characterization has been performed on a sample made of Type 316L stainless steel, removed from a boiling water reactor (BWR) steam dryer inner roof plate, which had suffered from intergranular stress corrosion cracking (IGSCC) after only a few years of operation. The aim of the characterisation was to increase the understanding of possible material related characteristics that could have affected the observed cracking.

Effect of Hot Cracks on EAC Crack Initiation and Growth in Nickel-Base Alloy Weld Metals

The differences in the EAC susceptibility between different weld geometries and weld metals have been distinguished by the doped steam test method. Pure weld metals of Alloy 182 and 82 are clearly more susceptible to EAC than the pure weld metals of Alloy 152 and 52, which did not show any crack initiation. The dissimilar metal welds (DMW) with diluted microstructures are less susceptible than the pure weld metals of Alloy 182 and 82. No crack initiation/extension from hot cracks was observed in any of the studied weld metals. At the hot crack tips no crack growth was observed in any of the studied samples. This is related to the segregated microstructure of the hot crack tips. In accelerated doped steam tests selective dissolution takes place and metallic Ni or NiO forms a continuous layer in the middle of the cracks surrounded by the Cr-rich oxide layer. Selective dissolution typical for EAC was not observed inside the hot cracks or at their crack tips. EAC initiation occurred in the Alloy 600 base metal of the DMWs and selective dissolution inside the EAC cracks in Alloy 600 was extensive. The results are discussed based on the selective dissolution creep model of EAC.

Static and cyclic crack growth of low alloy steels in high-oxygen water simulating HDR test conditions

Abstract Steel samples of reactor pressure vessel and piping steels from the German HDR programme have been tested in high oxygen water at different temperatures simulating HDR test conditions. The specimens have been exposed to sequences of static and cyclic loading or to purely cyclic loading. During the tests, threshold stress intensity values for stress corrosion cracking and crack growth rates with various cyclic loading parameters were determined. Extensive fracture surface and oxide layer investigations were also performed. Water chemistry parameters such as dissolved oxygen content, pH and conductivity were continuously monitored during the tests. Finally, the measured laboratory water chemistry parameters were compared to those measured in the HDR plant during full scale testing of components and the relevance of the results for normally operating plants is discussed.