Aki Toivonen

CORROSION STUDIES OF CANDIDATE MATERIALS FOR EUROPEAN HPLWR

The High Performance Light Water Reactor (HPLWR) design is one of the concepts chosen for Generation IV reactors; however, the material requirements for HPLWR offer challenges because of the extreme operating temperatures and pressures. Consequently, general corrosion rates were studied in water at 300 to 650°C at supercritical pressure using weight gain measurements. Oxide thicknesses were determined from cross-section samples. The compositions of the oxide layers were analyzed using scanning electron microscopy in conjuction with energy dispersive spectroscopy. The surface layers of selected samples were analyzed also by X-ray diffraction. The test matrix included ten materials from four alloy classes: ferritic/martensitic steels, oxide dispersion strengthened (ODS) steels, austenitic stainless steels, and nickel-base alloys. A high oxidation resistance was seen in Ni-base alloy 625, austenitic stainless steels with high Cr content (>18 wt% Cr), and an ODS steel containing 20% Cr at all applied test temperatures (300 to 650°C). The oxidation rates of austenitic stainless steels with lower Cr content, 15 to 18%, increase considerably at temperatures >500°C. The oxidation rates of 9% Cr ODS steels were moderate or high at all temperatures. Ferritic/martensitic steels showed high oxidation rates at all temperatures.

Comparison of the Electric Properties and ESCA Result of Oxide Films Formed on AISI 316L Steel in Simulated BWR Conditions during SSRT

A possible approach to describe the role of the environment in the phenomena behind crack initiation and crack propagation in stress corrosion cracking (SCC) is to assume that the transport of species through the oxide film on the material surface is one of the rate-controlling factors. The transport rates of ionic and electronic defects through the oxide film are, in addition to the environment, also affected by the stress and strain applied to the bulk material. In this paper, the surface oxide film formed on AISI 316L steel in slow strain rate tests (SSRT) in simulated BWR condition has been analyzed by using Electron Spectroscopy for Chemical Analysis (ESCA). The obtained film composition and structure have been combined with in-situ contact electric resistance (CER) measurements in order to evaluate the changes in oxide film electric properties during straining in the above environment. The results show that oxide film resistance of the strained part exhibits a maximum at around 2% of strain, which seems to correlate with a maximum in the Cr(III) concentration in the inner layer of the oxide. The implications of these results to SCC are discussed based on Mixed-Conduction Model (MCM).

Environmentally assisted cracking

Environmentally assisted cracking (EAC) is a complex phenomenon driven by the synergistic interaction of mechanical, chemical and metallurgical factors. The complex interplay between causative factors makes experimental measurements difficult, and the state of knowledge on EAC under supercritical water-cooled reactor (SCWR) conditions is not as well advanced as that of general corrosion. This chapter discusses the effects of the three key causative factors (environment, material, and mechanical) on the occurrence of EAC in supercritical water, focussing on candidate SCWR alloys and expected SCWR in-core conditions. Possible differences in mechanisms in the near-critical and higher temperature regimes are highlighted.

European Project “Supercritical Water Reactor – Fuel Qualification Test”: Summary of general corrosion tests

The main target of the EUROATOM FP7 project “Fuel Qualification test for SCWR” is to make significant progress towards the design, analysis and licensing of a fuel assembly cooled with supercritical water in a research reactor. The program of dedicated WP4 - Pre-qualification was focused on evaluation of general corrosion resistance of three pre-selected austenitic stainless steels 08Cr18Ni10Ti, AISI 347H and AISI 316L, which should be pre-qualified for application as a cladding material for fuel qualification tests in supercritical water. Therefore, the experiments in support of WP4 concentrated on 2000 h corrosion exposures in 25 MPa SCW at two different temperatures 550 and 500°C dosed with both 150 and 2000 ppb of dissolved oxygen content. Moreover, water chemistry effect was investigated by conducting tests in 550°C SCW with 1.5 ppm of dissolved hydrogen content. At first, corrosion coupons were exposed for 600, 1400 and 2000 h in JRC IET, VTT and SJTU autoclaves connected to recirculation loop allowing continual water chemistry control during the test. Following examination of the exposed specimens consisted of weight change calculations and detailed macro and microscopic investigation of oxide layers using SEM and EDX. With respect to general corrosion results, all tested steels showed sufficient corrosion resistance in SCW conditions taking into account the conditions foreseen for future fuel qualification test in the research reactor in CVR Rez. When the results of weight change calculations were compared for all three materials, it was found out, that the corrosion resistance increased in the following order: 316L<347H<08Cr18Ni10Ti. Results obtained in hydrogen water chemistry did not indicate any significant beneficial effect compared to tests in SCW with 150 or 2000 ppb dissolved oxygen content. Additional tests were dedicated to investigation of surface finish effect. In these exposures polished, sand-blasted and plane-milled surface finish technique were investigated. Beneficial effect of surface cold work in particular of sand-blasting was clearly demonstrated.

Radiation effects and mechanical properties

All in-core components in an SCWR will experience irradiation by α and β particles, neutrons and high-energy photons (γ-rays) resulting in damage at the atomic level in the form of ionization and microstructural degradation due to the development of vacancies, interstitials and voids. These microscopic defects induce changes in physical and mechanical properties such as hardening, ductility, swelling, radiation-induced segregation, and creep, and can increase the risk of cracking. In combination with thermal creep, these changes are a major factor in determining long-term component reliability. This chapter discusses the various forms of radiation damage relevant to SCWR concepts, as well as discussing thermal creep of candidate SCWR materials.

Environmentally Assisted Cracking Crack Initiation in Nickel-Based Alloy Dissimilar Metal Welds in Doped and Pure Steam and Pressurized Water Reactor Water

Environmentally assisted cracking (EAC) susceptibilities of Alloys 600, 690, and their weld metals with and without weld defects were benchmarked in pure and doped hydrogenated steam at 400°C and pressurized water reactor (PWR) water at 360°C on four-point bend (0.35% to 1.1% strain) and U-bend specimens (5% strain). Contrary to the expectations, no EAC initiation from existing weld defects was observed. As expected, Alloy 600 and its weld metals were much more prone to EAC initiation than Alloy 690 and its weld metals. Oxide structures remain the same in doped and pure steam and PWR water, although the environment has an impact on the EAC initiation times and oxidation rates. Because of the similar oxide structures, the EAC mechanism is assumed to be the same in all studied environments.

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.