Sami Penttilä

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.

Composition, Structure, and Properties of Corrosion Layers on Ferritic and Austenitic Steels in Ultrasupercritical Water

In situ electrical and electrochemical measurements during oxidation of ferritic steel P91 and austenitic steel AISI 316L(NG), as well as of their main constituents (Fe, Cr, and Ni) in ultrasupercritical water (500-700°C, 30 MPa) have been reproducibly performed. Features observed in those measurements were substantiated by ex situ results on the thickness, composition, and morphology of the formed oxide layers from weight gain measurements, scanning electron microscopic observations, and in-depth glow-discharge optical emission spectroscopic analyses of corrosion layers. The oxidation process was followed in situ by using the contact electric resistance and contact electric impedance techniques. Impedance spectra of the Ni-Ni and Cr-Cr contacts during oxidation have been reproducibly measured. They could be quantitatively interpreted by using general considerations of the corrosion process and the mixed-conduction model for oxide films. Preliminary estimates of the diffusion coefficients of principal ionic and electronic current carriers have been obtained and their relevance with respect to available data on Ni and Cr oxidation is discussed.

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.

Corrosion phenomena induced by supercritical water in Generation IV nuclear reactors

The various supercritical water-cooled reactor concepts being developed under the Generation IV International Forum are the natural evolution of the water-cooled reactor technology that has successfully supplied the majority of nuclear-based electricity since the dawn of commercial nuclear power generation. The materials challenges that must be addressed in the development of a supercritical water-cooled reactor are in most respects the same as those experienced by the current generations of water-cooled reactors. This chapter summarizes current knowledge of corrosion and environmentally assisted cracking phenomena under the conditions expected in the core of a supercritical water-cooled reactor, with an emphasis on recent advances in experiment and modeling.

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.

Supercritical Oxidation of Boiler Tube Materials

The advantage of using supercritical water systems for power generation is based on the increased thermodynamic efficiency when operating at higher temperature and pressure. High efficiency in power generation is not only desirable because of economical reasons but also for enhanced environmental performance. Steam oxidation has become an important issue for steam power plants as operating temperatures increase from current to 650#xb0;C and even higher. To achieve these higher steam values new materials are needed. This paper presents results of the oxidation performance of potential new alloys (FeCrAlY, NiCrAl and Sanicro 25) in a supercritical water autoclave environment at 650#xb0;C/250 bar.