The efficiency of ultra-supercritical (USC) steam power plants is limited by the materials properties, in particular, the steam oxidation resistance of the currently used steels at temperatures higher than 600 °C. Under these conditions, steam oxidation results in the development of thick oxide scales which spall and can accumulate in tube bends leading to blockage, overheating and premature creep rupture, as well as erosion of downstream components such as steam valves and turbine blades. Most published work related to oxidation testing is carried out at atmospheric pressure, with significantly less testing of austenitic steels in supercritical steam, and rarely including protective coatings. Indeed, the effect of high-pressure steam in the oxidation process is not quite understood at present. This paper covers a comparison of the behaviour of TP347HFG stainless steel at 700 °C under atmospheric pressure and 25 MPa, with and without slurry-applied diffusion aluminide coatings. The results show a very protective behaviour of the aluminide coatings, which develop a very thin Al-rich protective oxide, and no significant difference between the two environments. In contrast, the uncoated steel exhibited a different behaviour. Indeed, under atmospheric pressure after 3000 h, very thin scales, rich in Cr and not surpassing 5 to 10 µm in thickness, covered the samples along with some much thicker Fe-rich oxide nodules (up to 150 µm). However, under 25 MPa, a thick multilayer scale with a non-homogeneous thickness oscillating between 10 to 120 µm was present. A microstructural investigation was undertaken on the oxidised uncoated and coated substrates. The results suggest that pressure increases the oxidation rate of the chromia former steels but that the oxidation mechanism remains the same. A mechanism is proposed, including early detachment of the outer growing scales under supercritical pressure.

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.

The supercritical water reactor (SCWR) is one of the six reactor concepts being investigated under the framework of the Generation IV International Forum (GIF). One of the major challenges in the development of a SCWR is to develop materials for the fuel and core structures that will be sufficiently corrosion resistant to withstand supercritical water conditions and to gain thermal-hydraulic experimental data that could be used for further improvement of heat transfer predictions in the supercritical region by numerical codes. Previously, core, reactor, and plant design concepts of the European high-performance light water reactor (HPLWR) have been worked out in great detail. As the next step, it has been proposed to carry out a fuel qualification test (FQT) of a small-scale fuel assembly in a research reactor under typical prototype conditions. Design and licensing of an experimental facility for the FQT, including the small-scale fuel assembly, the required coolant loop with supercritical water, and safety and auxiliary systems, was the scope of the recently concluded project “Supercritical Water Reactor–Fuel Qualification Test” (SCWR-FQT) described here. This project was a collaborative project cofunded by the European Commission, which took advantage of a Chinese–European collaboration, in which China offered an electrically heated out-of-pile loop for testing of fuel bundles. The design of the facility, especially of the test section with the fuel assembly, and the most important results of steady-state and safety analyses are presented. Material test results of the stainless steels considered for the fuel cladding are briefly summarized. Finally, important outcomes and lessons learned in the “Education and Training” and “Management” work packages are presented.

This paper describes the methods and results of analytical TEM examinations and autoclave testing of two highly-irradiated flux thimble tube materials harvested from a commercial pressurized water reactor. The materials are cold-worked 316L, and accumulated 65 dpa and 100 dpa of radiation dose. To set the baseline for a broader study, the materials were examined in the as-irradiated condition and tested as O-ring specimens at relatively high constant loads in simulated PWR water conditions. Tests were also conducted with elevated hydrogen. For a given load, more rapid cracking was associated with higher radiation dose, and with the elevated hydrogen.

The presented work consists of a test setup study of a new pneumatic material testing device based on double bellows (DBs) loading device and with miniature autoclaves enabling applications at temperature and pressure up to 650 °C and 35 MPa, respectively. It has been demonstrated that it is technically feasible to carry out well defined and controlled material testing in the supercritical water (SCW) environment using this testing system. By using this type of system, it makes possible to investigate the intrinsic role of the applied stress on the deformation behavior of material in light water reactor (LWR) conditions and also in other harsh environments like SCW conditions. In addition, the compactness and versatility of the setup makes this system particularly attractive for deployment in a hot-cell for testing of irradiated materials.

The main target of the EUROATOM FP7 project “Fuel Qualification Test for SCWR” is to make significant progress toward the design, analysis, and licensing of a fuel assembly cooled with supercritical water in a research reactor. The program of dedicated Work Package (WP4)-Prequalification was focused on evaluation of general corrosion resistance of three preselected austenitic stainless steels, 08Cr18Ni10Ti, AISI 347H, and AISI 316L, which should be prequalified for application as a cladding material for fuel qualification tests in supercritical water. Therefore, the experiments in support of WP4 concentrated on 2000-hr corrosion exposures in 25-MPa supercritical water (SCW) at two different temperatures 550°C and 500°C dosed with both 150 and 2000 ppb of dissolved oxygen content. Moreover, the 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 hrs in Joint Research Centre-Institute for Energy and Transport (JRC-IET), VTT Technical Research Centre of Finland Ltd. (VTT), and Shanghai Jiao Tong University (SJTU) autoclaves connected to the recirculation loop, allowing continual water chemistry control during the test. The following examination of exposed specimens consisted of weight-change calculations and detailed macro- and microscopic investigation of oxide layers using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (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 that the corrosion resistance increased in the following order: $316L<347H<08Cr18Ni10Ti$. Results obtained in hydrogen water chemistry (HWC) 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 the surface-finish effect. In these exposures, polished, sand-blasted, and plane-milled surface-finish techniques were investigated. The beneficial effect of surface cold work in particular of sand-blasting was clearly demonstrated.