Sami Penttilä

Current development on advanced technology fuel (ATF) claddings is aiming at improved high temperature integrity of new candidate materials designed on existing zirconium cladding materials. To assess their performance in commercial use, characterization of newly proposed materials is essential. The primary requirement for the cladding materials is the ability to tolerate loss of cooling for a significant period without failing. The tests in this work were performed on different types of coated Zr-alloys in a high temperature steam furnace in flowing steam conditions at 1100 °C for 500 s, 1200 °C for 1800s, and 1300 °C for 300 s. In addition, autoclave exposures were performed in simulated pressurized water reactor (PWR) water chemistry to confirm the material viability in normal light water reactor (LWR) operating conditions. After autoclave and steam furnace tests, exposed specimens were characterized using a Zeiss Crossbeam 540 field emission gun-scanning electron microscope (FEG-SEM) equipped with a semiquantitative energy dispersive X-ray spectrometer (EDS). Oxide and coating layers were imaged using secondary electrons and elemental composition of the layers were determined using selected area (SA) and point analyses. In addition, elemental maps were collected from specific areas of interest. Most of the tested specimens indicated detached coating layer. Varying amounts of cracking in the coating materials were present. Some of the cracks extended even into the base material. Based on this study, further developments of ATF cladding solutions are needed through international collaboration.

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.

In this study, three alumina-forming austenitic (AFA) steels (Fe-18Ni-12Cr-AlNbC, Fe-18Ni-12Cr-Al, and Fe-18Ni-16Cr-AlNbC) were exposed to superheated steam (SHS) at 700 °C for 1000 h to study their oxidation behavior. The Fe-18Ni-16Cr-AlNbC alloy showed the best performance as it had the least weight gain due to a denser surface oxide layer and the lack of internal oxidation. The Fe-18Ni-12Cr-AlNbC alloy suffered from internal oxidation and had a greater weight gain than Fe-18Ni-16Cr-AlNbC, while Fe-18Ni-12Cr-Al experienced weight loss due to perhaps external oxide spallation, leaving a less protective Fe-oxide on the external surface.

The choice of materials is of great concern in the construction of Gen IV supercritical water reactors (SCWR), particularly the fuel cladding, due to the harsh environment of elevated temperatures and pressures. A material's performance under simulated conditions must be evaluated to support proper material selection by designers. In this study, aluminide and Cr-modified aluminide coated 304, as well as bare stainless steel 304 as a reference material, were tested in supercritical water (SCW) at 700 °C and 25 MPa for 1000 h. The results showed that all three samples experienced weight loss. However, the aluminide coated 304 had 20 to 40 times less weight loss compared to Cr-modified aluminide coated and bare stainless steel 304 specimens, respectively. Based on scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis results, spinel and hematite Fe2O3 formed on bare 304 after 1000 h in SCW while alumina was observed on both coated specimens, i.e., aluminide and Cr-modified aluminide surfaces. Oxide spallation was observed on the bare 304 and Cr-modified aluminide surface, contributing to a larger weight loss. Based on the results from this study, pure aluminide coating with Al content of 10–11 wt % demonstrated superior performance than bare 304 and Cr-modified aluminide coated 304.

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.

Concerns with greenhouse gas emissions and the uncertainty of long-term supply of fossil fuels have resulted in renewed interest in nuclear energy as an essential part of the energy mix for the future. Many countries worldwide including Canada, China, and EU are currently undertaking the design of generation IV supercritical water-cooled reactor (SCWR) with higher thermodynamic efficiency and considerable plant simplification. The identification of appropriate materials for in-core and out-of-core components to contain the supercritical water (SCW) coolant is one of the major challenges for the design of SCWR. This study is carried out to evaluate the oxidation/corrosion behaviors of bare alloy 214 and NiCrAlY coated 214 under SCW at a temperature of 700 °C/25 MPa for 1000 h. The results show that chromium and nickel based oxide forms on the bare surface after exposure in SCW for 1000 h. A dense and adhered oxide layer, consisting of Cr₂O₃ with spinel (Ni(Cr, Al)₂O₄), was observed on NiCrAlY surface after 1000 h in SCW.

Supercritical (SC) service at high operating values aims for good plant efficiency, but the waterside oxidation resistance can then become life-limiting. In this paper, selected materials and modeling options are compared for life assessment under waterside SC oxidation, particularly for thermal power plants that increasingly need to accommodate cyclic service, fast ramping, and low minimum loads to an extent to which the conventional design practices and materials solutions only partially accounted. For example, the life reduction by high-temperature oxidation and corrosion via lost load-bearing wall thickness is more easily accommodated than the impact on crack growth or material ductility.