Characterization of Austenitic Steels for High Temperature Power Plants
Authors: Karmiol Zachary , Chidambaram Dev 
Published: February 2020

Introduction

The use of supercritical water (SCW) as the heat transfer medium between the power source and the steam generating system is a method of attaining increased thermodynamic efficiency for both coal and proposed Gen IV nuclear reactors [1]. Operating at a core inlet temperature of 553K (280°C) and an outlet temperature of 823K (550°C), supercritical water reactors (SCWRs) are capable of increasing efficiency by ~ 40% compared to current reactors [2-4]. The high operating temperatures of SCWRs necessitate high operating pressures of ~25MPa to maintain the supercritical fluid state [4, 5]. Numerous material challenges stem from the use of SCW, as SCW is highly oxidizing in addition to the degrading high temperatures and pressures encountered in these systems [6-8]. It is necessary to characterize the effects of exposure to SCW for candidate materials.

Experimental:

Experiments were conducted in the supercritical water loop (SCWL) facility at UNR. This facility allowed for both exposure and mechanical testing in SCW. The two materials selected were the austenitic steels Nitronic-50 (N-50), which is also known as UNS S20910 and XM-19, and the more employed stainless steel 316 (SS-316). All samples underwent testing in their as-received conditions, only being rinsed with ethanol and D.I. water to remove surface contaminants prior to experimentation. The SCWL is capable of maintaining a temperature of up to 600˚C and pressures up to 30MPa. The surfaces formed on the austenitic steels as a result of exposure to SCW were characterized using scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy.

Results

Figure 1 shows the X-ray photoelectron spectra for both N-50 and SS-316 exposed to 425°C supercritical water. It can be observed that the same elements are present on each sample, however in different ratios. Further results from mechanical testing and surface chemistry will be discussed.

Figure 1: X-ray photoelectron spectra recorded on the surface of samples having undergone slow strain rate testing under SCW at 698K (425°C) and 27MPa conditions. Spectra were obtained with monochromatic Al Kα radiation at an accelerating voltage of 14kV and 300W, with the sample at an angle of 105° to the detector.

Acknowledgements: This study was supported by the Department of Energy, under contracts DE-NE0000454 and DE-NE0008236, and Nuclear Regulatory Commission (NRC) under award NRC-HQ-11-G-38-0039.

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Figure 1