In the framework of the development of new generations of nuclear reactors and more specifically Sodium Fast Reactors (SFRs), a more efficient concept of gas turbine with Brayton cycle is under study. It consists of a CO2 cycle utilizing the peculiar drop in the compressibility factor around its critical point (31 °C, 74 bars) to reduce the compression work. Coupled to the secondary circuit of a Sodium Fast Reactor (for instance Na or liquid Pb-Bi), the expected conditions of supercritical carbon dioxide at the inlet of the heat exchanger are about 550 °C and 250 bars and, as a consequence, the expected global cycle efficiency is about 45%, which is higher than the current Rankine cycle efficiency. Under these conditions and considering that, in nuclear plants, the life time of the material constituting the heat exchangers has to be, at least, twenty years, it appears obvious that the candidate structural material has to be chosen very carefully.
This study focuses on ferritic-martensitic steels T91, a relatively cheap steel (compared to austenitic steels and nickel alloys), with good thermal expansion and conductivity. However, its mechanical resistance is under question, especially in carburizing environments. This project aims at characterizing and modeling T91 corrosion and carburization in ambient pressure and high pressure supercritical CO2 environments. High pressure tests have been performed at the UW-Madison Thermal Hydraulics Laboratory. Samples are characterized using a set of electron microscopes (SEM, EDS, TEM) and specific techniques to detect carbon concentration profiles (GDOES). An oxidation and carburization model, based on the C4 model framework are currently under development to predict oxidation and carburization of steels.
Related publications
- J. Mahaffey, A. Couet, et al., “Effect of CO and O2 impurities on Supercritical CO2 Corrosion of Inconel Alloy 625,” Metallurgical and Materials Transactions A, 2018 (submitted).