Interaction of beryllium with 316H stainless steel in molten Li2BeF4 (FLiBe)

The increased demand in renewable energy resources has led to a renewed interest in nuclear reactors including a new generation of molten salt reactors. One of the issues encountered in handling of molten chloride or fluoride salts is the inability of metals to form and retain a protective oxide surface layer, gen-erally chromium oxide. The low chromium content, nickel-based alloy INOR-8, later marketed as Hastelloy N, was specifically developed to contain molten fluorides. This alloy had some deficiencies, but its com-position resulted in it incurring far less chromium depletion by the molten fluoride salts than alloys like 316 stainless steel. For the next generation of molten salt reactors, an alloy with pressure vessel code approval and higher temperature capability than Hastelloy N is required. Consequently, several reactor designers have reportedly chosen to build the reactor containment vessel from 316H stainless steel. In order to minimize corrosion of the stainless steel by the molten fluoride salt (2LiF-BeF2), it is proposed to add beryllium metal to the salt to react with impurities and provide a means to lower the oxidation potential thus making the salt less corrosive. However, there is a concern whether the beryllium would react with components of the stainless steel. To address this concern, static capsule tests were conducted in which selected amounts of beryllium were added to capsules containing 2LiF-BeF2 (FLiBe) salt and 316H stainless steel and Hastelloy N samples. Following exposure, the samples were cleaned and exam-ined using a variety of analysis techniques including, optical microscopy, SEM-EDS, XPS, LIBS, XRD and EPMA. Tensile samples were also exposed in capsules, and those were subjected to tensile testing. It was found that for the level of beryllium additions used in this study, intermetallic compounds were formed which could be detrimental to the long-term performance of the 316H stainless steel. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The increased demand for renewable energy resources has sparked a renewed interest in nuclear reactors, including a new generation of molten salt reactors. The handling of molten chloride or fluoride salts poses a challenge due to the inability of metals to form and retain a protective oxide surface layer. A specific alloy, Hastelloy N, was developed to address this issue, but it had some deficiencies. For the next generation of molten salt reactors, an alloy with higher temperature capability and pressure vessel code approval is required, leading some reactor designers to opt for 316H stainless steel. To minimize corrosion of the stainless steel by the molten fluoride salt, beryllium metal can be added to react with impurities and lower the oxidation potential. However, there are concerns about the potential reaction between beryllium and the stainless steel components.