The metallurgical troubles of Hastelloy-N in molten salt reactors

Abstract

Many nuclear energy companies, including two based in Canada (Moltex Energy and Terrestrial Energy), have become interested in reactors that use molten salts in place of water as coolant and as a medium to hold the fuel. They draw technical inspiration from the Molten Salt Reactor Experiment (MSRE), a reactor that operated from 1965 to 1969 at the Oak Ridge National Laboratory, Tennessee (ORNL). One of the challenges with molten salt reactors involves the metallurgical materials used to manufacture the various reactor components, which would have to work in highly corrosive environments at elevated temperatures. To solve this problem, the MSRE developed Hastelloy-N, an a y high in nickel and molybdenum but low in chromium, to manufacture most metallic parts of the reactor, including the MSRE reactor vessel, piping, and pumps for all coolant loops and heat exchangers. Hastelloy-N had superior corrosion properties to the previously favoured Incoloy (a high nickel alloy for hightemperature use), which was used in salt-containing systems for the old Aircraft Reactor Program. One of the problems experienced by Incoloy was that because of the choice of structural material, the molten salt had to be separated from any beryllium contamination. After the MSRE was shut down in 1969, a number of studies showed that Hastelloy-N had developed cracks, leading to a deterioration in mechanical properties such as creep strength. This review examines the literature and summarizes the technical problems associated with using Hastelloy-N as a nuclear construction material. These material properties are essential for assessing the feasibility of the molten salt reactor designs proposed for Canada.