Jack Wilson1,Michael Rushton1,David Goddard2,William Lee1,Simon Middleburgh1
Bangor University1,National Nuclear Laboratory2
Jack Wilson1,Michael Rushton1,David Goddard2,William Lee1,Simon Middleburgh1
Bangor University1,National Nuclear Laboratory2
Current zirconium-based fuel claddings are susceptible to high temperature steam oxidation in accident conditions such as LOCAs. This causes a build-up of hydrogen, as well as degrades fuel element mechanical properties leading to balloon-and-burst phenomena. This presents an engineering challenge for water-cooled reactors to be improved by the introduction of Advanced Technology fuels (ATFs). Coatings such as chromium are being investigated to serve as a barrier, to prevent the zirconium-steam reaction and delay the onset of cladding ballooning. However, challenges remain in the chemical interactions between chromium coat and the underlying Zr substrate. High Entropy Alloys (HEAs) are a new class of material which can be tailored to serve as a suitable interlayer between the current zirconium alloy substrates and chromium coating, improving in-reactor behaviour in normal conditions and accident scenarios.<br/>By careful composition selection of this thin HEA interlayer, a suitable joining can be made between the dissimilar metals Zr and Cr. By implementing an interlayer, it is possible to raise the eutectic reaction temperature, impede interdiffusion, and more importantly, suppress the formation of Laves phase ZrCr<sub>2</sub>, which is a brittle intermetallic and can lead to eventual spalling of the Cr-coat.<br/>Here we investigate key properties of HEAs to consider for an interlayer or coating: thermal expansion. Density functional theory calculations have been used to predict the stability and phonon properties of a wide range of increasingly complex alloy systems that can be used to predict the thermal expansion behaviour of them. Predictions are benchmarked to existing experimental data, where available, and targeted experiments on new alloy systems have been performed when necessary. We have shown that atomic scale modelling techniques can be used to reliably predict thermal expansion of a range of BCC high entropy alloys for the first time.