RADIATION RESISTANT HIGH ENTROPY ALLOYS FOR FAST REACTOR CLADDING APPLICATIONS

High entropy alloys are a recently developed novel class of materials in which no one element dominates. Instead four or more elements are used in near equal proportions. These alloys have been reported to have a wide range of attractive properties including high strengths at high temperatures, good corrosion resistance and ability to withstand irradiation damage. These properties make HEAs strong candidate materials for use as fuel cladding in sodium cooled fast reactors. These reactors can operate at higher temperatures, using less fuel and are safer than current water and gas cooled reactors.

However if HEAs are to be utilised in this highly aggressive environment there are two key needs that must be addressed. The first is to identify promising alloy compositions, manufacture them and characterise their mechanical behaviour. Secondly the mechanisms that lead to the excellent resistance to irradiation damage need to be understood. Through this we will establish whether the irradiation response is universal to all HEAs and devise strategies to predict other potentially better systems to investigate. This grant will use ion irradiations rather than neutrons as ion irradiations are cheaper and faster to carry out and allow for more rapid turn around in alloy development. How the structure of the alloys is changed by irradiation will be studied using advanced microscopy methods and the effect the irradiation has on the mechanical behaviour will be studied using novel micro-mechanical testing methods. Importantly these methods can be used to measure mechanical behaviour at temperatures over 900oC, so the mechanical properties at the operational temperature can be studied in both irradiated and unirradiated conditions.
Once the most promising alloys, with the best resistance to irradiation damage, have been identified their resistance to liquid sodium corrosion will be studied. In this way we will develop a novel alloy which can be used as fuel cladding in sodium cooled fast reactors.

Sodium cooled fast reactor are a proven nuclear fission reactor design which are a leading candidate for the next generation of nuclear reactors (so called GEN IV). These reactors will be more efficient and safer than the current gas or water cooled systems. However they are currently held back by a lack of suitable materials to use as fuel cladding. Zirconium cannot be used as the operational temperatures are too high. Steels have been used in several test systems. However they suffer from swelling under irradiation and a decrease in mechanical properties after irradiation and exposure to liquid sodium. High entropy alloys are a series of novel systems where no single element dominates as is the case in traditional alloys. Instead 4 or more elements are used in near equal amounts. Recent preliminary work has shown they alloys can be almost immune to the irradiation damage which severally degrades conventional alloys. This grant will accelerate the development of high entropy alloys and assess their ability to act under the extreme environments envisaged for a sodium cooled fast reactor. This development of safer nuclear reactors will allow the further deployment of nuclear power plants in an effort to reduce greenhouse gas emissions, and minimise the effect of climate change. This will have a positive impact of the well being of the global population.
Within the UK, high entropy alloys are of interest to a wide range of end users across the nuclear, aerospace and automotive industries. In particular the Culham Centre For Fusion Energy is interested in using HEAs as structural materials in future nuclear fusion devices and Rolls Royce are studying them for potential jet turbine applications. These are groups we already have good links with and we will disseminate our key finding directly to them, through our regular meetings as well as larger public workshops. In this way we aim to find synergistic links across industries and accelerate alloy design in this nascent area of research.
We will work with Oxford University Innovation, the University of Oxford's Technology Transfer Department, to exploit any commercial potential of our research. The testing techniques and modelling methodologies developed for high temperature micro-mechanical testing of irradiated materials are of interest to leading UK companies such as NNL for validating life extensions of nuclear reactors, EdF who are leading the current new build at Hinkley point and SME's such as Tokamak Energy who are developing compact nuclear fusion reactors. These are companies who are already involved in supporting research into nuclear materials and systems at Oxford University and we will invite them or their representatives to our open meetings to ensure they are exposed to the latest techniques we are develop (see "academic beneficiaries" section)