Rethinking materials limit in nuclear fusion technology: First in situ superconductor experiment
10 Mar 2026 - Rohini Gupta
Nuclear fusion - the process in which two light nuclei fuse, releasing huge amounts of energy - is best known as the reaction which powers the Sun, yet has garnered significant scientific interest and excitement due to its potential as a plentiful source of clean energy.
The most advanced and widely-researched fusion power design is the tokamak, in which a superhot plasma is confined by a powerful magnetic field. Leading next-generation tokamak concepts such as STEP (UK) and SPARC (USA) to choose REBa2Cu3O7-x (REBCO) high-temperature superconductors to generate the high fields demanded by their ambitious reactor designs. However, the deuterium-tritium fusion reaction produces high-energy 14.1 MeV neutrons that will inevitably irradiate these essential components, and understanding how this intense irradiation will impact the performance and lifetime of the cutting-edge superconductors is therefore crucial.
The Neutron Irradiation Laboratory for Electronics (NILE) at ISIS Neutron and Muon Source is one of only a few facilities worldwide capable of offering fusion-spectrum neutron irradiation for scientific research. Professor Susie Speller and PhD-student Kirk Adams of the Centre for Applied Superconductivity at the University of Oxford, in collaboration with Dr Will Iliffe of the UK Atomic Energy Authority approached the NILE team to explore how to use this capability to better understand the in-service performance of REBCO in fusion magnets.
The Oxford team have over the last decade pioneered in situ measurements of superconductors during irradiation. Their aim was to work with the UKAEA and ISIS teams to use the controlled flux of 14.1 MeV neutrons from NILE to study REBCO superconductors at cryogenic temperatures under the high-energy neutron bombardment present in an operational reactor. This would provide unparalleled insight into the durability, reliability and performance under realistic conditions. Their study is the first in situ experiment where a superconductor carrying current is exposed to neutron irradiation. Previous experiments have only been able to evaluate the superconductor performance after the neutron irradiation has finished.
Following a preliminary proof-of-concept experiment with existing equipment, it was decided that a new bespoke cryostat was going to be needed to maximise the neutron flux at the superconducting sample. This was a significant technical challenge, that required the sample to be housed millimetres from the NILE apparatus, while maintained at -230˚C to ensure that any irradiation-induced damage would not be healed by thermal recovery effects. Working closely together, the Oxford-UKAEA team designed a new cryostat that was then built by the ISIS Cryogenics team. The result was a new experimental setup which successfully reduced the sample-to-source distance from 5 cm to 1.5 cm and raised the neutron flux at the sample by over a factor of 100.
The superconductor was first tested in the absence of neutron irradiation to establish a performance baseline before exposure to the NILE neutron source. During irradiation, no instantaneous changes to the superconducting performance was observed under 14.1 MeV neutron fluxes of up to 7.3 x 107 neutrons per square-centimetre per second. However, post-irradiation measurements revealed a reduction in the critical current of the superconductor (the maximum current a superconductor can carry without energy losses).
Following careful analysis to exclude thermal effects, this observation confirmed degradation in the superconducting performance caused by the neutron-induced damage. Most significantly, this revealed the onset of degradation at a total neutron dose approximately 1000 times lower than any previously published study. This difference to the existing literature is attributed to the in situ nature of this experiment which avoids recovery processes at room temperature – a world-first for neutron irradiation testing of high-temperature superconductors.
Although follow-up experiments are planned to further investigate these findings, this important result suggests that superconducting materials degrade under neutron irradiation earlier than currently estimated and prototype tokamak designs may need to adjust accordingly.
Overall, this study has provided an important new insight into one important challenge faced by nuclear fusion technology. As fusion research continues to advance, particularly in tokamak development, these findings emphasise the need for continued material innovation and more precise experimental methods. Addressing such limitations will be essential to ensuring that nuclear fusion can fulfil its promise as a safe, sustainable, and abundant source of clean energy for the future.
A full report of this experiment is available to read here: https://iopscience.iop.org/article/10.1088/1361-6668/ae4548
