Neutron irradiation poses significant challenges for superconducting materials in high-energy physics applications and fusion reactors. This study investigates the effects of neutron irradiation on practical superconductors, focusing on REBCO-based high-temperature superconductors (HTS). We irradiated samples with varying neutron fluences up to 10²² n/m² at research reactors in the Belgian Nuclear Research Centre and Japan Atomic research Agency. Critical current measurements at variable temperatures were performed to investigate the effect of neutron irradiation on the superconducting properties of conductors.

This study provides crucial insights into the neutron irradiation tolerance of REBCO coated conductors, paving the way for their implementation in next-generation high-radiation environments accelerator projects with an estimated absorbed dose of over 100 MGy.

This work was performed under the GIMRT Program of the Institute for Materials Research, Tohoku University (Proposal No. 202012-IRKMA-0051, 202212-IRKMA-0507).

To investigate neutron irradiation effects on practical superconductors which must be recognized to design and construct D-T fusion reactors, irradiation tests were caried out in BR2 in Belgium and JRR-3 in Japan. A 15.5 T superconducting magnet and a variable temperature insert in a radiation control area at Oarai center in Tohoku University were used to study T_C, B_C2 and I_C of the irradiated samples. In case of Nb₃Sn wires, a parameter, I_C/I_C0, once exceeds 1.0 and decreases drastically to below 1.0, when the I_C/I_C0 is plotted against the neutron fluence of over 0.1 MeV. The main mechanism of the property change would be “displacement effect” by fast neutrons. On the other hand, a GdBCO tape showed poor superconductivity after small amount of neutron irradiation of 8.20x10²⁰ n/m² (> 0.1 MeV) in JRR-3. The irradiated sample was investigated carefully with a Ge detector 159 days after irradiation. As the results, two sharp peaks of gamma ray were observed at 41 keV and 102 keV and the production of ¹⁵³Gd of which half decay period is about 240 days was confirmed. It reveals that the nuclear transmutation of Gd degrades its superconductivity of GdBCO tape along with the displacement effect. Because a lot of thermal neutrons were captured, it is considered that a large amount of ¹⁵³Gd was produced, which was then transmuted into Eu. The superconductivity of YBCO tape did not change when irradiated by the same neutron fluence as the GdBCO tape.

The authors would like to express sincere thanks to Mr. Masanori Yamazaki and Prof. Takeshi Toyama at Tohoku University for their encouragement and contributions. The research samples of Nb₃Sn wires were supplied by companies in Japan, China and Korea. ReBCO tapes were provided by Shanghai Superconductor Technology Co., Ltd. The authors would like to express their gratitude to them.

This work was performed at the International Research Centre for Nuclear Materials Science in the Institute for Materials Research, Tohoku University.

Keywords: Neutron irradiation, Nb₃Sn, GdBCO, Nuclear transmutation

The advent of compact fusion reactors designs holds the promise of swift and affordable fusion energy [1,2]. The key technology that enables such reactors is high temperature superconductors (HTS)-based magnets, which produce extremely high magnetic fields and therefore can confine the plasma in smaller volumes. However, the compactness of these devices comes at a price: the magnet will be subject to neutron irradiation, introducing structural defects and degrade its superconducting properties[3,4]. Therefore, a full assessment of the radiation hardness of this fundamental component is needed. Computational methods can provide important insights in the microscopic mechanisms underlying the degradation process, therefore suggesting strategies to prolong the lifetime of HTS magnets.

In this presentation I will show our efforts in modelling the collision cascades started by impact of neutrons on primary knock-on atoms (PKAs) and their evolution on short timescales in YBa₂Cu₃O₇ (YBCO) with molecular dynamics (MD) simulations [5]. Based on Monte Carlo neutronics calculations, we obtained energy spectra of PKAs for specific reactor’s or experimental design, and we investigated with MD the collision cascades at several representative energies and PKA species employing an interatomic potential developed for radiation damage modeling [6]. Results are analyzed in terms of generated number of defects, defect morphologies, and temperature transients. As a step forward, we expanded the investigation of collision cascades in the energy-PKA element landscape including also electronic stopping power, providing the base for integration of damage from MD and binary collision approximation simulations for specific reactor’s designs. Preliminary results regarding the development of a machine learning interatomic potential for collision cascade simulations for YBCO and its sub-phases will also be shown.

[1] B. N. Sorbom et al., Fusion Engineering and Design 100, 378 (2015)
[2] A. Kuang, et al., Fusion Engineering and Design 137, 221 (2018)
[3] D. X. Fischer, et al., Superconductor Science and Technology 31, 044006 (2018)
[4] W. Iliffe, et al., Superconductor Science and Technology 34, 09LT01 (2021).
[5] D. Torsello, D. Gambino et al., Superconductor Science and Technology 36, 014003 (2023).
[6] R. L. Gray, M. J. D. Rushton and S. T. Murphy, Superconductor Science and Technology 35, 035010 (2022).

DG acknowledges financial support from the Swedish Research Council (VR) through Grant No. 2023-00208. This work is partially supported by COST action CA19108, by the Italian Ministry of Foreign Affairs and International Cooperation, grant n PGR01173, by Eni S.p.A.. DT carried it out within the Ministerial Decree 1062/2021, with funding from the FSE REACT-EU - PON Ricerca e Innovazione 2014-2020.

Keywords: HTS radiation damage, Fusion technology, Molecular Dynamics, Machine learning interatomic potentials

Among the key enabling technologies of compact fusion reactors there are high-temperature superconducting (HTS) magnets. Despite the great improvements recently achieved, one of the main challenges that still need to be faced is the evaluation of the impact of radiation in HTS tapes [1-4].
In this talk, we present results from Monte Carlo simulations performed with the PHITS code [5] on 3D CAD-imported models of the fusion reactors [6] and of HTS cables [7].
Our approach yields the expected neutron and secondary particles spectra impinging on the HTS material at the working conditions, the map of the deposited power and information about the damage in terms of displacement per atom (dpa) and Primary Knock-on Atom (PKA) spectra.
The output of such simulations is then used as input for detailed thermal and electromagnetic analysis of the cable and tape behavior at the operation conditions via FEM models [8], and for the study of the distribution and morphology of defects using Molecular Dynamics [4].

[1] W. Iliffe, et al., Superconductor Science and Technology 34, 09LT01 (2021).
[2] A. Kuang, et al., Fusion Engineering and Design 137, 221 (2018)
[3] D. X. Fischer, et al., Superconductor Science and Technology 31, 044006 (2018)
[4] D. Torsello, et al., Superconductor Science and Technology  36 (2023) 014003
[5] T. Sato, et al., Journal of Nuclear Science and Technology 50, 913 (2013)
[6] F. Ledda, et al., Fusion Engineering and Design (2024)
[7] F. Ledda, et al., IEEE Transactions on Applied Superconductivity (2024)
[8] S. Sparacio, et al., IEEE Transactions on Applied Superconductivity (2024)

This work is partially supported by COST action CA19108, by the Italian Ministry of Foreign Affairs and International Cooperation, grant n PGR01173, by Eni S.p.A.. DT carried it out within the Ministerial Decree 1062/2021, with funding from the FSE REACT-EU - PON Ricerca e Innovazione 2014-2020. DG acknowledges financial support from the Swedish Research Council (VR) through Grant No. 2023-00208.

Keywords: HTS tapes, HTS magnets, HTS radiation damage, Fusion technology