The Joint Evaluated Fission and Fusion (JEFF) Nuclear Data Library is a collaboration among NEA Data Bank participating countries with the aim to create a common set of evaluated nuclear data. This data is not only for fission and fusion applications but also for domains such as space and earth exploration, medical isotope production, and nuclear science. The JEFF project meets twice a year, in the spring and autumn, for the JEFF Nuclear Data Week. This week has the objective of promoting co-operation between experimentalists, evaluators, processing specialists and end-users of nuclear data.
The NEA hosted the Nuclear Data Week from 17-21 November 2025 in Boulogne-Billancourt, France. This event brought together experts from the JEFF nuclear data library project to provide and review the feedback on the JEFF-4.0 library, which was released in June 2025, after eight years of continuous effort. The event also served as a discussion forum towards future developments to expand capabilities that meet the needs of the user community.
The week featured technical sessions on fusion applications, experiments, thermal scattering laws, fission observables, decay data, theoretical models, nuclear data uncertainties, processing, and benchmarking. The numerous contributions from the experimental community in this edition fostered greater exchanges between experimentalists, theoreticians and evaluators, strengthening opportunities for future JEFF developments through new measurements and evaluations. Comprehensive feedback was provided by the community towards the improvement of library contents and through needs identification for a variety of applications.
Overall, more than 120 delegates participated in discussions throughout the week. The release of the JEFF-4.0 library helped strengthen co-operation within the international nuclear data community.
