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 can be applied in nuclear fission and fusion, as well as in domains such as space and earth exploration, medical radioisotope production, and nuclear science. The JEFF project meets twice a year during the JEFF Nuclear Data Week. This week enables co-operation between experimentalists, evaluators, producers, and end-users of nuclear data.
The NEA hosted the Nuclear Data Week from 13-17 April 2026 in Boulogne-Billancourt, France. For the first time, the JEFF technical sessions and the APRENDE (Addressing PRiorities of Evaluated Nuclear Data in Europe) project progress report were brought together. APRENDE represents the major European initiative for addressing the critical demand for accurate and advanced nuclear data. Its outcomes will play a key role in future JEFF developments through the identification of nuclear data priorities, the acquisition and evaluation of new experimental data, and its validation for a wide variety of application domains.
This edition also included a joint session held with the International Criticality Safety Benchmark Evaluation Project (ICSBEP) to enhance the collaboration on benchmarking activities that could benefit future JEFF nuclear data library developments.
During the week, participants attended technical sessions featuring updates on fusion applications, experiments, thermal scattering laws, fission observables, decay data, evaluations, nuclear data uncertainties, processing, and benchmarking. APRENDE work packages reporting was integrated within the technical sessions, informing about the applicability of the APRENDE project outcomes to future JEFF improvements.
Overall, more than 170 delegates participated in discussions throughout the week, paving the way towards the improvement of the JEFF-4.0 nuclear data library.
