Nuclear Energy Agency (NEA) – JEFF-4.0 nuclear data library is now available

The Nuclear Energy Agency (NEA) Data Bank has released the latest version of the Joint Evaluated Fission and Fusion (JEFF) nuclear data library. JEFF-4.0 has been developed by a community of experts over a period of eight years since the release of JEFF-3.3 in 2017.

The JEFF-4.0 nuclear data library combines the available experimental and theoretical knowledge of nuclear reactions into a standard format nuclear data file that serves a wide user community. JEFF-4.0 is a general-purpose library suitable not only for nuclear fission and fusion applications, but also for domains such as space and earth exploration, medical isotope production and basic science. The development of JEFF-4.0 included a significant improvement over JEFF-3.3 in modelling and simulation performance for light water reactors (e.g. reactivity versus burnup, boron-letdown, power maps, inventories) and a continued improvement for advanced reactor simulations.

Over the development period, the JEFF project has implemented many changes and updates to the library contents and to the way the data are produced. Highlights include the new evaluations of neutron-induced reactions on the major actinides U-235, U-238 and Pu-239, as well as the thermal neutron-induced fission yield for U-235, U-238, Pu-239 and Pu-241. The thermal scattering sub-library was improved for the key case of hydrogen in water and expanded significantly in co-operation with other initiatives. Decay data was augmented with the most recent outcomes of gamma-ray total absorption measurements. JEFF now provides a proton-induced reaction sub-library that has benefited from a careful review of worldwide evaluated data and an evaluation of activation data. There is a substantial integration of TENDL evaluations in JEFF-4.0 and the charged particle-induced reaction data are adopted from the TENDL project.

The JEFF-4.0 library was released in June 2025, recently announced at the 16^th Nuclear Data for Science and Technology Conference (ND2025), and is now publicly available for download from the NEA Data Bank website.

The NEA Data Bank has modernised its systems to enhance the support and development of the library, providing a fully reproducible processing and verification process following open science principles. All data is distributed through the new NEA Data Bank data management platform, complete with digital object identifiers (DOIs). The library release will be accompanied by a topical European Physical Journal A article collection presenting the library contents and most relevant benchmarking results.