The basic nuclear physics that are required in modelling and simulation of nuclear systems cannot be derived from first principles but must be carefully measured. Quantities such as the interaction probabilities between neutrons and different nuclei vary by orders of magnitude between different isotopes and incident neutron energies. For typical systems such as nuclear fission reactors, the incident energies of importance span values between thousandths of an electron volt to millions of electron volts and complete knowledge of all these areas is required. 

Since the earliest days of nuclear engineering, nuclear nations have performed experiments to carefully measure nuclear reaction quantities. These are essential for the design and construction of all nuclear reactors and national 'library' projects were established to collect all of these measurements. In the 1960s the EXFOR database was established as a collaboration between the US National Nuclear Data Center, the precursor to the NEA Databank, the IAEA and the Russian Nuclear Data Center, storing neutron-induced differential reaction measurements. Over time this database has grown to include hundreds of thousands of datasets that serves as an invaluable starting point for all nuclear data evaluation.