[en] The National Neutron Cross Section Center (NNCSC) has participated in the Electric Power Research Institute (EPRI) program of improving the nuclear data base to be used as input for the analysis of power reactors. The NNCSC contribution consisted of organizing and participating in specialists' meetings on the Thermal Neutron Cross Sections for the Actinides and Fission Products, participating in a meeting on the recommended value for the ²⁵²Cf anti nu, and keeping abreast of new measurements on the low energy resonance parameters of ²³⁸U. In addition, complete evaluations for seven isotopes of gadolinium have been produced for the next version of the data library. Definitive benchmark calculations using the Monte Carlo code RECAP have been carried out for two homogeneous uranium nitrate critical spheres (ORNL 1 and 2), two water moderated uranium metal lattices (TRX-1 and 2), and three plutonium nitrate spheres (PNL 2, 3, and 5). Results for the uranium spheres gave good agreement with experiments and with previous calculations. The calculation of the TRX lattice parameters, however, casts doubt on the validity of analytic leakage corrections used previously. Eigenvalues of the plutonium nitrate spheres are found to increase with increasing ratio of total ²⁴⁰Pu/²³⁹Pu captures