Assessment of the ENDF/B-VIII.1 nuclear data library for VVER reactor criticality safety, fuel depletion, and dosimetry

The ENDF/B-VIII.1 nuclear data library has been evaluated for VVER (Vodo-Vodyanoi Energetichesky Reaktor, i.e. water-cooled water-moderated power reactor) calculations in three application areas: criticality safety, fuel depletion, and reactor dosimetry. VVER reactors differ from square-lattice light-water reactor designs in fuel geometry, core structures, and structural materials, making dedicated nuclear data validation for this reactor family necessary. The study used MCNP6 for criticality and reactor dosimetry calculations and Serpent 2 for fuel depletion, together with benchmark data from the International Criticality Safety Benchmark Evaluation Project (ICSBEP) and Spent Fuel Composition Database (SFCOMPO), and a full-core VVER-1000 dosimetry model. Criticality validation of 343 VVER benchmark cases showed an average positive bias of + 399 pcm (1 pcm = 10 −5 in Δk/k), only 70 pcm higher than with ENDF/B-VIII.0 and well within statistical and modeling uncertainties. For fuel depletion, ENDF/B-VIII.1 successfully reproduced 312 post-irradiation assay values from the ISTC-2670 benchmark. Calculated isotopic inventories remained very close to ENDF/B-VIII.0 results, with 76% differing by less than ± 1% and 87% by less than ± 2%. Reactor dosimetry showed substantial improvement over ENDF/B-VIII.0. The calculated neutron fluence above 0.5 MeV increased by 12.5%, resulting in better agreement with measured responses from ex-vessel activation detectors. These improvements are attributed to revised cross sections and angular distributions for key nuclides, such as Fe-56 and O-16. Unlike ENDF/B-VIII.0, which significantly underestimated fast neutron fluence and was not recommended for reactor dosimetry, ENDF/B-VIII.1 demonstrates reliable performance across all three application areas and is therefore recommended for VVER reactor analyses.