A benchmark analysis was launched within the Work Package on Core Safety of the EU FP7 cross-cutting project supporting the European Sustainable Industrial Initiative (ESNII), named ESNII+, aimed at providing a quantitative estimation of the uncertainties affecting the re static neutronic parameters, including safety coefficients, of a Sodium-cooled Fast Reactor (SFR) low-void-effect core. The core is similiar to the one considered for the Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID). Established deterministic and stochastic neutronic codes, as well as different nuclear data libraries, were employed by eight European organizations to perform a complete core characterization, with the ultimate goal to achieve consensus on computational methods and associated databases to be employed for advanced-design SFRs safety analyses. The comparison of the results obtained by the participating institutions provided quantitative information about capabilities and limitations of the different approaches, and about library effects, by highlighting the sensitivity of safety parameters to both computational techniques and nuclear data. Selected results of the first phase of the benchmark are presented and analyzed, along with a discussion of the planned R&D activities needed to improve the present benchmark status.