[en] The irradiation embrittlement is the most important damaging mechanism in the assessment of the reactor pressure vessel lifetime. In spite of the improvements in the fluency determination using better computational techniques, as well as the corrected cross section values, significant discrepancies exist between calculations and measurements. A series of the VVER Mock-ups (engineering benchmarks) were carried out in LR-0 experimental reactor in the Nuclear Research Institute Rez to investigate the differential neutron spectra for the reactor pressure vessel dosimetry purposes. The obtained experimental results were compared with the calculated values. The differential neutron energy spectra, measured in the Mock-up provide a good test of the calculational models and data libraries, and provide more information than a set of reaction rates usually measured in many benchmarks. The VVER-440 Mock-up investigations on the LR-0 experimental reactor were done by NRI, Rez Skoda Nuclear Machinery, Plzen (Czech Republic), and Russian Research Centre Kurchatov Institute (RRC KI), Moscow (Russia). Later on, the Institute of Nuclear Research and Nuclear Energy (IRNE), Sofia (Bulgaria) and Institute of Theoretical and Experimental Physics (ITEP), Moscow (Russia) took part in the benchmark calculations. New VVER-1000 Mock-up investigations are supported by IAEA as the regional project RER/4/017, 'The Data Base for VVER-1000 Reactor Pressure Vessel'. The participants are NRI, RRC KI, IRNE and several Ukraine Institutes. (author)