Benchmark calculations were performed for the mockup of a WWER-1000 reactor developed on the test reactor LR-0 in Nuclear Research Institute Rez (Czech Republic). The experimental arrangement is azimuthally and axially shortened, but the radial dimensions and material compositions are identical with the power reactors. The neutron and photon spectra, attenuation coefficients, and spectral indices were calculated and compared with measurements in all representative points of the mockups. The calculations were performed using the MCNP4C Monte Carlo code. A detailed geometrical model was developed. Most calculations were repeated using different nuclear data libraries. In some cases it was found that the difference of measured and calculated values is higher than could be explained by the experimental and calculational error.