The Monte Carlo codes MCNPX and FLUKA were used to analyze the experiment realized at the Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research (Dubna) in December 2003. A 660 MeV proton beam was directed to a thick lead target producing spallation neutrons, which were probed at different places around the target with small activation detectors. The codes were used to study the systematic uncertainties and to predict the experimental results. Several spallation models included in MCNPX and FLUKA were checked against each other and the experiment. Both codes described successfully most of the experimental results.