In the frame of international projects spallation experiments are performed at the Joint Institute of the Nuclear Research in Dubna. The experiments with relativistic protons directed to thick targets are mainly focused on the research of the transmutation capabilities of spallation neutrons, but they also provide valuable data for benchmark tests of different spallation codes. In this work, Monte Carlo codes MCNPX and FLUKA are used to simulate two experimental setups (Phasotron and Energy Plus Transmutation). The influence of uncertainty in experimental parameters on the results is studied...