Abstract : The aim of this work is twofold : a study of the spallation reaction p+Au at 2.5 GeV and a study of neutron production out of thick targets. These two studies are part of a research program initiated within the framework of the European Spallation Source (ESS) project and of the french project of management of radioactive nuclear wastes by new approaches (GEDEON). Using both a high efficiency neutron detector and high granularity, high efficiency charged particle detectors, we have studied the evolution of the elementary spallation process as a function of the energy deposited by the incident proton in the nucleus (excitation energy). We have shown that the description of spallation by a two-step process is appropriate and have validated the use of the intranuclear cascade code of J. Cugnon (INCL2.0) coupled with a statistical deexcitation code (GEMINI). The production cross sections of light charged particles have been measured and the contribution of preequilibrium emission has been estimated. The formation of the ``direct'' composite particles has been implemented in the INCL2.0 code using a coalescence model. The neutron production (average multiplicities and distributions) has been studied as a function of the beam energy, of the target geometry and of the nature of the target material. We have shown that the gain in neutrons is dominated by the reaction probability when increasing the target thickness (for thicknesses lower than 2 or 3 lengths of interaction) and to a lesser extend by the development of secondary reactions. These two quantities have been investigated separately. The experimental data have been compared with the results of the HERMES transport code, validating its use for the neutron emission.