One of the main applications of the Hybrid Reactors (ADS - Accelerator Driven System) is the incineration of transuranics (TRU) by fast neutrons that emerge from a spallation source in a sub critical reactor waste burner [1, 2]. For this application, an accurate description and prediction of spallation reaction is necessary, including all the characteristics concerning spatial and energetic angular distributions, spallation products and neutron multiplicity. To describe the nuclear reactions at intermediate and high energies, Monte Carlo calculations have been used. The CRISP package considers the intranuclear cascade (INC) that occurs during the spallation process in a realistic time-sequence approach in which all particles inside the nucleus can participate in the cascade and the nuclear density fluctuations are naturally taken into account during the process. The occupation number of each single particle level is considered as a function of time and a more realistic Pauli blocking mechanism can be performed. None of the existing models have effectively used all those features. The evaporation of protons and alpha particles are taken into account making possible the correct prediction of fissilities of actinides and pre-actinides [3]. Another implementation is the NN single-pion production reaction. This reaction is especially relevant if one is interested in neutron or proton multiplicities, since the creation/emission of pions is directly related with the excitation energy of the residual nucleus. We will present some results obtained with the CRISP package for proton-nucleus reaction at intermediate and high energies. This package was obtained by the coupling of the MCMC [4] and MCEF [5] codes, with the introduction of some improvements, such as better Pauli blocking mechanism, which constrains the residual nucleus energetic evolution to the Pauli Principle from the ground-state to the final compound-nucleus formed at the end of the intranuclear cascade process, and introduction of the most relevant resonant excitation and the NN single pion production channel. The results of interest for ADS development are consistent with the experimental data at different proton energies. More detailed calculations are being performed for studying other features of proton-nucleus reactions and with different targets.

This paper reviews the physics of the spallation which is a nuclear reaction in which a particle (e.g. proton) interacts with a nucleus. Given to the high energy of the incident proton, in a first stage it interacts with the individual nucleons in an intranuclear cascade which leads to the emission of secondary particles (neutrons, protons, mesons, etc.). In a secondary stage the nucleus is left in an excited state and can de-excite by evaporation and/or fission. Given to the high number of secondary neutrons produced (~ 30 n/p for proton energy of 1 GeV), this reaction can be used as a source of neutrons, for example for ADS systems as external source to drive the sub critical reactor. The main codes used in the ADS target design and an example on the utilization of one of these codes (the LAHET code) for typical ADS target are given.

The progress of nuclear physics had allowed a better understanding of nuclear structure and reaction mechanisms, but also enables important applications of nuclear energy in our modern way of life. We can point out those ones related to power generation from nuclear reactors, providing an almost inexhaustive power source due to the huge quantities of nuclear fuel existent in our planet, which guaranties its utilization for thousands of years. The main interest is the so-called Hybrid Reactors (ADS - Accelerator Driven System), in which the reactor has a sub critical core and the chain reaction is sustained by an external source that comes from spallation reactions induced by high energy protons (about 1 GeV) from an accelerator, producing a large number of secondary particles (mainly neutrons). For those applications, precise knowledge about nuclear structure and nuclear reaction characteristics is needed.