ABSTRACT
Practical computer simulations of reactors use either deterministic or Monte Carlo codes. The deterministic codes are the most used for critical reactor simulations, while Monte Carlo methods are almost exclusively used for hybrid reactors. Since our emphasis is on the latter, we only give a short reminder of the deterministic methods. Both methods require extensive neutron reaction data which are obtained from various data bases. The quality of the calculations is often limited by that of the data. Prior to discussing calculation methods, it is fitting to discuss data files and their evaluation.
