[en] With an installed capacity of over 3.2 GW_e, the four Swiss Nuclear Power Plants (NPPs) supply 40% of the Swiss electricity demand. In view of future decommissioning liabilities, the nuclear utilities are required by law to ensure that adequate financial resources will be available in order to cover the costs arising. The Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) periodically performs and updates cost studies relevant to NPP decommissioning and radioactive waste management. These studies include the calculation of the nuclear activation in reactor components induced by their exposure to neutron radiation, because the radiological inventory directly effects the dismantling of the NPP and the relevant waste management. This report presents the results of a Nagra - ETH Zurich collaboration for the development, implementation and validation of a state-of-the-art activation neutronics scheme. Given the latest developments in neutron transport codes and the access to increased computational power, 3D Monte Carlo (MC) and hybrid methods linked to modern activation codes are used in order to yield the most reliable NPP activation estimates. The former NPP activation study was based on 1D deterministic neutron transport calculations. Bound to methods and computer power limitations, this study resulted, in some cases, in the underestimation of the effects of neutron streaming in areas outside of the pressure vessel (up to factors of 100 in the residual neutron induced activities). In other cases, over-conservative extrapolations led to the overestimation of the resulting activities. In this thesis, neutron streaming effects are accurately taken into account. The following issues are addressed: a) the suitability, effectiveness and efficiency of MC, deterministic and hybrid neutron transport methods for NPP activation studies; b) the development of 3D MC NPP-specific models for all Swiss NPPs; c) the performance of different variance reduction (VR) methods for accelerating the MC calculations; d) the suitability of state-of-the-art activation codes for reactor ex-core activation calculations; e) the validation/calibration of the simulation results, through the setup and analysis of foil activation campaigns within the NPPs. A modelling approach based on the MCNP5 code is developed and implemented for the modelling of each of the Swiss NPPs. The benchmarking between MC and hybrid VR methods shows that hybrid codes outperform the MC VR methods for accelerating the computationally demanding MCNP5 calculations by a factor of 10. Investigations on the use of deterministic calculations for the scope of the NPP activation analysis show that purely deterministic runs are prohibitively expensive in terms of both computer and memory requirements. Hybrid calculations aimed at global VR parameter optimization result in the sought-after global results with smaller computational expense. The performance of activation codes is also examined. Activation codes with the capability of multi-group flux-weighted update of the cross-section data are necessary for ex-core activation calculations. ORIGEN-S with a 44-group energy structure is shown to be the best option in terms of the overall computational cost/benefit. A fundamental part of the activation neutronics scheme is the validation of the calculations by full-cycle foil irradiation campaigns. The first campaign completed in the Goesgen Pressurized Water Reactor is presented. The calculation-to-measurement comparisons indicate an agreement within a factor of 2 for most of the foils. Given that the modelling approach aims to capture the neutron flux distribution within the facility over the NPP lifetime and that the total neutron flux drop from the core to the foil positions is up to the order of 1E+11, the comparison results confirm the validity of the proposed scheme. With the activation neutronics scheme presented in this work, Nagra and ETH Zurich demonstrate that 3D MC/hybrid neutron transport simulations and validation/calibration with foil activation measurements have the potential to become the industry-standard approach for the reliable NPP decommissioning planning