In the first year of the COSI project, WP1, the architecture of a co-simulation platform for power plants was designed. This architecture provides the possibility of studying the interaction among thermomechanical and automation processes, on-site electrical grids, and off-site transmission system for a power plant. Following the proposed architecture, in this year, WP1 starts to develop the first version of the co-simulation platform. In this regard, the architecture is developed further to solve some remaining issues, e.g. initialising, and then the first platform implemented using MATLAB/m-file. The initial survey of simulation tools in the COSI project shows that most of the nuclear power plants in Finland use Apros as the simulation tool for thermomechanical and automation processes. However, since Apros cannot simulate the detailed electrical system events, e.g. unsymmetrical faults such as a single phase-earth fault in the electric system, the detailed electrical power system models are simulated in different simulation tools while neglecting the interaction of them with thermomechanical and automation processes. To solve this challenge, the first version of the co-simulation platform includes a "Master Program" developed in MATLAB (m-file) environment to co-simulate Apros with power system simulators. Apros supports Open Platform Communications (OPC) and through this protocol, the Master Program can connect, read, write, and control Apros. Connecting to other power system simulators needs other appropriate protocols, which depend on the simulator's features. However, in the first version, Simulink will be the only power system simulator which is connected to the Master Program. The benefit of the cosimulation platform in different events will be studied in the third year of the project. In the case of a substantial benefit, the project will develop further this co-simulation platform to other power system simulator, e.g. PSCAD in the future.In order to prove the appropriate working of the architecture, small thermomechanical and electrical models have been developed. The preliminary results show that the co-simulation platform works as expected in the operation of APROS and power system simulators. The large-scale co-simulation using the exact NPP component models and off-site transmission power grids are done in WP2 and will be reported in the related deliverable.