Status of Development of GRS Code System AC² Part I: Modelling of Reactor Phenomena

System codes are widely used around the world to perform nuclear safety calculations. AC² is a system code that is capable of simulating the whole extent of an accident in a nuclear power plant, from the initiating event up to the release of fission products to the environment. The program consists of three main modules: ATHLET, ATHLET-CD and COCOSYS. The paper is the first part of the overview of the system code AC² and covers the main structures and models of ATHLET and ATHLET-CD, which simulate the relevant processes in the core and in the reactor circuit. The presented paper introduces not only the main features of the relevant models but also the basic directions of the future R&D. AC² can be used to simulate large plant accident scenarios as well as experiments in test facilities. Short examples are shown for both cases. (author)