The software AC² comprises the GRS system codes ATHLET, ATHLET-CD and COCOSYS, complemented by the interactive simulator software ATLAS and by the Numerical Toolkit (NuT) that provides access to external numerical libraries.

AC² is the GRS 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. AC² can be used to simulate large plant accident scenarios as well as experiments in test facilities. 

To allow multiphysical or multiscale simulations, AC² has been coupled successfully to various computer codes by means of dedicated coupling interfaces. The following figure depicts the essential interfaces that are realized for AC².

The capabilities and interaction of the main modules of AC² are illustrated in the figure below.