The thermal-hydraulic system code ATHLET (Analysis of THermal-hydraulics of LEaks and Transients) is being developed by GRS for the analysis of the whole spectrum of leaks and transients in PWRs and BWRs. The code is applicable for western reactor designs as well as for Russian VVER and RBMK reactors. The main code features are

- advanced thermal-hydraulics,
- modular code architecture,
- separation between physical models and numerical methods,
- pre- and post-processing tools,
- portability to the prevalent computer platforms.

ATHLET is composed of several basic modules for the simulation of the different phenomena involved in the operation of a light water reactor:
- thermo-fluiddynamics (TFD)
- heat transfer and heat conduction (HECU)
- neutron kinetics (NEUKIN)
- and control and balance-of-plant (GCSM)
together with the fully implicit numerical time integration method FEBE. Other independent modules (e.g. 3-D neutron kinetics or containment modules) can be coupled by means of a general interface.

The TFD module is based on both a five-equation system (mixture momentum equation with drift) as well as on a six-equation two-fluid model, enabling additionally the simulation of several non-condensable gases, dissolved nitrogen, and of boron transport. The system configuration to be simulated is modelled just by connecting basic thermo-fluid dynamic elements, called thermo-fluid and heat conduction objects. Multi-dimensional processes are simulated by parallel channels with cross flows.

The systematic validation of ATHLET is based on a well balanced set of integral and separate effects tests derived from the CSNI code validation matrices.