The thermal-hydraulic system code ATHLET (Analysis of THermal-hydraulics of LEaks and Transients) is developed by GRS for the analysis of the whole spectrum of operational conditions, incidental transients, design-basis accidents and beyond design-basis accidents without core damage anticipated for nuclear energy facilities. The code provides specific models and methods for the simulation of many types of nuclear power plants comprising current light water reactors (PWR, BWR, VVER, RBMK), advanced Generation III+ and IV reactors as well as Small Modular Reactors. ATHLET is available for both Windows and Linux operating systems. The latest code release version is ATHLET 3.1A.

A brief code description is provided by the ATHLET Overview.

The essential features of ATHLET are advanced thermal-hydraulic modelling ensuring the code’s versatile applicability, modular program architecture with separation of physical models and numerical methods and a flexible interface concept for external code coupling and user-provided model extensions. In addition, interactive code control and visualization is enabled by the GRS analysis simulator ATLAS.

ATHLET is composed of several basic modules for the simulation of the fundamental, multiphysical phenomena involved in the operation of a nuclear reactor, including thermal fluid dynamics, heat transfer and heat conduction, neutron kinetics as well as control and balance-of-plant, supplemented by a fully implicit numerical time integration method. Other independent modules such as 3D neutron kinetic codes, Computational Fluid Dynamics (CFD) codes or the GRS containment simulator COCOSYS can be coupled by means of suitable interfaces. For the analysis of beyond design-basis accidents with core degradation, the extended code ATHLET-CD is applicable.


The thermal fluid dynamic model of ATHLET is based on a six-equation two fluid model with fully separated balance equations for liquid and vapor. Alternatively, a five equation model with a mixture momentum equation and a full-range drift-flux formulation for the calculation of the relative velocity between phases is available. In order to improve the simulation of complex, multidimensional flow behaviour, the underlying one-dimensional modelling is continually extended by a fully three-dimensional set of balance equations. The range of working fluids covers light and heavy water with consideration of the transition between subcritical and supercritical fluid states. In addition, helium, liquid sodium, lead and LBE (Lead-Bismuth Eutectic) can be selected as coolant. Depending on the selected working fluid, dedicated models for the simulation of non-condensable gases, of dissolved nitrogen, and of boron or zinc-borate transport are also provided. Approved correlations are employed for the calculation of the one- and two-phase heat transfer in the reactor core, in heat exchangers and at walls.


The overall system configuration, which comprises the coolant piping network as well as metal structures and internals, is modelled by composing basic 1D elements, called thermo-fluid objects and heat conduction objects. Moreover, ATHLET provides dedicated models for the simulation of particular reactor components such as valves, coolant pumps, accumulators, heat exchangers, water-steam separators, steam and gas turbines, compressor units, steam condensers, single and double ended breaks, fills and leaks. Critical flow, e.g. discharge flow, is calculated by a dedicated one‐dimensional thermal non‐equilibrium model with consideration of the given flow geometry.


The code’s quality and capability is ensured by a continuous and systematic validation process that is based on a well-balanced set of integral and separate effect tests derived from the OECD/NEA/CSNI code validation matrices. In the frame of numerous national and international projects, benchmarks and International Standard Problems, ATHLET was successfully employed for pre-test and post-test calculations of both small and large scale experiments, e.g. from the test facilities UPTF, PKL and LSTF. With regard to passive safety systems, experimental data e.g. from ATLAS and INKA facilities were used to investigate the code's capability. For future Generation IV and Accelerator-Driven Subcritical systems, the code’s validation is underway against experimental data e.g. from TALL, KASOLA and MYHRRA test facilities.

Current Applications

ATHLET is internationally applied by technical safety organizations, regulatory authorities, research institutions and operating companies from more than 20 countries worldwide. Current applications are related to the increase of power and enrichment, core damage analyses, reactivity initiated events, periodic safety analyses, and probabilistic safety analyses. Moreover, passive safety systems of advanced light water reactors as well as new system components and working fluids of future Generation IV designs are investigated. Beyond nuclear reactor applications, ATHLET is employed for analyses related to conventional power plants and solar energy systems.

Release Versions

A history of recent code versions is given below. Please follow the links for a listing of new features and major model improvements.