The thermal-hydraulic computer code ATHLET is an advanced best-estimate one-dimensional two-phase system code. ATHLET has been developed by the Gesellschaft fur Anlagen und Reaktorsicherheit (GRS), Germany, for the analysis design basis and beyond design basis accidents (without core degradation) in light water reactors. The ATHLET structure is highly modular, and allows an easy implementation of different physical models. The code is composed of several basic modules for the calculation of different phenomena involved in the operation of light-water reactors:

— thermo-fluid dynamics,

— heat transfer and heat conduction,

— neutron kinetics,

— general control simulation module,

— specific modules as valve, pump, steam generator, pressurizer, leak and break, accumulator, quench front, feel, charge, boron concentration, non-condensable gases.

ATHLET provides a modular network approach for the representation of a thermal-hydraulic system. A given system configuration is simulated by a net of basic fluid dynamic elements, called objects. Their geometry and connections are done in an input deck. There are several object types, each of them applying for a certain fluid dynamic model.

ATHLET offers the possibility of choosing between different models for the simulation of fluid dynamics. In the current released code version, the basic fluid-dynamic option is a five — equation model, with separate conservation equations for liquid and vapour mass and energy, and a mixture momentum equation, accounting for thermal and mechanical non-equilibrium, and including a mixture level tracking capability.

As an option, there is a possibility to use a six-equation model, with completely separated conservation equations for liquid and vapour mass, energy and momentum, taking into account also non-condensable.

The spatial discretization is performed on the basis finite-volume approach. It means, the mass and energy equations are solved within control volumes, and the momentum equations are solved over flow paths — or junctions — connecting the centres of control volumes. The solution variables are the pressure, vapour temperature, liquid temperature and mass quality within a control volume, as well as the mass flow rate at a junction.

Two types of control volumes are available. Within the so-called ‘ordinary’ control volume a homogenous mass and energy distribution is assumed. Within the ‘non-homogenous’ control volume a mixture level is modelled. Above the mixture level steam water droplets, below the mixture level liquid with bubbles may exist. The combination of ordinary and non­homogenous control volumes provides the option to simulate the motion of mixture level through a vertical component.

A full-range drift-flux model is available for the calculation of the relative velocity between phases for the five-equation model. The model comprises all flow patterns from homogeneous to separated flow occurring in vertical and horizontal two-phase flow. It also takes into account counter-current flow limitations in different geometry.

Moreover, this fluid-dynamic option allows for the simulation of non-condensable gases, on the basis of the ideal gas formulation.

Another fluid-dynamic option in ATHLET consists of a four-equation model, with balance equations for liquid mass, vapour mass, mixture energy and mixture momentum. It is based on a lumped-parameter approach. The solution variables are the pressure, mass quality and enthalpy of the dominant phase within a control volume, and the mass flow rates at the junctions. The entire range of fluid conditions, from sub-cooled liquid to superheated vapour, including thermodynamic non-equilibrium is taken into account, assuming the non-dominant phase to be at saturation. The option has also a mixture level tracking capability.

For pipe-objects, on the basis of either a 5-equation or a 4-equation model, there is also the possibility to use method of integrated mass and momentum balances an option for fast­running calculations. With the application of the method, the solution variables are now the object pressure, the mass flows at pipe inlet and outlet, and the local qualities and enthalpies (4-equation model) or temperatures (5-equation model). The local pressure and mass flow rates are obtained from algebraic equations as a function of solution variables.

Furthermore, an additional mass conservation equation can be included for the description of boron transport within a coolant system.