Further development of the computer code ATHLET-CD

In the framework of the reactor safety research program sponsored by the German Federal Ministry for Economic Affairs and Energy (BMWi), the computer code system ATHLET/ATHLET-CD has been further developed as an analysis tool for the simulation of accidents in nuclear power plants with pressurized and boiling water reactors as well as for the evaluation of accident management procedures. The main objective was to provide a mechanistic analysis tool for best estimate calculations of transients, accidents, and severe accidents with core degradation in light water reactors. With the continued development, the capability of the code system has been largely improved, allowing best estimate calculations of design and beyond design base accidents, and the simulation of advanced core degradation with enhanced model extent in a reasonable calculation time. ATHLET comprises inter alia a 6-equation model, models for the simulation of non-condensable gases and tracking of boron concentration, as well as additional component and process models for the complete system simulation. Among numerous model improvements, the code application has been extended to super critical pressures. The mechanistic description of the dynamic development of flow regimes on the basis of a transport equation for the interface area has been further developed. This ATHLET version is completely integrated in ATHLET-CD. ATHLET-CD further comprises dedicated models for the simulation of fuel and control assembly degradation for both pressurized and boiling water reactors, debris bed with melting in the core region, as well as fission product and aerosol release and transport in the cooling system, inclusive of decay of nuclide inventories and of chemical reactions in the gas phase. The continued development also concerned the modelling of absorber material release, of melting, melt relocation and freezing, and the interaction with the wall of the reactor pressure vessel. The following models were newly developed or improved in the frame of this project: Oxidation in steam and air atmosphere, processes in the lower plenum, melt behaviour, melt relocation into containment after failure of reactor pressure vessel, calculation of the nuclide inventory, release of fission products from the core, transport of fission products in the primary circuit. As a result of these improvements the quality of calculations of experiments as well as postulated reactor transients and accidents has been advanced considerably. Furthermore, the general robustness of the code has been improved. During the reporting period four versions of ATHLET-CD were released and frozen respectively. In October 2012 the version ATHLET-CD Mod 2.2 Cycle C was released as a frozen development version. Main improvements were the module AIDA, which allows the simulation of the late phase effects after relocation of melt into the lower ple-num, the modelling of the nitride formation in case of air ingress and the new version ATHLET 3.0A. In August 2013 the version ATHLET-CD Mod 3.0 Cycle A was released. In comparison to the version 2.2C further extensions and improvements were performed, e. g. general improvements of the code robustness or improvements concerning the simulation of oxidation effects. In contrast to the development version 2.2C the version 3.0A is an official release version, which is available to all licensed users of ATHLET-CD. With the new version the users are able to simulate a complete severe accident sequence from the early phase until the possible failure of the reactor pres-sure vessel after the relocation of melt into the lower plenum. In December 2014 the development version ATHLET-CD Mod 3.0 Cycle B was released. In March 2016 ATHLET-CD 3.1A was frozen. With this version the release of melt from the reactor pressure vessel can be simulated after the failure of the vessel. Furthermore, GRS participated in the international OECD/NEA project ''Benchmark Study of the Accident at the Fukushima Dai-ichi NPS'' (BSAF). Using different codes the accidents in the units 1 to 3 at Fukushima were simulated and analysed in this project. In this framework, GRS performed analyses of the units 2 and 3 using the coupled version of ATHLET-CD/COCOSYS. The simulations could reproduce the most important aspects of the accident sequences and are in good agreement with the calculations of other participants.