Further development of the system calculation program ATHLET-CD for the simulation of accidents in the primary circuit

ATHLET-CD, which is part of the code system AC${}^2$, is developed to analyze nuclear accidents in pressurized and boiling water reactors. The development is financed by the German Federal Ministry for Economics and Energy (BMWi). The aim of the development is to create a tool to realistically simulate the phenomena occurring during an accident scenario in a water-cooled reactor. Through continuous development of the model basis of the code, ATHLET-CD can predict severe accident scenarios more realistically. ATHLET-CD is based on the thermohydraulic models of ATHLET. This allows the determination of the starting- and boundary conditions of the analyzed accident scenario, not only at the beginning of the calculation but also during the transient. ATHLET-CD can be coupled to the containment program COCOSYS. Through this coupling it is possible to cover the whole spectrum of the accident scenarios. ATHLET-CD consists of modules covering a wide range of phenomena that can occur during a severe accident: degradation of fuel- and control rods, melt relocation to the lower plenum and its behavior in the lower plenum, fission product release and transport in the cooling circuit, including fission product decay. Within the project RS1546 the following models were extended/newly implemented: - Models for core degradation, - Models for fission product behavior, - Models for oxidation in air, - Models for BWR structure component degradation, - Models for processes in the lower plenum. The improvement in the models for core degradation enable an adequate simulation of accidents with azimuthally asymmetric boundary conditions, as well as accidents in spent fuel pools. The developments for the lower plenum include an improved wall ablation model, an improved coupling of the lower plenum models to ATHLET and COCOSYS, and improvements regarding outside cooling of the reactor vessel. This improved the quality of the code, which is reflected also in the results of the simulation of many experiments. Within the project RS1546 a python coupling interface was created for easier coupling of external models. The new python interface also allows a better visualization of the core degradation processes and fission product transport phenomena. Many improvements were made regarding internal coupling between the parts of the AC${}^2$ package also. Generic input decks were developed for PWR, BWR and WWER reactors in order to verify the newly developed models under realistic accidental conditions. During the timeframe of the project a new ATHLET-CD version was released (ATHLET-CD Mod 3.1 Cycle A, July 2016), the works for another new release were started (ATHLET-CD 3.2). The most important changes in ATHLET-CD Mod 3.1 Cycle A were: - New lower plenum model LHEAD implemented, - Transport of melt from the reactor vessel to the cavity in COCOSYS after wall failure, - Considering the oxidizing/reducing atmosphere during fission product release, - Code performance and stability improvements. Several patches were released during the project which further improved the stability and performance of the code. ATHLET-CD 3.2 was prepared for release. The most important changes in ATHLET-CD 3.2 are: - Improved B${}_4$C-oxidation for absorber blades in BWR, - Improved air oxidation models, - Horizontal relocation of melt within the core region, - Improved fission product transport models, - Strongly improved models of the lower plenum module AIDA.