[en] In this project the long-term code development of GRS's simulation tools COCOSYS and ASTEC sponsored by the BMWi, for phenomena within the containment of light water reactors (LWR) has been continued. In correspondence to the top-level objective several important models in COCOSYS and ASTEC for phenomena which are currently under investigation have been updated and improved according to the progress of international research and development (R and D). A modification of the COCOSYS zone model has been elaborated and implemented in COCOSYS; this allows now the simulation of a complete flooding of containment rooms by introducing a new type of generic flow connection between two compartments. Through this flow connection a combined flow of water gas in parallel is possible. This work will be continued in a possible follow-up development project in order to consider in more detail the actual conditions of technical flow connections like doors, flaps etc. In co-operation with ITWM Kaiserslautern the coupling of COCOSYS with the pool model CoPool, which allows the detailed calculation of 3D convection patterns and temperature distribution in a deep water pool and which is under development at ITWM, has been realized. Furthermore, a model for metal fibre filter systems has been developed for COCOSYS for detailed investigations on the efficiency of filtered containment venting systems. This model has been successfully tested on the basis of two experiments from the ACE-A-series in which appropriate filter types had been used. Apart from this, the models for several other source term related phenomena have been improved: Consideration of the impact of steam concentration on the decomposition of ozone; the latter contributes to the formation of iodine aerosols; modeling of the radiolytic formation of nitric acid, which takes effect on the pH of the sump and thus on the iodine chemistry, within the atmosphere of the containment; consideration of the impact of humidity on the chemisorption of iodine on paint coatings; updates of the washdown-models. Moreover, some model differences in COCOSYS and ASTEC/CPA which had caused differences in temperature evolution in the wetwell within an application to a German BWR type '72 were clarified. With the objective to ensure the international acceptance and continuity or even to broaden the utilization of COCOSYS, it is regarded as necessary to consolidate the code with regard to quality of source coding, documentation, and user friendliness. This is one of the important objectives of the follow-up code development project.