Status of Development of GRS Code AC² Part II: Modelling of Containment Phenomena

The reliable computational simulation of relevant phenomena occurring during design basis and severe accidents in the containment of a nuclear power plant (NPP) is an ongoing challenge for the development of computer codes in order to assess potential consequences of accidents with regard to counter measures as well as new design features of modern plant types. The Containment Code System COCOSYS developed at GRS forms together with the reactor circuit and core degradation modules ATHLET/ATHLET-CD the future coupled code system AC² established for severe accident analysis in NPP. In order to keep the modelling at the state-of-the-art, continuative development and validation is required, supported by software quality assurance as an integral part of the validation and verification process. This is accomplished through participation to several national and international experimental research programs (OECD THAI, BIP, STEM), through which COCOSYS is assessed against separate effect tests as well as integral tests. The status of code development and validation is described here for topics that partly demanded anew attention in the aftermath of the Fukushima accidents. This regards COCOSYS models simulating thermal hydraulics of the containment atmosphere in presence of large water pools with temperature stratification, for aerosol and fission product behaviour and molten corium/concrete interaction (MCCI) after reactor pressure vessel rupture. An overview of the related model packages in COCOSYS is presented, highlights of current R&D work are outlined and an outlook on future R&D priorities is given. (author)