COCOSYS. New modelling of safety relevant phenomena and components

For the simulation of severe accident propagation in containments of nuclear power plants it is necessary to model severe accident phenomena and to assess the efficiency of severe accident measures under conditions as realistic as possible. Therefore the German containment code system COCOSYS is under development and validation at GRS. Main objective is to provide a code system on the basis of mostly mechanistic models for the comprehensive simulation of all relevant processes and plant states during severe accidents in the containment of light water reactors covering the design basis accidents, too. COCOSYS is being used for the identification of possible deficits in plant safety, qualification of the safety reserves of the entire system, assessment of preventive or mitigative severe accident management measures, support of integral codes, such as ASTEC, in Level 2 PSA studies and safety evaluations of new plants. A close link between development of new models for safety relevant phenomena, their validation and the application of COCOSYS for real scenarios supports the set-up of optimal solutions leading to a state-of-the-art code system. For example recent developments are related to an improved simulation of core-concrete interaction, new models for iodine chemistry and an improved simulation of passive autocatalytic recombiners. Within the COCOSYS/ASTEC validation project GRS is involved in several international projects like SARNET2, the OECD projects THAI, MCCI, BIP and PRISME. For example COCOSYS is actually used to check the capabilities of installed catalytic recombiners in a German KONVOI type nuclear power plant for additional scenarios not considered before. At present COCOSYS is in use for the licensing process of new EPR (European Pressurized Reactor) type plants on the industrial side. Extended questions rising up lead to new requirements on the modelling side, e.g. calculation of local dose rates close to measurement devices. After an overview of COCOSYS and recent developments the paper focuses on the improved modelling of catalytic recombiners, recent extensions of pyrolysis modelling and its application on DIVA experiments and new relevant models in the iodine chemistry module AIM-3. (orig.)