RE-EVALUATION OF PAR CONCEPT IN GERMAN PWR WITH REVISED PAR MODEL

摘要

Consideration and analysis of Beyond Design Basis Accidents (BDBA) and Severe Accident (SA) of NPPs is an essential component of the defence-in-depth approach used in nuclear safety. A set of Severe Accident Management (SAM) measures and guidelines is today applied to existing NPPs to be taken to prevent the SA or to mitigate its consequences. A basic concept for hydrogen mitigation inside the PWR containment of German plants by a large number of Passive Autocatalytic Recombiners (PAR) has been examined by GRS within the frame of projects sponsored by the German Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (BMUB) already in a period up to 2001. The PAR concepts have been realized in all German PWRs thereafter, while a re-evaluation of the concept was done at GRS more recently. Based on PSA level 2 results additional accident scenarios have been considered within the re-evaluation process. The GRS code COCOSYS has been applied again for the analyses with an updated input deck including the latest code models available, especially the new PAR model. Within recently finished OECD/NEA THAI projects several new tests related to hydrogen recombination using different PAR types (Areva, AECL, NIS) have been performed, which allowed a significant improvement of the PAR models used in COCOSYS and other codes, including CFD codes. The COCOSYS calculations were conducted according to the state-of-the-art of me computer code as well as based on experiences gained in the meantime of the validations on experiments. The analyses results for the German PWR underlined the efficiency of the implemented PAR concept even under more challenging severe accident conditions; still local hydrogen combustion processes could not be avoided in all cases and were analyzed with COCOSYS for the first time. Various ignition criteria were applied, like a presumed ignition of the gas mixture by a PAR to determine the most challenging conditions (worst case) with regard to the combustion process and the resulting pressure peak. A summary of the results of the re-evaluation of the PAR concept for a German PWR is presented together with recommendations on appropriate code application for the development of PAR concepts in large dry, multi-compartment PWR containments. Further reference is made to a recently published OECD/NEA SOAR report on "Hydrogen Management and Related Computer Codes" in which the German regulatory requirements related to hydrogen mitigation are summarized.