TRANSIENT AND FUEL PERFORMANCE ANALYSIS WITH VTT'S COUPLED CODE SYSTEM

VTT maintains and further developes a comprehensive safety analysis code system ranging from the basic neutronic libraries to 3D transient analysis and fuel behaviour analysis codes. Some of the computer codes have been developed at VTT, others have been acquired from internationally recognized sources. The code system is based on various types of couplings between the relevant physival phenomena. The main tools for analyses of reactor transients are presently the 3D reactor dynamics code HEXTRAN for cores with a hexagonal fuel assembly geometry and TRAB-3D for cores with a quadratic fuel assembly geometry. HEXTRAN has been applied to safety analyses of VVERtype reactors since early 1990’s. TRAB-3D is the latest addition to the code system, and has been applied to BWR and PWR analyses in recent years. The results from the 3D analysis can be used as boundary conditions for more detailed fuel rod analysis. For this purpose a general flow model GENFLO, developed at VTT, has been coupled with USNRC’s FRAPTRAN fuel accident behaviour model. FRAPTRAN/GENFLO is quite a promising tool that can handle flow situations that are far too complicated for the models that are usually seen with fuel codes.