Safety analyses using CFD code and its application to system code 'IMPACT-SAMPSON' for severe accident analysis

The Nuclear Power Engineering Corporation has developed the IMPACT software system. IMPACT's distinguishing features include interconnected hierarchical modules and mechanistic models covering a wide spectrum of scenarios ranging from normal operation to severe accident events, and high speed simulation on parallel processing computers. First, a fluid dynamics analysis code, PLASHY, was developed to provide the core program for other IMPACT software. The rapid boron dilution test, as the ISP-43, was analyzed with about 300,000 spatial meshes. The QUICK scheme for the momentum equations and the first order upwind scheme for the energy equation were employed for the analysis with reasonable result. The pressure oscillation in a PWR downcomer region was also analyzed with the third-order upwind difference scheme and 150,000 spatial meshes. The frequency dependency on the power spectrum density agreed very well with the test data. In the IMPACT, the PLASHY was applied to the flow induced vibration code, FLAVOR, as a fluid flow analysis module. In the FLAVOR, the moving boundary evaluated from the displacement analysis of the rigid body was handled in the fluid flow analysis by an Arbitrary Lagrangian-Eulerian method. The experiment performed by King, et al. was analyzed. The FLAVOR code could reproduce the first instability region of in-line oscillation due to symmetric vortex shedding. The validation analyses were also made on the 1/6th sector tests of the ABWR lower plenum in which the guide tubes for the control rod drives and in-core monitors were installed at full-scale. The results showed the capability for FLAVOR to be applied to the flow induced vibration of the tube array as well. The analysis code, CAPE, for boiling transition and the departure from nucleate boiling has also been developed, in which three-dimensional two phase flow behavior in a fuel bundle was analyzed by the PLASHY. The prediction precision of the critical power was -0.3% with 6.3% standard deviation for BWR fuel bundles and -0.6% with 7.03% standard deviation for PWR fuel bundles. The methodology applied to the PLASHY was also incorporated into SAMPSON code for severe accident analysis as a sub-module to solve flow behavior such as molten fuel relocation, steam explosion and hydrogen mixing and combustion. The quench test as the ISP-45 and the PHEBUS test as the ISP-46 were analyzed with reasonable result. The steam explosion test, KROTOS-44, was simulated very well with the VESUVIUS module in which the PLASHY was adopted as the base. (author)