Analytical study on integrity of BWR reactor internal structures against water hammer under RIA conditions

The integrity of the RPV head and reactor internals was assessed by means of fluid-structural analyses using a coupled method to evaluate the water hammer phenomenon arising from high burnup fuel failure under RIA conditions. The fluid viscosity effect on the water column burst as well as the complex three-dimensional flow paths caused by a core shroud and standpipes were considered in this study. The three analysis scenarios were designed to investigate the above mentioned influential factors separately. In the first scenario, a two-dimensional axisymmetric reactor vessel model without any reactor internals was modeled to assess the influence of the fluid dynamics in the NSC RIA regulatory evaluation. This model has an actual RPV geometry and can be simply separated from other influential factors in order to concentrate only on investigation of the fluid viscosity effect. In the second scenario, a two-dimensional axisymmetric reactor vessel model with major reactor internals, such as a shroud head and standpipes, was applied. The main purpose of this second scenario is to understand flu id flow behaviors in the actual complex flow path configuration. Based on the pressure distribution on the major structures, this model can also provide information to determine which reactor internals will be most impacted by high pressure. This information also supports an assessment of the integrity of these reactor internal components. Finally, in the third scenario, a coupled three dimensional fluid-structure interaction was considered by coupling the state-of-the-art CFD and FEM models. The coupled model permits the assessment of the integrity of the complex reactor internals configuration subjected to high fluid pressures while accounting for elastic -plastic material properties. For this scenario, the highest pressure imposed on the reactor internals was determined by transient flow analyses; this value was then used as an initial condition for the structural analyses. The following conclusions can be made as follows: The STAR-CD code was validated and shown to produce conservative results for rapid transient analyses. It was applicable for this study; The mechanical loads imposed on the RPV head could be reduced significantly by including the fluid viscosity effect. Fluid viscosity was an influential factor to dissipate kinetic energy; When the water column was obstructed due to the complex flow paths caused by the shroud head and standpipes, the water column was unable to reach the RPV head. In this case, the rising water column within the shroud dome was mostly affected by dynamic pressure and the effect of fluid viscosity was not obvious; Integrity of the RPV head and the shroud head was ensured with a sufficient level of margin even under these excessively conservative RIA conditions