2D fluid flow in the downcomer and dynamic response of the core barrel during PWR blowdown

As a part of the HDR program, methods for coupled fluid/structure dynamics are being developed. On the fluid side the 2D finite difference code YAQUI has been modified and adapted to describe the fluid dynamics in the downcomer of PWR's. On the structural side for determination of the dynamic core barrel response the code CYLDY2 has been developed. In this code the core barrel is treated as a thin cylindrical shell fixed at the upper end and ring stiffened at the lower end. The mass of the lower end ring also simulates a part of the core mass. Both models have been successfully tested. Coupling has been achieved for a simplified structural model proving the correctness of the coupling procedure. YAQUIR is a significantly modified version of the code YAQUI. The coupling of YAQUIR and CYLDY2 is performed by imbedding the structural model in the fluid model. Fluid velocities are parellel to the fluid/structure interface. The structure displacements define the time and space dependent thickness of the two-dimensional fluid layer. While coupling of the complete CYLDY2 model with YAQUIR is still underway, results have been obtained with a simple axisymmetric structural model. For an axisymmetric test case three forms of pressure fluctuations have been observed: 1) radial oscillations dominated by the local compressibility of the water, 2) axial compression/expansion waves in the water considerably different from those obtained for a rigid barrel, 3) bulk axial water oscillations dominated by the global compressibility of the core barrel. (Auth.)