Analysis of the RS16B experiment on fluid-structure interactions during PWR blowdown

For analysis of fluid-structure interactions during blowdown of a pressurized water reactor (PWR) the code FLUX2 has been developed. This code is based on a three-dimensional compressible potential flow model and a linear elastic structural model. For assessment of the validity of the code the computed results are compared with the measurements in the RS16B experiment (case DWR5) at Batelle (Frankfurt, 1977). In the experiment a sudden depressurization of a vessel (height 11.2 m, diameter 0.8 m) with internal structures is studied under PWR conditions. The initial pressure is 14.2 MPa, saturation pressure is 6.4 MPa. Of particular interest are the loadings on the core barrel. The core barrel wall is relatively thin (8 mm) in the upper third while thicker (18 mm) in the lower thirds. In FLUX2, bulk friction is included in form of a drag force proportional to the local fluid velocity with a constant friction coefficient. Its magnitude had to be taken relatively large; otherwise the pressure oxcillations are larger than found experimentally. The speed of sound is assumed to be a constant as it is appropriate for sub-cooled water. At the break, where two-phase flow appears, the pressure is prescribed to drop linearly from the initial value to saturation pressure within the experimentally found break time of 1 ms. The upper thin part of the core barrel is represented by the shell model CYLDY2 whereas the lower part is treated as a rigid body like the mass ring CYLDY2. (orig.)