Evaluation of hydrodynamic mass methods in reactor applications

Abstract

The hydrodynamic (or virtual) mass approach is evaluated by comparison of structural responses computed by the hydrodynamic mass method with those computed by MULTIFLEX code for a fluid-structure interaction problem with fluid compression effects taken into account. A sample problem used in that evaluation is a simplified 1-D PWR model which is first subjected to a LOCA type transient. The time history of structural displacement computed with the hydrodynamic mass approach is compared with MULTIFLEX results. The frequencies of structural oscillation of these two computations agree fairly well. But the amplitudes disagree by more than 50%, which is attributed to the effect of the fluid compressibility. For the seismic study, sinusoidal forces are applied to the floor at the vessel support. The system responses are expressed by the response functions or the maximum values of the barrel/vessel relative displacements as the applied frequency is varied. The response functions are computed by the hydrodynamic mass method and by MULTIFLEX for evaluation of the virtual mass method. For the pump pulsation study, sinusoidal pressure oscillations are applied at the pump outlet and the response functions are computed as above. For seismic transients, it is found that the hydrodynamic mass method is applicable in the low frequency range ω > 17 Hz. For pump pulsation analyses of ω ⪆ 150 Hz, the hydrodynamic mass method is applicable. Otherwise, the virtual mass method may yield quite different results than MULTIFLEX, because of fluid incompressibility assumptions.