[en] This report documents the results of mechanical inertance tests conducted on two vessels at the Heissdampfreaktor (HDR) facility located 25 km east of Frankfurt, West Germany. The specific purpose of the tests was to determine the modal properties (frequencies, mode shapes, and associated damping ratios) of the test vessels over a frequency range of 1 to at least 33 Hz using either an impulse hammer or shakers as a source of input excitation. Test vessel responses, measured by accelerometers, were processed by a digital based modal analyzer to obtain the modal properties. The results of these tests are intended for use by the US Nuclear Regulatory Commission (NRC) in developing standards for modeling such vessels and to comply with the guidelines of Appendix B, Section III of 10 CFR 50 concerning verification of mathematical models