Experimental modal analysis of the HDR core barrel

Within the framework of code development for predicting the dynamic response of PWR-internals to the blowdown pressure transient a series of 7 blowdown and 16 snapback experiments was performed at the HDR experimental facility at Karlstein/Main. The task of these experiments was to study the transient pressure loading and the corresponding transient response of a HDR core barrel mockup installed in a HDR-pressure vessel. The transient displacement and acceleration signals, measured on the core barrel, were evaluated with the computer code EVA yielding a set of 32 eigenmodes of the core barrel-reactor pressure vessel system and a set of 44 eigenmodes of the core barrel-water charge-reactor pressure vessel system. The particular objective of the investigations described in the paper was the modal survey of the core barrel performed immediatelly before and during the blowdown test V32 (initial water temperature 3110C, initial system pressure 111 bar). The results obtained confirmed the feasibility of an experimental modal survey under conditions of constant temperature and pressure, yielding a set of 16 eigenfrequencies, mode shapes and critical damping ratios of the core barrel. The model survey of the blowdown test V32 yielded a set of 16 eigenfrequencies and mode shape estimates, but there was no possibility to evaluate modal damping. An excellent consistency was obtained of eigenfrequencies related to time intervals shortly before and at the beginning of the blowdown; the mutual deviations are smaller than +- 0.5%. These eigenfrequencies are approximately 3-5% higher than those measured at room temperature. This increase is due to the temperature induced decrease of density in accordance with the theoretical expectations. (orig./GL)