The guide tube (GT) representation in the reactor-internal system model has been crude and inaccurate in the past because the lack of experimental data on a) the rotary stiffness of the hold down bolt joint and b) the rotary stiffness of the support pins. In addition, the slotted enclosure and the complex interior in the continuous section further complicated the modeling accuracy. In this paper, the static and dynamic characteristic of the domestically manufactured CAP1000 guide tubes are studied both experimentally and analytically. Using the stiffness data obtained from the tests and combined with the enclosure stiffness calculated from the detailed 3D finite element models, i.e., one for the intermediate section and one for the continuous section, a simplified model of the GT was established. Using the simplified model, the dynamic characteristics (natural frequency and modal shapes) were compared to the dynamic test results in the aerial condition. It can be concluded that the simplified guide tube model is believed to be the most accurate one, by far, to be used in reactor-internal system analyses.

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