8-grid 17 x 17 fuel assembly vibration in a D-loop test

For the Phase III, D-loop test of the 17 x 17 guide tube, an 8-grid 17 x 17 fuel assembly was supplied as an ancillary test component. In this test phase, anomalous vibrations of the fuel rod indicated fuel assembly vibrations at discrete frequencies and constant amplitude over sharply defined flow ranges. Such vibration had not been observed in previous tests. An investigation was undertaken to evaluate if the fuel assembly design required a modification to preclude this vibration. This report summarizes the nature of the anomalous vibration, the probable cause postulated, and the tests conducted to verify that hypothesis. It was concluded that the vibrations observed in the initial Phase III, D-loop test were a result of the specific test configuration and resulting flow conditions and are non-prototypical. No anomalous vibrations could be induced or were observed in subsequent tests of 8 grid assembly configurations which simulated actual in-reactor conditions. No modification of the fuel assembly design was found to be necessary. (U.S.)