Behavior of fuel rods owing to baffle jet in Mihama No. 1 reactor and method of evaluating momentum flux

This test was carried out from October, 1977, to May, 1978, by the request of Japan Atomic Energy Research Institute, and it was planned to confirm the fact that the break of fuel rods and grid springs occurred in PWR power stations was caused by baffle jet. It was found that damaged fuel rods were all placed at the position adjacent to the joint of core baffles and exposed directly to the jet flow from the joint. It was concluded from the experiment and the analysis that the cause of the failure of fuel rods is the abnormal vibration of the fuel rods due to the lateral baffle jet flow through the gap of baffle plate joint. In this test, the size of joint gap, the pressure difference at the both ends of the gap, and the axial distribution of the pressure difference were variously changed, and the vibration of fuel rods was measured. The test conditions were decided so as to simulate the actual reactor conditions in Mihama No. 1 reactor. The vibration of fuel rods was continued for about 50 hours, and the failure of the fuel rods and springs was examined. The testing apparatus and testing method, the test results, and the method of evaluating momentum flux are described. (Kako, I.)