Bifurcation of Water Column Oscillator Behavior Simulating Reactor Safety System: 1st Report, Experiment

The non-periodical oscillation was reported in Water Column Oscillator, which simulates a passive safety system of a future nuclear reactor, however, the mechanism of the oscillation is not clarified. The objective of this study is to elucidate the mechanism of non-periodical oscillation in Water Column Oscillator experimentally and analytically. The U-type rectangular duct is filled with the water in a certain degree of height. An end of the duct is sealed and the other is open to the atmosphere. The electromagnetic valve is installed at the top of the sealed side. A cover gas simulating vapor generation by residual heat is injected constantly into the gas phase of the sealed side. The water column oscillation with its natural frequency continues until the free surface in the sealed side, pushed by the gas, reaches at a certain height. When the height of the water level reaches H_L, the electromagnetic valve is open and the cover gas flows out from the U-type duct. The gas is ejected rapidly, and thus, the cover gas pressure decreases almost to the atmospheric pressure. The water level in the sealed side of the duct rises with water supply from the open side and reaches a certain height H_U. Then the valve is closed again and the increasing gas pressure pushes down the water surface of the sealed side. The water level oscillates repeating the above process. The analytical model which agrees well with the experiment is derived. The bifurcation-like phenomenon is shown in experiment, however, it can be clear that the phenomenon is not bifurcation but noise effects by the numerical simulation with noise effects. The variation of characteristics of oscillation are investigated in detail by using Poincare´ section (Y₂(N), U₂(N)), which indicate cover gas pressure and water level velocity at the beginning of the Nth valve-open instant. The parameter region where the system shows bifurcation can be discovered by the numerical simulation. The bifurcation is confirmed in the experiment.