Local Sodium Boiling behind Local Flow Blockage in Simulated LMFBR Fuel Subassembly

摘要

This paper deals with local sodium boiling in the downstream of a six-subchannel blockage in an electrically heated LMFBR fuel subassembly mock-up.The first series of experiments were conducted to measure temperature distributions in the downstream of the blockage under non-boiling conditions. The measured temper-ature rise due to the blockage agreed fairly well with the calculation by the LOCK code.The second series of experiments were performed to investigate local boiling phe-nomena. In the local boiling region, no flow instability was observed since the sub-channels near the wrapper wall were still filled with subcooled liquid. In the nearly bulk boiling region, however, considerable upstream voiding occurred and then the inlet flow decreased, leading to final dryout.The boiling caused a considerable increase in acoustic noise intensity. The root-mean-square (RMS) noise level of approximately 20 mbar obtained in the present local boiling experiments with sodium was much higher than that (approximately 0.5 mbar) in the ordinary nucleate boiling experiments with water. The peak observed in the hertz ranges was due to the repetition of bubble formation and collapse. In the kilohertz ranges, however, resonance peaks were superposed on a smooth curve with a broad peak at approximately 7 kHz.The frequency (2.9 and 20.2 sec-1) of bubble formation decreased with the increase of the bubble size at its point of maximum development. The product of the bubble frequency and the equivalent diameter was found to be constant.