並列タイトル等Local Boiling of Sodium in Downstream of Local Flow Blockage in a Simulated LMFBR Fuel Subassembly
一般注記This paper deals with local sodium boiling in the downstream of a six-subchannel blockage in an electrically heated LMFBR fuel subassembly mockup. The first series of experiments were conducted to measure temperature distributions in the downstream of the blockage under non-boiling conditions. The measured temperature rise due to the blockage agreed fairly well with the calculation by the LOCK code. When the experimental results are extrapolated to reactor conditions, the temperature rise due to the six-subchannel blockage is estimated to be less than 250$^{\circ}$C. The second series of experiments were performed to investigate local boiling phenomena. In the local boiling region, no flow instability was observed since the subchannels 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 was reduced, leading to final dryout. The boiling caused a considerable increase in acoustic noise intensity. The root-mean-square (RMS) noise level of approxmately 20 mbar obtained in the present local boiling experiments with sodium is 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 boiling caused a considerable increase in outlet flow fluctuation. The maximum RMS value of the fluctuation was 1.33 m/s. The peak observed in the hertz ranges was again due to the repetition of bubble formation and collapse. The frequency (2.9 s$^{-1}$ and 20.2 s$^{-1}$) 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.
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