The deformation of a hydrogen microbubble line and/or mesh in a viscoelastic fluid around a rising bubble was tracked from the original static position in order to discuss the mechanism of the typical phenomena such as the negative wake or the cusp shape. This new experimental method is essentially important because of the hysteresis-dependent nature of the viscoelastic fluid. This new method makes this study distinctive from a number of conventional studies of viscoelastic fluids focusing on the non-Newtonian properties and/or the instantaneous flow field. According to our experimental results, the flow mechanism responsible for the negative wake or cusp shape was attributed to the accumulation and release of the shear strain energy. Some residual displacements were observed after the bubble rising, which were almost completely reproduced as the internal dissipations in a Maxwell model modified with a non-linear spring.

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The loss-of-reactor-level (LORL) is a typical accident condition causing a severe accident (SA) in sodium-cooled fast reactors (SFRs). In the loop-type SFR Monju, important cause of the LORL is the second coolant leakage at the low elevation of the primary heat transport system (PHTS), which occurs in cold standby in a different loop from that of the first coolant leakage in rated power operation because of excessive declining of the sodium level. This study developed an evaluation method for the LORL with considering countermeasures to prevent LORL: i.e., pumping sodium up into reactor vessel (RV) and interruption of sodium flowing out by siphon effect. To evaluate the effectiveness of the countermeasures, the transient behavior in the RV sodium level was analytically studied in representative accident sequences. The representative sequences with lowest sodium level were selected by considering combinations of possible coolant leakage positions. The evaluation result clarified that the LORL can be prevented by conducting the above-mentioned countermeasures to maintain the RV sodium level sufficient for the operation of decay heat removal system, even after the second coolant leakage of PHTS.

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