In the design of future sodium-cooled fast reactors, a design measure to prevent severe recriticality events even in the case of core disruptive accidents is considered. This design adopts an inner duct within the fuel subassembly that should allow molten fuel ejection out of the core region. The effectiveness of this design depends on the failure time of the duct and significantly depends on heat transfer from the melting core materials to the duct. In a previous study by the authors, heat transfer from a molten fuel/steel mixture to the inner duct was evaluated by computer model simulation for an in-pile experiment performed in an impulse graphite reactor (IGR) focusing on demonstration of design effectiveness. In the present study, possible uncertainties in the assumption and model parameters in the previous study were evaluated so that the validity of the main conclusion of the previous study could be confirmed and re-enforced. This confirmation consisted of the evaluation of the necessary fuel-to-steel heat transfer area, the effect of the hydrodynamic fragmentation of steel droplets, steel-vapor condensation heat transfer onto the duct surface and fuel crust formation. Furthermore, possible effects of variations in fuel design and transient scenario on the heat transfer were evaluated by changing the steel volume fraction as the initial boundary condition. It was concluded that, in the previous study, the realistic situation was appropriately represented and the conclusions were enforced. An additional set of analyses showed that possible underestimation of heat transfer from the fuel/steel mixture to the duct could be enhanced at a low steel volume fraction. Future model improvement is preferable for this characteristic.

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  In order to realize a quick mass production of PuO₂/UO₂ (MOX) particles via vaporization of Pu/U mixed nitrate solution, an innovative denitration process is required. In this process, one of the important problems is the safety control of radioactive materials in the solution. However, it is known that violent boiling (i.e., geysering) of liquid sometimes occurs in a long, heated vessel. In this study, we experimentally examined the boiling characteristics of water in a vertical glass tube with a closed lower end opening to an upper plenum at the top. In our experiments, we measured the transient statistic water pressure and the water temperature at four elevations along the glass tube. The effects of the water temperature of the upper plenum and heat input into the water on the boiling of water were investigated. Flow visualization shows that the bubbles are first formed on the heater surface, and then they detach and start to rise in the water. They suddenly coalesce and form a large slug bubble in the test tube. Measurements of local water temperatures showed that in the case of violent boiling, the temperatures and pressure fluctuate at the same frequency.

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  A mixture of plutonium nitrate and uranyl nitrate is co-converted to MOX powder by the microwave heating method developed by JAEA. The heating efficiency is very important for improving the energy-saving performance in this conversion process. In this study, the heating efficiency was measured using both experimental and engineering-scale microwave ovens in water, nitric acid and a mixture of plutonium nitrate and uranyl nitrate as a function of distance between the specimen and the base of the oven. In addition, the distribution of electromagnetic field strength and its absorption concentration in the solution were numerically evaluated by an electromagnetic field analysis code, MWS 2009. The experimental results could almost be explained by the numerical analysisresults. When the distance of the specimen and the base of the oven was beyond 1/4 wavelength, the efficiency became constant because the influence disappeared.

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