The basic examination on the development of long-life control rod for prototype reactor Monju. Evaluation of the porous-plug properties in liquid sodium

Sodium-bond double porous-plug is one of the candidates in development of the long-life life control rod for prototype reactor MONJU. Gas hold pressure and sodium penetration pressure of the porous-plug in liquid sodium are an important factor which determines the filling up of sodium into porous-plug and the gas plenum length in control rod pin. In this study, the fundamental characteristic of porous-plug was clarified by experiment in high temperature sodium. Moreover, the characteristic equation of design for a porous-plug was evaluated based on the obtained data. The sintering metal porous-plug and the wire net porous-plug types which were manufactured by Type 316 stainless steel were used for the experiment. The nominal diameters of pores in the porous-plug are 70 μm to 400 μm. Sodium temperature is 466 K to 923 K in consideration of the operating condition of a control rod, and the gas hold pressure and sodium penetration pressure were measured. The obtained main results are as follows. (1) The basic characteristic on gas hold pressure and sodium penetration pressure of the porous-plug in liquid sodium was clarified experimentally. (2) When wetting occurred between sodium and porous-plug, the gas hold pressure was not influenced by sodium temperature change. The gas hold pressure was changed with the differences of the surface tension and diameter of porous-plug corresponding to each temperature. (3) Experiment values of gas hold pressure and sodium penetration pressure in sodium were in good agreement with the calculation result by a capillary tube model, and it was shown clearly that this model can apply to evaluation of the plug characteristic. And then, it has been evaluated that a bubble test in alcohol is applicable to an estimate of plug hold pressure in sodium. (4) The realization of the porous-plug was evaluated based on the evaluation equation in consideration of parameters, such as the length for control rod pin and the flow rate of coolant. As result, in the case of lower porous-plug for control rod pin, the sodium charging into a plug was possible at static pressure equivalent to the length of control rod. At upper porous-plug, the gas plenum in the control rod pin was pertinently formed by controlling the flow rate of a coolant under operation. (5) The evaluation equation of design for porous-plug was recommended based on the obtained data in this study. (author)