Norihiko Sagawa

Transport and Deposition of Metals in Sodium-Stainless Steel Systems, (III):Deposition of Cesium in Stainless Steel Capsules under a Large Temperature Gradient

With the object of gaining an insight into cesium behavior in its relation to oxygen concentration in sodium-stainless steel systems, a deposition experiment was carried out with use made of stainless steel capsules containing 20 g of sodium and a trace amount of radioactive cesium. The capsule was vertically set to establish a steep axial temperature gradient, descending forward the bottom, and maintained during a period considered ample for the sodium oxide contained in excess of saturation to settle on the capsule bottom. The oxygen concentration in the sodium was varied by adjusting the temperature of the capsule bottom. The radial distribution of cesium across samples sectioned from different parts along the length of the capsule tube were determined on annular samples extruded from sections of the capsule after chilling for rapid solidification of the sodium. The activation energy for cesium deposition was obtained from partition coefficients —defined as the ratio of cesium concentration on the stainless steel surface and that in the bulk sodium. The resulting data on activation energy indicate that cesium radioisotope deposits in the temperature region below 440dc presumably by physical adsorption on the stainless steel surface when it has previously been exposed to high temperature sodium, and that this surface adsorption appears to be insensitive to differences in oxygen concentration in the sodium.

An Experimental Study of Spray Cooling in Nuclear Reactor Containers

As a contribution to the safety analyses of condition following loss-of-coolant accidents in light water reactor, experiments were performed to obtain data on container spraying and on accompanying heat transfer characteristics, by means of a one-tenth scale model container. The loss of coolant was simulated injecting high temperature saturated water into the model container in which air had been enclosed at atmospheric and room temperature, and then the steam-air mixture was cooled by sprays of water. The rate of heat absorption by the spray was obtained from both stationary and transient data, and it was confirmed that the absorption becomes incomplete as the container pressure is lowered. The evaporation of the water collected at the bottom and of that from sprayed walls of the container was derived from container pressure variation in time, by measuring its rate of change immediately following stoppage of the spray; the rate of evaporation was found more active as the rate of spray cooling increases. ...

Transport and Deposition of Metals in Sodium-Stainless Steel Systems, (VI) Diffusion of Deposited Radioisotopes into Stainless Steel

Abstract The diffusion of radioactive manganese, zinc and cesium into stainless steel tubes exposed during 700~1,500 hr to liquid sodium containing the radioisotopes was studied by measuring the residual radioactivity on samples with their surface layers successively removed by electropolishing. Plots of the logarithm of the residual radioactivity in the tube wall drawn against the square of the thickness polished off from the surface were found to fall along straight lines, which changed their slopes in three steps. That of outermost step was most steep, with the residual radioactivity indicating a drop of about one order of magnitude within a layer of a few microns from the surface. The ensuing step presented a much gentler slope, from which the diffusion coefficients were determined. The ratio obtained between the diffusion coefficient and the approximate jump frequency of the diffusing radioisotope was correlated with the reciprocal of absolute temperature, which indicated that the diffusion mechanism...

Transport and Deposition of Metals in Sodium-Stainless Steel Systems, (V) An Experiment on Thermal Diffusion of Zinc, Antimony and Cesium in Liquid Sodium

Thermal diffusion constants were determined over a temperature range of 400°–700°C in liquid sodium alloyed with trace amounts of zinc, antimony or cesium. Purified sodium containing a radioisotope of one of the three solute metals was filled by suction in a capillary made of stainless steel. The capillary was then carefully seal-welded to prevent the sodium from oxidation; the sealed capillary was placed along the core of a heat conductor rod; a steady linear temperature gradient was produced along its length by heating its head and cooling its foot. After a predetermined period of this thermal diffusion run, the capillary was quickly chilled to solidify the sodium and sectioned into pieces. The specific radioactivity observed in the sodium in the sectioned piece were plotted in log-log scale against the absolute temperature prevailing in the piece during the thermal diffusion run, and the thermal diffusion constant was obtained from the inclination of the straight lines drawn through the plots. All thre...