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The construction and performance of a sodium sulfur cell with dynamic sodium and sulfur electrodes are described. The cell was constructed with a sodium feed into a [3"-alumina tube and a sulfur feed into an annular sulfur electrode. Lowresistance graphite felt was tightly packed around the ~"-alumina tube. Sodium pentasulfide was removed from the sulfur electrode. The cell was stably charged in the two-phase region and a high charge acceptance of 95% was obtained. The cell capacity and the discharge voltage increased with the sulfur and sodium feeds. The internal resistance was decreased by thinning the sulfur electrode and using a single zone of low-resistance graphite felt.

The sodium sulfur battery has a high theoretical specific energy, 760 Wh kg Ϫ1 , with a low rate of self-discharge. Hence, considerable effort has been devoted to its development for large-scale energystorage applications, such as in load leveling systems for electric power plants. [1][2][3] The number of sodium sulfur cells needed to store the required energy, however, becomes very large in such systems. These cells are arranged in series and parallel chains, and each cell must have the same charge and discharge characteristics, high efficiency, and good reliability. Therefore, much work has focused on making the cell characteristics uniform and improving the energy efficiency by optimizing the cell structure, especially regarding the sulfur electrode. [4][5][6] Typical sodium sulfur cells under development consist of sodium and sulfur electrodes separated by a solid electrolyte, ␤Љ-alumina. Liquid sodium and mixtures of liquid sulfur, sodium polysulfide, and graphite felt fill and the respective electrode containers. During repeated charging and discharging, sodium ions pass through the wall of the ␤Љ-alumina, react with sulfur or sodium polysulfide at the surface of the graphite felt in the sulfur electrode, and then form different ionic species. Therefore the charge and discharge characteristics of the cell depend strongly on the distribution of the active materials in the sulfur electrode.Cell characteristics can be effectively evaluated by measuring the active material distribution. However these cell characteristics are generally evaluated from cell voltage and current during charge-discharge operations. 4-6 Because the active materials, sodium and sodium polysulfide, react readily with moisture and oxygen in air at room temperature, the distribution of the active material cannot be measured accurately by destructive methods. 7 It should instead be obtained by a nondestructive method. The behavior of active material has been observed using X-ray radiography. 8 But this method cannot clarify the distribution of active material on a cross section of the cell.To solve these problems, in our previous report, we have observed a cross section of the cell at room temperature using X-ray computed tomography (CT). 9 In the present report, an in situ X-ray CT system with a high-energy linear accelerator was applied to image cross sections of sodium sulfur cells during charge and discharge at 350ЊC. The cell charge-discharge characteristics were evaluated using the tomograms. Sodium Sulfur CellSodium sulfur cell reaction.-The discharge reaction of the sodium sulfur cell is written as follows 2Na ϩ xS r Na 2 S x (x ϭ 5, 4, 3)[1]The cell is discharged from the two-phase region composed of sulfur and sodium pentasulfide to form sodium trisulfide. A reaction model of the sodium sulfur cell is shown in Fig. 1. During discharge, the sodium ions penetrate the ␤Љ-alumina, reacting with the sulfur or sodium polysulfide in the graphite felt of the sulfur electrode. During the charging operation, the sodium polysulfide is decom...

Working from the lessons of the Fukushima Daiichi nuclear accident, we have been developing the following various safe technologies for boiling water reactors (BWRs), a passive water-cooling system, an infinite-time air-cooling system, a hydrogen explosion prevention system, and an operation support system for reactor accidents. The objective of the study reported here was development of the passive water-cooling system. The above technologies are referred to as ‘Inherently Safe Technology’. The passive water-cooling system works without electricity for the first 10 days after an event to remove a relatively large mount of decay heat from the core. The system consists of a condenser and a steam turbine-driven pump for transferring water from a suppression pool to the reactor. Steam from the reactor pressure vessel is condensed in the condensation tubes of the condenser, and the condensate flows out into the suppression pool in the primary containment vessel (PCV). The water temperature at the condensation tube outlet is lowered to less than the saturated temperature at the partial steam pressure of the maximum PCV design pressure to prevent the PCV failure. The condenser is located at a lower level, e.g., underground, for easier access and for supplying cooling water to a condenser pool without electricity during an event. The lower level condenser pool has an advantage that it can be seismically designed. To evaluate our concept of the water-cooling system, heat transfer tests were conducted using full-scale U-shaped single tubes with three diameter sizes under a wide range of pressure and inlet steam velocity conditions. The heat transfer data were obtained at system pressures of 0.2 to 3.0 MPa (absolute) and inlet steam velocities of 5 to 56 m/s. The heat transfer data with this wide range of pressure and inlet velocity conditions include thermal hydraulics conditions for a passive containment cooling system (PCCS) and some of the data can be extrapolated to isolation condenser (IC) conditions. We also confirmed thermal hydraulics conditions to determine the practicality of our new concept.

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