Hiromi Tokoi

Evaluation of Sodium Sulfur Cell Characteristics Using an In Situ X‐Ray Computed Tomogram System

An in situ X-ray computed tomogram system to observe a sodium sulfur cell was constructed with a 0.14 g/cm 3 density resolution and 0.17 mm spatial resolution. Density distributions in the radial cross sections of the sodium sulfur cell were obtained by the system during charge and discharge operations at 350°C. The typical behavior of sodium supplied to the surface of the solid-electrolyte tube was observed. The experimental density changes of the sulfur electrode agreed with the theoretical values during charge and discharge operations except in one part of the cell.

Measurements of velocity profiles of gas in a rapidly rotating cylinder

The azimuthal and axial velocity of the source‐sink flow in a rapidly rotating cylinder were measured by using the back scattered mode of the laser‐Doppler velocimeter. A rotating optical system was devised for measuring the gas flow with a high angular velocity of 5.2×102 rad/s. The incident laser beams were introduced into the cylinder through the rotating optical system which synchronously, but independently, rotating with the cylinder. The Doppler‐shifted signal was observed from the light back‐scattered by small paraffin mist particles of about 1 μm in diameter. The accuracies of the measurements were within ±5% for the azimuthal flow‐velocity component and ±8% for the axial one. The E1/4 layers formed on the sidewall and the feed radius were clearly observed due to the small Ekman number E=2.2×10−5. The theoretical results (boundary layer theory and numerical calculation) agree quantitatively with the experimental ones, except in the case of the feed radius.

Estimation of Sodium Sulfur Cell Characteristics Using Computed Tomograms from High Energy X‐Rays

The density distributions in the axial and radial cross sections of two typical sodium sulfur cells were obtained by a high-energy x-ray computed tomography (CT) system with a linear accelerator. The tomograms of the low-resistance cell showed a uniform density distribution of sulfur and sodium pentasulfide in the graphite felt of the sulfur electrode, while those of the high-resistance one were indicative of a nonuniform density distribution. The difference in the cell characteristics between the cells could be accounted for by the difference in the density distribution of the active material in the sulfur electrode. The low discharge voltage of the high-resistance cell, calculated using an internal-parallel-cell model, agreed well with experimental results. The tomograms of cells taken by x-ray CT can be applied to evaluate sodium sulfur cell characteristics.

Experiments on Extinguishing Sodium Fires by Jet-spraying of Liquid Nitrogen

高温 の金 属 ナ トリウ ム(Na)は,大 気 中 の酸 素 と接 して 燃 焼 す る。 大気 中で のNaの 発 火温 度 は,Naの 性 状 や 雰 囲 気条 件 な どに 影響 され,120~320°C といわ れて い る1)。 プ ー ル状 で 燃焼 して い る時のNa火 炎 の高 さは 数cmと 短 い が,酸 化 ナ ト リ ウ ム(Na2O)や 過 酸 化 ナ ト リ ウ ム (Na2O2)の 有 害 な エア ロ ゾル を大 量 に発 生 す る。 大気 中 に 湿分 が あ る とエ ア ロ ゾル は,水 酸 化 ナ トリウム(NaOH)に 変化 す る。 燃 焼Naの 消 火 には,乾 燥 砂 あ るい は炭酸 ナ トリウム や 塩 化 ナ トリウ ム を主成 分 とす る金 属 火災専 用 の粉 末消 火剤 が用 い られ て きた。 最 近 で は,新 しい 消火 法 としてNaの 燃 焼 領域 に窒素 ガ スや 液体 窒素 を注 入す る方法 が検討 され て い る2,3)。 本 研 究 は,燃 焼Naの 消 火 法の 更 な る高度 化 を 目指 し, 液 体窒 素 の 噴霧 に よる 消 火基 礎 実 験 を実 施 した もの で あ る。 そ の特 徴 は,燃 焼Naに 不活 性 な液 滴 を噴 霧 す る こ と に よ り,(1) 液 滴 がガ ス 化 す る際 にNaか ら気 化 熱 を奪 い燃 焼 面 を 冷却(冷 却 効 果),(2) 不 活性 な ガ スで燃 焼Naを 覆 い 酸 素 の供 給 を遮 断(窒 息 効 果),さ らに(3)液体窒 素 を直 接注 入 す る場 合 に比 べ,構 造 材 へ の熱衝 撃 を緩 和 で きる こ とで あ る。 しか し,実 験 の結 果,燃 焼Naに 液体 窒素 を噴霧 す る と炎 の 消失 直 後 に,Naの 突 発 的 な飛 散 現 象 を生 じる こ とがわ か っ た。 以下 に,消 火 基礎 実 験 の概 要 と,Na飛 散 要 因 の調査 ・検討 した結 果 を示 す。

A high temperature sodium test facility for development of passive safety devices in FBRs

In a large fast breeder reactor (FBR), passive safety devices are able to introduce a negative reactivity effect passively to prevent a core disruptive accident during an anticipated transient without scram. A high temperature sodium test facility was developed to investigate and develop the passive safety devices. This facility can be used for liquid sodium tests at a maximum pressure of 1 MPa and 700° C or for sodium boiling tests up to 1,000°C. Preliminary tests using a gas expansion module unit were carried out with the core conditions of a large FBR. Experimental results were obtained as follows. (1) The gas expanding process is similar to an isothermal one. (2) The gas expansion speed increases gradually due to sodium flow resistance and inertia. (3) Since forced convection by the gas expansion promotes heat transfer, the heat transfer increases rapidly at the start of expansion. (4) The above experimental results are used to propose an empirical correlation.