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水プール直接貯蔵方式における使用済燃料冷却特性の検討

Study on cooling characteristics for spent fuel in direct water pool storage system

藤井 正; 堀 徹; 此村 守; 堺 公明; 小川 伸太; 大木 義久; 大久保 良幸

not registered; not registered; not registered; not registered; not registered; Oki, Yoshihisa; not registered

実用化戦略調査研究において概念設計を進めているナトリウム冷却大型炉では、燃料取扱設備の簡素化を図る候補概念として、水プール直接貯蔵方式を検討している。 本概念では、取扱う集合体の崩壊熱量が比較的高い条件(実証炉の炉外貯蔵方式の約4.5倍である18KW)となるため、通常時でのプール浸漬過程や、プール移送中の事故を想定した上部からの注水過程における冷却特性に関する知見が不足している。このため、高発熱燃料の冷却特性を把握することが、プラント概念の成立性を左右する熱流動課題の一つとして指摘されていた。今回、単ピン体系での試験装置を用いて、燃料ピンの熱的条件、集合体出口形状、浸漬速度等をパラメータとした試験を実施し、冷却特性に関して、実機設計に反映できる以下のような基礎的データを取得した。・通常浸漬時の沸騰挙動の可視化やヒータピンの温度変化等により、通常浸漬時の冷却形態を同定した。・ヒータ初期温度と出口部閉塞率が、発熱部全体の冷却終了時間に対する支配因子となる。・試験中のヒータ温度上昇の最大値は、通常浸積時で約4K、注水時に約6Kと小さく、有意な温度上昇を伴うことなく冷却できる。・通常浸漬時には、出口部閉塞率を大きくした条件でも、試験体上部圧力が下部圧力を上回ることはなく、発熱部への水位上昇は阻害されない。

The conceptual design study of the large-scale sodium-cooled reactor is in progress in the "Feasibility Study on Commercialized Fast Reactor Cycle Systems (F/S)". The direct water pool storage system is being examined as a candidate concept to simplify the fuel handling facility for the sodium-cooled reactor. In this concept, the decay heat of a fuel subassembly is relatively higher (18kW which is about 4.5 times of the Ex-vessel Storage Tank system in the demonstration FBR). Therefore, the information about the cooling characteristics of the fuel subassembly are lacking in cases of submergence process at a normal operation and cooling water injection process from upper part of the subassembly at a transfer accident. Accordingly, the understanding of the cooling characteristics of the fuel subassembly in higher decay heat condition was pointed out as one of the thermal hydraulic problems which influence the realization of the plant concept. Using the single heater pin equipment, fundamental tests were conducted with the parameters of the thermal conditions of a fuel pin, the outlet shapes of it, the submergence speeds and so on. Then, following basic data were acquired to be reflected in the actual plant design. (1)Cooling modes of the normally submergence tests and water injection tests were identified by visualization of the boiling behavior in the test section and the temperature change of the heater pin. (2)The initial temperature of the heater surface and the blockage size of the outlet of test section were dominating factors to the cooling completion time. (3)Maximum temperature rise of the heater surface was about 4K in normally submergence tests and 6K in water injection tests, respectively. Therefore, the heater was well cooled without significant temperature rise.(4)In the normally submergence tests, the pressure of the upper part of the test section did not exceed the lower part pressure and a water level rise in the test section was not obstructed ...

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