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「常陽」MK-III性能試験炉心の湾曲反応度解析

Evaluation of core bowing reactivity of Joyo MK-III performance tests core

高松 操 ; 黒羽 隆也*; 吉田 昌宏 

Takamatsu, Misao; Kuroha, Takaya*; Yoshida, Akihiro

高速実験炉「常陽」では、高速炉の固有安全性の実証を目的として、安全特性試験の実施に向けた研究を行っており、その一環として、過渡時のフィードバック反応度の評価制度の向上に関する研究を進めている。フィードバック反応度のうち、炉心湾曲反応度に関しては、設計により将来炉の受動的安全性を向上させることが可能であること等から、その評価手法確立を目的として研究を進めてきた。 「常陽」では、これまで、炉心湾曲反応度を計算する解析システム"MERBA(MEchanical behavior and Reactivity shift caused by core Bowing Analysis code system FOR Joyo)"をMK-II炉心用に整備し、解析を実施した結果、MK-II炉心で観測された出力係数の出力依存性(原子炉主力上昇・下降時に出力係数が変動する現象)が、計算された原子炉出力変動時の炉心湾曲反応度により、定性的・定量的に説明できることを確認した。 一方、MK-III炉心では、中性子束及び原子炉出入口冷却材温度差の増加等により、炉心湾曲及びそれによる反応度挙動を決定する因子がMK-II炉心と異なる。そこで、MK-III炉心の炉心湾曲及び炉心湾曲反応度の特性を把握するため、MK-III炉心用に"MERBA"を整備し、平成15年7月に初臨界を達成する予定のMK-III性能試験炉心の解析を行った。 その結果、MK-III性能試験炉心では、継続使用する反射体の残留変位により、炉心燃料集合体が拘束され、MK-II炉心と同様に、原子炉停止状態において、炉心中心方向に倒れた状態にあり、原子炉出力上昇に伴い、炉心外側に傾いていく(フワラリング)ことから、負の反応度が投入されることがわかった。

A study of passive safety test using Joyo has been carried out to demonstrate the inherent safety of sodium cooled fast reactor. In this study, emphasis was placed on the improvement on feedback reactivity calculation accuracy. Especially, the core bowing reactivity has been one of the main targets of this study because it can be designed to provide negative reactivity in the event of power excursion. The core bowing reactivity was calculated by "MERBA" (MEchanical behavior and Reactivity shift caused by core Bowing Analysis code system for Joyo)" which had been developed for Joyo MK-II core. Through the operation of MK-II core, measured power reactivity coefficient shows a power dependency on reactor thermal power, which is considered to be caused by core bowing effect. In order to investigate the mechanism of this phenomenon, the measured power dependency on power reactivity coefficient were compared with the calculation by "MERBA". It was confirmed that the power dependency on the power reactivity coefficient of the MK-II core could be explained by a core bowing reactivity. In the upgraded MK-III core, neutron flux, the coolant temperature difference between reactor inlet and outlet, etc. will be changed and these may effect on the core bowing reactivity. Therefore, the core bowing reactivity of MK-III performance tests core was evaluated using "MERBA" which was modified to analyze MK-III core. As a result of "MERBA" calculation, the core bowing behavior of MK-III performance tests core was almost similar to MK-II core; the fuel subassembly was initially bent toward the core center at zero power, because the middle spacer pad of fuel subassembly was suppressed by reflectors which have rather large permanent distortion. During the thermal power increase, the fuel subassembly was bent to outward and the negative reactivity was inserted by the fuel movement to outward.

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