サブチャンネル解析コードASFRE-IIIの整備;燃料ピン熱伝導モデルの改良

Improvement of single-phase subchannel analysis code ASFRE-III; Modification of fuel pin heat conduction model

成田 均; 大島 宏之

Narita, Hitoshi; Ohshima, Hiroyuki

高速炉燃料集合体局所異常事象や変形バンドル内熱流動現象の評価においては、燃料ピンの健全性を判断する観点から、被覆管を含む燃料ピン内の温度分布を精度良く予測することが重要となる。そこで本研究では、燃料ピンの詳細な温度分布の評価を可能とすべく、動燃で開発・整備を進めている単相サブチャンネル解析コードASFRE-IIIの燃料ピン伝熱モデルの多次元化を図った。三次元熱伝導方程式を導入する際には、計算負荷の増大、流動・伝熱現象の時定数の差異を考慮して、径方向を陰的に、周および軸方向を陽的に離散化した。また、径・周・軸各方向の熱伝導計算を自由に組み合わせて計算が実施できるよう、入力にて任意にそれらを指定するフラグも設定した。これにより解析対象とニーズに合わせて、一三次元計算を自由に選択することができる。本モデルを7本ピンバンドル体系の局所流路閉塞事象解析に適用し、妥当な解が得られることを確認した。

In the safety assessment of fast reactors, the evaluation of thermal hydraulic phenomena in a locally blocked fuel subassembly and in a deformed fuel pin bundle is one of most important issues from the viewpoint of the integrity of the fuel pin and it requires to predict temperature distribution in the fuel pin including the cladding tube. In this study, the fuel pin heat conduction model of single-phase subchannel analysis code ASFRE-III, which has been developed at PNC, was modified to treat multi-dimensional heat conduction phenomena in the fuel pins under specific conditions, e.g., a fuel pin partially contacted with porous blockage. In the discretization of three-dimensional heat conduction equation, the heat conduction term in the radial and peripheral directions was implicitly treated as the same way of the original model. The ones in the axial and peripheral directions were explicitly discreted considering both the difference of the time constant between flow and heat conduction and the efficiency of the algorizum modification. This model can calculate not only three-dimensional heat conduction in a fuel pin but also each direction effect separately only by selecting the related flag in the input data. The model verification was made through the analysis of thermal hydraulic phenomena in a locally blocked subassembly.