Failure evaluation analysis of reactor pressure vessel lower head of BWR in a severe accident

事故時のBWR圧力容器下部ヘッドの破損評価解析

加治 芳行  ; 勝山 仁哉  ; 山口 義仁 ; 根本 義之  ; 阿部 陽介  ; 永瀬 文久 

Kaji, Yoshiyuki; Katsuyama, Jinya; Yamaguchi, Yoshihito; Nemoto, Yoshiyuki; Abe, Yosuke; Nagase, Fumihisa

東京電力福島第一原子力発電所における過酷事故進展及び炉内状況の推定に役立てることを目的として、溶融燃料の落下に伴う原子炉圧力容器下部ヘッドの破損挙動を評価するための研究を進めている。本講演では、解析に必要な原子炉圧力容器構成材料の高温引張・クリープデータ等の材料特性データや熱流動解析及び熱流動・構造連成解析に用いる下部ヘッド詳細解析モデルについて説明するとともに、種々の事故シナリオにおける熱流動・構造連成解析を行い過酷事故時の原子炉圧力容器下部ヘッドの破損挙動を評価した結果について発表する。

In existing severe accident code, rupture of reactor pressure vessel (RPV) lower head after melt down of core is analyzed using the simple model like the Larson-Miller model. It is difficult to evaluate the local deformation and rupture behavior for the actual lower head with such a simple model. Therefore, in order to predict the real position of molten fuel outside pressure vessel, it is necessary to evaluate rupture time and rupture behavior of RPV lower head of BWR precisely.Re-evaluation of materials data such as mechanical properties, creep deformation/rupture properties is made for low alloy steel, Ni-based alloy and stainless steels based on past research activities. To expand materials database and verify the creep constitutive equation and rupture model, we started obtaining the materials data under uniaxial and multi-axial stress conditions at high temperature near melting point. To investigate the inhomogeneous temperature and stress distribution by geometrical complex of BWR lower head, the detailed 3D model of RPV lower head with control rod guide tubes (CRGTs) and shroud supports are constructed and the 3D thermal hydraulic analysis of simulated molten pool and creep deformation analysis of lower head are performed using ANSYS Fluent / Mechanical finite element code. It is found that the possibility of failure mode for BWR lower head are both the penetration failure which is melt-through or drop-away of the guide tube, local rupture and global rupture of lower head by creep deformation mechanism and the melting collapse mechanism due to different boundary conditions.