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

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

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.