TRAC-BF1 three-dimensional BWR vessel thermal-hydraulic and ANSYS stress analyses for BWR core shroud cracking

TRAC-BFl transient analysis for a General Electric boiling water reactor (BWR/2) was performed for a main steam line break (MSLB) event and a recirculation line break (RLB) transient to determine the susceptibility of the core shroud to geometric alteration when intergranular stress corrosion cracking (lGSCC) is present at the upper H3 and lower H7 core shroud weld locations. The TRAC-BF I model developed for the MSLB transient utilized a I-D vessel component model with both main steam lines rupturing at transient initiation. A detailed ANSYS model was developed to find the forces imposed at the upper H3 weld positions. The TRAC-BFl model developed for the RLB transient utilized a 3-D four azimuthal sector vessel component. FORTRAN programs were developed to calculate the forces at the lower (H7) weld location. The results of both transients followed the typical thermal-hydraulic behavior pattern expected during the transients; however, the extreme forces exerted on the core shroud could displace or rotate the shroud during these two postulated transients. Such shroud movement could impede control rod insertion and impair the safe shut down of the reactor. The TRAC-BF 1 pressure response and the ANSYS lift forces demonstrated that when IGSCC is present at the upper H3 core shroud weld, a lift force of 95 925 Pa (13.91 psi) is generated during the double MSLB transient which is great enough to overcome the weight of the vessel internals resting on the core shroud, causing the core shroud to lift. Additionally, the finite element ANSYS stress analysis demonstrated that when IGSCC is not present in the upper core shroud weld, stresses of 1757.3 MPa (254880.0 psi) are induced in the filler material during a main steam line break. This is great enough to overcome the ultimate strength of 1723.7 Ma (250000 psi) of the filler material and cause failure of the weld. The TRAC-BFl RLB transient results determined that during the RLB event a force of 401.7 kN (90 3061bf) is generated at the lower H7 core shroud weld which is great enough to cause displacement of a segment of the core shroud support structure when IGSCC is present.