Failure Analysis of Motorized Bellow-Sealed Valve of Sodium Loop in Fast Breeder Test Reactor

A motorized bellow-sealed valve in the steam generator leak detection circuit of fast breeder test reactor developed sodium leak while carrying out functional testing after providing temporary power supply to the valve. A systematic analysis of failed bellows along with the stem was carried out to understand the possible cause of the failure. Visual examination indicated that the bellows had suffered extensive twisting before failure. The cause for extensive deformation of the bellows is attributed to the failure of anti-rotation mechanism and high friction between the nut and the screw. This resulted in torsional loading on the bellows. Detailed finite element analysis showed that the bellows were safe during normal operating condition and, however, experienced extensive deformation due to the torsional loading. Systematic microstructural studies using scanning electron microscopy and defect and residual stress analysis using X-ray diffraction (XRD) were carried out. The microstructure of stem and bellows exhibits the presence of extensive slip bands and twins, which indicated that the failure occurred due to extensive plastic deformation. Large variation in hardness and defect density as revealed by XRD analysis from different locations of the failed bellows supports the above observation. The presence of large number of twins in different locations shows that the component has experienced severe stress due to torsional loading.