Theoretical Explanation for the Acceptability of Steam Generator Tubesheet-Shell Junction under Manufacturing Deviation

摘要

In 500 MWe Prototype Fast Breeder Reactor (PFBR), steam generator (SG) is a critical component as its availability decides the capacity factor of the plant. Nuclear heat generated with in reactor assembly is carried by secondary sodium to the SG. SG is a shell and tube type heat exchanger, where sodium is on the shell side and water/steam is on the tube side. The presence of the sodium and water on either side of the shell of SG will make the system very much important from the structural integrity considerations. The hot sodium (525°C) enters at the top of SG and leaves at the bottom at lower temperature (355 °C). The material of construction is modified 9 Cr-l Mo (G91) steel.The shape of SG tubesheet-shell junction with the dished end is optimized based on the detailed thermo mechanical calculations using 3D finite element method. While manufacturing the SG tubesheet header, a small circumferential notch (3.3 mm × 3.1 mm) has been found in the conical shell. Even at the cold end of SG (bottom header), the possibility of thermal shocks at the notch under transient cannot be avoided. Hence the notched geometry is not favored from the fatigue strength considerations.This paper discuss about the measures carried out to accept notched header tubesheet-shell junction for the SG.