Comparison of SS 304 LN and 316 LN as choice of structural material for primary pipe in fast breeder reactors

In pool type FBRs primary sodium pumps (PSP), operating in parallel, circulate the sodium through the core to remove the nuclear heat. The pumps suck the sodium from cold pool and supply it to a spherical header at the bottom; subsequently the sodium flows through pipes from the spherical header into the grid plate, before entering the core subassemblies. Under normal operating condition, each pipe is subjected to 0.8 MPa internal pressure of sodium flowing at 670 K. As the pipe is located in the cold pool, it is at an isothermal temperature of 670 K during normal operation. The pipe is subjected to hot shock during two thermal transients Viz. offsite power failure (160 times) and loss of Steam water system (47 times). Both these events lead to Safety Grade Decay Heat Removal (SGDHR) operation and the cold pool reaches 791 K during offsite power failure and 816 K during loss of steam water system. Even though the normal operating temperature for the primary pipes conveying sodium from spherical header to grid plate is below creep regime, creep damage occurs due to the governing hot shocks discussed above. One of the criteria for selecting the material is the creep fatigue damage for the longer design life (40 years for the Prototype Fast Breeder Reactor under construction and 60 years for the future reactors). The material choice for out of core reactor assembly components are generally austenitic stainless steels (SS316LN or SS304LN) except for the top shield. In this paper, the choice between SS 304 LN Vs SS 316 LN for primary sodium pipe with respect to the governing failure mode of creep-fatigue interaction is addressed.