Analysis of the optimization of the secondary hot piping for a sodium fast reactor

Mod. 9Cr-1Mo steel (T91) is a candidate material for Sodium Fast Reactor (SFR) components and in particular for secondary hot piping. As compared to austenitic stainless steels used in the past reactors, 9Cr-1Mo steel's good conductivity and low thermal expansion let the possibility to reduce the size of the loops and thus to gain on the costs. In order to validate this choice, it is necessary, firstly to verify that this alloy can resist the planned environmental and operating conditions, secondly to check its supply, fabrication and welding possibilities and finally to ensure that the existing design codes cover mechanical design rules. A large R and D program on mod. 9Cr-1Mo steel has been undertaken in France, in order to characterize the behavior of this material and of its welded junctions in operating conditions representative of SFR. In this program, a numerical analysis on secondary hot piping design has been carried out using a stainless steel 316L(N) (used in the previous SFRs Phenix and Super Phenix) and a mod. 9Cr-1Mo steel. The aim of this study was to optimize the secondary hot piping by minimizing the size of the loop and by comparing both candidate materials. This analysis deals with the secondary piping considered for the European Fast Reactor (EFR) and the design has been made for realistic operating conditions of EFR for a period of 60 years. The analysis is based on the creep-fatigue damage and the application of the RCC-MR rules. The results show that the use of mod. 9Cr-1Mo steel has generally an advantage for moderate temperature (below 525 deg. C). On the contrary, when the temperature is more important, stainless steel 316L(N) presents lower damage than 9Cr steel. Indeed, thanks to advantageous thermal properties of mod 9Cr-1Mo steel, the stress state due to mechanical and thermal loading for this material is 20 to 30% lower than this of 316L(N) stainless steel. But at high temperatures this benefit is too low to compensate for the lower creep properties of 9Cr steel. (author)