Development of low carbon 9Cr-1Mo-V-Nb steel for fast breeder reactor

In a fast breeder demonstration reactor, it is demanded to make its construction cost same as that of LWRs, therefore, the design of increasing the capacity and reducing the cost has been carried out, and as one of its means, the use of 9 Cr steel has been investigated. Since the coefficient of thermal expansion of 9 Cr steel is small as compared with austenitic stainless steel, the occurrence of thermal stress is low, and the equipment can be designed to compact size. Also in 9 Cr steel, the sensitivity to stress corrosion cracking is low, therefore, the safety in water and sodium environment in the secondary system becomes high. The steam generator system, in which the preheat of water, evaporation and the superheat of steam are carried out with one steam generator of once through type instead of conventional separate type, has been examined. The authors have carried out the development of the thick plates for the tube plates and shell plates of steam generators, at the same time with the investigation concerning the high temperature strength of 9Cr-1Mo-V-Nb steel. The target of the development, the investigation of the composition, the performance of the steel material made on the actual scale, and the allowable stress for the thick plates up to 550 mm are reported. (Kako, I.)