Effects of Nickel Addition on Microstructural Evolution and Mechanical Properties of Reduced Activation Martensitic Steels Irradiated in the ATR-A1

Abstract

Validity of nickel isotope tailoring (NIT) method to be able to generate a large amount of transmutation helium in 9%Cr-2%W reduced activation ferritic (RAF) steel for fusion reactor structural material was investigated and discussed through mechanical properties and microstructural evolution after fission neutron irradiation. Miniature tensile and Charpy impact specimens of RAF steels and 1% nickel added RAF steel were irradiated in the ATR-A1 to evaluate irradiation hardening and shift in ductile-brittle transition temperature (� DBTT). The amount of transmutation helium in all the RAF steels with and without nickel addition was calculated as only about 0.6 appm. After irradiation at 621 K, theDBTT for the 1% nickel added steel was similar for the JLF-1. After irradiation at 543 K, however, theDBTT for the 1% nickel added steel was significantly larger than that for the JLF-1. Microstructure observations revealed that irradiation-induced dislocation loops in the 1% nickel added steel were finer and denser than in the RAF steel without nickel addition, suggesting that the nickel addition to the RAF steels directly affected nucleation and growth processes of dislocation loops and enhanced irradiation hardening and embrittlement. Therefore, there is a limit in the NIT method as a simulation method for understanding effects of helium on irradiation embrittlement of RAF steels. The RAF steels used in the present study were JLF-1, Mod.JLF-1/LSM and Mod.JLF-1/LSM/Ni. (The ''LSM'' means low concentration of Si and Mn compared with the JLF-1.) The chemical compositions and the heat treatments of these steels are shown in Table 1. Mod.JLF-1/LSM/Ni contained 1 weight percentage of natural nickel. Neutron irradiation was carried out in the two sub-capsules of the ATR-A1, AS4 and AS16, in the Advanced Test Reactor (ATR) at Idaho National Engineering and Environ- mental Laboratory for 132.9 Effective Full Power Days (EFPDs). 11) Irradiation temperature and displacement dam- age were estimated for the two sub-capsules as at 543 K to 2.2 dpa and 621 K to 3.4 dpa. These sub-capsules were equipped with gadolinium shields of 1.7 mm in thickness to reduce the thermal neutron flux in order to suppress trans- mutation effects. The amount of transmutation helium was also suppressed due to the thermal shields. Greenwood et al. calculated the concentration of transmutation helium as 0.6 appm in Mod.JLF-1/LSM/Ni and 0.5 appm in the other RAF steels without nickel addition, respectively. 12)