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Transactions of the American Nuclear Society, Vol. 116, San Francisco, California, June 11–15, 2017

Nuclear Science User Facilities: Structural Materials—III

Irradiation Response of the Ferrite Phase in CF3 Cast Stainless Steel

Yong Yang, Zhangbo Li

University of Florida, 549 Gale Lemerand Dr., Gainesville, FL, 32611, yongyang@ufl.edu

INTRODUCTION Extending service of Light Water Reactors (LWRs) to

beyond 60 years will demand a high integrity of materials and components in reactors. The accurate evaluation and prediction of materials performance under anticipated operation conditions are of particular importance for ensuring the safe operation of nuclear power plants over their extended lifetime. The duplex structure stainless steels including cast austenitic stainless steels (CASS) and welds of austenitic stainless steels are widely used in LWRs. The CASS alloys are mainly used in LWRs primary pressure boundary and for some limited core structural components. The type 308/309 welds are used for welding the reactor components made of 304 and 316 stainless steels and as a cladding layer on the inner surface of reactor pressure vessel. The core support columns made of CF8 CASS in a B&W design PWR are expected to be exposed to a fluence of between 1x1021 and 1x1022n/cm2, E>1.0 MeV, and the austenitic stainless steel welds can potentially receive a fluence up to 5x1022n/cm2, E>1.0 MeV [1]. In addition to the thermal aging effect, the radiation induced fracture toughness in those duplex structure stainless steel components located in the high neutron fluence regions in LWRs hasn’t be fully evaluated. Previous studies showed that the spinodal decomposition in the delta ferrite phase in CASS or type 308/309 welds is the primary embrittlement mechanism, and the G-phase precipitates were also identified but with less impact on the reduction of fracture toughness [2]. It is generally agreed that the irradiation induced fracture toughness decrease in CASS and weld steels saturates after 10 dpa (~1x1022n/cm2, E>1.0 MeV). Nevertheless, there are not enough high dose data points to confirm the lower bound in measured fracture toughness as the reported doses are limited to 15 dpa [1]. The work in this paper is to present a more comprehensive study on the microstructural evolution in the ferrite phase in a CF3 CASS, and the results provide a scientific basis for extrapolating the change of fracture toughness to a high dose range.

EXPERIMENTALS The chemical composition of cast stainless steel CF-3

(13.5% d ferrite) is 8.12Ni/0.92Si/0.012P/0.005S/0.57Mn/ 0.009C/0.052N/19.49Cr/0.35Mo and balance Fe in weight percentage. Neutron irradiation was conducted in the Bor-60 reactor at 320°C with a dose rate of 10-6 dpa/s. The microstructure characterizations were focused on the spinodal decomposition and nano-sized G-phase precipitation in the ferrite phase by using atom probe

tomography (APT). 3-D reconstruction and analysis were performed using the Interactive VisuAlization Software (IVAS 3.6.6). The APT equipment and Focus Ion Beam are located at the Center for Advanced Energy Studies (CAES) in Idaho Falls, ID.

RESULTS AND DISCUSSIONS The chemical compositions of the ferrite phase in four

CF3 discs were measured using APT, and the results show a good chemical homogeneity among those disc specimens. Fig. 1. shows the Cr atom distribution evolution at incremental irradiation doses. The wavelength and amplitude of Fe-Cr spinodal decomposition are estimated by using the Fe and Cr element frequency distribution and radial distribution function, respectively, and they are listed in table. 1.

Fig. 1. Cr atom distribution indicates spinodal decomposition in the ferrite phase of irradiated CASS specimens.

Table 1. Spinodal decomposition wavelengths (l) and amplitudes (DC). dose (dpa) 5 10 20 40 l (nm) 15.08 17.60 19.60 20.01 DC (at.%) 5.88 6.77 16.41 20.01

Figure 2. shows the Mn/Ni/Si rich G-phase precipitates in the ferrite phase of irradiated CF3, and it can be seen that the precipitates gradually coarsen as the irradiation dose increases. The ionic concentration analysis also shows that

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Transactions of the American Nuclear Society, Vol. 116, San Francisco, California, June 11–15, 2017 Transactions of the American Nuclear Society, Vol. 116, San Francisco, California, June 11–15, 2017

Nuclear Science User Facilities: Structural Materials—III

the enrichment of Mn/Ni/Si in the precipitates increases with the irradiation dose.

Fig. 2. Mn/Ni/Si rich G-phase precipitates in the ferrite phase of irradiated CF3 steel.

Our study shows the spinodal decomposition in ferrite phase of irradiated CASS starts to saturate after 20 dpa as both the wavelength and amplitude only slightly increased between 20 and 40 dpa. This saturation is due to the depletion of Cr in the Fe rich zone and increased diffusion path length for the Cr to segregate from Cr depleted zone into the Cr rich zone. In contrast to the spinodal decomposition, the coarsening of G-phase precipitates hasn’t reached the saturation even at 40 dpa. The correlation between the microstructural changes and the material’s fracture behavior is yet to study in the future.

ACKNOWLEDGEMENTS This research was sponsored by the U. S. NRC under

contract #NRC-HQ-14-G-0014. This work was also partially supported by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07-051d14517 through the National Scientific User Facility RPT program.

REFERENCES 1. EPRI Technical Report, “Materials Reliability Program:Thermal Aging and Neutron Embrittlement Assessment of Cast Austenitic Stainless Steels and Stainless Steel Welds in PWR Internals (MRP-276)”, (2010). 2. O. K. Chopra and A. Sather, Initial Assessment of theMechanisms and Significance of Low Temperature Embrittlement of Cast Stainless Steel in LWR system, NUREG/CR5385, (1990).