This document summarizes a study that examined the post-irradiation microstructure of 316 stainless steel and Hastelloy N alloy samples exposed to molten FLiBe salt in the MIT research reactor. The alloys were characterized using XRD, SEM, EDS, TEM. For 316 stainless steel, gamma-phase FeNi formed in the surface layer due to chromium depletion, and grain boundaries contained many irregular chromium and molybdenum rich precipitates. Hastelloy N maintained its FCC nickel phase structure but developed molybdenum rich phases in grains and grain boundaries, with oxides and carbides on the surface and dislocation loops from irradiation.

1. Post-Irradiation Examination of Structural Alloys Exposed to
Molten FLiBe Salt in MIT Reactor
G. Zheng*, D. Carpenter, M. Ames, G. Kohse, L. Hu
Nuclear Reactor Laboratory, Massachusetts Institute of Technology
*gqzheng@mit.edu, 138 Albany Street, Cambridge, MA 02139
2016 MRS Fall Meeting, ES5: Materials Research and Design for A Nuclear Renaissance, Boston, MA, Nov.27-Dec.2, 2016
Motivation
Materials and In-core Experiment
FHR combines
the advantages
of latest
technologies
(MIT, UC-
Berkeley, UW-
Madison) 7LiF-BeF2
ES5.13.04
Fluoride salt-cooled High-temperature nuclear Reactors (FHRs) is emerging as a leading reactor
concept among all Gen IV nuclear reactors because it offers, among other benefits, a high degree of
passive safety, high thermal efficiency, and low spent fuel. One primary challenge in the development
of the FHR is to select a structural alloy that is required to be reliable at 700°C, and to be compatible
with molten Li2BeF4 (FLiBe) salt, as well as to be stable in high neutron flux environment. Among many
candidate alloys, nickel-based Hastelloy N® and iron-chrome-based 316 stainless steel have been
selected as the most promising structural alloys for FHR.
Acknowledgements
FHR advantages:
• passive safety
• high outlet temperature
• low pressure salt coolant
• high temperature solid fuel
• high efficiency power cycles
• and more
C Cr Cu Mn Mo N Ni P S Si Fe
0.0225 16.8250 0.3795 1.5305 2.0115 0.0510 10.0250 0.0310 0.0016 0.3090 68.8134
C Cr Mn Mo Ni Si Fe others
0.08 7.00 0.80 16.00 71.00 1.00 5.00 1.05
Hastelloy N® (UNS N10003, HAYNES International)
• Developed in1960s for MSRE at ORNL
• High resistance to fluoride salts and air-side oxidation, corrosion attack <25μm/year at 704ºC
• High neutron flux-induced embrittlement due to high nickel percentage
• ASME Section III code qualified for nuclear system, widely applied In high temperature systems
• Corrosion attack is ~10μm/year at 650ºC
316 Stainless Steel (UNS S31600, North American Stainless)
Graphite crucible Ni capsule Thimble in core Installation MITR
• 700±3ºC in graphite crucible, Ni-lined crucible, 316ss-lined crucible for 1000 hours
• thermal neutron, 8.8x1019n/cm2; fast neutron (E>0.1MeV), 4.4x1020n/cm2
• radiation activity of post-irradiation samples, 3.8x10-3-1.1x10-2 Ci/g (Mn-54, Co-58, Co-60)
Post-Irradiation Examination
10 20 30 40 50 60 70 80 90 100
(220)
(200)
(110)
316ss-316ss
(111)
FeNi  phase
due to Cr loss
(211)
(220)
(200)
(110)
(111)
2 theta
316ss-graphite -ferrite phase
due to high C
10 20 30 40 50 60 70 80 90 100
(222)
(220)
(311)
(200)
Hastelloy N-Nickel
(111) FCC nickel matrix
and FeNix
(222)
(311)
(220)
(200)
(111)
2 theta
Hastelloy N-graphite
Quanta 3D FEG (INL CAES)Rigaku SmartLab (INL CAES)
FIB, SEM, EDSXRD STEM, EDS, TEM
Tecnai F30 (INL CAES)
X-ray diffraction patterns
 Cr depletion induced formation of gamma-phase FeNi  No comparable phase change, FCC(Ni) as Cr(0-7wt%)
Focused ion beam
(a) selection, (b) Pt deposition, (c) milling, (d) extraction, (e) mounting,
(f) thinning by using FIB technique for 316ss-G lamella preparation
e-transparent lamellae, (a) 316ss-G, (b) 316ss-
316ss, (c) Hastelloy N-G, (d) Hastelloy N-Ni
Scanning transmission electron microscopy
This work was supported by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho
Operations Office Contract DE-AC07-051D14517 as part of a Nuclear Science User Facilities experiment.
The authors are very grateful to Joanna Taylor, Jatuporn Burns, Allyssa Bateman and Dr. Yaqiao Wu for
providing technical supports and facility trainings at the Center for Advanced Energy Studies (CAES) at INL.
Hastelloy N-nickel Hastelloy N-graphite
316ss-316ss 316ss-graphite
Mo-rich phases
on surface and
at GB
Oxide, carbide
particles on
surface
irradiation-
induced
dislocation
loops in grains
A large number
of irregular
precipitates
formed in grain
and GB in
addition to RIS
and structural
defects
Summary
• Microstructure of post-irradiation/corrosion 316 stainless steel and Hastelloy N was characterized
• Stable FCC (Ni) phase (0-30wt%Cr-Ni) in Hastelloy N, new γ-phase of FeNi in 316 stainless steel surface layer
• 316 stainless steel: a large number of irregular Cr, Mo-rich precipitates in grain and GB, and RIS
• Hastelloy N: Mo-rich in grain and GB, oxides and carbides on surface, irradiation-induced dislocation loops