Assessment of Embrittlement of VHTR Structural Alloys in Impure Helium Environments Page: 2 of 63
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Assessment of Embrittlement of VHTR Structural Alloys
in Impure Helium Environments
The environmental degradation of intermediate heat exchanger (IHX) materials in impure helium
has been identified as an area of research with major ramifications on the design of very high
temperature reactors (VHTR). It has been reported that in some helium enviroments, non ductile
failure is a significant failure mode for Alloy 617 for long-term elevated-temperature service.
Non ductile failure of intermediate exchangers can result in catastropic conquences; however, the
knowledge of creep crack initiation and creep crack growth (CCG) in candidate alloys is very
limited. Current codes and code cases for the candidate alloys do not provide specific guidelines
for effects of impure helium on the high temperature behavior. The work reported here explores
creep crack growth characterization of Alloy 617 and Alloy 800H at elevated temperatures in air
and impure helium environments, providing information on the reliability of these alloys in
VHTR for long-term service. Alloy 617 was found to exhibit superior tensile properties,
hardness, and CCG resistance compared to Alloy 800H. For Alloy 617 test at 700 0C, a notable
increase in the resistance to crack growth was measured in air compared to the helium
environment; CCG results for Alloy 800H suggest that air and helium environments produce
similar behavior. Testing of grain boundary engineered (GBE) Alloy 617 samples revealed that
this technique produces superior mechanical properties in many respects; however, GBE samples
exhibited inferior resistance to creep crack growth compared to the other Alloy 617 samples
tested under similar conditions. Grain size is noted as a confounding factor in creep crack growth
Helium is the coolant that has been chosen for the Next Generation Nuclear Plant (NGNP), a
very high temperature helium-cooled reactor (VHTR) for generating electricity and co-
generating hydrogen using the process heat from the reactor. The helium coolant in high
temperature reactors inevitably contains low levels of impurities during steady-state operation.
The primary impurities are small amounts of H2, H20, CH4, CO, CO2 and N2 from a variety of
sources in the reactor circuit. These impurities are problematic because the structural alloys used
in the heat exchangers can experience significant long-term corrosion by these gaseous
impurities present at elevated temperatures. The maximum primary helium coolant temperature
in the high temperature reactor is expected to be at 850-1000 C, although early prototype
designs may use a lower maximum temperature. Currently, the primary candidates for
intermediate heat exchangers are Alloy 617, Haynes 230, Alloy 800H and Hastelloy X. The
corrosion may involve oxidation, carburization or decarburization depending on temperature,
oxygen partial pressure, carbon activity, and alloy composition. These corrosion reactions can
substantially affect long-term mechanical properties such as crack-growth rate and fracture
toughness, creep rupture, and fatigue. Although there are some studies on the effects of
impurities of helium coolant on creep rupture and fatigue strength, very little is known about the
effects of impurities on the creep crack initiation and crack growth rate in candidate alloys at
elevated temperatures. Stress-assisted grain boundary oxidation is one mechanism of creep crack
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Crone, Wendy; Cao, Guoping & Sridhara, Kumar. Assessment of Embrittlement of VHTR Structural Alloys in Impure Helium Environments, report, May 31, 2013; United States. (digital.library.unt.edu/ark:/67531/metadc836772/m1/2/: accessed June 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.