Assessment of Embrittlement of VHTR Structural Alloys in Impure Helium Environments Page: 3 of 63
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growth, which may result in brittle failure of the candidate alloys during long term elevated
temperature service. However, current codes for the candidate alloys do not provide specific
guidelines for effects of impure helium on the high temperature mechanical behavior.
This proposal aims to evaluate the role of impurities in helium coolant on the stress assisted grain
boundary oxidation and creep crack growth in candidate alloys at elevated temperatures. The
objectives of the project are listed as follows:
m To evaluate the stress assisted grain boundary oxidation and creep crack initiation and
crack growth in the temperature range of 500-850 C in a prototypical helium
m To evaluate the effects of oxygen partial pressure in helium on the stress assisted grain
boundary oxidation and creep crack growth in impure helium at 700 0C, and 850 C
m To characterize the microstructure of candidate alloys after long term exposure to an
impure helium environment in order to understand the correlation between stress assisted
grain boundary oxidation, creep crack growth, material composition and impurities in the
- To evaluate grain boundary engineering (GBE) as a method to mitigate the stress assisted
grain boundary oxidation and creep crack growth of candidate alloys in impure helium.
To accomplish the proposed objectives, the project consisted of three tasks as follows:
Task A: Creep crack initiation and crack growth rate measurements of candidate alloys
Task B: Evaluation of the effects of oxygen impurities in helium on the stress-assisted
grain boundary oxidation and creep crack growth.
Task C: Evaluation of the effects of grain boundary engineering on the mitigation of
stress-assisted grain boundary oxidation and creep crack growth.
2. MATERIALS AND METHODS
2.1 Materials and Specimens
Two candidate alloys were studied in detail: Alloy 617 and Alloy 800H. Alloy 617 is the leading
candidate alloy for intermediate heat exchangers. It has been reported that Alloy 617 can be used
at temperatures as high as 982 0C. Alloy 800H is currently approved under ASME code for
nuclear service for up to 760 C and could be immediately used in a lower temperature reactor.
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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/m1/3/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.