The environmental degradation of intermediate heat exchanger (IHX) materials in impure helium has been identified as an area with major ramifications on the design of very high-temperature reactors (VHTR). It has been reported that in some helium environments, non-ductile failure is a significant failure mode for Alloy 617 with long-term elevated-temperature service. Non-ductile failure of intermediate exchangers can result in catastrophic consequences; unfortunately, the knowledge of creep crack initiation and creep crack growth (CCG) in candidate alloys is 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 in impure helium environments, providing information on the reliability of these alloys in VHTR for long-term service. Alloy 617 was found to exhibit superior CCG resistance compared to Alloy 800H. For Alloy 617 tested at 973 K (700 °C), a notable increase in the resistance to crack growth was measured in air compared to that measured in 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, although the technique produces superior mechanical properties in many respects, the 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 resistance.

Investigating creep crack growth under extreme environmental conditions is a challenging yet essential undertaking for the assessment of structural lifetimes of critical components subjected to extreme working conditions for long periods of time. For example, there is currently a need to evaluate structural materials for Next Generation Nuclear Plants (NGNPs) which will operate very high temperature helium-cooled reactors (VHTRs) for generating electricity and co-generating hydrogen using the process heat from the reactor. The primary helium coolant is expected to operate at temperatures at or above 750 °C. In order to evaluate candidate materials for the intermediate heat exchangers, such as Inconel 617 and Alloy 800H, we have developed a creep crack growth (CCG) test apparatus which was designed to test compact tension specimens at temperatures up to 850 °C in controlled environments, including impure helium environments, following ASTM standard E 1457–07. Details of the design of the CCG apparatus will be discussed.

Next Generation Nuclear Plant (NGNP) designs for very-high-temperature reactors (VHTR) employ intermediate heat exchanger (IHX) for which the material demands are extreme. Currently, Alloy 617 and Alloy 800H are considered to be among the candidate materials for the high-temperature, helium-cooled environments that are planned for these systems. The primary helium coolant is expected to operate at temperatures at or above 750 °C, and creep crack growth (CCG) of these candidate alloys is of particular concern for their reliability in VHTRs for long-term service. Using an apparatus that was designed and constructed in-house, CCG testing was conducted on compact tension specimens at temperatures up to 850 °C in controlled environments, including air and impure helium, following ASTM standard E 1457-07. Overall, our CCG testing revealed that Alloy 617 exhibits superior resistance to creep crack growth compared to Alloy 800H. Trends observed in the mechanical behavior and microstructure of the candidate alloys as a function of environment will be discussed.