Fig. 5-4 Creep tests were conducted in a simulated high-temperature gas-cooled reactor helium environment. The solid lines are regression curves obtained from application of the Larson-Miller parameter to all data points. The dashed lines are the design allowable creep-rupture stress of the design allowable-limits.

Fig. 5-5 Two mechanisms of material degradation are considered, first: propagation of surface crack due to gas corrosion, second: inside crack originated from hard precipitates at the triple bond points of grain boundaries. Stress vs time to rupture in Fig. 5-4 indicates that no abrupt decrease in rupture stress occurs due to the above mentioned degradation.

Creep rupture characteristics up to 100,000 h in helium are the most important basic items of material performance to be considered in the high-temperature structural design of the HTTR. It requires over 12 years to obtain creep data up to 100,000 h in a helium environment. Thus, it is prudent to estimate the rupture strength of 100,000 h based on experimental data of 30,000 h. Creep and rupture tests were conducted on Hastelloy XR (a modified version of conventional Hastelloy X), which is used for the intermediate heat exchanger of the HTTR. These tests were conducted at 800, 900, and 1000 degrees cent. in simulated high-temperature gas-cooled reactor helium. The results up to about 50,000 h showed no significant degradation and confirmed the above design allowable creep-rupture stress of the design allowable-limits in Fig. 5-4. As shown in Fig. 5-5, two factors cause the degradation of material during creep in an impure helium environment. The first is the environmental effect. Helium is inert, however, the HTTR coolant contains impurities such as CH4, CO2, CO, H2O, and H2 generated from the core material graphite. These impurities may corrode structural material and promote the growth of surface cracks under tensile creep-stress; though Hastelloy XR has good corrosion resistance. The second is the intrinsic factor in the material. If some brittle phases precipitate at grain boundaries during long-term exposure, inside cracks occur. These cause significant degradation of creep properties. The results of creep-tests have demonstrated the long-term performance of Hastelloy XR in HTTR helium at high temperatures.

Reference Y. Kurate et al., Long-term Creep Properties of Hastelloy XR in Simulated High-temperature Gas-Cooled Reactor Helium, J. Nucl. Sci. Technol., 32, 1108 (1995).

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