Dynamic Deformation in Nuclear Graphite and Underlying Mechanisms

Area of Science:

• Materials Science
• Nuclear Engineering
• Solid Mechanics

Background:

• Nuclear graphite properties are crucial for reactor performance and safety.
• Radiation-induced defects and pre-existing flaws like Mrozowski cracks affect graphite's mechanical behavior.
• Understanding time-dependent deformation under irradiation is vital for nuclear applications.

Purpose of the Study:

• To investigate the role of pre-existing defects (point defect clusters, Mrozowski cracks) in nuclear graphite (IG-110) under irradiation.
• To analyze the influence of different ion types (Au2+ and C2+) and irradiation conditions on graphite's mechanical response.
• To elucidate the mechanisms of radiation-induced deformation and embrittlement in nuclear graphite.

Main Methods:

• Irradiation of IG-110 graphite with 2.8 MeV Au2+ and 8 MeV C2+ beams at specific fluences.
• In situ mechanical loading of microscopic specimens within a transmission electron microscope (TEM).
• Ex situ indentation-based creep loading experiments.

Main Results:

• In situ TEM revealed localized plasticity around Mrozowski cracks, forming slip or ripplocation bands.
• Slip bands were observed near defect-free regions only under very high stresses.
• Ex situ self-ion irradiation led to embrittlement and reduced creep, while heavy ion irradiation showed opposite effects.
• Hypothesized that large gold ions induced swelling and defect mobility channels.

Conclusions:

• Pre-existing defects play a critical role in the dynamic stress relaxation of nuclear graphite during irradiation.
• The type of irradiation significantly alters the mechanical response, including embrittlement and creep.
• Further research is needed to fully understand the complex interplay between the radiation environment and graphite's mechanical properties.