4.20.2.1 Pure Copper

Copper is widely used where high electrical or ther­mal conductivity is required. Pure copper is defined as having a minimum copper content of 99.3%. Copper with oxygen content below 10 ppm is called ‘oxygen — free.’ ‘Oxygen-free, high conductivity’ (OFHC) grade copper has room temperature electrical conductivities equal to or greater than 100% International Annealed Copper Standard (IACS), where 100% IACS = 17.241 nO m at 20 °C.3 Copper grades with the ASTM/SAE unified number system (UNS) designation C10100, C10200, C10400, C10500, and C10700 are classified as OFHC copper. Grades C10400, C10500, and C10700 have significant silver content, which creates activa­tion hazards. Only C10100 and C10200 are considered for fusion systems.

The use of unalloyed copper is often limited by its low strength. Copper can be strengthened by various processes, for example, cold working, grain refine­ment, solid solution hardening, precipitation hard­ening, dispersion strengthening, etc. While these approaches can significantly increase the strength, they can also lead to a pronounced reduction in con­ductivity. The challenge is to design a material with the best combination of strength and conductivity.

Cold work can significantly increase the strength of pure copper and has a relatively moderate effect on conductivity.4 However, cold-worked copper can be softened at relatively low temperatures (^200 °C) because of its low recrystallization temperature.5 A recent study has shown that ultrahigh-strength and high-conductivity copper can be produced by introducing a high density of nanoscale twin bound — aries.6 The tensile strength of the nano-grained cop­per can be increased by a factor of 10 compared to conventional coarse-grained copper, while retaining a comparable conductivity. The potential of high — strength, high-conductivity bulk nano-grained cop­per in nuclear energy systems, however, has not been widely explored.

Alloying in copper can significantly improve mechanical strengths and raise the softening tempera­tures. However, additions of alloying elements also reduce electrical and thermal conductivity. Among the three alloying strengthening mechanisms, namely, solid solution hardening, precipitation hardening, and dispersion strengthening, solid solution hardening has the most detrimental effects on the conductivity4 and is the least favored mechanism to obtain high- conductivity, high-strength copper alloys.