Which one of the following metals is alloyed with sodium to transfer heat in a nuclear reactor ?

Potassium is the metal alloyed with sodium to create a liquid metal coolant for nuclear reactors. This alloy, commonly known as NaK, is a eutectic mixture that typically consists of approximately 77% to 78% potassium and 22% to 23% sodium by mass [1]. NaK is particularly valued in nuclear engineering because it remains liquid at room temperature, with a melting point as low as -12.6 °C to -12.8 °C [1]. It possesses excellent heat transfer properties, high thermal conductivity, and a high boiling point, making it suitable for compact, high-performance systems like fast breeder reactors and space nuclear power applications [3]. Historically, NaK was used in the Experimental Breeder Reactor-I (EBR-I), the first reactor to generate usable electricity. While highly efficient for heat transfer, it is chemically reactive and requires special handling to prevent violent reactions with air or water [1].

Sources

1. [1] https://en.wikipedia.org/wiki/Sodium%E2%80%93potassium_alloy
2. [3] https://ntrs.nasa.gov/api/citations/20110015702/downloads/20110015702.pdf