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Application of CB752 Alloy in High-Temperature Heat Pipe Components for Small Nuclear Reactors and Comparison with Rare, Corrosion-Resistant Refractory Metals
High-temperature heat pipes, heat exchange ducts, and in-core thermowells for small modular reactors and space nuclear power systems operate under demanding conditions: sustained high temperatures (700–1100°C), exposure to corrosive liquid alkali metals, and cyclic neutron irradiation. These applications require materials that offer high-temperature creep resistance, alkali metal corrosion resistance, radiation resistance, and stable hermetic sealing. While austenitic stainless steels and nickel-based superalloys suffer from insufficient high-temperature strength and severe alkali metal corrosion, and rare refractory metals like zirconium, tantalum, and molybdenum have their own respective limitations, the CB752 niobium-based alloy stands out. Thanks to its comprehensive properties—including resistance to liquid lithium and sodium corrosion, dimensional stability at high temperatures, neutron irradiation resistance, and good ductility/weldability—CB752 has become a core material for high-temperature heat pipes in space and terrestrial micro-reactors, offering an irreplaceable balance of performance and cost-effectiveness under extreme nuclear operating conditions.
Standard industrial pure metals frequently fail under in-core heat pipe operating conditions. Although 316L and Inconel-series nickel-based alloys are conventional materials for nuclear power, prolonged contact with liquid lithium or sodium at temperatures above 750°C leads to dissolution corrosion (causing wall thinning and leakage) and high-temperature creep deformation (blocking heat pipe channels). Pure iron and carbon steel exhibit poor radiation resistance and are prone to embrittlement and swelling at high temperatures. Pure copper and aluminum lack sufficient high-temperature resistance, making them suitable only for ambient-temperature auxiliary heat exchange piping. While pure niobium offers excellent resistance to alkali metal corrosion, its strength in an unalloyed state is too low; it lacks the necessary pressure-bearing and creep-resistance capabilities for heat pipes, making it prone to bulging and deformation during long-term service in high-pressure applications and unable to meet design requirements for in-core pressure limits.
Comparative analyses against four rare refractory metals used in nuclear applications—zirconium, tantalum, molybdenum, and hafnium—clearly demonstrate the advantages of CB752 for nuclear heat pipe applications. Zirconium metal (Zr-4 alloy) is the mainstream cladding material for pressurized water reactors, but its upper operating temperature limit is only 450°C; it reacts violently with liquid alkali metals above 600°C, making it entirely unsuitable for high-temperature heat pipe applications. Tantalum offers superior resistance to lithium and sodium corrosion and excellent radiation resistance, but its high density (16.65 g/cm³) results in excessive self-weight for the tubing; furthermore, the scarcity and high cost of the raw material make the total cost of a heat pipe assembly 4 to 6 times that of CB752, resulting in extremely poor economic viability. Molybdenum meets temperature requirements but is prone to localized corrosion in contact with liquid lithium; it is brittle at room temperature, prone to micro-cracking during bending, susceptible to rapid weld embrittlement under neutron irradiation, and lacks long-term reliability regarding heat pipe hermeticity. Hafnium offers the best overall corrosion and radiation resistance but suffers from extremely low production volume and processing difficulties, limiting it to the manufacture of tiny, millimeter-scale tubes and making large-scale production of large-diameter heat pipes impossible. In CB752, the tungsten content enhances high-temperature creep resistance, while zirconium suppresses the segregation of interstitial impurities and reduces the tendency toward radiation-induced embrittlement; this alloy retains the alkali-metal corrosion resistance of pure niobium while overcoming issues such as insufficient strength, excessive costs associated with rare metals, or excessive brittleness.
Among niobium-based alloy systems, CB752 boasts a far more mature manufacturing process: while Nb-1Zr offers corrosion resistance comparable to CB752, it lags significantly in high-temperature creep and pressure-bearing performance; multi-component Nb-Ta alloys offer superior corrosion resistance but involve complex compositions and difficult smelting processes, resulting in low tubing yields; and high-strength Nb-W series alloys exhibit high weld sensitivity—prone to porosity and leakage in welds—which compromises the hermetic integrity required for heat pipe manufacturing. CB752 tubing can be manufactured via seamless rolling or precision welding, offering high airtightness reliability; it exhibits no creep deformation under sustained pressure at 1,000°C, an annual corrosion rate of less than 0.002 mm in liquid lithium, and minimal shifts in the ductile-to-brittle transition temperature following prolonged neutron irradiation—making it highly resistant to radiation-induced brittle fracture and ideally suited for the requirements of sealed heat pipe heat-exchange systems.
Currently, CB752 heat pipes are utilized in space nuclear power satellite systems and terrestrial micro-molten salt reactor heat-exchange test loops, effectively replacing molybdenum alloy test pipes and costly tantalum-based small-diameter tubing; this has resulted in a fivefold extension of the maintenance cycle for in-reactor heat-exchange systems and a significant reduction in leakage failure rates. Existing limitations include the accumulation of trace interstitial impurities and a gradual decline in toughness during prolonged, intense irradiation. Future development will focus on three areas: first, employing ultra-high-purity melting techniques to strictly control interstitial elements (oxygen, nitrogen, and hydrogen) and further enhance long-term stability under irradiation; second, applying inner-wall passivation treatments to strengthen resistance to extreme alkali metal corrosion; and third, optimizing rolling processes for large-diameter, extra-long seamless pipes to reduce production costs, thereby facilitating the gradual replacement of various rare refractory metals with CB752 as the standardized material for next-generation high-temperature nuclear heat pipes and supporting the commercialization of small-scale nuclear reactors.
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