Why are Refractory Metals Used in Nuclear Reactions?

Refractory metals are metallic elements of the periodic table with unique properties such as extremely high melting points. However, they can remain solid at room temperature. These characteristics make these materials ideal for research-based and critical nuclear reactions.

What Are Refractory Metals?

Refractory metals possess a melting point above 2,000° Celsius (3,632° F) and are solid at room temperature. They are also extremely resistant to wear and tear elements such as creep deformation, which is a change in shape that ordinary metals undergo when subjected to stress.

Five established refractory metals, including niobium, molybdenum, tantalum, tungsten, and rhenium. However, other metal elements and compounds are sometimes included in this group, including chromium, tungsten alloys, and a molybdenum alloy containing titanium and zirconium, known as TZM.

In short, a refractory metal is a material or substance that shows exceptionally high resistance to high temperatures.

Heat Resistance of Refractory Metals

The heat resistance of the refractory metals of tungsten is a good example of one of these properties. It is so powerful that even when heated to a temperature of 1000°C (1832°F), it still exhibits twice the resistance of the item to be ironed at room temperature. This makes it useful for applications such as rocket cones, as long-lasting filaments in incandescent light bulbs, and as an additive for steel used in welding and other high-temperature applications. Stainless steel also contains tungsten to increase its resistance to refractory metal corrosion for industrial piping where corrosive and strong chemicals are used.

Refractory Metals in Nuclear Reactions

Until the mid-1940s, refractory metals were only used as alloying elements to improve the mechanical characteristics of non-ferrous steel alloys based on copper and nickel in the electrical industry. Molybdenum and tungsten compounds have also been used to produce hard alloys.

The technical revolution, associated with the active development of aviation, the nuclear industry, and rocket science, has found new ways of using refractory metals. We list here just a few recent applications:

• They are used for assemblies of a thermo-reactor and a rocket engine: nozzles, tail skirts, turbine blades, and nozzle flaps.
• Vanadium is the basis for manufacturing thin-walled tubes for the fuel elements of a thermonuclear reactor in the nuclear industry.
• Because of their low activation characteristics, refractory metals are also preferred as components for fusion reactor structural materials.
• Refractory metals are used as control rods in nuclear reactors, as catalysts, and for their chemical properties. Components of refractory metals are seldom cast because of their high melting point. However, the powder metallurgy process is used. Pure metal powders are compacted, heated with an electric current, and then cold-worked with annealing steps.

Refractory Metals in Various Industries

Other properties of refractory metals target their uses in several specific industries. For example, molybdenum has a very predictable coefficient of expansion, making it an essential metal in thermocouples and computer heat sinks. At the same time, rhenium's unique chemical reactivity gives it applications in processes such as hydrocracking, the breakdown of petroleum into simpler molecules. Niobium is used in the design of nuclear power plants and is an ideal metal for superconductors because it absorbs very few neutrons. Finally, tantalum is used in aerospace and surgical equipment due to its inertness when in contact with body fluids and tissues.

Moreover, refractory metals are widely used in many industries and in everyday life. For example, they are used to manufacture incandescent light bulbs, mobile phones, computers, and nuclear reactors. In a broader concept and practical application, vanadium, hafnium, ruthenium, chromium, zirconium, and osmium are also refractory metals. They are also used as alloying elements in alloys with first group metals to improve a set of operational or technological properties.

Use of Refractory Metals for Manufacturing

Since refractory metals wear at a very low rate, they are also widely used in manufacturing components that need to exhibit long-term abrasion resistance, such as bearings and nozzles. In addition, many of these components are used in high-performance types of machinery, such as the aerospace industry or the manufacture of semiconductor electronic components. Tungsten alloys, such as TZM Niobium and chromium, are most often used for these applications. Chromium also falls into the oxidation resistance category of refractory metals, as it is a highly durable coating for bearings.

Refractory Metal Alloys

Pure metals are, of course, used in production; for example, pure molybdenum and tungsten are used in the radio electronics industry, chemical engineering, or in the production of heat treatment furnaces. But most of them are subject to brittle fracture at high temperatures and have relatively low heat resistance. Therefore, the use of alloys of these metals is much more interesting from the point of view of improving the functional properties.

Refractory alloys based on tungsten

A representative of such alloys is an alloy of tungsten and Niobium BB2 with a heat resistance temperature of up to 1200°C. Furthermore, tungsten alloys are alloyed with rhenium to improve corrosion resistance and refractoriness. And to increase wear resistance with thorium.

Refractory niobium-based alloys

Due to their high corrosion resistance, high heat resistance (up to 1300°C), and good performance under neutron irradiation, Niobium, and its alloys have found wide application in the manufacture of products for the nuclear industry. By way of example of niobium-based alloys, mention may be made of VN2, VN2A, and VN3 alloys.

Molybdenum based alloys

Molybdenum and its alloys are probably the most commonly used refractories. In industry, alloys alloyed with zirconium, boron, titanium, and Niobium are often utilized: TsM3, TsM6, TsM2A, and VM3 alloys.

Methods to improve the heat resistance of alloys

The heat resistance of refractory alloys, as mentioned above, is increased by alloying with higher melting point elements, which form solid substitute solutions in the alloy. Greater efficiency in increasing heat resistance, and in some cases, wear resistance, can be achieved with precipitation hardening of the alloy with the formation of carbides (ZrC, NiC), nitrides (TiN), and oxides (ZrO 2).

All refractory metals have low heat resistance; therefore, intermetallic and ceramic coatings are used to protect them at temperatures above 400°C. For molybdenum and tungsten, silicon-based coatings (MoSi 2 & WSi 2) are used. Almost all metals are solids under normal conditions. But at certain temperatures, they can change their aggregation state and become liquid.

Conclusion

As explained earlier, a refractory metal is a material or substance that shows exceptionally high resistance to high temperatures.

Refractory metals have been known since the end of the 19th century. They were useless then. The only industry they were used in was electrical engineering, then in minimal quantities. But everything changed dramatically with the development of supersonic aviation and rocket technology in the last century during the 50s. The production needed new materials capable of withstanding high loads at temperatures above 1000ºC.