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Why Do Nuclear Reactors Require Alloy Steel?

What is Alloy Steel?

Steel that has been alloyed with additional elements, which are alloying elements to enhance strength, hardness, wear resistance, and toughness, is known as alloy steel. No more than 5% of the alloy steel’s material composition normally comes from the additional alloying elements added to the steel’s basic iron and carbon structure.

Alloy steel offers the attributes one seeks, whether the project calls for increased corrosion resistance, machinability, strength, or another trait. With additional heat treatment, alloy steels can offer several advantageous characteristics, such as enhanced resistance to corrosion, increased toughness, superior hardness, and strength.

High alloy steel has alloying components that account for more than 8% of its composition, excluding carbon and iron. Since most steel only devotes a small percentage of its composition to the extra elements, these alloys are less prevalent. The most often used high alloy, with at least 10.5% chromium by mass, is stainless steel. With a chromium oxide coating to prevent rusting, this ratio increases the corrosion resistance of stainless steel.

Other elements are just minimally added to low alloy steel, giving it slight advantages in hardenability, toughness, and free machining. The strength and formability of low alloy steel can be increased by reducing the carbon content to about 0.2%.

What are the Common Steel Alloying Elements?

When it comes to steel, a variety of other components can be combined with the basic substance, allowing the buyer to experiment with variations until the ideal alloy is discovered. The following are typical alloying components:

  • Manganese:

The steel alloy becomes less brittle and easier to hammer when combined with trace levels of sulfur and phosphorus.

  • Chromium:

A small amount (0.5%–2%) can aid in the alloy’s hardening; greater amounts (4%–18%) can have the additional benefit of inhibiting corrosion.

  • Vanadium:

With just.15%, this element can improve grain structure, strength, and heat resistance. The steel alloy hardens significantly when combined with chromium while keeping its formability.

  • Nickel:

This alloying ingredient will increase the steel’s strength by up to 5%. It offers exceptional corrosion resistance of around 12%.

  • Tungsten:

Increases heat resistance, raising the melting point. Enhances the steel’s structural makeup as well.The desired outcome determines the procedures for alloying and processing alloy steel. The necessary mixture of components is first fused for eight to twelve hours at temperatures above 1600°C. After that, the steel is annealed at more than 500 °C to purge impurities and change the chemical and physical properties.

Before repeating the annealing and descaling procedure, the mill scale (a combination of iron oxides), a byproduct of the annealing process, is first cleaned from the surface of the steel with hydrofluoric acid. The steel is finally melted and cast before being rolled and shaped into its final form. Since “alloy steel” refers to various steel kinds, its applicability varies.

Due to their great strength, machinability, affordability, and availability, low alloy steels are employed in various sectors. They can be found in ships, pipelines, pressure vessels, oil drilling platforms, military vehicles, construction equipment, and structural elements.

High-alloy steels can indeed be expensive to produce and challenging to work with. However, they are perfect for structural components with automotive applications, chemical processing, and power generation equipment due to their high hardness, toughness, and corrosion resistance.

What is a Nuclear Reactor?

A device that contains and regulates ongoing nuclear chain reactions is known as a nuclear reactor. Reactors are employed for various tasks, including creating medical isotopes for cancer treatment and imaging, propelling aircraft carriers and submarines, and performing research.

The big tank reactor vessel is filled with fuel and a tiny neutron source. Fuel is composed of heavy atoms that split when they absorb neutrons. Each atom that divides releases additional neutrons, which induces other atoms to split. The neutrons initiate this process. Large amounts of energy are released as heat each time an atom divides. The coolant, most frequently just ordinary water, transports the heat out of the reactor. A turbine uses the heated coolant to turn a generator or driving shaft. Nuclear reactors are merely unusual generators of heat.

Components of a Nuclear Reactor:

All of the nuclear fuel and all of the heat are produced in the reactor’s core. It has structural components, control systems, and low-enriched uranium (5% U-235). Numerous fuel pins can number in the hundreds of thousands in the core.

The substance that moves through the core and transfers heat from the fuel to a turbine is known as the coolant. It might be liquid sodium, heavy water, helium, or something else. Water is the norm in the US power reactor fleet.

Like a fossil fuel plant, the turbine converts the heat from the coolant to electricity. The building that isolates the reactor from its surroundings is known as containment. These are typically composed of high-density, steel-reinforced concrete shaped like domes. Some plants require cooling towers to dispose of the extra heat that cannot be turned into energy because of the rules of thermodynamics. They simply release pure water vapor.

Why is Alloy Steel used in a Nuclear Reactor?

Anyone involved in the nuclear industry knows that steel plays a fundamental role in protecting people and resources. In any event, even industry insiders must understand how to make their practices more secure in this era. This objective considers the composition of alloy steel and other super combinations. It is crucial to know what matters to atomic designers and look into the ideal pairings for an atomic application. Learn about an atomic reactor’s components before discovering how alloy steel properties are utilized in them.

Reactor pressure vessels (RPVs) are used in pressurized water reactors to house the nuclear fuel, moderator, control rods, and coolant. High-pressure liquid water cools and regulates them (e.g., 16MPa). Water boils at around 350°C (662°F) at this pressure, and the input temperature is roughly 290°C (554°F). As the water runs through the reactor core, the temperature (coolant) is raised to about 325°C (617°F). As can be observed, the reactor’s subcooled coolants are around 25°C (distance from the saturation).

The pressure vessel housing the reactor core and other vital internals are known as the reactor pressure vessel. It is a cylinder vessel with a flanged and gasketed top head and a hemispherical bottom head.

While the top head is bolted to the cylindrical shell using the flanges, the lower head is welded to the cylindrical shell. To refuel the reactor during predetermined outages, the top head is detachable.

High-quality low-alloy carbon steel is used to build the reactor vessel’s body, and to prevent corrosion, all surfaces that come into contact with reactor coolant are covered with a minimum of 3 to 10 mm of austenitic stainless steel (304L, for example)

Materials for reactor pressure vessels: 

The RPV is typically considered the most important lifetime-limiting (and irreplaceable) component for a nuclear reactor. It is the crucial safety boundary between the reactor and the outside environment. Low-alloy Mn-Mo-Ni steel that has been quenched and tempered makes up the reactor pressure vessel.

The most popular type of steel nowadays is low-carbon steel, sometimes referred to as mild steel, because it is reasonably inexpensive and has material characteristics suitable for a wide range of uses.

Low-carbon steel is malleable and ductile due to its 0.05–0.25% carbon content. Mild steel is simple to shape and tough despite relatively low tensile strength. Low activation capability is one of the unique requirements for reactor vessel materials, especially because of the Co-60 formation.

The RPV becomes brittle after prolonged radiation exposure, which lowers its fracture toughness. Heat exchangers and pipelines are made of alloys based on nickel. Due to the size of these pressure vessels, the primary cost consideration for the material.

Alloying agents:

Pure iron is too soft to be utilized for construction, but small amounts of other elements, such as carbon, manganese, or even chromium, considerably improve the mechanical strength of iron. Different microstructures and characteristics are created by the cooperative action of alloying components and heat treatment. The following four primary alloying elements are:


Chromium makes these steels harder and stronger. The concentration specified for most grades is around 2%, and it appears that this concentration achieves the optimal balance between toughness and hardness. Chromium is regarded as indispensable and plays a significant part in the hardening mechanism. Chromium adds to enhanced strength at higher temperatures.


In steel, nickel does not produce any carbide compounds. It continues to be dissolved in the ferrite, toughening and reinforcing the ferrite phase.


Molybdenum (added to steel in amounts ranging from 0.50 to 8.0%) increases its resistance to high temperatures. Due to its high melting point, molybdenum improves hardenability and strength, especially at high temperatures. The degree to which molybdenum boosts steel tensile and creep strengths at high temperatures makes it special.

Between 16 and 25% of chromium is present in austenitic stainless steels, which are employed as a corrosion-resistant clad. They can also contain nitrogen in solution, increasing their relatively strong corrosion resistance. The most popular grade is AISI 304 stainless, consisting mostly of non-iron metals such as nickel (between 2% and 10.5%) and chromium (15–20%). There is excellent resistance to various atmospheric situations and numerous corrosive media in 304 stainless steel. These alloys are typically described as ductile, hardenability, and weldability in cold forming.

Extra-low carbon versions of the 304 steel alloy are often used in the nuclear industry and are known as type 304L stainless steel. Despite having slightly worse mechanical qualities than the common 304 grade, this grade is nonetheless commonly utilized because of its adaptability. Due to welding, detrimental or hazardous carbide precipitation is reduced to a minimum in 304L due to its lower carbon content. Therefore, 304L does not require annealing and can be used “as welded” in situations with severe corrosion. In intermittent service up to 870 °C and continuous service up to 925 °C, grade 304 offers good oxidation resistance. Grade 304L is frequently used in heavy gauge components since it doesn’t require post-weld annealing. Some examples of stainless steel in use:

  • 304L stainless steel type
  • The steel of type 08Kh18N10T is stainless.

The most crucial parts of nuclear power facilities are the reactor pressure vessels. The reactor pressure vessel, which houses the reactor core, is directly important for safety. The material of the reactor pressure vessel is subjected to neutron radiation during nuclear power plant operation, particularly fast neutrons, which causes localized embrittlement of the steel and welds near the reactor core. Radial neutron reflectors are positioned around the reactor core to reduce this material damage. The core baffle and the heavy reflector are the two fundamental types of neutron reflectors. Heavy reflectors are more effective than core baffles at reducing neutron leakage from the core (particularly of fast neutrons) because they have a higher atomic number density.

These standard components are included in the reactor itself:

Moderator and Coolant

The moderator’s job is to reduce the fast neutrons’ energy from a few MeV to exactly 0.025 eV. The molecules of the moderator material must be roughly the same size as the neutrons to slow down these neutrons as effectively as possible. Because hydrogen has the smallest atomic size, H2O is the most logical option. Graphite, heavy water, salt, and carbon dioxide are common moderators. Suppose the moderator material has a large heat capacity to absorb the heat from the reactor, like water. In that case, it can also operate as a coolant with a low neutron absorption cross-section.


Neutrons occasionally escape the reactor core. Using reflectors, these neutrons are stopped. The reflector must possess the same material qualities as the moderator, except that it must be solid. The most typical reflector materials are graphite, beryllium, or austenitic stainless steel.

Control Rod

The control rod’s job in the reactor is to take in neutrons. Reactor scrams, which include inserting control rods into the reactor, are carried out if the neutron population rises to an unmanageable level. The control material is often constructed of B4C distributed in a 304-type stainless steel matrix or a hafnium matrix, and it takes the form of blades placed through the fuel assembly in the shape of a cruciform. Below are listed below: boron, cadmium, hafnium, and other suitable materials with high neutron absorption cross-sections.

The Prototypical Reactor

If anyone pictures a typical reactor, it is seen that the entire facility is full of groupings made of hardened steel. A variety of manufacturers meets the needs for alloy steel. Pipes for various procedure-related vessels are commonly constructed from austenitic-treated steel.

Fuel & Coolants

To keep the plant from overheating, uranium oxide balls are stored in cylinders. Coolant is continuously pumped around the center of the plant.

Pressure tubes

These hold coolants or gasoline. Some plants use larger containers to house their mediator or coolant. In any event, tempered steel is frequently used to make these cylinders or vessels.

Steam generators

A pressurized coolant uses the heat from the reactor to create steam, which operates the turbines. A few steam generators are often present in reactors.

Arbitrators or moderators

The mediator, which may be made of graphite or water, sits in the reactor’s core and calms the produced neutrons’ splintering.


Control poles located in the reactor’s core were designed to limit or speed up atomic reactions. They are made of materials that may effectively absorb neutrons, such as boron, cadmium, or hafnium.

Contamination Vessels

The largest stainless steel structures protect the reactor from outside interference. More importantly, these strong steel tanks protect workers from radiation.

To prevent breaking Austenites, Super Alloys, and New Steel Classes, the steel must contain no more than 3% ferrites if a segment exhibits ferritic erosion.

Austenitic materials perform well in heated environments when consumption is common, as every atomic architect is aware. Even though both types of steel may be present in a typical reactor, this treated steel can withstand warmer concentrations than ferritic steel. Consider a fourth-generation reactor that operates at 1,000 degrees Celsius to help the user understand the different alloy steel grades and types. Under these constraints, austenitic steel generally outperforms ferritic steel.

Whatever the case, a constant neutron attack can weaken the two metals. Indeed, in the circumstances similar to those mentioned above, even so-called super composites (nickel-based) may produce some problematic memories. They can form helium bubbles when exposed continuously to neutrons and erosion and eventually become incapacitated. Steelmakers must keep up as atomic engineers develop additional high-temperature reactors. It appears promising to use a family of ferritic-martensitic tempered steels. This type of steel was partially grown with the assistance of the fossil energy division. It appears to withstand atomic temperature restrictions, consumption, and other upcoming effects well.

The Prognosis for Alloy Steel

Despite any contradictory claims, atomic vitality is increasing. This suggests that future reactors will always need more grounded steel components. Most atomic scientists agree that interest in clean atomic energy will only grow in the coming decades. To meet ever-increasingly stringent requirements, alloy steel manufacturers create new compounds as demand rises. This includes siphon castings, consumption-safe weight vessels, tube sheets, and more grounded steam generators. These areas are crucial to the atomic industry. Unmistakably, the steel industry will take on a fundamental role in the future for workable energy as the world demands more secure, increasingly affordable energy options. The future of atomic energy appears to be splendid because of the skill of international steel suppliers.


Constant development in nuclear materials has the potential to dramatically increase the safety of the nuclear reactor while it is in operation. A significant amount of time passes between discovering new nuclear material and its employment in nuclear reactors. Before being approved for use in a reactor, this novel material must undergo extensive testing in national labs and academic institutions. This is significant since a nuclear reactor has an average lifespan of 30 to 40 years, and these materials must last for that long. Failure to do so could cost money. As a result, developing new materials in the nuclear industry has lagged behind other industries.

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