Reactor Pressure Vessel Steel Grades

Author:

Piotr Sompoliński

Date added:

NuScale have already tried and failed to contract several Small Modular Reactors (SMRs) in US and Europe, but the idea is potent enough to dismiss claims the SMR's are too costly to ever be commercialized. Speaking of cost and materials - what metal alloy will be used in future reactor plants? While next-generation Generation IV reactors require advanced nickel alloys, water-cooled SMR's designed by NuScale rely on steel. In this article, we will explore the challenges steel faces in nuclear reactors, the steel grades historically used, and the most promising candidates for future projects.

Challenges for Steel in Nuclear Reactor Pressure Vessels

The Reactor Pressure Vessel (RPV) is a critical component of nuclear water reactors. It encapsulates the reactor core, including all nuclear fuel, and is crucial to plant's safety. Since RPVs are not replaced throughout a plant's operational life, they must maintain integrity for decades, despite prolonged exposure to radiation, high temperatures, and extreme pressures.

In modern Light Water Reactors (LWRs), including Small Modular Reactor (SMR) designs, pressure vessels must withstand:

  • Temperatures reaching 300°C
  • High mechanical stresses
  • Corrosive effects of water
  • Neutron radiation exposure

These challenges demand alloys with strength, corrosion resistance, and radiation tolerance. Additionally, cost is a factor—while nickel alloys offer excellent properties, their expense makes them impractical for widespread use in commercial reactors. This is why steel is used instead.

Early Reactor Steels – Carbon and Ferritic Steels

Early Use of Carbon Steel: In the initial years of nuclear power, commercial RPV's were made of low-carbon normalized and tempered steel. Then low-alloy steels with manganese, molybdenum, and nickel additions were introduced. For example, ASTM Type A212-B steel was used in the american Indian Point-1 reactor.

1960s and 1970s – Low-Alloy Ferritic Steel: By the 1970s, most commercial nuclear reactors had RPVs made from quenched and tempered manganese-molybdenum ferritic steel. Japanese nuclear reactors, including Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs), commonly used A533 Grade B Class 1 steel, except of Tsurugi power plant (using A302-B steel) and JPDR-2 (using a normalized and tempered variant of A302-B steel).

In West Germany, RPVs were fabricated from local ferritic steel grades 22NiMoCr37 and 20MnMoNi55 (1.6311). Ferritic steel was more resistant, but not perfect - an German RPV made of 22NiMoCr37 steel had shown stress corrosion cracking and underclad cracking.

Carbon steel was not completely abandoned—Swedish Ågesta reactor used a carbon-manganese steel similar to ASTM A212-B.

The Future: Austenitic Stainless Steels?

SMRs represent a potential breakthrough in nuclear energy, and many designs continue to utilize water-cooled LWR technology, bringing renewed attention to optimal RPV materials. Which steels are being considered? A key technical report submitted by NuScale to the United States Nuclear Regulatory Commission (NRC) provides some insight.

According to the document, NuScale’s SMR design features a lower reactor pressure vessel constructed from austenitic stainless steel SA-965 Grade FXM-19, with welding materials E/ER209 or E/ER240. This material, also known as S20910, XM-19, or Nitronic 50, is commonly used in petrochemical, chemical, and marine industries. But why choose austenitic steel over ferritic steel?

NuScale's report states:

The data show that austenitic stainless steels have superior ductility and are less susceptible to the effects of neutron and thermal embrittlement than ferritic materials.

In addition to XM-19, other austenitic stainless steels used in nuclear applications include steel 304, steel 347, and steel 316. NuScale selected Nitronic 50 due to its superior strength.

It's worth noting that austenitic steel RPVs have been used in nuclear research reactors for decades. One notable example is the Advanced Test Reactor (ATR) in the U.S., which has been operational since 1967—over 58 years of neutron exposure! The ATR’s RPV is made from AISI 304 stainless steel. If SMRs become widespread, it will mark the first large-scale commercial use of austenitic stainless steel in nuclear power plants.

Will SMRs be commercialized?

Several years ago, Poland aimed to deploy its first SMRs by 2029. However, NuScale’s failure with its pilot project in the U.S. raised doubts, leading to setbacks in its partnership with Polish company KGHM. Despite this, the concept remains viable, and hopes are raised by the recent agreement between Westinghouse and Korea Hydro & Nuclear Power, two key SMR developers, which may result in lower EPC costs.

One thing is certain – the development of nuclear technology allows us to see steel, this fascinating material, in a new light. Steel 304, Steel 316, and XM-19 are all extremely durable materials successfully used as cost-effective alternatives to expensive nickel alloys. They are widely applied in the petrochemical, chemical, marine, and food industries.

If you are interested in any of the remarkable materials discussed here, feel free to text me.

Tables

Comparison of chemical compositions:

Steel Grade C: Si: Mn: P: S: Ni: Cr: Cu: Mo: V: Others:
A533-B, C1.1 <0.25 0.15-0.40 1.15-1.50 <0.035 <0.04 0.4-0.7 - <0.01 0.45-0.60 <0.05 -
A508-B, C1.2 <0.27 0.15-0.40 0.5-1.0 <0.025 <0.025 0.5-1.0 0.25-0.45 <0.1 0.55-0.70 <0.05 -
A508-B, C1.3 <0.25 0.15-0.40 1.20-1.50 <0.025 <0.025 0.4-1.0 <0.25 <0.1 0.45-0.60 <0.05 -
20MnMoNi55
(1.6311)
0.17-0.23 0.15-0.30 1.20-1.50 <0.012 <0.008 0.5-0.8 <0.2 <0.12 0.40-0.55 <0.02 Al: 0.01-0.04


Mechanical properties at room temperature:

Steel Grade Tensile Strength: Yield Strength: Elongation: Relative Reduction in Area:
A533-B, C1.1 >550 MPa >345 MPa >18% -
A508-B, C1.3 >550 MPa >345 MPa >18% >38%
20MnMoNi55
(1.6311)
560-700 MPa >390 MPa >19% >45%

Bibliography

Suzuki, K.1, Reactor Pressure Vessel Materials, International Atomic Energy Agency, October 1998

Use of Austenitic Stainless Steel for NPM Lower Reactor Pressure Vessel, NuScale Power, LLC, December 2022

Post author

Piotr Sompoliński

CSO Virgamet

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