High-performance steel for fusion application

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2009

An international consortia was formed to build a Tokamak reactor that would demonstrate the feasibility of fusion power for generating electricity.

KEY CONCEPTS
• A high-strength steel is required to manufacture shield modules for use in a fusion reactor.
• A cast stainless steel of the CF3M grade is found to be suitable for use in this application.
• Small percentages of manganese and nitrogen were added to improve the physical properties of the cast stainless steel.

Growing energy demands worldwide are leading to the evaluation of all possible options. One technology for generating electricity that has not been cultivated is fusion. This process involves the combination of nuclei from two hydrogen isotopes (deuterium and tritium) to form a helium nucleus, a neutron and a tremendous amount of energy. Fusion is the same reaction that powers stars such as our sun.

The goal in developing fusion is that the process can be self-perpetuating. In other words, the amount of energy released from fusion has the potential to exceed the amount needed to fuse the nuclei in the first place. Much attention on attaining this goal has centered on the use of the Tokamak reactor that generates a magnetic field suitable for containing an energy plasma heated to more than a million degrees Celsius. The plasma is needed to perpetuate fusion.

An international consortia has been organized with the objective of harnessing the power of fusion for use in generating electricity. The group is known as the ITER Fusion project and was formally initiated in 2006, although research and design has been underway over a much longer time frame.

The first goal of ITER is to build a Tokamak reactor to demonstrate the feasibility of fusion power. The group decided to build the reactor in France using components manufactured in the other participating countries of China, the European Union, India, Japan, the Republic of Korea, Russia and the U.S.

One of the challenges is to develop materials that can withstand the harsh conditions in the Tokamak reactor. Jeremy Busby, research scientist at the Materials Science and Technology Division of Oak Ridge National Laboratory in Oak Ridge, Tenn., says, “We were asked by Mike Hechler at the US-ITER office to provide assistance in the development of a high-strength steel that can be used in the preparation of shield modules for the reactor. These modules are analogous to very large engine blocks but contain many cooling channels.

Typical dimensions for the blocks are 1.5 meters-by-1 meter-by-0.5 meter, according to Busby. The steel must withstand an operating temperature of approximately 200 C over an 8- to 15-year lifetime.”

A shield module is shown in Figure 3. This design is very complicated with a large number of holes needed. Busby adds, “The geometry of this particular component makes the use of traditional wrought stainless steel very difficult and expensive. Much of the difficulty is due to having to drill holes in the steel and needing to remove and lose approximately 30% of the steel.” In addition, Busby indicates that big enough blocks of wrought steel are not made to meet the requirements for the shield modules.


Figure 3. A shield module for a fusion reactor contains many cooling channels and is very complicated to prepare. Cast stainless steel reduces the manufacturing cost and provides a material with comparable physical properties compared to traditional wrought stainless steel alloys. (Courtesy of Oak Ridge National Laboratory)

ITER is requiring that the U.S. group produce 100 of these shield modules. Each of these shields weighs three to four tons. An alternative approach is required to improve the efficiency of the process. Busby comments, “Our motivation in this project was economic and fabrication-related.”

CAST STAINLESS STEEL
An alternative that Busby and Hechler looked into is cast stainless steel. Busby says, “Casting offers alternatives without the detrimental aspects of machining costs and dealing with waste materials.”

Cast stainless steels in general do not provide the performance features of their wrought counterparts. Busby adds, “Cast stainless steels exhibit tensile strengths that are 60 to 80 megapascals lower than wrought stainless steels.”

Key cast stainless steel characteristics that need to be improved include impact properties such as toughness. In addition, Busy indicates that the steel must be weldable, and the corrosion performance must be good because the cooling channels in the shield modules are filled with distilled water.

In particular, the steel must resist stress corrosion cracking, which can be caused by exposure to heating water/steam at temperatures in excess of 50 C. This type of corrosion can form deep cracks in the steel. The microstructure of the cast stainless steel must also tolerate radiation without corroding or losing tensile properties.

Other ITER-related projects tried using cast stainless steel approximately 10 years ago. Busby revealed that the alloy used did not exhibit acceptable strength.

The main problem with cast stainless steel is its microstructure. Busby says, “Cast stainless steels exhibit larger grain sizes and different phases such as delta ferrite lead to lower strength.”

The ORNL-team was able to introduce two elements to improve the strength of the cast stainless steel. A small percentage of manganese was added to improve the austenite stability of the steel. This is important because austenite stainless steels exhibit improved physical properties and better corrosion resistance. In addition, the manganese eliminates the different types of microstructures that also improve the physical properties of the steel.

Nitrogen was added to improve the tensile strength. Busby comments, “Addition of 0.1% nitrogen leads to a 50 megapascals increase in the tensile strength of the steel.”

A cast stainless steel of the grade CF3M has been developed that appears to meet the requirements of the ITER project. Testing has been conducted on small batches that shows required improvements in fracture properties, tensile strength, microstructure properties, welds, impact properties, corrosion performance and radiation resistance.

Future work will focus on scaling up the cast stainless steel to the dimensions required for the ITER project. Busby says, “We need to validate that the stainless steel will provide the required properties when used in the manufacture of parts with geometry relevant to the shield modules.”

Cast stainless steels also have use in other machinery applications that operate under severe conditions. Busby mentioned that the cast grade CFC8 has been used in heavy engine components.

The CF3M grade is comparable to 316 wrought stainless steel, which is one of the most widely used alloys. This means that this cast grade could theoretically be useful in any application that requires 316 stainless steel.

Further information on the work done in development of the cast stainless steel can be obtained by contacting Busby at busbyjt@ornl.gov. 

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.