A new alloy, Alloy 617, was just added to the American Society of Mechanical Engineers (ASME) “Boiler and Pressure Vessel Code”, which sets the standards for nuclear reactors in the United States and determines what materials are acceptable for power plant construction. Alloy 617’s inclusion in the code means that it can now be used in the design and construction of the next generation of nuclear reactors currently being developed, including molten salt, high-temperature, gas-cooled, and sodium reactors.

The alloy is useful because of its ability to maintain strength at high temperatures while resisting corrosion. It is also remarkable for its lack of creep—i.e., unlike many other metals, it doesn’t change shape over time at high temperatures, an essential quality for new reactor designs that operate at more than twice the temperatures of around 290℃  seen in typical light water reactor designs.

A team at Idaho National Laboratory (INL), in collaboration with partners at Argonne and Oak Ridge national laboratories, along with consultants in the industry and international partners have successfully developed Alloy 617, and stewarded it through the arduous process required for acceptance into the ASME code.

This is the first addition of a new alloy to the code in 30 years, and is an important step in the development of the next generation of nuclear reactors. New high-temperature reactor designs have far fewer approved materials available than do more traditional reactor designs.

As Richard Wright, a Laboratory Fellow Emeritus at INL who headed the project made clear, “In contrast to light water plants, the commercial fleet, where you might have 50 or 100 materials that you could use, there were exactly five you could use for high-temperature reactors.” This distinction is essential to understanding the impact that this development will have on new reactor design and construction.

Alloy 617 is composed of nickel, chromium, cobalt, and molybdenum, and is qualified to be used in design and construction for reactors that operate at up to 950℃. Older high-temperature materials were only approved up to 750℃, so the approval of this alloy will open space for new designs that require higher temperatures.

The project took 12 years and cost the Department of Energy $15 million to complete, not including the work it took prior to the start of the project, or the contributions from outside of DOE. After the alloy was developed and tested, it needed to go through the ASME approval process. One reason that the project took so long was that the last alloy approved for the code was 30 years ago. As Wright pointed to this as one cause for the time the process took. “A big challenge for us was simply reinvigorating this process that hadn’t been used in 30 years.” In order to enter the ASME code the alloy went through a working group, a subgroup, and in front of the full committee which meets four times a year. This process took 3 years and culminated in the alloy’s approval in the fall of 2019.

In addition to its importance to future reactor designs, Alloy 617 has applications for natural gas power plants, as well as for a range of other high-temperature activities. The 12 years that it took for this project to see its culmination shows that decisions being made now will determine the technical capabilities of energy production decades into the future.

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