Alloy 617 - a combination of nickel, chromium, cobalt and molybdenum - has been approved by the American Society of Mechanical Engineers (ASME) for inclusion in its Boiler and Pressure Vessel Code. This means the alloy, which was tested by Idaho National Laboratory (INL), can be used in proposed molten salt, high-temperature, gas-cooled or sodium reactors. It is the first new material to be added to the Code in 30 years.

Alloy 617 was subjected to repeated fluctuations in temperature or physical stress to provide data for the ASME code case (Image: INL)

The Boiler and Pressure Vessel Code lays out design rules for how much stress is acceptable and specifies the materials that can be used for power plant construction, including in nuclear power plants. Adhering to these specifications ensures component safety and performance.

INL spent 12 years qualifying Alloy 617, with a USD15 million investment from the US Department of Energy. A team at INL, in collaboration with groups at Argonne National Laboratory and Oak Ridge National Laboratory, as well as industry consultants and international partners, has now received approval from ASME for the alloy's inclusion in the Code. Designers working on new high-temperature nuclear power plant concepts now have more options when it comes to component construction materials.

"It's a pretty substantial accomplishment," said Richard Wright, an INL laboratory fellow emeritus who headed the INL part in, and overall management of, the project. "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."

Unlike light water reactors, which operate at around 290°C, the proposed molten salt, high-temperature, gas-cooled or sodium reactors will run two or more times hotter. So, determining what happens to Alloy 617 over time at a given temperature was critical.

Any measurements had to be done on different batches of Alloy 617 to account for slight variations in composition and manufacturing. Some of the tests were quick, like measuring how much stress the material could take before it breaks. However some of the tests, like those involving creep (the tendency of a substance to change shape over time), take years.

"These time dependent properties get to be really tricky to measure and understand," Wright said. ASME permits a threefold extrapolation factor for time.

After gathering the data on Alloy 617, the researchers came up with conservative figures to go into the ASME specification. They then submitted this proposal for balloting, starting the next phase of the process to get the material into the Code. Volunteers from industry, national laboratories and elsewhere serve on the various ASME working groups, subgroups and committees, which meet to consider changes to the standards four times a year.

"We started moving it through the balloting process," Wright said. "It took three full years." The final approval came in late-2019.

Now that Alloy 617 has been included in the ASME Code, designers of high-temperature nuclear plants have a new material that offers an expanded operating range. The previously allowed high temperature materials could not be used above about 750°C, according to Wright.

"Our newly qualified material can be used in design and construction up to 950°C. As a result, it could enable new higher temperature concepts." he said.

Researched and written by World Nuclear News