A team at Idaho National Laboratory (INL) in the USA in collaboration with groups at Argonne and Oak Ridge national laboratories, as well as industry consultants and international partners, has for the first time in 30 years had a new material, Alloy 617, accepted into the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
INL has nine electromechanical load frames, four servo-electric frames, 11 servo-hydraulic frames, 21 different creep frames and two custom-built stress-relaxation systems for testing new materials like Alloy 617.
To help engineers and others pick the right material, ASME publishes its Code, which 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.
Alloy 617 is a combination of nickel, chromium, cobalt and molybdenum, which can be used in advanced nuclear plants because it allows higher temperature operation. “It’s a pretty substantial accomplishment,” said Richard Wright, an INL laboratory fellow emeritus, who headed the INL part and overall management of the project.
Designers working on new high-temperature nuclear power plant concepts now have 20% more options when it comes to component construction materials. “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,” Wright said.
Alloy 617 took a long time to make its way into the standard because of physics, and the way materials are added to standards. Each aspect took years to complete.
Regarding the physics, Wright said the issue related to creep – the tendency of a substance to change shape over time. Creep only becomes an issue starting at about half the melting point of a material. But at higher temperatures, creep will be a factor in new proposed reactors. Unlike light water reactors that operate at around 290 degrees Celsius (about 540 degrees Farenheit), 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 overtime at a given temperature was critical. However, it was not an easy task.
“These time-dependent properties get to be really tricky to measure and understand,” Wright said. 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, such as measuring how much stress the material could take before it breaks. But some, such as those involving creep, took years.
ASME permits a threefold extrapolation factor for time. In other words, to qualify a material for 100,000 hours, or 11.4 years of operation, researchers must gather data for 33,000 hours – or 3.75 years. That mark is about the minimum for which a new material must be qualified, as power plants are designed to run for decades.
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, INL said.
At ASME, there has to be unanimous agreement on changes to standards. So, when changes are introduced, proponents of the change provide supporting data, answer questions posed about the data, educate committee members if needed, and otherwise try to move the process along.
The researchers working on the Alloy 617 Code inclusion started with the high-temperature working group, then the appropriate subgroup followed finally by achieving consensus from the full committee. Volunteers from industry, national laboratories and elsewhere serve on the various working groups, subgroups and committees, which meet to consider changes to the standards four times a year.
The process for Alloy 607 took three full years, Wright said. The final approval came in autumn 2019.
The last new high-temperature material had been added to the Code in the 1990s, and that partially explains why getting approval took so long, he said. “A big challenge for us was simply reinvigorating this process that hadn’t been used in 30 years.” The entire Alloy 617 project took 12 years to complete at a cost to the Department of Energy of $15 million. Adding prior work and non-DOE contributions, brought the total material research expenses into the tens of millions of dollars and decades.
The previously allowed high-temperature materials could not be used above about 750 degrees Celsius (around 1380 degrees Farenheit). “Our newly qualified material can be used in design and construction up to 950 degrees Celsius. As a result, it could enable new higher temperature concepts,” Wright said.
Photo: INL has nine electromechanical load frames, four servo-electric frames, 11 servo-hydraulic frames, 21 different creep frames and two custom-built stress-relaxation systems for testing new materials like Alloy 617.