A new material for high-temperature nuclear reactors
In depth

A new material for high-temperature nuclear reactors

Some nuclear reactors function at very high temperatures, and for this reason they need to be built with specific heat-resistant materials. The current choice of possible materials is quite small for these reactors, which limits possibilities. There is always ongoing research to look for more options and extend their limits regarding temperature.

The Boiler and Pressure Vessel Code

The Boiler and Pressure Vessel Code (BPVC) is a standard that regulates the design and construction of boilers and pressure vessels. It specifies the materials that can be used for construction of nuclear power plants with high-temperature reactors. Adhering to these specifications ensures component safety and performance.

Alloy 617

Alloy 617 –a combination of nickel, chromium, cobalt and molybdenum–  was tested at the Idaho National Laboratory (INL) in the U.S. and has been approved by the American Society of Mechanical Engineers (ASME). This means that the alloy can be used in molten salt, high-temperature, gas-cooled or sodium reactors. It is the first new material added to the BPVC in 30 years.

Alloy 617 can be used in molten salt, high-temperature, gas-cooled and sodium reactors

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

Years of work to qualify the new material

INL spent 12 years qualifying Alloy 617, with a USD15 million investment from the US Department of Energy (DOE). A team at INL, in collaboration with other national laboratories as well as  industry consultants and international partners, received approval from ASME for the new 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.

Designers working on new high-temperature nuclear power plant concepts now have more options when it comes to component construction materials.

According to INL, there were only five materials available for high-temperature reactors, whereas light water nuclear plants can choose between 50-100 materials. Unlike light water reactors, which operate at around 290°C, molten salt, high-temperature, gas-cooled or sodium reactors need double or more temperature. This means that determining what happens to Alloy 617 over time at a given temperature was critical.

Many different batches of Alloy 617 were tested 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.

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 tests to qualify alloy 617 lasted several years

According to INL, the previously allowed high temperature materials could not be used above about 750°C and the new qualified material can be used in design and construction up to 950°C. This could enable higher temperature concepts.

Source: World Nuclear News