Study points to need for standardization in helium generation rate calculations
Researchers from the University of Michigan and Oak Ridge National Laboratory have found that predictions of helium generation rates through neutron transmutation in fission and fusion reactors vary widely and include dependencies on the choice of nuclear data library and calculation method.
“Helium production has a strong influence on high temperature embrittlement and dimensional stability for structural materials in both fusion and fission systems. As industries move to assess and deploy their technologies, having a standard method of analysis with a sense of uncertainty is needed for comparisons across materials systems and environments,” said Stephen Taller, an Alvin M. Weinberg Fellow and R&D staff scientist at ORNL.
As described in a paper published in Journal of Physics: Energy, the team ran FISPACT-II, a multiphysics software for advanced nuclear simulation, to calculate how helium generation rates varied with neutron flux spectrum and the alloy composition of materials being irradiated.
Neutron energy impacted helium generation rates, as expected, and the researchers found that the presence of certain elements—such as carbon, nitrogen, and nickel—could significantly impact helium production predictions.
According to an article published by the University of Michigan’s Department of Engineering, “This finding has implications for the industry, particularly where and how they get their alloys to build fusion and fission energy systems as different metal producers have different composition control.”
The researchers also tested two variables that they weren’t expecting to significantly change the predictions, with surprising results.
The first was the choice of a nuclear data library to provide information such as interaction cross sections. The team found “differences often as high as 231 percent, while sometimes as extreme as 859 percent.”
The second was the choice of helium generation calculation method within the simulation, such as whether rates were calculated at each time step or across time periods. This also was found to cause significant variance.
“We were really surprised by how much the nuclear data library and calculation method impacted the final answer. It’s so significant that it’s like forgetting to add salt in a savory dish, resulting in a sweet final dish,” said Kevin Field, a professor of nuclear engineering and radiological sciences at University of Michigan.
They found that using a calculation method that adds up all helium rates and damage rates over the time period and then finds the ratio provided the most reliable results achieves a consistent calculation across different neutron energies, compared with other methods, which tended to either underestimate helium or miss transmutation rate changes over time.
The researchers propose standardizing how the field performs these calculations, suggesting that best practices should include “reporting the calculated range of expected helium production rather than a single point value, specifying the unique heat designation and trace elements of the alloy and documenting both the reactor’s raw and digitally upscaled neutron spectra.”
“If used, our reporting methods will improve the experimental and modeling fidelity of the nuclear materials databases being generated both domestically and internationally, driving the rapid deployment of advanced nuclear,” said Field.
