A team of researchers at the Massachusetts Institute of Technology (MIT) (Cambridge, Massachusetts, USA) and the Lawrence Berkeley National Laboratory (Berkeley, California, USA) have found that exposure to radiation can improve the resistance of certain alloys that could be used in fission or fusion reactors.1
According to the researchers, radiation usually degrades the materials exposed to it. In turn, this hastens their deterioration and can require the replacement of key components in high-radiation environments, such as nuclear reactors. But for certain alloys, the opposite turns out to be true: they have found that instead of hastening the material’s degradation, radiation actually improves its resistance, potentially doubling its useful lifespan.
They believe the findings could be a boon for some new, cutting-edge reactor designs, including molten-salt-cooled fission reactors, as well as new fusion reactors.
Experimental Phase
Michael Short, a professor of nuclear science and engineering at MIT, says the result was a bit of serendipity. Initially, the researchers were looking to quantify the opposite effect by determining how much radiation would increase the rate of corrosion in certain alloys of nickel and chromium, which can be used as cladding for nuclear fuel assemblies.
The experiments were difficult to carry out, because it is impossible to measure temperatures directly at the interface between the molten salt, used as a coolant, and the surface of a meal alloy. As a result, the research team had to figure out the conditions indirectly by surrounding the material with a battery of sensors.
Their tests, however, showed signs of the opposite effect. Corrosion, described by the researchers as the main cause of materials failure in the harsh environment of a reactor vessel, seemed to be reduced, rather than accelerated, when it was bathed in radiation. In this case, the radiation source was a high flux of protons. “We repeated it dozens of times, with different conditions,” Short says. “Every time, we got the same results.”
In their experiments, the simulated reactor environment involved the use of molten sodium, lithium, and potassium salt as a coolant for both the nuclear fuel rods in a fission reactor, as well as the vacuum vessel surrounding a hot, swirling plasma in a future fusion reactor. Typically, corrosion can take place rapidly in areas where hot molten salts are in contact with the metal. But with these nickel-chromium alloys, the researchers found that the corrosion took twice as long to develop when the material was bathed in radiation from a proton accelerator, which produced radiation levels similar to what would be found in the proposed reactors.
According to MIT’s Short, being able to more accurately predict the lifespan of reactor components could reduce the need for preemptive, early replacement of parts.
Research Findings
In their research, the team initially irradiated the metal in contact with molten salt at 650 °C, which is a typical operating temperature for salt in such reactors. From there, using transmission electron microscopy to examine the affected alloy surfaces, they believe they learned the mechanism causing the unexpected effect.
According to the researchers, the radiation tends to create tiny defects in the structure of the alloy, and these defects allow atoms of the metal to diffuse more easily. From there, they flow in to quickly fill any voids created by the corrosive salt. In effect, the radiation damage promotes a sort of self-healing mechanism within the metal.
There had been hints of such an effect a half-century ago, Short says, when experiments with an early experimental salt-cooled fission reactor showed lower-than-expected corrosion in its materials. However, the reasons for that remained a mystery until this new work. “It took us a lot longer to make sense of it,” Short adds, even after his team’s initial experimental findings.
Future Implications
The discovery could be relevant for a wide variety of proposed new designs for reactors, which could be safer and more efficient than existing designs, Short contends. Several designs for salt-cooled fission reactors have been proposed, including one by a team led by Charles Forsberg, a principal research scientist in the nuclear science and engineering department at MIT.
The team’s findings could also be useful for several proposed designs for new kinds of fusion reactors being actively pursued by startup companies, which hold the potential for providing electricity with no greenhouse gas emissions and far less radioactive waste. “It’s not particular to any one design,” Short says. “It helps everybody.”
The research work was supported by the nuclear power company Transatomic Power Corp. (Cambridge, Massachusetts, USA), as well as the U.S. Department of Energy (Washington, DC, USA). Further information is available at MIT’s website and in the peer-reviewed scientific journal Nature Communications.
Source: MIT, news.mit.edu.
Reference
1 “To Engineers’ Surprise, Radiation Can Slow Corrosion of Some Materials,” MIT News, July 9, 2020, http://news.mit.edu/2020/radiation-slow-corrosion-materials-0709 (Sept. 16, 2020).