For many materials, irradiation causes damages or accelerates structural degradation. Hence, nuclear reactors and other types of equipment that operate in a highly radioactive environment are required to have their key components replaced regularly. However, a team of researchers from the Massachusetts Institute of Technology (MIT) and the Lawrence Berkeley National Laboratory (LBNL) in the US has discovered that exposure to radiation can actually benefit some alloys that can be used in a nuclear reactor. Instead of hastening degradation, radiation has the effect of increasing (if not doubling) the operational lifespan of these alloys by enhancing their resistance. The team’s findings are detailed in a paper that has just been published in the journal Nature Communications.
Michael Short, an MIT professor of nuclear science and engineering and one of the authors of the paper, said that the discovery was made purely by chance as his team was conducting an experiment to determine the relationship between the level of radiation and the rate of corrosion in several alloys. The materials that were being experimented on include a nickel-chromium alloy. They were all potential candidates for the outer layer (or cladding) of fuel rods that comprise the core of a nuclear reactor. However, the result of laboratory testing showed the opposite effect.
Material corrosion, which is induced by the radioactive environment, is often cited as one of the main reasons behind the failure of a nuclear reactor. However, the MIT-LBNL team immediately observed a contradictory result in its experiment. The team originally expected that irradiation would hasten corrosion in the selected alloys. Instead, the rate of corrosion appeared to slow down noticeably.
Short said that his team conducted the irradiation simulation dozens of times and under different conditions, but the outcome was the same. Bathing the alloys in radiation helped reduce the rate of corrosion.
In the simulated reactor environment created by the MIT-LBNL team, the coolant was a molten salt mixture that included sodium, lithium, and potassium. This type of coolant could be used with fission reactors of the next generation as well as a future fusion reactor that would have a vacuum vessel to contain a churning plasma. The general understanding is that most metals will suffer rapid corrosion when they come into contact with salts, especially those that are melted at high temperatures. However, this is not the case with the nickel-chromium alloy. Under the simulated reactor environment, the corrosion took twice as long to occur for this material.
In the experiment, the nickel-chromium alloy was in contact with the molten salt that had a temperature of 650 degrees Celsius and bombard with proton radiation at the same time. Afterward, the surface of the material was examined using transmission electron microscopy. The MIT-LBNL team surprisingly discovered the following: Irradiation creates tiny surface defects that allow metal atoms to spread out more easily and fill in the larger cavities caused by the corrosive effect of the molten salt. In other words, irradiation drives a self-healing mechanism within the alloy.
Short pointed out that this effect was observed 50 years ago in the operational trials of the experimental salt-cooled fission reactors. At that time, scientists also found that the corrosion problem for this type of nuclear reactor was not as severe as they had anticipated. However, they did not understand why this was so.
The experiment, along with the follow-up study, has helped make sense of this unique process. This discovery, in turn, can now be applied to the design of new nuclear reactors. The findings of the study can also lead to methods that more accurately measure the operational lifespan of the key components for a nuclear reactor. The timely replacement of components, together with the development of more durable materials that reduce the frequency of maintenance, will make nuclear reactors safer and more efficient in the future.
Additionally, the MIT-LBNL team believes that this new revelation will be useful in many other fields aside from nuclear power.
(News source: TechNews. Image credit: the Massachusetts Institute of Technology.)