Steel, the main structural material used in nuclear reactor vessels, can get damaged from radiation exposure. Understanding how this radiation damage occurs is important for the design of reliable, safe nuclear fission and fusion reactors of the future. But the exact mechanisms for how various types of radiation-induced defects and defect structures arise in these steels have remained elusive, though this has been under study for decades.

A team of engineers has now accurately revealed how shockwaves form a particular defect, interstitial ferritic loops, in body-centered cubic (bcc) iron, the main component of a particular class of steels. The results will allow researchers to tune the microstructure of these steels, leading to higher toughness and better radiation-resistance.

When steel is irradiated with energetic particles, the burst of energy displaces tens of thousands of atoms in the material’s crystal lattice, boosting local temperature to thousands of degrees Kelvin. The atoms fall back in place when the material cools down, usually within picoseconds. But some atoms get displaced and leave behind vacancies which can cluster to form larger voids that lead to swelling, and to atoms arranged in a circle-like arrangement that causes brittleness. Experiments have shown that such loop-like defects form in steel especially after radiation exposure. But the exact mechanism behind their formation has continued to be debated.

“We need to know how to reduce radiation-induced swelling,” says Qing Peng, a researcher in the laboratory of nuclear engineering, and Fei Gao, a professor of radiological sciences, at the University of Michigan. “So we need to understand how loops are generated and how they move.”

Previously, researchers have proposed five different ideas to explain how these loops are generated. But none of them offered a complete, perfect picture. All the mechanisms proposed were thermally driven and show that it takes a relatively long time of over a nanosecond to generate loops, Peng says. They also assume some kind of defect to already pre-exist in the crystal. “Our simulation shows a totally new mechanism,” Peng says. “It’s the first to show the whole dynamics where you start with a perfect crystal, then you form the defects.”

Part of the problem with previous simulations is that they use a small set of atoms and low energies. Peng, Gao, and their colleagues from Michigan, Hunan University, and Rensselaer Polytechnic Institute have created a computer model of a lattice of 220 million iron atoms. The simulation shows that when a high-energy particle slams into the lattice, a powerful shockwave tears through it. Millions of iron atoms get displaced and millions fall back into place as the wave dissipates. But the simulation also shows hundreds of atoms left out of place. It also shows that a handful of loops are created in the initial shockwave in just around 13 picoseconds.

The analysis also shows that the formation of the interstitial loops depends on the redistribution of the kinetic energy imparted by the shockwave. This indicates that higher incidence energy or larger atom mass would result in more nucleation of these interstitial dislocation loops.

This paper addresses a well-known problem, says Michael Jenkins, Emeritus Fellow in materials at Oxford University. What is new here is “the very large scale of the molecular dynamics simulations, which allow large cascades to be simulated to longer times and so allow possible new loop formation mechanisms within the cascade to be identified.”

The new mechanism also provides insights for engineering microstructures in steels via ion implantation, high-energy particle bombardment, or shockwave compression.

Video note: The video in this piece is a rendering of an atomistic simulation that captures the formation of a certain type of loop defect (seen as pink loops around pink spots in the animation) that appears in steel after exposure to radiation. Purple spots are empty places left behind by an atom, and pink spots are atoms that are in between other atoms in the lattice. The green loop defects can travel through the crystal, and their directions and speeds are denoted with red arrows.

Read the article in Nature Communications.