Kai Nordlund applies computational science to studying materials irradiation at the Division of Materials Physics at the University of Helsinki. At the Accelerator Laboratory, they have equipment to increase the kinetic energy of an individual atom roughly 10⁹ times from its normal state. With this capability it is possible to investigate how materials behave.

“We want to know what happens when fast-moving atoms collide with target material. The same method is used especially in the semiconductor industry,” Nordlund explains. In order to be able to study the tolerance of materials to radiation, we need to understand what happens at the atomic level. No radiation damage occurs unless atoms are accelerated beyond a certain energy level, approximately 10 electron volts (eV). When this limit is exceeded, radiation damage grows with increasing energy.

“Radiation damage arises in the nuclear reactors of today, and will arise in the fusion power plants of the future. It is important to understand, what radiation damage really is and how it changes the properties of the material.”

Nordlund’s research group is clearly at the global forefront in computer simulation of radiation damage. The group, comprising about 20 researchers, is constantly developing new methods and models for use by themselves and other scientists. Nordlund estimates that there are roughly 1000 researchers who perform basic research on computer simulations of radiation damage, and maybe a couple of thousand more in applied science.

But why do they study radiation damage of materials? Clearly, the most important area of application is the computer chip. When computer chips are manufactured, their electronic properties are modified by means of impurity atoms.

“The function of all modern computer chips is based on impurity atoms, which are implanted inside the chip. We use a particle accelerator to shoot impurity atoms (dopants) into the material, and these atoms provide the chip components with desired electrical properties. A doped chip is more conductive than, for example, pure silicon,” says Nordlund.

We can also create optoelectronic materials, i.e. light-emitting materials, and irradiate them to speed up emission. Optoelectronics is used in, for instance, telecommunications, where speed is essential. Radiation damage can essentially increase the pulse frequency of specific pulsed lasers, roughly by a factor of 10–100.

Fusion energy is a challenge

On the other side of the coin there are adverse effects of radiation damage. For example, in a nuclear reactor the damage is almost always detrimental, because irradiation makes a metal brittle and finally breaks it.

“Naturally, this is undesirable in a nuclear reactor,” says Nordlund. “It is very important to understand the nature of the damage and how it develops with time, so as to be able to avoid it. Modern nuclear reactors behave quite reliably, but when new types of fusion and fission reactors are being designed we need to know, which materials can be used to guarantee that the materials of the walls can sufficiently withstand the stream of particles. This is a critical problem especially in fusion reactors.”

A fusion reaction occurs when light atomic nuclei are fused together with the release of large amounts of energy; it is fusion reactions that serve as the energy source for the sun and other stars. Man has tried to harness fusion for energy production as a safe, efficient and environmentally friendly source, yet so far without success. It is difficult to create a fusion reactor, because a fusion reaction needs either a very high temperature or a high pressure.

“It is possible that fusion energy will never successfully work the way we want, because the radiation damage is too great. And even if it were to work in principle, components would last such a short time that from an economic point of view it would not be reasonable to use them,” says Nordlund.

But how do materials withstand extreme conditions? If we want to increase the power of a fusion or fission reactor, we are forced to use higher temperatures. The problem is that most materials cannot withstand extreme heat. Steel is normally very hard and durable material, but once it is heated above a certain temperature, from 300 to 600–700 ^◦C depending on the steel, it begins to soften extremely radically and becomes useless. This happened when the World Trade Center towers fell down in 2001: hot, burning fuel heated the steel and made it soft, and the towers came crashing down.

Plasma is an extreme condition

Plasma is a state of matter in which the atoms have lost some electrons, i.e. become ionized. A fusion reactor functions with plasma heated to extreme temperatures as high as 100 million ^◦C, which is hotter than the core of the sun. In spite of the heat, the fusion reactor wall construction should remain only at a few hundred ^◦C.

“Both radiation and heating occur inside the fusion reactor: radiation is continuous, but must be kept as low as possible. Heating should not occur, but possible disruption in fusion plasma might cause heat leakage to become in contact with the wall and start melting it,” Nordlund explains.

Special materials that are heat-resistant have long been under development. For example, graphite heat tiles in a space shuttle are heat-resistant and conduct heat extremely well. Graphite is a mineral that is a pure form of carbon. In a space shuttle these heat tiles work, but in a fusion reactor they do not work well enough.

It was shown by empirical testing as far back as 30 years ago that graphite- and carbon-based tiles cannot endure the heat in a fusion reactor.

“At first, nobody could understand why. Based on simple mathematical theory, the tiles should have endured radiation, but experiments continuously showed otherwise. Ten years ago we succeeded in explaining why: the breakdown of carbon-based materials at the lower part of the fusion reactor is unavoidable and it is impossible to stop carbon's erosion completely. Until today, carbon has been used in the reactor due to its high melting point and light mass, but we should abandon carbon altogether and replace it by another material, for example, by tungsten.”

Sparked by research on sparks

“At the moment one of the biggest scientific challenges for us is to understand sparking and electric arcs. Everyone knows that sparking occurs, but the phenomenon itself is poorly understood, especially if it occurs in a vacuum,” says Nordlund.

A common example of electric sparks is found in car ignition plugs, where sparks are deliberately produced to ignite the fuel. The best known example of an electric arc is lightning, and one of the controlled forms is the welding flame. Both of theses can be explained by almost the same physical phenomenon: plasma is created between two conducting materials, and as a result, the electric field is allowed to discharge.

“What is still a great mystery is how to predict the starting of sparking. We know which computational tools are needed to solve the problem. However, simulations are challenging and demanding, because there are numerous uncertainty factors. Hopefully within five to ten years, we will know much more about sparking and our knowledge will enhance developing spark-resistant materials.”

Atoms create craters, too

“One intriguing topic of research is to understand the formation of impact craters. We all know that craters are formed on the surfaces of moons and planets. They also form when projectiles are shot at, for example, clay. If a single atom with a suitable amount of energy hits heavy metal, for example, a gold atom hitting on a gold surface, a crater is formed,” explains Nordlund.

Image: Professor Kai Nordlund at the Accelerator Laboratory of the University of Helsinki. Nordlund is interested in what happens in materials when they are exposed to radiation. © Tiina Raivo.

Tiina Raivo