Steel to capture the sun

Temperatures of over hundred million degrees centigrade and high energy neutrons and alpha particles that blast everything to shreds. What materials can withstand the harsh conditions in fusion reactors? TU Delft researcher Inês Carvalho set out to discover.

The doughnut-shaped vacuum vessel used to contain the plasma. (Photo: ITER)
The doughnut-shaped vacuum vessel used to contain the plasma. (Photo: ITER)

With dozens of images with interlocking mosaics, swirls, and veins, the thesis of Ines Carvalho looks at first glance to be about marble. The captions give away that these are in fact electron microscopy images of steel; of Eurofer97. This is a tough material designed about twenty years ago to withstand the environment inside a nuclear fusion reactor.

If an electricity producing fusion reactor is to become reality one day, much more work is needed on Eurofer97 and other materials such as tungsten, that are candidate materials to make the intestines of this impressive apparatus that mimics the stars. Fusion is the process that powers active stars, like our sun. Solar energy is based on the fusion of deuterium and tritium. This reaction leads to the formation of neutrons and helium nuclei, and the release of 17.6 MeV of energy per reaction.

The problem is that even these materials, designed for the purpose, are not indestructible. They do suffer from massive radiation. How much so was the topic of Carvalho's thesis, which she will defend later this month.

Besides iron, Eurofer97 consists of an amalgam of elements, amongst which are chromium and nickel, which give the steel superior properties. Strong as it may be, the metal suffers, witness the tiny black dots that we see in the electron microscope, disfiguring the steel like cavities. "These are crystal defects," said Carvalho. "Iron atoms are being struck away by incoming alpha particles and neutrons at very high velocities."

In designs for fusion reactors, such as the ITER (International Thermonuclear Experimental Reactor) which is being built at the moment in the south of France, the plasma – the heated deuterium and tritium - will be kept away from the reactor walls by means of magnetic fields. "The preferred design is a doughnut-shaped vacuum vessel used to contain the plasma with magnetic fields, making the plasma particles run in spirals without touching the walls," said Carvalho.

The plasma can reach temperatures of over 100 million degrees centigrade. But thanks to the magnets, dealing with the heat is not the biggest challenge. The irradiation is. Long term irradiation causes Eurofer97 to become brittle. Other materials used for reactors suffer similarly.

Fifty times the irradiation of a fission reactor

"The physics underlying defect formation in fusion reactors is also acting in existing fission reactors," said Carvalho's supervisor, materials scientist Dr. Jilt Sietsma. "What you have to realise is that the material irradiation in a fusion reactor will be up to 50 times stronger than that in existing fission reactors. For clean, safe and sustainable energy from fusion reactors to become a success, these issues of deteriorating materials need to be solved."

Carvalho studied the process of the deteriorating metal alloy at the Nuclear Research and Consultancy Group (NRG) with samples irradiated in the nuclear reactor in Petten (The Netherlands) for two years. The High Flux Reactor in Petten has a radiation source that is much stronger than the source of the Reactor Institute Delft (RID). It would have taken decades to create similar samples in Delft.

With the electron microscope one can see defects of the size of at least some tens of metal atoms, resulting in the little pit holes on the pictures in the thesis. To discover defects of only one or several atoms, Carvalho used a different technique. At the Reactor Institute she bombarded the steel with positrons. Positrons are the antiparticles of electrons. When these two hit each other, they annihilate, sending out two gamma rays. If there are many cavities in the steel, it will take the positrons and electrons a little longer to bump into each other. By observing the annihilation pattern one thus gets insight into the condition of the steel.

The research of Carvalho continues in slightly altered form. In collaboration with the Dutch Institute for Fundamental Energy Research in Eindhoven (DIFFER), Sietsma and positron-annihilation expert Dr. Henk Schut of RID, will look into the deterioration of Eurofer ODS, an improved Eurofer97-version that has oxide particles added to it. Thus continues the quest for a steel that can capture the sun.