Scientists discover how tungsten fails in nuclear fusion reactors

Researchers in the Electron Microscopy and Materials Analysis Research Group have become the first to simulate the process that nuclear irradiation has on tungsten in a nuclear reactor.  The work, led by Dr Rob Harrison (pictured), will help in the quest to find the best materials to use in the construction of reactors.

UNIVERSITY of Huddersfield scientists have become the first in the world to discover the key to one of the most puzzling phenomena in nuclear materials science – how, when tungsten transmutes into rhenium, it forms clusters in nuclear reactors, later becoming hard and brittle and prone to failure. 

The breakthrough will help in the quest to find the best materials to use in the construction of fusion reactors.

For over a decade, scientists in research institutes around the world have attempted to replicate the deterioration effect of rhenium and simulating the process that nuclear irradiation has on tungsten in a nuclear reactor.  Now, the Electron Microscopy and Materials Analysis Research Group (EMMA), at the University of Huddersfield, have succeeded using their unique group of instruments combining ion irradiation with transmission electron microscopy.

It was long thought that because of its high melting point, tungsten would be an ideal material for use in fusion reactors, but the anomaly of the change has baffled scientists and engineers for many years.

“Some tungsten atoms will transmute into rhenium by absorbing neutrons followed by nuclear decay reactions changing the atomic structure of the nucleus,” said one of the EMMA team, Dr Rob Harrison.

As rhenium is of a similar size and mass to tungsten, it was considered that the rhenium produced would not be a problem for the material.

However, when under neutron irradiation in nuclear reactors, the rhenium atoms aggregate into clusters and precipitate in the tungsten crystal structure to form new crystals with differing structure, similar to how a solid precipitates in a liquid.

As Dr Harrison explains, a phenomenon that has confounded many then followed: “The formation of these precipitates has drastic effects on the material’s mechanical properties.  As the new rhenium clusters have a different crystalline structure, they cause the material to become harder and more brittle, making it more prone to failure.

The Huddersfield team now feel that they might be able to come up with a way to negate the problem and engineer materials that will adapt better.

“We played with the type of ion that we were irradiating with,” said Dr Harrison.  “The other studies were using really heavy ions and that causes lots of damage inside the material – mixing up the atoms inside the material like a soup – and the rhenium atoms won’t have had chance to coalesce.  They will just constantly be getting knocked around.

“But we used a light ion, which causes a different kind of damage – sporadically through the material.  We think that gives the rhenium atoms the opportunity to coalesce and grow.”

“By arriving at a fundamental understanding of how this precipitation happens, we may be able to engineer materials that negate it,” said Dr Harrison.

The facilities available to the EMMA researchers include the Microscope and Ion Accelerator for Materials Investigation (MIAMI) facilities.  Developed with an award of £3.5 million from the Engineering and Physical Sciences Research Council, it has dual ion beams and is one of the world’s leading facilities of its kind.

• The work is explained in an article, entitled Intermetallic Re phases formed in ion irradiated WRe alloy, which appears in the Journal of Nuclear Materials, published by Elsevier.