Nuclear mattersMetal mist will not choke off controlled nuclear fusion
Fusion reactors — experimental reactors called tokamaks — are doughnut-shaped devices that contain ionized gas, or plasma, at temperatures of more than 100 million degrees; it was feared that fine metallic dust generated inside the containment vessel would choke off the controlled nuclear fusion; scientists now find this is not the case
Toxic metal dust, which physicists feared could choke their efforts to attain controlled nuclear fusion, may instead be disintegrated by the powerful forces inside fusion reactors.
Experimental reactors called tokamaks are doughnut-shaped devices that contain ionized gas, or plasma, at temperatures of more than 100 million degrees. The latest is a multibillion-dollar project called ITER, set to be completed in southern France in 2019. It is designed to fuse together two heavy isotopes of hydrogen to release vast quantities of energy while producing no carbon emissions.
Stephen Battersby writes that doubts have lingered over whether the fusion reactions could poison themselves. That’s because the plasma bites little pieces out of the beryllium metal walls of a tokamak’s reactor vessel, generating fine metallic dust.
The toxic stuff could “clog up holes in the instruments inside ITER”, says Michael Coppins of Imperial College, London. If it gets into the inner core of hot plasma, it could even choke off the fusion reaction.
Coppins realized that this is not like ordinary industrial or household dust. The dust grains swept up in existing tokamaks are often spherical, implying that they had been molten at some stage. “It occurred to me that the dust in this plasma is not solid particles but liquid droplets,” he told New Scientist.
Battersby writes that liquid drops should be much less of a problem than solid dust. When one of them reaches the outer layers of the plasma ring, electrons in the plasma will bombard it and build up a strong negative charge. This will produce repulsive forces within the droplet. “If it is of sufficiently small size, these forces will lead to total disintegration very swiftly,” says Coppins.
Given the rapid rate of electron bombardment within ITER’s energetic plasma, he calculates that all but the largest beryllium drops will disintegrate in less than a nanosecond. That means they will be destroyed in the plasma’s outermost layers before they can reach and pollute the fusion reactions deeper inside.
Some relatively large droplets much bigger than a micrometer across may be able to reach the inner core, but with luck there will be too few of them to pollute the plasma fatally.
-read more in M. Coppins, “Electrostatic Breakup in a Misty Plasma,” Physical Review Letters 104 (2010) (DOI: 10.1103/physrevlett.104.065003) sub. req.)
