Most of my posts on this blog about nuclear power reactors deal with fission reactors. Occasionally, I switch over to nuclear fusion reactors. Fusion power reactors don't exist yet but billions and billions of dollars over decades have been spent in the quest for fusion power. If fusion power can be achieved, it should have many important advantages over nuclear fission for power generation. There are major projects like the ITER research reactor being built in France to test theories about how to achieve controlled nuclear fusion. On the other hand, there are at least half a dozen projects in the U.S. to create small fusion reactors that would be cheaper than nuclear fission reactors.
Generally, fusion reactors are designed to compress and heat an ionized gas called a plasma until lighter atoms fuse into heavier atoms and a great deal of energy is released. The plasma is trapped in a vessel and manipulated by intense magnetic fields to prevent it from touching the sides of the vessel. None the less turbulence can cause the plasma to do just that. In order to deal with this problem, plasma approaching the sides of the vessel is directed to what is called a diverter. The diverter usually a solid material like a block of carbon or tungsten that is cooled by water to dissipate the heat in the plasma that touches it. However, when the plasma hits the diverter, it causes damage. This requires frequent maintenance or replacement. The need for frequent maintenance means that the reactor cannot function reliably for long periods of time without needing attention.
In order to deal with the higher temperatures that will be present in a commercial nuclear fusion power reactor, the problem of diverter damage and maintenance must be solved. It has been proposed for decades that it might be possible to use some sort of liquid metals such as lithium, or tin to create a system to function as a diverter. With sufficient velocity of flow of the liquid metal, the heat from the incident plasma could be carried off without causing damage. The problem with this solution is that when the plasma strikes the diverter, it is converted to a neutral gas. This gas must be quickly evacuated from the reactor so that it won't dilute the plasma. Up to this point, there has been no liquid metal diverter design that could maintain high flow rate while exhausting neutral gas.
Scientists at the National Institute for Fusion Science in Japan have now proposed a design for a shower of liquid tin that would flow down just inside sections of the vessel to function as a diverter. Tin is a good choice because it has a low vapor pressure, is cheap and safe for such use. Their design will remove neutral gas before it reaches the wall of the vessel. This new design should be able to dissipate ten times the amount of heat that is now possible to cool with solid diverters being used in research fusion reactors. The scientists believe that their approach to heat diversion will be a practical solution to the problem of disruptive plasma flow in future fusion reactors.
