Nuclear Fusion Reactors

Nuclear Fusion
Source: Max-Planck Institut für Plasmaphysik

I’ve spoken about nuclear fusion before, in the context of nuclear weapons. Today, I’m going to talk about the more peaceful application of nuclear fusion: the nuclear fusion reactor.

Fusion power

Us humans have been dabbling with combustion engines for a few centuries now, but that is just child’s play. Every day the universe shows us a better alternative for generating power: the sun.

Every star in the universe is a giant nuclear fusion reactor that slowly fuses through elements, starting with hydrogen and working its way up to who knows which exotic elements. Whenever atoms are fused, part of their weight is converted to energy (until you get to the radioactive ones anyway). The amount of energy is not that big per atom, but for a large enough reaction mass, the result is far more spectacular.

If we could harness the power of nuclear fusion, we’d solve all of our energy problems in no time. For half a century or so, a lot of scientists have been investigating exactly that.

Of course, we already have working nuclear fission reactors, but those produce far more radioactive waste and requires uranium.

A fusion reactor

Nuclear fusion requires atoms to fuse, which isn’t easy. You see, atoms naturally repulse each other. When a star is formed, a large cloud of hydrogen falls in on itself until the density is high enough to overcome this repulsion and start nuclear fusion. The resulting heat starts a chain reaction of nuclear fusion which ignites a new star.

Reproducing that kind of atomic violence in a reactor is not that easy. A reactor needs to overcome the atomic repulsion, and for enough atoms to produce more energy than goes into it. Oh, and on an economically viable scale as well. Let’s look at some of the ways this might work.

The most promising way nuclear fusion can be achieved in a reactor, is by heating a plasma of deuterium/tritium (two isotopes of hydrogen) atoms to temperatures high enough to achieve fusion. No material exists that can withstand these kinds of temperature, though, so containment is a problem.

Tokamak reactors

The basic idea of many fusion reactors is to use magnetic fields to contain the high energy deuterium/tritium plasma. One way to do this would be to use a tubular coil and put the plasma in there. Unfortunately, the plasma will flow out of the end of the tube. By bending the tube into a torus (a.k.a. a donut shape), this problem would be overcome. That is what a Tokamak reactor does.

Unfortunately, bending a coil into a torus changes the magnetic field, which causes particles of your plasma to drift to the sides. Tokamaks use a slightly altered coil to prevent this: the coil is slightly bent like it would be on a barber’s pole.

This does not solve all problems. Eventually, the mixture inside the torus will still start to drift, meaning you have to end and then restart your fusion reaction periodically.

Stellarator reactors

A stellarator fusion reactor is similar to a tokamak in that it uses magnetic containment. The magnetic fields are different, though. The image above this post shows the configuration of the fields in a stellarator.

The idea originated from the same donut shape. Instead of the tokamak approach, though, the first idea was to use a figure eight design. The figure eight did not quite work either, and the above alternative was designed using computer aided design.

Supposedly, the stellarator design will reduce the need for restarting the nuclear fusion reaction. Of course, all this is partially theoretical at this point.

Practical fusion

In theory, tokamak and stellarator reactors are possible. In practice, though, we haven’t built one that’s actually a viable power plant. A stellarator in Germany is looking good, but that’s about halfway between the drawing table and actual working power plant.

A working nuclear fusion reactor is still a few decades away, at least. Even if we get that far, though, the question remains if this is a viable way to produce power. Contrary to nuclear fission plants, a nuclear fusion plant produces far less nuclear waste, but how does it hold up against wind turbines, solar panels, and hydroelectric dams.

The combination of wind turbines, solar panels, and hydroelectric dams with improved energy storage devices could well make a nuclear fusion plant a very expensive form of power generation. And if the price-per-watt is too high, it won’t work.

However, if energy storage remains a problem for the next few decades, then nuclear fusion in tokamaks or stellarators might well be a part of the future.

Conclusion

All  in all, nuclear fusion is not all it’s cracked up to be. It isn’t viable yet, and we don’t know if it ever will be. Even then we don’t know if it will stack up against other alternatives.

I guess we’ll see.

Author: Martin Stellinga

I'm a science fiction and fantasy writer from the Netherlands

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