Question: I’ve been reading about how they’ve been trying for decades to harness fusion, essentially a limitless source of green energy. To put it bluntly: What’s taking so long? — RO, New Mexico
Answer: The fundamental difficulty is shown in the video. This is a simplified version of fusion using only two protons. In a real fusion reactor the “fuel” is usually deuterium or tritium — both isotopes of normal single-proton hydrogen, and containing one and two neutrons respectively.
As the two protons approach each other the electrostatic force (yellow arrows) grows exponentially, and under normal conditions this would cause them to rebound from each other before they even got close. But if they are driven together fast enough, and can reach a separation of about one proton-diameter, the strong nuclear force (yellow orange band) kicks in and overwhelms the electrostatic force, pulling the protons together. That’s fusion.
FYI, the strong nuclear force is mediated by particles aptly named gluons. And they don’t just affect protons — neutrons are also subject to their force. Here’s an animation showing what happens in a near-miss scenario:
Protons and neutrons don’t repel each other, so high speeds aren’t needed for fusion. All they need to do is wander within a proton-diameter of each other and BAM! — you get fusion. So having some extra neutrons involved adds more strong force to overcome the electrostatic force, and reduces the speed at which you need to slam the fuel particles together. Deuterium and tritium have those neutrons and are suitable as fuel.
Even so, particle speeds near 1000 km/s are required, and to get them moving that fast a temperature of around 100 million °C is required. That high a temperature introduces many engineering hurdles that are active areas of research. No known material can withstand that temperature, so the fuel must be burned where it will not contact the containment vessel. Magnetic and inertial confinement are two possibilities being investigated.
Fission reactors, by contrast, are easy. The uranium fuel they use (U-235) is already unstable and ready to release its energy. Here’s why:
Those 92 protons constitute a lot of repulsive electrostatic force. The 235 neutrons (contributing their strong force) manage to hold the nucleus together — but just barely. It might seem like, with that many neutrons, the strong force would dominate the electrostatic force and hold everything together. But when you look at the nucleus in 3D you see that neutrons are only close enough to a few protons to exert their strong force. Protons, in contrast, exert their repulsive electrostatic force throughout the entire nucleus:
Uranium has the largest nucleus found in Nature. Larger nuclei can be created, but quickly break apart into smaller more stable constituents. Uranium is right on the edge, so all it needs is a single neutron to become unstable and fission. The fission fragments aren’t always symmetrical, as in the graphic, but however the split the two fragments will repel each other at high speeds (aka temperatures), providing the energy needed to boil water, spin turbines, and generate electricity:
So fission is easy but fusion is tough. I do expect fusion energy to become available at some point. When it does, it’ll totally replace fossil fuels, and ultimately fission reactors as well. Fusion will lead to a major reduction in greenhouse gases and global warming. When that will happen is uncertain. The joke in the business is that “fusion has always been just 30 years away.” That’s been the mantra since 1920.
Significant advances have been made in the last few years. The basic processes of fusion are understood. What remains to be solved are the engineering problems that come with high temperatures and pressure. The physicists have done their work — now it’s up to the engineers.
Next Week in Sky Lights ⇒ The Problem With Bohr’s Atom