Fuel for Thought, Part 2: Fission and Fusion

Jonathan Strickland

© Timothy Fadek/Corbis

Right now, we use the process of fission in our nuclear power plants. This process involves taking a heavy element -- enriched uranium to be specific -- and forming it into rods. As the uranium decays, it radiates energy (and neutrons). Part of that energy is given off as heat. Immerse the rods in water and it will convert the water into steam. The steam then turns steam turbines, which are connected to electrical generators. That's how a nuclear power plant generates electricity -- it's a really fancy facility designed to turn water into steam.

But nuclear power plants have some serious drawbacks. One of those is safety -- you can't let the uranium continue to react inside a nuclear reactor because it will heat up to a temperature so intense that it can melt through the reactor walls. This is the nuclear meltdown everyone has heard about and it's a seriously bad thing. The radiation is deadly and it will last for thousands of years. Control rods -- rods made of material that can absorb neutrons -- help control the rate of the nuclear reaction inside a reactor.

Let's assume everything goes as planned in the nuclear power plant and you remove the rods in plenty of time. Even then, you've only used up about three percent of all the useful uranium in the rods. You're wasting the vast majority of the potential for that uranium. And you have to store the spent uranium somewhere remote where people aren't likely to come into contact with it for thousands of years.

That's where the Waste Annihilating Molten Salt Reactor would take over. This proposed technology would immerse the uranium into a molten salt, which would more effectively manage the heat emitted by the uranium. You could use that heat to continue to convert water into steam and turn turbines to generate electricity. Using up more of the uranium would also decrease the amount of time the waste products would be harmful to humans. It still would take a few hundred years before the waste would be inert but that's better than thousands of years.

We also talk about fusion power in the episode. Why would we be interested in fusion? For starters, fusion power could create energy in the form of heat just like nuclear fission but without all the nuclear waste. You'd still generate electricity by converting water into steam but you wouldn't have to worry about disposing spent uranium or other radioactive materials afterward.

And nuclear fusion is how stars generate energy. They convert hydrogen into helium (at a temperature of millions of degrees). It's all about the fundamental forces of the universe. Those forces are the strong nuclear force, weak nuclear force, electromagnetic force and gravity.

The strong nuclear force is the most powerful but only comes into play at incredibly tiny distances -- it's what glues protons together in a nucleus even though protons have a positive charge and like charges repel each other. That means that over very short distances the strong nuclear force is more powerful than the electromagnetic force. But first you have to get those protons close enough for the strong nuclear force to take over. To do that, you need a lot of energy.

Stars have an advantage due to gravity. The intense gravity generated by a star's mass compresses the hydrogen gas far more than we could manage here on Earth. The particles heat up and all that energy due to compression and heat strips the hydrogen of its electrons, turning the gas into a plasma (a plasma is a gas that has free flowing electrons in it). The compression coupled with the high-energy plasma forces protons closer together until the strong nuclear force takes over and BAM, hydrogen is now helium.

All of that is awesome, but here's the part that's important for those of us who want energy out of the deal. The mass of the new helium atom is less than the collective mass of the two hydrogen atoms that combined to make helium. Where did that leftover mass go? It converted into energy. Einstein tells us that energy is equal to the mass of a particle times the speed of light squared. That means a tiny bit of mass is equivalent to a huge amount of energy. And that's why stars are so darn powerful -- they are enormous nuclear power plants.

Here on Earth, it's pretty challenging to get nuclear fusion to work. We don't have the benefit of a star's gravity to help us compress a plasma. We have to use things like electromagnets or lasers to immobilize a plasma. Even then, we have to heat the plasma to incredible temperatures -- even hotter than those found in the core of the Sun -- to get it to work. That means we've got to pour a lot of energy into the system to get anything to happen. So far, we've spent more energy fusing atoms together than we've generated from the process. That's a losing proposition. But there are labs in several countries racing to find a way to make fusion a positive-energy endeavor.

I have high hopes that fusion will work out within the next decade or so. But some scientists joke that the technology is always "twenty to fifty years away," which is sort of like saying "I'll take the trash out tomorrow." It's a future that's never gets any closer.

In Part Three of the series, I'll talk about some interesting ways that we might use to offset our energy needs using garbage. It's not Mr. Fusion, but it could be just as awesome.