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Information: Introduction to Thorium the fuel of the future

Introduction to Thorium the fuel of the future.

This is intended to be a location for discussion and education about the value of thorium as a future energy source. Despite the fact that our world is desperately searching for new sources of energy, the value of thorium is not well-understood, even in the “nuclear engineering” community.

The fundamental basis for considering nuclear energy over chemical energy is the binding energy released in each case. Chemical energy is released when the electron configuration of atoms is rearranged through a chemical process (combustion, digestion, etc.) Electrons are bound to nuclei with binding energies measured in electron volts (eV).

The protons and neutrons in an atomic nucleus, on the other hand, are bound with energies measured in millions of electron volts (MeV). Thus, rearranging the nucleus of an atom (through fusion or fission) releases roughly a million times more energy than chemical energy release.

There are four basic nuclear “fuels” found in nature: deuterium, lithium, thorium, and uranium. Deuterium is an isotope of hydrogen that is found wherever hydrogen is found (such as water). Lithium is a light metal found in lake evaporates. In a traditional fusion reactor, lithium is converted to tritium (another hydrogen isotope) and then fused with deuterium, releasing energy and additional neutrons.

But fusion is fundamentally difficult because positively charged particles tend to repel each other strongly, and only extraordinary temperatures, magnetic confinement, and complicated engineering can coax them to fuse. Despite all this effort, the goal of economical fusion energy is distant and perhaps unreachable, even if the physics can be conquered.

Fission of uranium or thorium, on the other hand, is much easier because neutrons are used to induce destabilization and splitting of the nucleus. The neutron is uncharged, so there is no magnetic repulsion to contend with in the fission process. No magnetic confinement or vacuum chambers are required either. The downside of fission is the generation of unstable, neutron-rich fission products that seek stability through successive beta decay.

Fission of natural uranium requires the construction of reactors that maintain high neutron energies (fast-spectrum reactors) throughout their operation. This is because the fission of plutonium-239 (the result of neutron absorption in uranium-238, the dominant isotope) does not produce enough neutrons to sustain the process unless it is bombarded by high-energy neutrons.

Fission of natural thorium, on the other hand, is much easier because its absorption product (uranium-233) produces enough neutrons from collision with a slowed-down (thermal) neutron to sustain the fission reaction, given that the reactor is designed to be frugal with its neutrons. This feature, and the abundance of thorium worldwide, give thorium a profound advantage over the other nuclear fuels for sustained energy generation.

Thorium is abundant in the Earth’s crust and widespread across the United States and around the world: The major distinguishing factors between thorium and fusion are feasibility and power density.

Controlled thermonuclear fusion is intensely difficult. The more you learn about it, the more you realize just how difficult. It’s easy to point to the Sun as an “existence-proof” for fusion, but the fusion type and conditions in the Sun are totally different than what we try to do on the ground. The fundamental reason that fusion is so difficult is that positively-charged nuclei repel each other. Strongly. To give them enough energy to overcome this repulsion, you must get them very very hot. So hot that measuring temperature in degrees kind of breaks down, and we go to measuring temperature in electron-volts.

The typical temperature needed in a deuterium-tritium fusion reactor (the easiest one to build) is about 10,000 electron-volts, or about 200 million degrees Fahrenheit. Even at these temperatures, fusion of nuclei is still very improbable. Most interactions simply scatter (deflect) away from one another. Only once in a great while do you have a head-on collision that doesn’t scatter, and then you can have a fusion reaction and energy release. This is why the other two components of the Lawson criteria (the basic blueprint of fusion) come into play: density and confinement.

You have to have the nuclei hot enough, you have to have enough of them, and you have to keep them there long enough for fusion. Up to this point, we have not built a fusion machine that maintains these conditions in sufficient quantity to release more energy than it consumes. We may someday, but we haven’t yet.

Even if we did build such a machine, it would have a very low power density. This is because fusion plasmas are a pretty good vacuum, and the amount of fusion power taking place (per unit volume) is pretty low. So you need a very big machine. And a fusion machine is not a simple machine.

It’s essentially a very, very good vacuum chamber, surrounded by intensely power superconducting magnets, held together by a huge steel superstructure to keep it from ripping itself apart. (the magnets don’t like each other) As if this wasn’t enough, it also needs to be a nuclear breeder reactor, converting the neutrons from D-T fusion into more tritium.

This is done by surrounding the inner chamber with lithium (the precursor of tritium) and beryllium (as a neutron multiplier). And all the extraction systems to remove gaseous tritium generated in the breeding blanket. Contrast that with a thorium reactor, which operates at relatively low temperatures (<1000 K), has no magnets, vacuum chamber or high-pressure systems, and no huge superstructure holding it together.

A liquid-fluoride thorium reactor is very power dense, compared to fusion, meaning that it physically has a smaller “footprint” and could conceivably be built small enough to fit in submarines or trailers. You won’t be able to do that with a fusion machine.