How close are we to nuclear fusion?
Date: Friday, March 04, 2016 @ 17:54:58 EST

From Q: Will we know how to fuse atoms (in order to produce electricity) in a large scale before 2030? Is it a good idea to study Nuclear Physics to help this field?

A (Clint Law): There are three good answers...

1) No one knows and
2) Sooner than you may think and
3) Right now

How can three seemingly contradictory paths be correct?

No one knows... (option #1 -- Magnetically confined fusion)
As of right now, there is no known way to create a continuous controlled fusion reaction that lasts a long time (let's say weeks to be suitable for commercial scale power production). Tokamak reactors are where all of the (non-weapons) money is going; they are controlled and can work for many seconds in very large implementations (like ITER).

The problem is, magnetically confined fusion is fundamentally flawed for power production. This is because there is no force to stop particles moving directly towards the outer wall, which means you eventually lose enough particles the reaction stops. (Stars, the best known example of continuous fusion use gravity to confine the fuel, which solves this problem.) If we continue this course of research exclusively, we are waiting for a breakthrough, not an engineering solutions, which may never come. Or maybe someone will devise a way to add radial confinement to a tokomak tomorrow and we'll have reactors producing electricity within a decade or two. Personally, I doubt this will be proven suitable for power production within anyone alive's lifetime.

Sooner than you may think... (option #2 -- Inertially confined fusion and possibly other types of fusion)
There are other ways to press particles together and create fusion, both of which have a potentially bright future, limited mainly by engineering problems. The first is inertially confined fusion, where you compress a sphere of light nuclei using lasers (or something else, lasers are inefficient of course), creating a short pulse of fusion. This solves the radial problem, but now you have to worry about two big engineering problems: efficiently compressing the target and rapidly replacing the target. If you can master that, you could potentially create a fast pulse fusion reaction. However the energy produced is small, so this isn't a particularly good source for commercial power production. As a side note, this sort of fusion is what is used in thermo-nuclear weapons (aka hydrogen bombs), so it has a well established ability to create large amounts of power.

There are other potential sources of fusion power. The most promising in my opinion is fusion of very light elements with either alpha particles emitted from a radioactive source or energetic protons (i.e. hydrogen). The problem here is one of low fusion power output, scarce materials, and high energy required to initiate each fusion event. The recent cold-fusion claim, by Rossi in Italy, is supposedly a reactor making use of proton-light nuclei fusion.

If I had to guess, I would say that some sort of non-magnetically confined fusion will become a feasible commercial power source within about 50 years.

Right now... (option #3 -- Wind/Hyrdo/Biomass/Solar power and some current nuclear power implementations)
The obvious part of this is that, by definition, any renewable energy source on Earth is using fusion power (from the Sun) in some secondary (solar) or tertiary (wind/hyrdo/biomass) form. Oddly, because of the engineering involved, direct solar power is typically the least efficient and most carbon-intensive.

The not-so obvious part is that fusion that takes place in conventional (fission-based) nuclear power plants are already being used in very select applications. This is through a secondary process, where you fuse one or more neutrons onto an existing atom, which creates some radioactive element. You then get the fusion energy out slowly as the newly formed atom decays. Common examples of this type of fusion power source that are used in radio-thermal generators (typically for spaceflight applications) include:
  • Pu-238, made from U-238, with a half-life of 87.7 years, producing .5 watts per gram (for many years, then slowly decaying)
  • Po-210, made from Bi-209, with a half-life of 138 days, producing 140 watts per gram (for a few dozen days, then rapidly decaying
Hypothetically, tritium (H-3) could fall into this category, if you start with regular hydrogen and capture a couple of neutrons. In practice, this process is extremely rare, hard to produce, and therefore costs a ridiculously lot of money. All commercially available tritium is produced by neutron induced fission reactions with lithium or boron, not neutron capture from hydrogen. (Note: fission production is still incredibly costly at ~$100,000 per gram)

Read many other good answers here: How close are we to nuclear fusion?

Thanx to Peter Gluck for the heads up in his post today: EgoOut blog

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