A Few Thoughts On Fusion [crisis du jour]

One of Ace’s posts yesterday was about recent experiments in nuclear fusion and a claimed breakthrough from a fusion lab that the energy released in the fusion experiment was greater than the energy that was used to create the reactions. I have some thoughts on that claim.

First, a little of the science. The nuclear power plants currently generating electricity for us are powered by nuclear fission, which is the splitting apart of large nuclei (like uranium) into smaller pieces, releasing energy in the process. The experiment in yesterday’s post was about the reverse process, nuclear fusion, in which smaller nuclei (like hydrogen) are fused together into larger nuclei (like helium), releasing energy in the process.

Much of the civilian pursuit of fusion energy has taken place using ‘magnetic confinement’ schemes, in which the hot plasma is suspended by wrapping it in strong magnetic fields. By contrast, the experiment mentioned in yesterday’s post uses ‘inertial confinement’, where a brief pulse from an astonishingly powerful laser crushes a pellet of frozen hydrogen, causing the hydrogen to fuse together to make helium and release energy in the process. It’s actually kind of a nano-bomb … and, accordingly, these inertial confinement fusion experiments are typically funded (in the U.S.) by the Department of Defense rather than by the Department of Energy.

Every fusion experiment rates its efficiency only from what happens in the reactions themselves: how much energy did we get out of the fusion reaction compared to the energy it took to make this plasma (in the magnetic confinement case) or fire this laser pulse (in the inertial confinement case). On that score, yesterday’s report was genuinely encouraging. We’re still a very long way from producing electricity from fusion, but it is a milestone worthy of note.

However, as Ace’s post yesterday and the subsequent comments pointed out, we’re still nowhere near making fusion a practical power source. The milestone yesterday was concerned only with the reactions – what went in and what went out. In order to make fusion a practical energy source, far more energy has to be expended in making the reactor, building its containment building, and so on. Concerns like these were by design not part of yesterday’s announcement, but they will have to be addressed whenever – if ever – fusion technology advances to the point where it could become a commercially viable energy source.

Addendum 1: Some of the commenters yesterday were waxing scientific about the First Law of Thermodynamics and about the Carnot Cycle. But: fusion reactors (using either the magnetic or inertial confinement schemes) are not heat engines and they don’t work in a cycle like Carnot had conceived. In addition, the First Law of Thermodynamics holds in fusion reactions because of the energy released via E=mc² . The products of the reaction have less total mass than the reactants do, and that excess mass is carried away as energy.

Addendum 2: The Sun (and all stars) have been pumping out energy from fusion reactions for gazillions of years. So what makes fusion so difficult here on Earth? In a nutshell: because things usually expand when heated. Hot and huge is easy, cold and confined is easy, but hot and confined is astonishingly difficult. In the sun and stars, gravity provides the confinement. Our sun is massive, more than a million Earths combined, and the crushing gravity in its core fuses hydrogen into helium rather easily. We don’t have that confinement scheme available to us on Earth, so we have to bootleg the confinement some other way, and that turns out to be a vexing problem both for the energetics of fusion confinement and for the engineering challenges it poses.

posted by Open Blogger at 01:15 PM

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