Think of a pile of bowling balls in a deep well. They don’t fly out because the force of gravity holds them in. If you roll an extra bowling ball to the edge of the well and let it drop in, energy is released when it hits the pile at the bottom. Atomic nuclei can be compared to the well. The neutrons and protons that make it up are the bowling balls, and the “gravity” is the far more powerful “strong force.” Roll some of these “bowling balls” into the well and energy will be released, just as in a real well. The process is called nuclear fusion, and it’s the source of energy that powers the sun. We’ve been trying to produce energy by repeating the process here on earth for a good many years now, but were only “lucky” enough to succeed in the case of thermonuclear weapons. We’ve been stymied in our efforts to harness fusion energy in less destructive forms. The problem is the Coulomb, or electrostatic force. It’s what causes unlike charges to attract and like charges to repel in the physics experiments you did in high school. It’s much weaker than the strong force that holds the neutrons and protons in an atomic nucleus together, but the strong force has a very short range. The trick is to get within that range. All atomic nuclei contain protons, and protons are positively charged. They repel each other, resisting our efforts to push them up to the edge of the “well,” where the strong force will finally overwhelm the Coulomb force, causing these tiny “bowling balls” to drop in. So far, only atomic bombs have supplied enough energy to provide a “push” big enough to result in a net release of fusion energy.
To date, we’ve tried two main approaches to supplying the “push” in a more controlled form; magnetic fusion and inertial confinement fusion, or ICF. In both approaches the idea is to heat the nuclei to extreme temperatures, causing them to bang into each other with enough energy to overcome the Coulomb repulsion. However, when you dump that much energy into a material, it tends to fly apart, as in a conventional explosion. Somehow a way must be found to hold it in place long enough for significant fusion to take place. In magnetic fusion that’s accomplished with magnetic lines of force that hold the hot nuclei within a confined space. Some will always manage to escape, but if enough are held in place long enough, the resulting fusion reactions will release enough energy to keep the process going. In inertial confinement fusion, as the name would imply, the magnetic fields are replaced by the material’s own inertia. The idea is to supply so much energy in such a short period of time that significant fusion will occur before the material has time to fly apart. That’s essentially what happens in thermonuclear weapons. In ICF the atomic bomb that drives the reaction is replaced by powerful arrays of laser or particle beams.
Both of these approaches are scientifically feasible. In other words, both will almost certainly work if the magnetic fields can be made strong enough, or the laser beams powerful enough. Unfortunately, after decades of effort, we still haven’t managed to reach those thresholds. Our biggest ICF facility, the National Ignition Facility or NIF, has so far failed to achieve “ignition,” defined as fusion energy out equal to laser energy in, by a wide margin. The biggest magnetic fusion facility, ITER, currently under construction in France, may reach the goal, but we’ll have to wait a long time to find out. The last time I looked there were no plans to even fuel it with deuterium and tritium, (D and T, heavy isotopes of hydrogen with one and two neutrons in the nucleus in addition to the usual proton) until 2028! The DT fusion reaction, shown below with some of the others, is the easiest to harness in the laboratory. For reasons I’ve outlined elsewhere, I doubt that either the “conventional” magnetic or inertial confinement approaches will ever produce energy at a cost competitive with the alternatives.
There are, however, other approaches out there. Over the years, startup companies have occasionally managed to attract millions in investment capital to explore these alternatives. Progress reports occasionally turn up on websites such as NextBigFuture. Examples may be found here, here and here, and many others may be found by typing in the search term “fusion” at the website. Typically, they claim they are three or four years away from building a breakeven device, or even a prototype reactor. So far none of them have panned out, but I keep hoping that eventually one of them will pull a rabbit out of their hat and come up with a workable design. The chances are probably slim, but at least marginally better than the odds that someone will perfect a perpetual motion machine.
I tend to be particularly dubious when I see proposals involving fusion fuels other than the usual deuterium and tritium. Other fusion reactions have their advantages. For example, some produce no neutrons, which can pose a radioactive hazard, and/or use fuels other than the highly radioactive tritium, which occurs in nature only in tiny trace amounts, and must therefore be “bred” in the reactor in order to keep the process going. Some of the most promising ones are shown along with the more “mainline” DT and DD reactions below.
D + T → 4He (3.5 MeV) + neutron (14.1 MeV)
D + D → T (1.01 MeV) + proton (3.02 MeV) 50%
D + D → 3He (0.82 MeV) + neutron (2.45 MeV) 50%
H + 11B → 3(4He); Q = 8.68 MeV
H + 6Li → 3He + 4He; Q = 4.023 MeV
3He + 6Li → H + 2(4He); Q = 16.88 MeV
3He + 6Li → D + 7Be; Q = 0.113 MeV
The problem with the seemingly attractive alternatives to DT shown above as well as a number of others that have been proposed is that they all require significantly higher temperatures and/or confinement times for fusion “ignition” to occur. Take a look at the graph below.
The horizontal axis is in units of the “temperature” of the fuel in thousands of electron volts, and the vertical shows the “cross-section” for any of the reactions shown in units of “barns.” The cross-section is related to the probability that a particular reaction will occur. It is measured in units of 10−24 cm2, or “barns,” because, at least at the atomic scale, that’s as big as the broad side of a barn. Notice that the DT reaction is much higher at lower temperatures than all the others. Yet we failed to achieve fusion ignition on the NIF with that reaction in spite of the fact that the facility is capable of focusing a massive 1.8 megajoules of laser energy on a fusion target in a period of a few billionths of a second! Obviously, if we couldn’t get DT to work on the NIF, the other reactions will be difficult to harness indeed.
In short, I tend to be dubious when I read the highly optimistic progress reports, complete with “breakthroughs,” of the latest fusion startup. I tend to be a great deal more dubious when they announce they will dispense with DT altogether, as they are so sure of the superior qualities of their design that lithium, boron, or some other exotic fuel will work just as well. Still, I keep my fingers crossed.
