Fusion Milestone? Yes, But Not The One Reported

This is the reaction chamber at the National Ignition Facility. (public domain image)

“Nuclear fusion milestone passed at US lab.” That’s how the BBC reported it. The science editor, Paul Rincon, wrote:

The BBC understands that during an experiment in late September, the amount of energy released through the fusion reaction exceeded the amount of energy being absorbed by the fuel – the first time this had been achieved at any fusion facility in the world.

Popular Science titled its report, “The National Ignition Facility Just Got Way Closer To Fusion Power.” Right under that headline, one reads:

In a major first, an experiment in the California lab got more energy out of its fuel than went into the fuel. We’re one step closer to ignition, when the reaction becomes self-sustaining.

Before you start having dreams of clean, limitless power, however, you need to know what actually happened at the National Ignition Facility. So let’s start from the beginning to find out what all the hype is about.

It has been known for some time that mass can be converted into energy and vice-versa. For example, when a radioactive nucleus decays, the products of the decay have less mass than the radioactive nucleus had to begin with. In other words, mass is “lost” when a radioactive nucleus decays. Of course, it isn’t really lost. It is converted into energy. It turns out that there is a lot of energy in mass, so if we could harness the conversion of mass into energy, we would have nearly limitless power. Nuclear power plants were built to make this a reality.

In today’s nuclear power plants, a large nucleus (like uranium-235) gets bombarded by a neutron and splits apart into two smaller nuclei and a few more neutrons. This process is called nuclear fission. The total mass of the two nuclei and neutrons is less than the total mass of the uranium-235 and the neutron that hit it, so the extra mass is converted to energy. That energy can then be used to generate electricity. Nuclear fission has its drawbacks, however. The fuel is difficult to make, the products of the reaction are radioactive, and there is always a possibility that the reaction will get out of control, such as what happened at Fukushima and Chernobyl.

Nuclear fusion, on the other hand, is a different means by which mass can be converted into energy. In this process, two light nuclei are pushed together to produce a heavier nucleus. Once again, the mass of the products is smaller than the mass of the two light nuclei that started the process, so mass is converted to energy. The nice thing about nuclear fusion is that the fuel is easy to make, and the products are not radioactive or toxic. Also, we know that it can work, because the sun is powered by nuclear fusion.

The problem is that in order to get nuclear fusion going, you need to push the light nuclei together so that they are very, very close. That takes an enormous amount of energy. The sun has a huge gravitational field that gets the job done, but we can’t make such a field. We have to do something else. There are two main methods that are being investigated. In a process called magnetic confinement, we use magnetic fields to push the nuclei together. In a process called inertial confinement, we use lasers to push the nuclei together.

In principle, both methods should work just fine. If we could transfer energy very efficiently to the two nuclei, it should take less energy to push them together than what is produced by the reaction. As a result, a nuclear fusion reactor should be able to produce energy. The problem is that we can’t transfer the energy efficiently. In fact, so much energy is wasted in both methods that right now, it still costs more energy to keep the reaction going than we get from the reaction itself. Obviously, then, neither process can be used as a power source.

This is why I was amazed when I first read the BBC story. The way the story is written, you would think that the energy-wasting problem has been eliminated, and the scientists at the National Ignition Facility (NIF) have finally gotten more energy from the reaction than they put into it. I was rather shocked by that, however, because inertial confinement (the process used at the NIF) has lagged behind magnetic confinement in progress for years. As a result, I went to a more reputable scientific source and found that the BBC and Popular Science are not giving a realistic view of what happened.

Science magazine has a better explanation of the breakthrough. It reports that the NIF still hasn’t come close to getting more energy from the reaction than they put into it:

NIF’s laser input of 1.8 MJ is roughly the same as the kinetic energy of a 2-tonne truck traveling at 160 km/h (100 miles/h). The output of the reaction—14 kJ—is equivalent to the kinetic energy of a baseball traveling at half that speed. Numerically speaking, the gain is 0.0077.

In other words, the energy output is miniscule compared to the energy needed to keep the reaction going. So what was the breakthrough? Well, if you look only at the energy that was absorbed by the fuel and ignore all the energy that was wasted heating up everything else in the process, then there was more energy produced than what was absorbed. Now don’t get me wrong. That’s a great first step, but it is not the breakthrough that the BBC hyped. In fact, it’s still a long, long way from the results achieved by magnetic confinement. Right now, magnetic confinement reactors have gotten back as much as 70% (0.70) of the energy they put into their reaction. In other words, magnetic confinement is about 100 times more efficient than inertial confinement at this stage of the game.

So do the results at the NIF represent a breakthrough? Absolutely. Do they mean clean, limitless energy is right around the corner? Absolutely not. Right now, if fusion is going to become a viable power source, I think that magnetic confinement (or some hybrid model) will end up being the method that gets it done.

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