Dear Dr. Zoomie – I read something in the paper recently about fusion power and reaching “ignition” – they sounded really excited, but I’m not sure why. Can you tell me why this is a big deal? Or is it? Thanks!
So let me start by saying that when I was in 7th grade (in 1973) I somehow got a nuclear engineer on the phone (a landline, of course) and asked him a bunch of questions about nuclear energy and fusion reactors. The last question I asked him was when we might see a fusion reactor online, pumping energy into the grid. “Probably around 20-30 years from now” was the answer. Which leads to the standing joke about fusion energy – that it’s about 20-30 years in the future…and always will be. But let’s back up a little….
In fission, which is what happens in our current nuclear reactors, atoms are split and, in splitting, they release a lot of energy – about 200 million electron volts (200 MeV) per fission. This is a lot more energy than is released by burning fossil fuels – breaking a chemical bond releases several electron volts, so fission releases close to 100 million times as much energy per reaction when compared to combustion. The problem is that splitting an atom produces two radioactive fission products – so fission reactors produce a lot of radioactivity; as we saw after Chernobyl and Fukushima, this radioactivity can cause problems if there’s an accident. Fusion, however, is different.
With fusion, instead of splitting atoms, we’re slamming them together hard enough that they stick together – this is most easily done with hydrogen. The physics involved requires that the atoms slam together at very high speeds – which requires high temperatures. In the Sun, where fusion reactions take place naturally, the temperatures are on the order of tens to hundreds of millions of degrees – here on Earth, we try to get as close as we can. The problem is that even the slightest contact of the plasma with any object (such as the walls of the fusion chamber) immediately cools it too much to sustain the fusion reactions, so we need a way to contain the plasma. Fusion also produces very little radioactive waste, it produces none of the plutonium that’s formed in nuclear reactors, and it’s fueled by the most common element in the universe – hydrogen. So what’s the catch?
Well…remember that high temperature needed to create a fusion reaction? That’s the biggest catch. First, we need to raise the temperature of the hydrogen to millions or tens of millions of degrees. But not only that – we also have to keep it at that temperature, which means keeping it from even the slightest contact with the walls of the reactor. That means confining it with magnetic fields, or slamming it with incredibly powerful laser beams, where the power of the beams helps to keep the plasma confined to give the atoms time to fuse together. Thus far we’ve done a great job of confining most of the plasma – what we haven’t been very good at is, well, most of the rest.
What it comes down to is time – we don’t want to cause fusion momentarily, we want to get the hydrogen fusing, and to keep it fusing for an extended period of time. We want what’s called “ignition,” and that’s what Reuters and other news agencies reported was achieved at Lawrence Livermore National Laboratory’s National Ignition Facility for the second time in less than a year (https://www.reuters.com/business/energy/us-scientists-repeat-fusion-power-breakthrough-ft-2023-08-06/). Fundamentally, what it comes down to is that the fusion reaction lasted long enough to produce more energy than went into operating the equipment that created the fusion. And that’s going to call for a little explanation.
To start, fusion reactions work best in a vacuum – that way all of the energy goes into heating the fuel (more on this in a moment) and the fuel only reacts with fuel. But it takes energy to run the pumps needed to create and maintain a vacuum. Energy is also required to heat the fuel to the millions of degrees needed to initiate fusion. One type of fusion reactor uses high-power lasers to implode a tiny pellet of frozen hydrogen; another approach uses radio-frequency radiation to heat the hydrogen to amazingly high temperatures, coupled with a strong magnetic field to keep the hot plasma from touching the walls of the container (which would immediately col it down below fusion temperatures. All of this requires energy too – simply creating the conditions to allow and sustain fusion sucks up a lot of juice – that’s where Livermore is at right now.
But there’s more than this. We still have to figure out how to get energy out of a fusion reaction. With fission, the energy goes into heating the fuel, where it’s transferred to the reactor coolant and, from there, it makes steam that turns turbines to produce electricity. But what’s the game plan with fusion? However it is that we end up extracting the energy, it’s likely to require running machinery (pumps, fans, or whatever) as well as the basic “hotel loads” of the plant itself – keeping the lights on, running instruments and control systems, and all that. Not to mention that every time energy changes form some is lost – no process is 100% efficient. And on top of all this, it’s got to produce surplus energy – enough to be worth the bother.
Livermore’s achievement is important – they generated 3.15 megaJoules (MJ) of energy after zapping the fuel with 2.05 MJ of laser energy. But that was over in a tiny fraction of a second – putting fusion power on the grid will require doing this over and over and over and…you get the idea…for days and then for weeks and months at a time, zapping multiple fuel pellets every second that whole time. So this is an important step – but commercial fusion power is likely still a few decades in the future. But this time we can hope that this projection won’t continue to be pushed back.
Image Reference:
- Photo captured by Lawrence Livermore National Laboratory / Damien Jemison.