Nuclear Fusion power – how long will we have to wait ?

Just before Christmas, it was announced for the first time that researchers at the National Ignition Facility at the Lawrence Livermore National Laboratory in the US had successfully achieved “ignition” within a nuclear fusion reaction. This generated a lot of excitement in the press, not all of it well-informed, so I thought it might be interesting to explore this topic in more detail. What is nuclear fusion ? Why was this news so significant ? When will we begin to benefit from these experiments ? 

Nuclear fusion is a reaction between the tiny fundamental particles that make up all elementary matter – protons, neutrons and electrons. Unlike chemical reactions – which occur between elements, rather than within them – or nuclear fission reactions, which happen when the nucleus of a heavy, unstable atom breaks apart, releasing energy and smaller particles –  fusion reactions occur when light atoms are forced together to become heavier ones. When two such nuclei fuse, the amount of energy required to hold together the new, combined nucleus – the binding energy – is lower than the binding energy required to hold together the two nuclei individually. This surplus binding energy can be converted to heat, which we can use to generate electricity. This is the process that powers the sun, and all the other stars in the universe. 

 

Diagram of D-T fusion. Protons are red, neutrons are black.

 

The lightest element in nature is hydrogen. Regular hydrogen (sometimes called “protium”) has no neutrons, just one proton and one electron. This unique configuration means that there is no binding energy within the nucleus to release in a fusion reaction. However, there are two variants – isotopes – that can be fused. A hydrogen atom with one neutron added is called “deuterium”. It represents a small proportion of the hydrogen on earth, but there is more than enough for us to use. Hydrogen with two neutrons is called “tritium”.

 

Hydrogen Isotopes
Hydrogen Isotopes

Tritium is an unstable (i.e. radioactive) configuration, decaying with a half-life of around 12 years into stable helium-3. Because of this short half life, it is found only in trace quantities in nature, in the upper atmosphere, where it is produced when cosmic rays interact with the gases found there. Most of the world’s man-made supply of tritium comes from CANDU nuclear fission reactors. In these unusual and innovative fission reactor designs, operating almost exclusively in Canada, deuterium is used as a moderator. The tritium is produced when the deuterium nuclei absorb an additional neutron within the reactor. This can be separated and extracted. Regular nuclear reactors around the world will produce trace quantities of tritium due to the presence of small amounts of deuterium within the light water coolant.

Assuming we have a supply of fusible nuclei, how do we kick off the fusion reaction ? This is the really hard part. With a fission reaction, it’s a lot easier :  in simple terms, all you have to do is put the fissile fuel and a moderator in close proximity to each other and the reaction will sustain itself until it runs out of fuel, producing energy (and radioactive waste). Indeed, these conditions have occurred in nature, and we’ve had the ability to produce these reactions in a controlled fashion since the 1940s.

But with fusion it is far more difficult. Nuclei in general are positively charged, and will repel each other – imagine trying to push together two magnets at the same pole. To put them close enough together to fuse, you need to push them hard enough to overcome the repulsion. This has happened in nature since the dawn of time (the heat from the sun comes from nuclear fusion of hydrogen), but in conditions of enormous pressure and gravity. We can’t create gravity on demand, so to compensate we need to heat the fuel up – in fact we need to heat the hydrogen nuclei to 100 million degrees C. 

It should go without saying that this is technically very difficult to accomplish. Every substance known to man will disintegrate if exposed to these conditions, long before the temperature is reached. This is one of the several major engineering challenges that needs to be overcome. You need to produce enormous amounts of heat, contain it within an enclosed space, and end up with more energy coming out than you put in, and do all of that at the scale required to produce electricity reliably. Nuclear fusion researchers use the so-called “Q Factor” to describe the energy characteristics of a reaction. 1 indicates breakeven; more than 1 indicates that more energy was created than put in. All experimental fusion reactions up until now have had a Q value less than 1. 

The other major challenge is the production of fuel for the reactor. You’ll often hear the press talk about how the supply of fuel is effectively limitless, but this is not quite true. Deuterium occurs readily in nature, but the fusion of deuterium with itself (D-D fusion) to produce a net energy gain is not thought to be within reach in the short term. Instead, the immediate target is the fusion of deuterium with tritium, as shown in the diagram at the top of this article. As noted above, tritium does not occur in nature, so it must be synthesized somehow. For now, we can use tritium from CANDU reactors, but in the longer term, the plan is to produce it by installing a “blanket” of lithium metal within the fusion reactor. The isotope lithium-6 decays to release and tritium and helium (technically, an alpha particle) when it absorbs a neutron. However, that effectively means that lithium now becomes reactor fuel, and it must be enriched to be useful. Lithium is a very useful substance elsewhere, not least within rechargeable batteries, so using it for fuel will put more strain on supplies.

Breeding tritum from lithium as a side effect of the D-T fusion reaction
Breeding tritum from lithium as a side effect of the D-T fusion reaction. Protons are green, neutrons are blue.

Another thing you might hear from the press is that the reaction produces no radioactive waste. This isn’t true either. The tritium required to fuel the reaction is radioactive, and it is possible that it may leak into the environment. Tritium, which behaves chemically like any other form of hydrogen, is difficult to contain at the best of times, and if it combines with oxygen in the atmosphere you will end up with radioactive water. However, the short half life (12 years) means that most of the tritium will decay fairly quickly, limiting the impact of any leak. 

In addition, as with a fission reaction, there are lots of neutrons flying everywhere inside a fusion reactor. These neutrons can be absorbed by the reactor vessel itself, causing its atoms to become radioactive – a well-understood process known as neutron activation. The resulting radioactive isotopes tend to be short lived, but represent a danger to humans working with the reactor, and as such, it complicates the operation and eventual decommissioning of the reactor.

Scientists around the globe are working, mostly in concert, to identify solutions to solve all of these problems. These are the biggest technical problems ever attempted to be solved by mankind. The prize is huge. Second generation fusion technology, which can fuse deuterium within itself, will have an environmentally friendly energy source with an inexhaustible fuel supply lasting billions of years (well beyond the lifetime of the planet we’re living on!). We will be limited only by how quickly and easily we can build the reactors. 

The big news that was announced a few weeks ago was that the first controlled reaction with a Q value greater than 1 had occurred. The National Ignition Facility’s research focuses on inertial confinement fusion – essentially, encapsulating the fuel inside special pellets and firing a series of lasers at it. The pellets need to be manufactured to high precision, and the timing and power of the laser pulses tightly controlled. This simple description trivializes what is a fiendishly difficult problem, with many decades of trial and error under research conditions to identify the precise characteristics to make it work. 

Demonstrating Q > 1 is an important milestone for fusion. But we should be clear that it has taken six decades of research to reach this point, and it is but one out of a large number of milestones that we need to reach in order to deliver on the promise of fusion power. 

The inertial confinement fusion technique used at the NIF is one of two major strategies to try to demonstrate viable fusion power. The other involves magnetic confinement, in a device called a tokamak, which was first invented by the Soviets in the 1950s (a rare example of Soviet innovation!). Rather than firing a laser at a capsule of fuel, a tokamak heats up and charges the fuel as a plasma within a large chamber, with massive magnets used to keep it from touching the walls. So far, the JET (Joint European Torus) project in Culham has been able to achieve Q = 0.67. 

 

Composite image showing JET when idle, and on the right, with plasma undergoing fusion.
Composite image showing JET when idle, and on the right, with plasma undergoing fusion.

JET’s successor is called ITER, and it is currently under construction at Cadarache in the south of France. ITER is an international collaboration between most of the developed countries in the world. It will support plasmas ten times larger than JET and aims to demonstrate sustained fusion at Q = 10. In addition, it will be used to perform experiments on the design of lithium blankets that can be used to breed the required tritium, and develop general processes around the operation and maintenance of a fusion reactor.

ITER has been under construction for several years and is expected to be switched on in 2025, although it is not clear what impact the Russian invasion of Ukraine may have on this (Russia is part of the ITER coalition and is supplying a number of critical components for the reactor). A further ten years will be required to demonstrate net-gain fusion. That will take us to the second half of the 2030s – assuming it all works. 

From there, the ITER experiments are expected to lead to a prototype fusion electric generation plant, called DEMO. Current estimates have DEMO construction beginning in 2040, suggesting operation at some point in the 2050s. A number of years of trouble-free operation of DEMO would be expected before governments around the world would push for the construction of commercial fusion plants. Assuming the usual delays, we’re really looking at 2070 for the first commercially-operated fusion power plant to come online, just under 50 years from now. 

As a non-expert, the key difference which is apparent to me is that the ITER project has a set of milestones that will result in the delivery of a prototype fusion-driven power station. It’s not clear to me how this will be accomplished by the NIF. An inertial confinement reactor would have to have processes to manufacture the fuel pellets and put them into the reactor where the lasers can hit them.

Overall, however, while I think it is right that governments around the world continue to focus on fusion research, we need to be realistic about the fact that it will be nearly the end of the century before we have mastered the various technical challenges and have the means to build fusion reactors at will. It’s important that milestones, such as the one at the NIF, do not mislead policymakers or the public into believing that the solutions are around the corner. The danger here is that we could be distracted from making the difficult, but necessary, decisions to deliver nuclear fission reactors as part of the strategy to end the use of fossil fuels. That process must start today, not in a lifetime.

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