S6C3: Nuclear Power & Hydrogen

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a) Splitting and merging

To make the most of this section, if you are unfamiliar with the atomic nucleus or think you may need a refresher, I strongly advise reading through S1 Section 1.c on the atom and S1 Section 1.d on the fundamental interactions, including the weak and strong nuclear forces, beforehand continuing.

There are three different ways in which a strongly bound atomic nucleus can release energy and all of them have to do with the uncoupling of some bonds and reforming of other, stronger bonds with the difference between the strength of these bonds equating the amount of energy released.

The first of those is through the natural, apparently random, emission of a particle out of the nucleus. The technical term is radioactive decay and it was first discovered at the end of the 19^th century though not understood until many years later since, at the time, the atom was not thought to contain sub-atomic particles.

When the emitted particle is a helium-4 atom (without the 2 electrons), consisting of 2 protons and 2 neutrons, it is called alpha-decay. This is the main source of helium on Earth.
In beta-decay, a proton gets converted into a neutron, or vice versa. In the first scenario called β⁺ decay, the change is accompanied by the emission of a neutrino and positron holding the electric charge, and in the second scenario called β^– decay, it is an electron and antineutrino that are ejected. The hypothesized reason for this is the minimization of binding energy which drives the nucleus towards a more “stable” configuration. This process entails a change in the identity of the chemical element since the number of protons in the nucleus changes, even though the total number of nucleons remains unaltered, hence it results in an atomic transmutation.
Gamma-decay is quite different and yields no change in the composition of the nucleus. It consists in the emission of high-frequency electromagnetic radiation called gamma ray by high-energy electrons decaying to a lower energy state.

The period between the two world wars saw rapid progress in the understanding of what happens within the atom with nuclear fusion being advanced as the explanation behind stellar power and, in the 1930s, the atom was split by man for the first time and the fission process theorized a few years later. Fusion is the merging of two nuclei and fission is the opposite, the splitting of a nucleus into two smaller nuclei. And both events can release energy. The reason for this is, as we explained in S1 Section 7.b, and I will quote from it: “fusion takes place with lighter elements such as hydrogen and helium whereas fission is carried out on heavy nuclei like uranium and plutonium”. The top of the hill, the most stable configuration, is Iron-56 (comprising 26 protons and 30 neutrons). This can be computed mathematically and fits with what we can observe in nature since stars die with an iron core.

Here, on Earth, the temperature and pressure required for fusion to take place are still beyond the capabilities of our time – if you want to get a sense of the required conditions, please refer to S3 Chapter 4 on the Sun. However, it quickly became apparent to scientists that a small amount of energy liberated in one atomic fission could be used towards triggering another one, and therefore that a chain reaction could be initiated. Uncontrolled, it can generate an atomic explosion we are sadly familiar with. For purpose of civilian power generation, the reaction thus has to be controlled.

b) Nuclear power plants

The job of a nuclear power plant is to produce electricity and, as we have seen in the previous two chapters, a reliable way to achieve this is to heat up water into pressured steam and run it through a steam turbine coupled with an electric generator – a process we described in S6 Section 1.f on thermal power plants.

In the case of nuclear power plants, the job of heating up is carried out by fission nuclear reactors using plutonium-239 or uranium-235 as fuel. The latter is naturally occurring but at very low concentration so when uranium ore is mined, most of the content is uranium-238 and, through isotope separation, the ore is enriched to a higher concentration of U-235. Plutonium, on the other hand, is the fuel of choice for weapons (that would be the Pu-239 isotope) and can also be produced by the neutron capture of U-235 atoms as part of a fission reaction.

During a fission reaction, a nucleus absorbs neutrons and this can lead to a split into two smaller nuclei and some free neutrons travelling at high speed. If those intersect with other fissile nuclei, then another fission event can take place, etc. The heat produced by the ongoing, violent energy release is used to generate pressured steam. As for the control aspect, the key is to manage the statistical number of neutrons that will cause further fission. On one hand, this can be done by using neutron moderators in the form or water or graphite rods to slow down the free neutrons to speeds where the likelihood of them causing fission events increases – this moderator thus increases reactivity. On the other hand, to reduce reactivity, neutron absorbers such as xenon-135 or boron, also called neutron poisons, would be used. I include links to the highly technical Wikipedia entries for neutron moderator and neutron poisons at the end of this chapter if you wish to read more on this topic.

With all this heat being produced, the reactor needs cooling and the primary method for this is water. Which is why, in addition to the volume required to drive the steam turbines, nuclear power plants are always built along rivers or freshwater reservoirs.

As of 2024, the civilian installed capacity amounted to only 374 GW and they generated about 9% of the global electricity supply. With the exception of France where nuclear contributes to more than 70% of electricity production (and it used to be higher), the low uptake is down to two fundamental reasons: public opinion and construction cost. The cost side has to do with the huge array of back-up systems and security measures to avoid accidents, that do happen in rare occasions, the long time needed to build a plant, which means a lot of interests have to be paid on the loans used to finance construction, and the lack of economies of scale since so few reactors are built each year.

Regarding public opinion, the fault is partly with the media who, apparently, love a good catastrophe. It is also partly with the public who is unable to run the maths and prefer the very-low probability risk of a catastrophic accident to the higher-probability risk of individual accidents. The analogy here would be the fear of flying in an airplane but not of climbing in a car, a mode of transport in which the odds of dying or becoming severely handicapped are statistically much higher. In that respect, the number of death per MWh of electricity generated by nuclear power plants should be compared to that created by burning fossil fuels such as coal in thermal power plants. And there, there is no debate, because we should take into account fatalities from air pollution in the computations and also remind ourselves of the mind-numbing amount of energy released in nuclear reactions: one kilogram of U-235 will release approximately three million times more energy than one kilogram of coal.

c) Future technologies

So, what’s in store for nuclear power in the coming decades?

The first angle of attack, to improve adoption, will be the deployment of a systematic, reliable logistic chain and facilities to store the radioactive waste, mostly originating from spent fuel. This was already covered in S5 Section 9.f so I will not repeat it there other than to say the most promising method would be to bury it deep underground and that construction of the first such facility, the “proof of concept”, is currently underway in Finland.

The second thrust is the reliable strategy of seeking incremental improvements: technological evolution rather than revolution. A lot of research and development work is carried out to design new reactors with improved efficiency or less drawbacks than the previous generations. These efforts are handily grouped under the umbrella term of Generation IV reactors, because we are now onto the third generation. I will only expand on two of the most promising technologies, the Very-High Temperature Reactor (VHTR) and Sodium-cooled Fast Reactor (SFR) but I invite you to read about a few others, you can find your way there using the Wikipedia link for Generation IV Reactor enclosed at the end of this chapter.

VHTR are gas-cooled, typically with helium, an inert gas which, besides the lack of reactivity, also has the added benefit of not becoming radioactive when exposed to free neutrons. By operating at temperature ranges of 750-950 °C, the heat can be used in industrial processes, including the production of hydrogen with sulphur, iodine and water as the reactants and hydrogen (H₂) and oxygen (O₂) as the products. This is called the sulphur-iodine cycle.
SFR is cooled by liquid sodium. There are two main advantages in using sodium as a coolant; the first is its lower propensity to moderate neutrons, which would be an undesirable property in a fast reactor where the fission uses high-velocity neutrons called “fast neutrons” as opposed to “thermal neutrons” that have been slowed down. The second is the heat absorption property of this chemical element which has a liquid state of matter spanning 785°C, from 98°C up to 883°C. In a fast reactor, there is there no need for neutron moderators and the fast-neutron-based reactions can be used to convert the more common isotope uranium-238 into fissile plutonium-239 as part of a process called breeding. It also results in less nuclear waste, in particular the more problematic radioactive transuranic elements.

Despite what I wrote earlier about nuclear fusion, this doesn’t mean there is no hope of fusion technology ever coming to fruition. Here, a quick aside is necessary to clarify that cold fusion is a myth without any basis in our current physical models so I will not write anything more about it.

To be sure, there are experimental fusion reactors, the problem is that their energy output is still negative, i.e. more energy is required to initiate and sustain the fusion reaction than is released from it. The main international project pursuing the technology is called ITER, which stands for International Thermonuclear Experimental Reactor. The fuel for fusion would be two hydrogen isotopes: deuterium (²H) and tritium (³H) and, when fused, the product is a helium-4 atom plus one free neutron and, crucially, 17.6 MeV of energy. The maths work out to 11 million times more energy density per gram of fuel than coal. Tritium would need to be bred but deuterium can readily be extracted from water.

The fusion reactions take place in heated plasma at temperatures where no solid can exist, therefore the plasma is contained via electromagnetic fields generated by powerful magnets in a torus-shape (like a donut, the variety with a hole in the middle) device called tokamak. Construction of the largest experimental is ongoing with first operations now scheduled for the mid-2030s, after several years of delay.

d) The hydrogen markets

In a bid to reduce the emission of carbon dioxide and other carbon-based compounds with greenhouse effects, hydrogen could play a major role alongside the rise in installed renewable energy sources capacity and, plausibly, more responsible customer behaviours translating into a reduced demand for goods and services entailing a significant carbon footprint, which could also be achieved through carbon-emission-related taxation. The main applications for hydrogen in the global economy are #1 oil & gas refining, #2 as feedstock in the production of chemicals, in particular ammonia, #3 steel manufacturing, #4 as fuel in long-distance freight, and #5 for seasonal energy storage. Let’s take a quick look at each of those in turn.

Today, hydrogen is already used as part of hydrocracking, whereby the carbon-carbon bonds of long-chain hydrocarbons are split and short-chain, more valuable compounds are formed – something we mentioned in S6 Section 1.d. The addition of H₂ in the chemical reaction allows the transformation of aromatics and olefins into molecules of the naphthene and alkane families with proportionally more hydrogen atoms. In parallel, the presence of hydrogen can lower the sulphur content of the refined petroleum products through a catalytic process called hydrodesulphurization in which sulphur (S) reacts to yield hydrogen sulphide (H₂S) that can be extracted.

In the chemical manufacturing sector, the main use of hydrogen is for the production of ammonia, and the major application of this product is overwhelmingly as fertilizer in the agricultural sector thanks to its nitrogen content. The formula for ammonia is NH₃ and, using iron as a catalyst, it uses gaseous nitrogen from the atmosphere (N₂) and H₂ as reactant: N₂ + 3H₂ = 2 NH₃.

The steelmaking industry is one of the largest emitters of greenhouse gases (GHG): in 2020 it was about 7% of global emissions and as much as 11% for CO₂. Naturally, it is thus a prime candidate for decarbonization and hydrogen-based direct reduction of iron ore is the main technique which can be used towards that end. In this, the oxygen content of iron ore is removed without having to melt the ore, thus saving on fuel to maintain temperatures above 1,200°C. Combined with 3 molecules of hydrogen (H₂), three hematite molecules (Fe₂O₃) undergo several reactions to yield three iron molecules (Fe ) and three of water (H₂O).

For the purpose of transportation, hydrogen can be used either in the form of fuel cells, a technology we will look at in the upcoming chapter on batteries (refer to S6 Section 5.c), or in jet engines and rockets. Adoption would help cutting down on greenhouse gas emissions since fuel cells only exhaust water and in an internal combustion engine there is no CO₂ emission but some nitrogen oxide is produced. On land and seas, fuel cells are the obvious technology, however in aviation both options are contemplated and have their respective merits and issues. We will cover the main challenges to the adoption of hydrogen as fuel for transportation in section f).

Finally, using electricity for electrolysing water, it is possible to think of hydrogen as energy storage, which can later be used to produce electricity through combustion or via fuel cells. Obviously, there are significant losses involved in such a roundtrip, about 60-70% gets wasted in total, but it can make sense in certain situations, mostly in the context of renewable energy sources, to even out electricity generation and partially make up drops in volumes during periods of low output.

e) A colourful production landscape

The previous section started out with the words “in a bid to reduce the emission of carbon dioxide and other carbon-based compounds with greenhouse effects”; this served to frame the purpose of pushing for the adoption of hydrogen as a fuel, feedstock or storage medium. The implication is that we ought to look carefully at the process of hydrogen production to determine if GHG emission savings have been made and quantify those. In other words, we need to ensure we are actually solving a problem, and not making it worse or perhaps creating another one as a by-product.

As it stands, the bulk of the hydrogen produced relies on methane as feedstock. When reacting with water, it yields three hydrogen molecules and carbon monoxide (CO), what we call syngas, and this reaction requires a significant energy input – it is endothermic. Syngas can be used in the production of methanol and ammonia but when the objective is to maximize hydrogen output, the carbon monoxide is made to react with water to generate hydrogen and carbon dioxide. The first reaction is named steam methane reforming (SMR) and the second water-gas shift reaction (WGSR).

This standard method of producing hydrogen is nicknamed “grey hydrogen”. Fortunately, the CO₂ emission can be cut down significantly when captured and stored away from the atmosphere, typically in natural underground geological formation. For instance, this method is employed in the oil production industry by injecting carbon dioxide into depleted reservoirs with insufficient pressure where the gas mixes with the oil and alters some of its fluid properties, including reducing viscosity, thereby making oil easier to extract. We will cover carbon capture and storage (CCS) in a bit more details in section g), nonetheless it should be noted that #1 not all carbon dioxide is captured, #2 the carbon capture process is also energy intensive, #3 the storage part, including when used for enhanced oil recovery, is also energy intensive, and #4 as a result the production of this “blue hydrogen” turns out to be quite expensive.

When considered from the perspective of the entire production and transport chain, the most desirable process should therefore involve renewable energy. And we happen to know a way because, as we have seen in Chapter 2, we can produce electricity from sources such as sunlight and wind, and this electricity can be used to split water molecules into oxygen and hydrogen in a process known as electrolysis. I include a link to the Wikipedia entry in the last section if you wish to know more about this. The issue for this coveted “green hydrogen” economy, as it often is, has to do with cost: the cost of developing large scale projects and the ultimate cost per unit of hydrogen produced and brought to market. For the time being, the main projects are located in places with strong solar irradiation, significant wind, and close to water reserves. Thus, countries like Mauritania, Australia and Saudi Arabia are where the main development efforts are currently being undertaken.

In this kaleidoscopic terminology, white refers to naturally occurring hydrogen, black to that produced from coal, and pink (or red) is powered by nuclear energy.

f) The mountains to climb

Even assuming the cost of green hydrogen was to come down to a somewhat competitive level compared to the blue or even the grey variation, there remain several significant challenges on the way to a wide adoption and the flourishing of a clean hydrogen economy. The first of these is the time and investment required to bring some of the green hydrogen projects to fruition – we are talking several billion dollars apiece here.

Then there is the storage aspect. Indeed, at atmospheric pressure, its energy density is only about 0.01 MJ/L (Megajoule per litre) whereas it is around 35MJ/L for jet fuel. This would make it impractical to transport it, let alone filling tanks on a truck or plane with it. Therefore, it needs to either be compressed or liquefied. When compressed at 69 MPA, the energy density jumps to around 5MJ/L, which is just over half the 9MJ/L of natural gas when compressed at 25MPa whereas LNG requires a temperature of -162 °C, which is already quite challenging to handle, and delivers 22MJ/L. As it happens, to liquefy hydrogen, the gas needs to be cooled to just over 20°K, that is -253°C, and in that format the energy density is around 9MJ/L, still several times below refined petroleum products.

In practice, this means much higher costs to transport the same amount of energy, a full-fledged logistic chain handling either compressed gas or capable of maintain cryogenic temperatures. And, when used as transport fuel, there also needs to be a dedicated refuelling infrastructure and either tanks would have to be bigger, which is less of an issue for freight ships, or trucks, buses and cars would need to stop more often to refuel.

Furthermore, for an aircraft, this energy density matters very much since it partially translates into additional weight, and it takes more fuel to lift fuel. Still, the benefit of not emitting GHG high up in the sky warrants a push towards this type of fuel, but don’t expect the same ticket price for your hydrogen-powered flights, at least in the near future. The solution here, of course, is to fly less. Reduce, reuse, recycle.

Finally, in a somewhat ironic turn of events, the theoretically promising market of land transport has already shifted trajectory towards electrification. This required changes in engine and transmission technology as well as the build-out of charging stations. For the time being, it is hard to see how hydrogen would dislodge either petrol or electric vehicles and there probably isn’t a place for a third type of fuel, even if fuel cell batteries using hydrogen are able to piggyback on the EV batteries format to make their way into vehicles. Unless national regulators decide to force the issue. My personal viewpoint on the issue, for what it is worth: reduce! And there, autonomous vehicles are the best way forward, as explained in S5 Section 2.e titled “the autonomous paradigm shift”.

g) Trivia – Carbon sequestration

The removal of CO₂ from the atmosphere, or the preventing of it being released in the atmosphere in the first place, and its storing away falls under the umbrella term of carbon sequestration. As it stands, the main technology is the CCS mentioned earlier in this chapter and, technically, the main process involves a chemical solvent binding to the CO₂ molecule, the filtering of the resulting compound, and then its heating up to unbind it before it can be purified and either stored away or used in industrial applications.

This is expensive, it requires energy, and not all CO₂ can be sequestered this way so it should be considered as one option but certainly not the main one to decarbonize our atmosphere and lifestyle. Why is it getting so much airtime then? Simply put, because this is the method that has the less negative impact on the deep-pocketed oil & gas industry, look no further. And the success rate of CCS plants and projects in terms of sequestration efficiency is thus far unimpressive.

Without going into details, the main alternatives are as follows:

Planting, or should I write, replanting trees. Improving tree cover means more surface available for photosynthesis, which absorbs CO₂ and releases O₂. You may want to re-read S1 Section 7.f if you do not recall the chemical basis of the process.
Somewhat related to this are technologies looking to grow biomass through photosynthesis and then either bury it or use it to create biofuels or energy. Of course, the key here is not to release more carbon dioxide in the atmosphere than has been removed in the first place. The tag name for these technologies is BiCRS, which stands for Biomass carbon removal and storage.
Better farming practices where ploughing doesn’t release as much stored carbon from the ground and bare fields are planted with cover crops acting as carbon dioxide extractors through photosynthesis.
Direct air captures, which is analogous to CCS except that it deals with CO₂ already present in the atmosphere. As of today, we do not have the technologies to make this energy-intensive process both affordable and a negative-carbon proposition. More R&D is required.