Fission to Fusion: Part 2
Adam Kall, Co-Founder & Director of Science
5 minute read
In Fission to Fusion Part 1 I walked through the very highest levels of nuclear fission and fusion, but then focused on fission for most of the column. This time we’re going to focus on fusion, the often-described future of energy and the key to unlocking the utopian Star Trek existence of a post-scarcity economy. In a nutshell, the basic argument is that fission energy requires these large and hard-to-come-by uranium atoms that, after producing energy, remain in a radioactive state that has to be contended with for thousands of years. Compared to that, fusion promises to use simple seawater in a reaction that releases far more energy per unit of input material and has an end result that is just helium and no radiation. As I’ll explain in this column, these ideals of fusion energy have continued to run into issues of reality, and there may be a new form of energy generation based on the same principles but far more appropriate for creating a future of abundance.
Fusion can technically occur for any element lighter than iron. For elements heavier than iron you need a quantum process involving up quarks and down quarks (yes, there’s quantum physics involved here, don’t worry about it), which I may write a column on in the future, but safe to say we wouldn’t be generating energy from trying to slam two thorium atoms together. Instead, the best case for generating power would be taking two deuterium atoms, which are hydrogen atoms with both one neutron and one proton, and combining them to create a helium atom with two protons and two neutrons.
Fusion of two deuterium atoms combining to create a helium atom.
This is the best case because the mass of a single deuterium atom is 2.014 amu (atomic mass units), but the mass of a helium atom is 4.002603 amu. Looking closely, that means each combination of deuteriums to helium is missing 0.025397 amu, which is mass converted to energy in the binding process. This difference between the theoretical sum of fusion parts and the mass of the outcome is greatest in this case, and while atomic mass units are incredibly small, you could still convert 6.3 grams of matter to pure energy per kilogram of deuterium fused. That is equivalent to 157 Gigawatt hours, or 157 nuclear fission reactors running for one hour given that they each produce one gigawatt on average. To top off this miraculous result, deuterium makes up 0.0312% of seawater, and is relatively easy to extract, making the fuel source for this power essentially limitless.
Utopian future.
It is easy to see how fusion has inspired visions of a utopian future, but to get the full picture we need to look beyond the large power generation and start to ask what it takes to combine deuterium atoms like we described. The first thing to note is that we know this fusion is possible because it is what happens inside our Sun (the Sun also takes hydrogen and combines it to make deuterium as a first step). The reason the Sun can achieve this is because it is so mind-bendingly huge that the pressures in its core allow temperatures to reach 27 million degrees fahrenheit, forcing these atoms together. It would be a bad idea to add a sun’s worth of mass to Earth [citation needed], so we instead look towards advanced machines and magnets to force the atoms together. This causes the first issue, which is that these machines are not cheap, and the magnets require a lot of energy to create the pressures needed. So now instead of easily producing the electricity of 157 nuclear reactors, you have a machine that costs the same as a nuclear reactor and takes about as much or more power than it generates to sustain the reaction.
In fact, if you’re focused on combining deuterium with deuterium, we have not yet shown a way to combine them without taking more energy (again the Sun is huge so it doesn’t spend energy to achieve the temperatures). Instead, the best we humans can do is combine deuterium with tritium, another isotope of hydrogen with two neutrons instead of one.
Fusion of a deuterium atom and tritium atom to create a helium atom with a spare neutron.
This makes the tritium more unstable and thus requires less energy to force together, while also producing a little less energy and a spare neutron on the other side. This spare neutron will go on to bounce around and eventually get absorbed by an atom (the magnets don’t contain it since it has no charge). The real problem is in how we get tritium. As an unstable isotope, it is not found in nature in significant amounts, so our best sources are either the heavy water coolant pools of nuclear fission reactors, or in a process that creates the unstable conditions to create tritium, but which would also create radioactive byproducts. In an ironic twist of fate, the best method we have discovered so far to create clean and cheap fusion energy takes an enormous and expensive machine that still produces radioactive waste in the end. Scientists will keep working to improve the process and eliminate these issues, but they are major concerns with the promises of nuclear fusion.
Recognizing all those difficulties in creating nuclear fusion, it’s worth noting that humanity has already succeeded in harnessing the power of nuclear fusion as a means of producing energy. We already have power plants that generate energy through nuclear fusion, create no waste in the process, and have recently claimed the title of the most cost-effective method of energy production. The trick was not to try and recreate the Sun on Earth, but simply use solar panels to collect the abundant energy that already reaches us from that far-off fusion process.
Photovoltaic panels, or PV panels, are a remarkable technology. Nearly all other forms of power generation rely on some process that heats something and then transforms that thermal energy into electrical energy with the use of a turbine. This always results in energy loss as the heat moves into places we don’t want it to go, and also requires a large power plant in order to achieve efficiencies of scale. This is why burning gasoline in a big power plant is more efficient than every home having its own at-home generator. PV panels break both of these standards by generating energy through the Sun and directly charging the solar cells. Since there are very minimal scale efficiencies, PV panels could be used to charge a cell phone on a hike, increased to power a home, or a large number be placed in a big field to play the role of a standard power plant. This scalability is revolutionary, as instead of needing millions of dollars and years to build a big utility-scale power plant, your average homeowner can invest $30K and get clean, free energy for the next 25 years.
Solar panels in a utopian future.
It is my opinion that the future of cheap and abundant energy will rely on nuclear fusion, but we will instead continue to rely on the Sun, which already makes the energy without anyone asking. A future human civilization will likely surround the Sun in a Dyson swarm, made of many solar collecting satellites, that could absorb a tiny percentage of the Sun’s total output and vastly exceed the current power output of humanity by several orders of magnitude. This far-off potential, and the more immediate use cases, rely on the clean space environment that KMI is working to create and sustain every day. While I said in Fission to Fusion Part 1 that KMI would not be using a nuclear fission reactor for our spacecraft, we will be making use of solar panels, meaning fusion energy is still a key part of powering our mission of Keeping Space Clear for All.
Recommended column to read next: Fission to Fusion: Part 1