Fission to Fusion: Part 1
Adam Kall
Director of Science and Co-Founder
8 minute read
On an average day at KMI there are a dozen conversations and projects occurring that relate to the various operational aspects of a spacecraft, from location determination to thermal control, how often it pings home, and how often the batteries need recharging. Central to nearly all of this is electricity, which powers the computers, thrusters, reaction wheels, and basically everything on the spacecraft. Typically this key resource is easy enough to generate as we can use solar panels, which in space are able to reach much closer to their peak efficiency given the lack of atmosphere and clouds obscuring the Sun. This might lead you to think we have an easy solution to all our problems: If the spacecraft needs more power, just add more solar panels and absorb more of the Sun’s energy. Except this clearly non-engineering solution quickly runs into a problem, the solar panels can take up a lot of volume and since about a third of the orbit is behind the Earth, we still need batteries that cost a ton and weigh a lot. While there isn’t a magic solution to producing electricity in space, it got me thinking about the “magic” electricity solutions proposed on Earth. Even though KMI is not going to be using nuclear materials in our spacecraft, as that isn’t the right approach to our problem, I still find the topic of nuclear power sources on Earth a fascinating area to look at.
Nuclear power almost always refers to nuclear fission, but the core of the word “nuclear” is really talking about the nucleus of an atom and how they can change to create energy. If the nucleus is splitting to give off energy, then it is “fission” meaning “to split,” while if the nucleus is combining with another nucleus it is called “fusion” meaning “to merge things together.” I’m discussing both here at the beginning because in the context of creating power they exist on the same gradient of elements, just on extremes of mass. If we’re talking about lighter elements, such as hydrogen and helium, then net energy is released when they fuse, but it would take more energy than released to split the atoms (think the Large Hadron Collider as one example of splitting smaller nuclei). This is true with decreasing efficiencies until you hit Iron, which won’t generate net energy whether you’re fusing or splitting it (I tried typing the phrase “fissing it” and that just doesn’t look right). As you look at elements with nuclei heavier than iron, you start to see a positive net energy return from the fission process, and a negative energy return when fusing into heavier elements. So basically the best ways to generate nuclear energy are either with fusion of very light elements, like what the sun does, or fission of very heavy elements, like the nuclear power plants many of us are familiar with.
Nuclear fission occurs when a nucleus splits, either through natural instability or forced by a neutron collision. Either way the atom splits into two smaller atoms which are more stable, but that may require some leftover neutrons to be released, resulting in the potential for more nuclear fission in a chain reaction. If each collision of a neutron with a nucleus causes less than one additional collision on average, it is called subcritical, and the reaction will fade away. If one additional collision is caused on average for each collision, it is a critical chain reaction and will sustain indefinitely until fuel runs out, which can take a very long time. Finally, if more than one additional collision is caused by each collision on average, it is called supercritical and you get an atomic bomb through the exponential increase in energy released. The actual energy from this reaction can be thought of like a mouse trap being triggered. Without going too deep into the physics of it, a nucleus has forces that want the nucleus to stay tightly packed (strong nuclear force) and forces that want to violently throw it apart (electromagnetism). Because the strong nuclear force is, as the name implies, stronger at close subatomic distances, it is able to hold the atom together. However, with large atoms like Uranium, there exists the potential for a fast moving particle to hit the atom and provide just enough separation for the electromagnetic repulsion to win, and when it wins it isn’t a subtle and cordial separation, but a violent, hot, and fast separation that generates energy.
The nuclear reaction theorised by Meitner and Frisch starting the nuclear reaction observed by Hahn and Strassmann.
Clearly, if the goal is to generate a long-term power source, a nuclear power station would want to utilize sustained critical reactions to generate power. However, plenty of things can occur that either increase or decrease reactivity, so it isn’t as simple as making a critical mass of uranium and chucking it into a box to generate power. Each station needs controllers managing processes like cooling pumps and control rods that adjust the criticality of the reaction to keep it balanced and producing power. It is possible to build a reactor that is naturally subcritical, so if power or control is suddenly lost the system works to bring the reaction under control. Unfortunately, these reactors cost more money to build and have had an issue in the past with accidentally shutting down, so humans in their brilliant hubris designed cheaper reactors that, in the case of loss of control, would naturally tend towards meltdown.
Melting down is just one of the major downsides of nuclear fission that must be considered, but it actually isn’t the most concerning on a regular basis. Instead the major concerns, as well described in this Kurzgesagt series, are nuclear weapons proliferation, nuclear waste generation, and then finally the nuclear disasters and meltdowns:
Nuclear weapons proliferation is defined as the spread of nuclear weapons, fissionable material, and weapon-applicable nuclear technology and information to nations not recognized as “Nuclear Weapon States” by The Nuclear Proliferations Treaty. This was created to serve the purpose of spreading nuclear technology without spreading nuclear weapons. Sadly, it has had limited success as five countries have developed their own weapons with the help of reactor technology. It is also difficult to distinguish a covert nuclear weapons program from the peaceful use of nuclear energy. The unfortunate reality is that nuclear fission will always be connected with nuclear weapons technology.
Nuclear waste and pollution is another major downside. Spent nuclear fuel is radioactive and loses its harmfulness only after thousands of years have passed. There are various proposals for how to deal with the problem, like the French La Hague site that has been reprocessing nuclear waste since 1976, burying it deep in mountains, or permanently destroying it. The universal problem these solutions have is that they cost a lot of money, and frequently have ongoing maintenance costs for thousands of years. All it would take is a future generation, long after the danger of the storage site has been forgotten, to stop the tradition of maintenance for another disaster to unfold. There have been attempts to make signage and statements that express just how dangerous the waste is and to stay away, but every cliche horror movie starts by some rebellious teens ignoring warning signs and releasing a monster.
Finally, major nuclear disasters have resulted in loss of life and enormous economic damage, dating all the way back to 1945. These accidents caused the release of radiation and radioactive particles into the environment. The particles act as carcinogens in humans and other mammals, so cleansing affected areas typically involves drastic measures, including crop burning, slaughtering of farm animals, and digging up meters of topsoil to be moved to some other unfortunate location. Massive areas of land in Russia, Ukraine, and Japan have had to go through this liquidation process and are deemed unfit for human habitation for decades to come. Click here for a brief history of nuclear accidents.
These serious issues must be considered in the context of the wider question, which is electricity generation safety. Electricity, as a product to humanity, has enabled many benefits to health and life, so I’ll make the, hopefully not controversial, statement that having electricity is a net positive thing for humanity. Given this, we also need to acknowledge that every form of power has some kind of negative associated with it, such as cost, land use, or a direct threat to human health and safety. We then as a society need to make the trade-off between safety and the other factors to find the safest form of viable power generation, while also acknowledging that we currently generate power in a multitude of ways, some of which are very dangerous. When comparing nuclear fission with other widespread forms of electricity generation, it becomes a quite reasonable trade-off, even with the previous list of concerns and dangers.
The first big benefit to point out is that nuclear fission does not generate greenhouse gasses. Each year nuclear fission prevents 470 million metric tons of CO2 from entering the atmosphere. This is because the electricity need is non-negotiable, the power would have to come from other power planets, predominantly coal and fossil fuels, if the nuclear power plants weren’t operational. In total, adding up all the years of preventing CO2, nuclear power has prevented over 60,000 million metric tons, or over 2 years worth of all global emissions, and that is with only about 10% of power demand being satisfied by nuclear power. This may seem like an irrelevant point as the world transitions to renewable energy, but that transition is occurring slowly, and renewables have the major issue of base load that nuclear power could help solve.
Base load is the concept that as power demand grows and shrinks throughout the day and throughout the year, there is some annual or daily minimum that demand will never drop below. Put another way, there is an amount of power generation that needs to be constant. This is a problem for renewables, as they turn off when the sun is down, the winds aren't blowing, or the dam reservoirs drop too low. Storing extra power for when renewables aren’t producing could help, but that becomes very expensive, very quickly. Alternatively, nuclear power thrives under constant load, where the criticality of the reactions are kept steady and will not fluctuate to meet demand. According to the EIA, the lowest hourly demand in the US is right around 300 million kilowatts, or 300 gigawatts. The average nuclear plant in the US generates about 1 gigawatt, so the math says we should aim for about 300 nuclear plants across the country (currently there are 54). Alternatively, coal plants also generate a consistent base load, it just comes with CO2 and radioactive material spewing out into the air. No you didn’t misread that, coal is radioactive too.
Source: U.S. Energy Information Administration, U.S. Hourly Electric Grid Monitor
Note: Data shown represent the average aggregate U.S. hourly load (Eastern Standard Time) by day of the week for the months indicated between 2015 and 2019.
The biggest misconception about nuclear power isn’t that it produces hazardous waste, because it does, but that other forms of fossil fuel generation don’t. The key difference is that when a nuclear plant creates solid waste it is concentrated and can be stored in a concrete sarcophagus with scary warnings on the side. Meanwhile coal contains trace amounts of arsenic, lead, thallium, mercury, uranium, and thorium, which turn into fine radioactive ash that is released into the air after the coal is burned. Once mixed into the air, it becomes very difficult to separate back out, and because we can’t slap a giant hazard sign in the sky around every coal plant, it is easy to forget about this danger. The World Health Organization estimates that each year there are 7 million deaths across the globe that are attributed to pollution from fossil fuels. To reiterate, it isn’t that nuclear power is 100% safe, because it isn’t, we just have to weigh that against how safe our other methods of producing power actually are.
What if there was a power generation method which was 100% safe and could generate unlimited cheap energy from anywhere in the world? Well, that’s a common claim from proponents of fusion power, and in part 2 we’ll look at what fusion is and whether it can live up to the hype of being humanity’s electricity savior.
Recommended column to read next: Goodness Gracious, Green Balls of Fire