Read · 4 min
We're studying nuclear reactions! But they're unlike the chemical reactions with which we're familiar. We don't care about electrons, atomic orbitals, or bonds. Instead, we want to know what happens in the nucleus. It's the exciting realm of nuclear chemists! Now, we'll explore fusion reactions. Our following explanation will examine nuclear fission.
What is Nuclear Fusion?
Do you remember how we defined fusion? It's when two nuclei unite and generate a heavier element. How do we recognize the reaction? Energy! Most often, it's kinetic energy. So, our temperatures are increasing. It comes from the mass defect. There's a discrepancy between products' and reactants' masses. The difference comes from the conversion between mass and binding energy. To get there, we use Einstein's famous relativity equation.
How Does Fusion Happen?
In fusion, we are almost always working with tiny atoms. Why? Any fusion reactions beyond the transition metal isotopes iron-56 or nickel-62 are endothermic. So, they need energy. But, we want to release energy. We like to go so small that we generally only talk about converting hydrogen to helium. Why? Consider the nucleus. It's a packed ball of positive charge. What happens when two nuclei approach each other? There is immense electrostatic repulsion. A higher atomic number equals more repulsion. So, to limit it, we use tiny atoms.
But, even hydrogen and helium have electrostatic repulsion. How do we overcome it? We use unfathomable temperatures. Most hydrogen fusion reactors are at . That's so hot that the hydrogen is a plasma. Every single hydrogen atom gets ionized. Even still, only about half the ions have enough energy to react. The atoms are moving so fast that they overcome any electrostatic repulsion. When they get close, the strong nuclear force takes over. Remember, the strong nuclear force only acts over the length of a femtometer. So, they have to get close. But sometimes, they do. What is the result? The two nuclei become attracted to one another and fuse.
Nuclear Fusion in Nature
With temperatures like , it is hard to imagine fusion occurring in nature. But it does! The principal source of energy for stars is hydrogen fusion. But, a star's core is only . How can it have such a low temperature? It has massive pressure. The order of magnitude in a star's core is about atmospheres. That astronomical!
For fun, let's talk more about stars. They are massive concentrated clouds of gas. What keeps the gas from diffusing? Gravity! It's constantly pulling the gases towards the star's center. But, the core is hot and pressurized. The internal pressure resist gravity's pull. So, the star reaches equilibrium. But, it needs to maintain its temperature and pressure. It does that via hydrogen fusion. Hydrogen fusion releases a lot of kinetic energy. So it maintains a stable temperature. But, after millions or billions of years, the star runs out of hydrogen. So, it starts to fuse helium into carbon. The helium-carbon fusion emits a lot of energy. So, the star expands. What happens after it uses up its helium? That depends on the star's mass. Smaller stars collapse under the weight of their own gravity. They have super high densities. Larger stars explode into supernovas. Depending on the size, gravity turns them into neutron stars or black holes.
Artificial Nuclear Fusion and Fusion Power
I've already mentioned that we can do hydrogen fusion on earth. What are its advantages over fission? There are a couple. Hydrogen is way more abundant than any of the radioactive elements. In nuclear reactors, fission reactions use the actinide elements. These elements are rare! But, hydrogen is accessible. Also, fusion produces less waste. Many fission products are radioactive. But, they can't sustain a nuclear reaction. So, we have to bury them in huge caverns underground to limit exposure.
Historically, the dream was that fusion would solve all our energy problems. What happened? It takes a lot of energy to heat something to . We can't produce a reactor efficient enough to generate an energy surplus. What are the most optimistic expectations? Some scientists believe we'll have efficient fusion reactors starting in the 2030s. More conservative estimates place it in the 2050s. But, we've been working on this technology since the 1940s. So, many nuclear scientists have their doubts. But, there's another problem. Renewable energy prices continue to decrease. So, even if we can produce fusion power, there are huge questions about cost-efficiency. It's might be more cost-effective to put solar on everyone's houses. It costs a ton of money to build fusion nuclear reactors.
