plutonium, uranium, and how a metal can be used as a power source



Title, how can such a small amount of metal be so powerful/potentially destructive? How is the power extracted? What makes it so dangerous? Any other interesting facts?

In: Physics

They’re radioactive. Radioactive decay produces heat. This heat can be used to heat up water and produce steam. That steam can then be used to drive turbines.

Basically nuclear power is a steam engine that has replaced the coal with nuclear fuel.

When you burn coal for energy, you harness the energy stored in the molecular bonds. Nuclear energy harnesses the energy from inside the atoms. The protons and neutrons are held together, nuclear power breaks the atom and releases that power. It is far greater than molecular bonds.

The nuclei of atoms hold a lot of energy, in significant part because they are full of charged particles (protons) that repel each other pretty strongly – so the nuclear forces have to be *very* strong to hold atoms together. Since the forces are very strong (compared to the electromagnetic force, which holds the energy of chemical bonds), breaking or forming nuclear bonds can release huge amounts of energy compared to breaking/forming chemical bonds (which is what you would do if you burn fuel for energy).

As for how to extract that energy from radioactive/fissile elements like plutonium, there are two main approaches:

**Radio-thermal generators**: Radio-thermal generators, or RTGs for short are rather simple: they hold a radioactive material, which produces heat as it decays (since those nuclear bonds are being broken down over time). It’s essentially the nuclear analog to a slow-burning fire. RTGs are usually only used for powering things like spacecraft that need a self-contained power source, since the materials you need for good RTGs are often highly radioactive and difficult to produce.

**Nuclear fission**: This is what you commonly think of with nuclear power. Essentially, some (but not all) radioactive materials are *fissile*. Being fissile means that on top of decaying by themselves, you can also make them decay in a way that releases energy by hitting them with the right particle (usually a neutron). For materials like plutonium, the fission that happens when you hit them with a neutron makes it release more neutrons, which can then hit other plutonium atoms, fissioning them and releasing more neutrons and so on – this is what is called the chain reaction. Because every atom split can cause more than one other atom to split, this is a process that accelerated exponentially as long as there are enough atoms available to split, which is what makes nuclear power dangerous (and enables nuclear bombs) – you can release a lot of energy *very quickly*. In order to produce safe nuclear power, you need to control the amount of neutrons available to start fission events to keep fission at a steady rate (at which point you are producing continuous heat, which you can use to boil water to run turbines to produce electricity).

Gonna try to stick to eli5 as much as possible.

The reason why they are so powerful/destructive is because they release alot of energy. Now everything that exists is made up of energy, but in order for it to destroy something that energy needs to be released. There are many types on energy, the one released by Uranium in powerplants is Heat for example.

But why does Uranium release so much energy?

Its because mass is energy. Mass is what everything physical is made out of, mass is “stuff”. If something is more massive, it contains more energy. On earth we can refer to mass as weight, so you can say “1kg of mass”, but that only works on earth. If you take 1kg of Uranium to the moon it will still be the same amount of Uranium, but weigh less since the gravity is weaker. This is important because i will be using weight to explain what happens, just know that mass and weight are not the same.

So, what happens with Uranium in a bomb?

You split it. You use a start energy, and split the uranium. Why does this release energy? Its because the mass that remains is less. The extra mass has been converted into heat.

To illustrate, lets say you have 1kg of Uranium. Now you split all those uranium atoms, as in 1 atom becomes 2 atoms. If you did that with all Uranium atoms, gathered them up and weighed them again after splitting you would expect that to weigh 1kg again right?

But it doesnt. It weighs less, our 1kg of mass now weighs less. Lets say it weighs 990grams. What happened to those 10 grams of mass?

They became heat. But wait. Heat is a type of energy, and we used energy to split the Uranium. Wont that energy split even more Uranium?

It will! That is a chain reaction. In power plants and bombs you use this fact. The energy that splitting Uranium releases is enough to split even more Uranium. If you control this energy you get a powerplant. You release just enough so you can use it for power.

If you do not control, and let it just do its thing, you get a nuclear bomb. Kaboom!

Basically they are so heavy that they fall apart and release heat in the process, that heat can be used to drive turbines and generate electricity. There are ways to make them fall apart faster releasing even more heat in the process.

isotopes of plutonium and uranium are fissile material which means we can split them into multiple smaller elements. The reason this releases so much energy has to do with the average binding energy in the nucleus of an atom. this is essentially the energy that keeps the nucleus of an atom together and is different for each isotope.


where does this binding energy come from?

this energy comes from the difference in the weight of the atom and its individual parts. We know the weight of an individual proton and an individual neutron. you would assume that if we multiply the weight of a proton by the number of protons, do the same for the neutrons and add the together you would get the mass of the nucleus. However, experiments have showed that there is a difference between the theoretical mass (calculated as i explained) and the actual mass of nuclei. the difference in mass gets converted to the binding energy via Einstein’s equation E=mc^2.


During fission we split atoms, this is typically done by bombarding a nucleus with neutrons. the neutron is absorbed by the nucleus which becomes unstable and then splits into typically 2 smaller nuclei plus 2-3 neutrons.

How does fission release energy?

we take a mass balance of what came out (2 small nuclei + 2-3 neutron) and what we put in (1 big nucleus and 1 neutron) and subtract them. Because different isotopes have different binding energies we see a difference in the mass between products and reagents. This difference again gets converted to energy via Einstein’s equation.

Important to note is that the difference is mass is absolutely tiny like 10^-28 kg tiny, but c^2 is extremely large plus this is only for 1 atom. there are about 6.23*10^23 atoms in one mol (1 mol of u235 would be 235 gram). This means that the potential energy released per mol of atoms from a fission reaction is orders of magnitude greater than for a chemical reaction.

below is a link which shows a graph of the average binding energy per nucleon for all the different isotopes. This graph is important because the slope of this graphs indicates whether fission will release energy or fusion will release energy, and how much ( steeper = more energy released). A positive slope => fusion releases energy, negative slope => fission releases energy. As you can see the left side of the graph (positive slope => fusion releases energy) is much steeper than the right side which means per atom fusion releases much more energy than fission which is one of the reason to develop fusion reactor (besides being safer and not producing nuclear waste).

link graph binding energy per nucleon: [,and%20are%20less%20stable%20because%20the…%20More%20](,and%20are%20less%20stable%20because%20the…%20More%20)

Because of E = mc^2. A huge amount of energy for a tiny amount of mass.

Most of the energy in an atom is stored in the bonds that hold neutrons and protons together. Those neutrons and protons were fused together inside a supernova.

It’s basically stored star power. And that energy gets released when you separate those protons and neutrons.