
Nuclear power provides ~20% of the US electricity supply. To understand how it works, we need to first understand the nucleus of an atom.
Atoms
We tend to think of atoms as solid balls linked together with chemical bonds forming molecules. The reality is that the "hard" shell that represents an atom is a representation of the electrons that are whizzing around the centrally placed nucleus. The "solid" ball is a poor representation; instead, think of a ping-pong ball. Most of the ball is empty; in the case of an atom, the shell(s) (mass of electrons) does not contribute much to the mass, as electrons are very low in mass. The bulk of the weight is in the nucleus where the proton(s) and neutron(s) reside. These each have a mass of 1 unified atomic mass units (also called Dalton). So, hydrogen, the simplest atom has 1 proton and 1 electron. It is the number of protons that determines the element, so 1 proton is hydrogen represented as 1H. If there is also a neutron, then the element is still hydrogen, but it is a different isomer. We call these deuterium atoms 2H or D, and if we produce water from it (D2O rather than H2O) we have "heavy water" (the two hydrogens being replaced with the heavier mass deuterium atoms).
Atom/Isotope | Half-Life |
---|---|
Uranium 235 (235U) | 0.7 x 109 YEARS |
Plutonium | 23,924,000 YEARS |
Radon 222 | 3.8 days |
Polonium 218 | 3 min |
There are many elements that are radioactive. For some, only certain isotopes are radionuclide (they are radioactive). However, some will have very long half-lives; others decay very quickly. During the decay, the nucleus splits into two or more atoms producing heat (the kinetic energy of these new atoms) and neutrons to continue the reaction. Given the long half-life of 235U, we can't wait for natural decay for electricity production. Instead, we induce the splitting of the atoms with a neutron. Basically, when the 235U (uranium) is induced into decay via capturing a neutron, the atom breaks apart violently, producing 2 fragments that are the new atoms and several neutrons. Thus, when at the critical state, 1 neutron is allowed to induce another 235U to decay. To increase the probability of this occurring the moderator has the role of slowing the neutrons down. As all of the fragments are moving quickly; they have lots of kinetic energy, or in the case of atoms and molecules we can say they have a high temperature. Thus, we can create steam, as discussed in the video, to turn a turbine (and spin a generator). If you wanted less fission, the control rods can be lowered — absorbing the neutrons and controlling or stopping the heat and thus the electricity produced.

How a Nuclear Reactor Works
Watch the following video (beginning at minute 1:30) to learn how nuclear energy creates electricity. What you watch here is fair game for the exam.
MR. ANDERSON: Hi. It's Mr. Anderson, and this is AP Environmental Sciences video 25. It's on nuclear energy. You're probably familiar with the Richter scale. It's a log scale by which we measure the size of earthquakes. But you're not familiar with the INE scale, or the International Nuclear Event Scale. It's also a log scale, and we use it to measure the size of nuclear accidents. We've only hit 7 twice.
The first time was in 1986 in Chernobyl. We had a collapse and a meltdown of the reactor. 31 people died from exposure to radiation. In 2011 in Fukushima, we also hit a level 7. We had three other reactors meltdown after an earthquake and tsunami. In the US, the highest we've ever gone is a level 5 at Three Mile Island. It released a little bit of radioactive material into the surrounding area. But it scared people. These accidents scare people, and radiation scares people because we can't see it.
And so the amount of energy we're getting from nuclear reactors has remained static for decades, but it's starting to be revisited again. And the reason why is there's also something in the environment that is scary, and it's also invisible. And that's carbon dioxide. If we look at the amount of carbon dioxide being produced by nuclear power plants, it's on the level of the same as wind generation or hydropower. If we compare that to gas and oil and coal, there's way more carbon dioxide being created. So new technology and a decrease in carbon emissions could see a resurgence of nuclear energy.
Where does the energy come from? It comes from the fusion of radioactive material, generally uranium-235. So as it decays, it breaks down into two fragments-- barium and krypton. And as it does that, it gives off energy, and it gives off neutrons that can trigger more fission and more radioactive 235. So the way this is controlled, unlike in a weapon, it's controlled in a reactor. Most of the reactors in play right now are light water reactors or normal water reactors.
What you do is you put fuel rods inside it. And as they decay, it produces a little bit of energy. And that energy inside the water heats it up. And we can use it to generate steam and then generate electricity. Now when it melts down, this goes out of control, and we get a release of that radiation into the environment. And so by having it in water, we can contain some of that energy. And we can also use control rods. These are actually going to take in some of those neutrons. And by lowering them between the fuel rods, we can slow down the reactor.
Now the disadvantages are pretty apparent. Nuclear waste is going to be created, and it can be around for thousands and thousands of years. So we have to keep track of that. Each of the radioactive materials has a different half-life, but it's going to be on the order of thousands of years. And also we have these accidents where we can have explosions, malfunctions, and it releases that radiation into the environment and can cause things like thyroid cancer.
Why do we still have it? Well, the advantage is that it creates a huge amount of energy. And it can do that without increasing the amount of carbon emissions in the environment. So if we look at uranium-235, now we're looking just at the nucleus. And so we're looking at the protons and the neutrons. And so if we were to hit one of those uranium atoms with a neutron, what it'll do is it'll break in half. It breaks apart into these two fragments. And as it does that, it releases a certain amount of energy. You can see it's also liberating three of these neutrons. And each of those have the potential to hit another uranium-235, and we can break it down.
So it's not an out of control chain reaction like this that we might see in a nuclear bomb, but it goes slow over time. And so if we look at what those fuel rods are like, most of the uranium is actually going to be uranium-238. A few of it is uranium-235. And so as those neutrons are given off, by having it in water, we can absorb some of that energy, and we can control that radiation. And also, we can lower these control rods. They absorb the neutrons, and so we can slow it down.
So if we look at a typical light water reactor, we're going to have the fuel rods and the control rods in the core. We're then going to heat up a fluid. And that fluid is going to be in a closed system. So as it moves through these pipes, it returns back where it was. But it's bringing with it a huge amount of heat. Now that heat moves into a separate loop. And so in this loop, what we're doing is we're heating up the water. It's forming steam up at the top. And then that steam is moving through a generator, so we're generating electricity.
And then finally, we still have a lot of heat right here. So before we pump it back in, we have to get rid of some of that heat. And so we're going to do that by pumping the water and another loop into a cooling pond. And so as long as we have energy contained within those fuel rods, we can generate electricity. But what happens when we decay too much of that uranium-235? Now it becomes waste. It's still radioactive, but it's not generating enough electricity for the plant to go. And so now we've generated waste. So that's one form of nuclear waste.
But we're also generating a little bit of heat over here into the environment as well. And so how do we deal with that waste? Well, how do we deal with the fuel rods? We're going to put him in a pool. And as we put them in the pool, we're going to absorb some of that energy here. But eventually, we're going to have to put it in some kind of a container. And a lot of these are on these concrete slabs. And we've got that nuclear waste contained inside there. There's no real long-range plan of what we're going to do with his nuclear waste. And it's going to be a problem that we'll have to deal with generations down the line.
If we look at how long this could occur, you have to understand what a half-life is. A half-life is going to be the amount of time it takes for half of the material to decay or to break apart. And so if we look at time 0, let's say the half-life is one year, at time 0, we'd have 100% of the radioactive material. At time 1, we would have 50% of it. In other words, half of it would have decayed. In another year, it would be half of that and a half of that and a half of that and a half of that.
And so in an AP Environmental Science class, you should be able to calculate the half-life. And let me give you a problem. Let's say radium has a half-life of 1,500 years. How long will it take for 250 kilograms of the radium to decay down to less than 10 kilograms? And so we're saying the mass of radium at the beginning is 250 kilograms at time 0. And so in one half-life, in other words, in 1,500 years, we would have decayed half of it down to 125. In another 1,500 years, we'd be down to 62.5. And you can just keep doing this. And you can see at 7,500 years, we're less than 10 kilograms left. But you can see a lot of that is still going to be radioactive.
Now what happens in accidents, something happens where we're not able to contain this core. And so if we look at Chernobyl, they were testing the reactor, and it got out of control. It heated. We have a melting or an explosion that actually collapsed the roof and released a lot of radiation. If we're looking at Fukushima, it's like three levels of protection that failed. We have an earthquake, but we also have this giant tsunami. And if we're looking at Three Mile Island, it was a problem with a valve but also a problem with user error as well.
And so all of these, for the most part, are human error, either we had a mistake at the reactor or had a mistake in the design. And what it does is it releases some of this radioactive material into the environment. So, for example, radioactive iodine can cause thyroid cancer. So we eat it in our food. It causes cancer years down the line. And we're going to see this-- wherever there's a nuclear accident, we're going to have increases in thyroid cancer after that.
So if we look at these accidents, so this is at-- here's Three Mile Island, here's Chernobyl. So we had the heyday of nuclear reactor creation during this oil crisis. But then after these accidents, you can see the amount of reactors that we have has remained static. And you can say even though we could produce this amount of energy, we're producing less of that. And the reason has to do with this fear of radiation and the fear of accidents as well.
And so what's the future hold for nuclear power? Well, they're going to be increases in new technology. Thorium reactors are going to be working much better than uranium, light water reactors. And we can have these third-generation reactors where we can actually reuse some of that waste. And then finally, we have to reduce carbon emissions, and nuclear energy is going to be part of that discussion.
So could you pause the video and fill in the blanks? Let me do that for you. Nuclear energy is the fusion of something like uranium-235. We break it apart into fragments. We also get energy and some neutrons that can cause fission in other atoms. We've got the fuel rods. That's where the radioactive material is, but we also have these control rods. Disadvantages, nuclear waste. It takes a long time due to the half-life of these radioactive materials for the waste to go away.
We can have accidents that increase the amount of cancer. Thyroid cancer is an example of that. But the advantages, again, nuclear power can help us reduce the amount of carbon dioxide in the environment, reduce global warming. And that's why it's being revisited. And I hope that was helpful.
Below is a static representation of a nuclear power plant. The caption provides a short explanation.

Advantages of Nuclear Energy
There are two primary advantages of nuclear energy: low cost (once the plant is constructed) and no greenhouse gas emissions - unlike many other major electricity generation sites!
Disadvantages of Nuclear Energy
Nuclear Power is quite controversial and comes with a number of concerns.

- Cost. It is very expensive to build the plants, but remember, once built, it is relatively inexpensive to produce electricity.
- Safety. Nuclear disasters like the ones in Chernobyl (Ukraine), Three-Mile Island (Pennsylvania), and Fukushima (Japan)
- Waste storage. Currently, all nuclear waste is stored on the site where it is generated (initially in pools then in dry casks). In the U.S. there was a plan for a permanent disposal site at Yucca Mountain in Nevada but it was ultimately not used due to objections by Nevada. Thus, we do not currently have an appropriate storage plan for the waste —this will be a more important issue when we continue to close down nuclear power plants and have much more waste to store.
- Proliferation. The key risk here is that facilities originally constructed for nuclear energy production could be used to create nuclear weapons.
Future of Nuclear
We currently have 96 reactors in the US producing electricity in 56 nuclear power plants in 29 states, far more reactors than any other single country! The oldest one running is the same age as I am (~50'ish)! They are licensed for 40 years with a 20-year extension possible. As most were built in the 1970s and 1980s, soon we will be looking at dismantling a large number of nuclear reactors. Cheap natural gas is accelerating the time frame to decommissioning nuclear plants.
The figure below shows the trend of our nuclear contribution. Given the age of the existing reactors, we can expect a decline in the number of reactors in the next decade. A good question is; Will we build new ones? In an era of climate change expectations, nuclear is an excellent means to produce electricity without greenhouse gases. However, after Fukushima (earthquake and resulting Tsunami in Japan), this event stopped new nuclear construction in many nations with some moving away from nuclear power completely. The few "new" nuclear reactors have been in construction for a long time due to many delays. One of the large advantages of nuclear power is no greenhouse gas emissions. So closing down nuclear utility sites after ~2025 will cause an increase in greenhouse gas emissions if we replace nuclear power with power produced by fossil fuels (such as natural gas). We do have one nuclear reactor activated in 2016 in Tennesee, it was the first new plant in the new century with two more under construction in Georgia expected to be completed in the 2020s. However, 17 nuclear reactors have been closed and are being decommissioned. So, nuclear power is likely at its peak production of electricity in the U.S.

Nuclear Power Plants Around the World

Many nations are moving away from nuclear energy (despite the lack of greenhouse gas emissions) and are in the process of phasing out or reduce nuclear power.