Introduction to nuclear energy

An icon of a nuclear reactor with an atom symbol on a cooling tower and a lightning bolt on the reactor.

In the late 1930s, we discovered that some particularly large atoms found in nature can be split into two, releasing a shocking amount of energy as heat. Because the energy emerges from the atomic nucleus, we call it nuclear energy.

When these atoms are arranged properly in a machine called a nuclear reactor, each splitting nucleus can induce its neighbors to split in turn, creating a controlled chain reaction. Reactors can convert the released nuclear heat into electricity, shaft horsepower (to power ships), building heating, desalinated water, hydrogen, and many other things useful to human civilization.

Today, about 430 commercial nuclear power plants worldwide produce around 400 GW of electricity, enough to power 400 million average households. About one-fifth of the USA’s electricity comes from nuclear power, which represents about half of the country’s zero-carbon electricity.

Nuclear energy is controversial due to concerns about radiation. Public support varies geographically and over time, but as of 2023, polls show that a majority of people support expanding nuclear power.

What are the key capabilities of nuclear energy?

(Click on each of the headings for more details.)

Because nuclear fuel contains millions of times more energy per mass than anything else, it is possible to keep all the byproducts accounted for and out of the biosphere, in strong contrast to fossil and biofuels which release much of their combustion wastes into the air, causing severe health and environmental problems. The following table shows how long a 100 Watt light bulb could run from using 1 kg of various fuels. The natural uranium undergoes nuclear fission and thus attains extremely high energy density (energy stored in a unit of mass).
MaterialEnergy Density (MJ/kg)100W light bulb time (1kg)
Wood101.2 days
Ethanol26.83.1 days
Coal32.53.8 days
Crude oil41.94.8 days
Diesel45.85.3 days
Natural Uranium (LWR)5.7x105182 years
Reactor Grade Uranium (LWR)3.7x1061,171 years
Natural Uranium (breeder)8.1x10725,700 years
Thorium (breeder)7.9x10725,300 years

Energy densities of various energy sources in MJ/kg and in length of time that 1 kg of each material could run a 100W load. Natural uranium has undergone no enrichment (0.7% U-235), reactor-grade uranium has 5% U-235. By the way, 1 kg of weapons grade uranium (95% U-235) could power the entire USA for 177 seconds. All numbers assume 100% thermal-to-electrical conversion. See our energy density of nuclear fuel page for details.

Splitting atoms is a carbon-free process, so nuclear power is a global solution to climate change. While some processes in the overall lifecycle are currently carbon-emitting, the net result is that nuclear is nearly as low-carbon as you can get. Once we electrify construction and mining equipment and power it all with nuclear and other zero-carbon processes, the overall carbon will trend to zero.

graph showing
              carbon intensity of energy sources in grams CO2-equivalent per
              kilowatt electric generated. Fossil and biomass are bad, in the
              400-800 range. Solar PV is 40. Hydro is 24. Nuclear is 12. Wind is
              11. There is an arrow saying that nuclear is among the lowest
              carbon forms of energy we know.
Source: Schlomer S.,, 2014: Annex III: Technology-specific cost and performance parameters. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the 5th Assessment Report of the IPCC. (Unlabeled version here)

One loading of fuel lasts 18+ months in a reactor, and they generally operate for that long non-stop. No cloudy days or calm nights will prevent nuclear energy from being delivered to those who depend on it. While uncommonly done due to current market structures, today's nuclear reactors are perfectly capable of ramping their power up and down daily, to the tune of 2-5% full power per minute!
Graph showing that nuclear reactors can load follow
Nuclear plants can load follow, and will if we set up markets to encourage it. (OECD-NEA)
This can be an important complement to low-carbon but uncontrollably-intermittent power sources like wind and solar.

We have enough nuclear fuel resources to power the world for literally billions of years with advanced reactors. Even with conventional reactors, peak uranium is far off.

Bar graph showing how many years you could power the world using nuclear fuel from different sources. It starts with mined uranium in conventional reactors going just a few years to crustal uranium in breeder reactors going for 4 billion years.

Humans use a lot of energy, and we’re using more every day. Between 2000 and 2010, the world total energy consumption rose by an astounding 29% [1]. Choices about our consumption of energy are fundamental to the primary geopolitical and environmental struggles of our day. Nuclear energy is a strong candidate for supplying our energy while alleviating these struggles.

What are the downsides of nuclear energy?

Of course nothing’s perfect. Long-standing questions and concerns abound regarding nuclear energy. Click for details.

When heavy atoms split and release energy, the two smaller atoms remaining (called fission products) are often left with some extra energy to give off. This energy is released over a period of time (the longest-lived waste lasting 100,000+ years) in the form of energetic particles called radiation. The high radiation is hazardous and must be kept isolated from the biosphere. We have not yet agreed on what should be done with this high-level nuclear waste.

Waste solutions

We know how to deal with nuclear waste safely. The Finns simply chose to go ahead and solve their nuclear waste issue and built the repository at Onkalo. We have good experience with deep geologic disposal in salt deposits that have been stable for 250 million years. Research in deep borehole technology is also looking promising. Finally, if we close the fuel cycle and recycle spent fuel, then it decays to safe levels in several hundred years rather than hundreds of thousands. Furthermore, despite the fear, few people, if any, have ever been injured by stored commercial nuclear waste.

We have a detailed page dedicated to nuclear waste here.

The radioactive fission products are hottest when a reactor first shuts down. In effect, you can’t shut a reactor completely off. This decay heat must be cooled or else the containment structures that hold the fuel and waste can breach, releasing radiation into the biosphere. Accidents at Fukushima and Three Mile Island were caused by this effect. Unstable reactor design and operation at Chernobyl led to a power excursion and widespread dispersal of radioactive material. So, people worry about reactor safety.

A graph showing
power of a nuclear reactor before and after shutdown.
Safety solutions

Nuclear energy has actually saved over 1.8 million lives by displacing air-pollution related deaths that would have occurred had fossil or biofuel plants been built instead of the clean-air nuclear ones [2]. This includes the health effects of the nuclear accidents. So they’re like airplanes; when one goes down, it is a major disaster and huge story, but when you look at the data, it is clear that nuclear reactors are one of the safest ways known to produce energy. And advanced designs can make them even safer.

Nuclear safety and risk details

The first application of fission was as an atomic bomb. While nuclear reactors and atomic bombs are significantly different machines, there is some technology overlap, especially in fuel cycle facilities like enrichment and reprocessing plants. So, some people argue that having reactors around might make it easier to spread nuclear weapons.

Proliferation solutions

It is important for nuclear facilities to monitor nuclear material. That said, advanced designs are being developed that reduce reliance on enrichment. Actually, nuclear reactors are useful for peacefully destroying nuclear weapons, and between the late 1990s and 2013, fully 10% of the US electricity was generated in nuclear reactors using dismantled ex-Soviet nuclear warheads in the Megatons-to-Megawatts program.

Read more about proliferation »

Nuclear reactors are generally large and complex, with lots of reinforced concrete and nuclear-grade quality assurance programs. As a result, they tend to be expensive to build. Once they’re built, the fuel and operating costs are relatively cheap, but the capital cost is a major hurdle.

Cost Solutions

If carbon dioxide is ever treated as a pollutant, then nuclear reactors will become much more competitive. But there is definitely room to improve! Research is ongoing in many venues to reduce the cost of nuclear reactors. Countries that chose a standard design and built many of the same have succeeded in bringing costs down.

Read more about economics

A nuanced reality

Nuclear fission’s ability to responsibly produce global-scale, 24/7, (nearly) carbon-free energy is unmatched among known technologies.

Next-generation reactor designs exist that can further reduce waste, improve safety, increase proliferation resistance, and reduce costs. Even if someone doesn’t support current nuclear, it is difficult for them to disregard all possible improvements. We humans have made impressive accomplishments before.

Of all the known energy resources, nuclear is perhaps the most passionately debated and least understood. Our goal is to explain what makes some people so excited and supportive, and what makes others so passionately opposed. There are many sides to each story. Let’s explore them deeper.

More intro: A primer on energy, greenhouse
gas, intermittency, and nuclear

Fission and Fusion

There are two fundamental nuclear processes considered for energy production: fission and fusion.

  • Fission is the energetic splitting of large atoms such as Uranium or Plutonium into two smaller atoms, called fission products. To split an atom, you have to hit it with a neutron. Several neutrons are also released which can go on to split other nearby atoms, producing a nuclear chain reaction of sustained energy release. This nuclear reaction was the first of the two to be discovered. All commercial nuclear power plants in operation use this reaction to generate heat which they turn into electricity.
  • Fusion is the combining of two small atoms such as Hydrogen or Helium to produce heavier atoms and energy. These reactions can release more energy than fission without producing as many radioactive byproducts. Fusion reactions occur in the sun, generally using Hydrogen as fuel and producing Helium as waste (fun fact: Helium was discovered in the sun and named after the Greek Sun God, Helios). This reaction has not been commercially developed yet and is a serious research interest worldwide, due to its promise of nearly limitless, low-pollution, and non-proliferative energy. Read more at our fusion page.

Where to go from here

Take a look at the navigation bar on the top of the page (or click the line-icon if you’re on a small screen). You’ll find information on all sorts of relevant topics. To get started, check out the what is a nuclear reactor? page.

Other highlights include:

Our goal

To answer this common sentiment:

"Honestly, my gut feeling is that I’m not in favor of it, but I don't know hardly anything about it."

"I second that."

"Energy is part of a historic process, a substitute for the labor of human beings. As human aspirations develop, so does the demand for and use of energy grow and develop."