What is nuclear energy?

A lamp

Everything around you is made up of tiny objects called atoms. Most of the mass of each atom is concentrated in the center (which is called the nucleus), and the rest of the mass is in the cloud of electrons surrounding the nucleus. Protons and neutrons are subatomic particles that comprise the nucleus.

Under certain circumstances, the nucleus of a very large atom can split in two. In this process, a certain amount of the large atom’s mass is converted to pure energy following Einstein’s famous formula E = MC2, where M is the small amount of mass and C is the speed of light (a very large number). In the 1930s and ’40s, humans discovered this energy and recognized its potential as a weapon. Technology developed in the Manhattan Project successfully used this energy in a chain reaction to create nuclear bombs. Soon after World War II ended, the newfound energy source found a home in the propulsion of the nuclear navy, providing submarines with engines that could run for over a year without refueling. This technology was quickly transferred to the public sector, where commercial power plants were developed and deployed to produce electricity. Read more about the history of nuclear energy.

Nuclear Energy Today

Nuclear reactors produce just under 20% of the electricity in the USA. There are over 400 power reactors in the world (about 100 of these are in the USA). They produce base-load electricity 24/7 without emitting pollutants (including CO2) into the atmosphere. They do, however, create radioactive nuclear waste which must be stored carefully.

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.

Click here to see animations of fission and fusion reactions.

Energy density of various fuel sources

The amount of energy released in nuclear reactions is astounding. Table 1 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 very 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

Table 1 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.

Capabilities of Nuclear Power


Table 1 sums the sustainability of nuclear power up quite well. However, there is quite a bit of talk about nuclear fuel (Uranium) running low just like oil. Technically, this is a non-issue, as nuclear waste is recyclable. Economically, it could become a major issue. Today’s commercial nuclear reactors burn less than 1% of the fuel that is mined for them and the rest of it or so is thrown away (as depleted uranium and nuclear waste). The US recycling program shut down in the ’70s due to proliferation and economic concerns. Today, France and Japan are recycling fuel with great success. New technology exists that can greatly reduce proliferation concerns. Without recycling, the 2005 Uranium Reserves “Red Book” published by the U.N. IAEA suggests that there are over 200 years of Uranium reserves at current demand. There is also a very large supply of uranium dissolved in seawater at very low concentration. No one has found a cheap-enough way to extract it yet, though people have come close. Nuclear reactors can also run on Thorium fuel.


In operation, nuclear power plants emit nothing into the environment except hot water. The classic cooling tower icon of nuclear reactors is just that, a cooling tower. Clean water vapor is all that comes out. Very little CO2 or other climate-changing gases come out of nuclear power generation (certainly some CO2 is produced during mining, construction, etc., but the amount is about 50 times less than coal and 25 times less than natural gas plants. Details coming soon). The spent nuclear fuel (nuclear waste) can be handled properly and disposed of geologically without affecting the environment in any way.

They’re safe too. In March, 2013 the former NASA scientist James Hansen (of the 350 ppm limit fame) published a paper showing that nuclear energy has saved a total of 1.8 million lives in its history worldwide just by displacing air pollution that is a known killer2. That includes any deaths nuclear energy has been responsible for from its accidents.


With nuclear power, many countries can approach energy independence. Being "addicted to oil" is a major national and global security concern for various reasons. Using electric or plug-in hybrid electric vehicles (PHEVs) powered by nuclear reactors, we could reduce our oil demands by orders of magnitude. Additionally, many nuclear reactor designs can provide high-quality process heat in addition to electricity, which can in turn be used to desalinate water, prepare hydrogen for fuel cells, or to heat neighborhoods, among many other industrial processes.

Problems with Nuclear Power

Nuclear Waste

When atoms split to release energy, the smaller atoms that are left behind are often left in excited states, emitting energetic particles that can cause biological damage. Some of the longest lived atoms don’t decay to stability for hundreds of thousands of years. This nuclear waste must be controlled and kept out of the environment for at least that long. Designing systems to last that long is a daunting task — one that been a major selling point of anti-nuclear groups.

Dramatic accidents

Three major accidents have occurred in commercial power plants: Chernobyl, Three Mile Island, and Fukushima. Chernobyl was an uncontrolled steam explosion which released a large amount of radiation into the environment, killing over 50 people, requiring a mass evacuation of hundreds of thousands of people, and causing up to 4000 cancer cases. Three Mile Island was a partial-core meltdown, where coolant levels dropped below the fuel and allowed some of it to melt. No one was hurt and very little radiation was released, but the plant had to close, causing the operating company and its investors to lose a lot of money. Fukushima was a station black-out caused by a huge Tsunami. Four neighboring plants lost cooling and the decay heat melted the cores. Radiation was released and the public was evacuated. These three accidents are very scary and keep many people from being comfortable with nuclear power.


Nuclear power plants are larger and more complicated than other power plants. Many redundant safety systems are built to keep the plant operating safely. This complexity causes the up-front cost of a nuclear power plant to be much higher than for a comparable coal plant. Once the plant is built, the fuel costs are much less than fossil fuel costs. In general, the older a nuclear plant gets, the more money its operators make. The large capital cost keeps many investors from agreeing to finance nuclear power plants.


  1. Office of Air Quality, “Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units – Final Report to Congress,” EPA-453/R-98-004a, 1998 (Online)
  2. P. Kharecha and J. Hansen, “Prevented Mortality and Greenhouse Gas Emissions from Historical and Projected Nuclear Power,” Environ. Sci. Technol., 2013, 47 (9), pp 4889–4895 (Press release)
  3. A. Gabbard, "Coal Combustion: Nuclear Resource or Danger," (online).