What is nuclear energy?

Overview of Nuclear Energy

A lamp Nuclear energy comes from mass-to-energy conversions that occur in the splitting of atoms larger than Iron or joining atoms smaller than Iron. The small amount of mass that is lost in either of these events follows Einstein’s famous formula E = MC2, where M is the small amount of mass and C is the speed of light. 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.

Nuclear Energy Today

Nuclear reactors produce about 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 any pollutants into the atmosphere (this includes CO2). They do, however, create radioactive nuclear waste that must be stored carefully.

Fission and Fusion

  • Fission is the splitting of a large atom such as Uranium or Plutonium into two smaller atoms, called fission products. Also released during such a fission are several neutrons (that enable a chain reaction) and substantial energy. This nuclear reaction was the first to be discovered. All commercial nuclear power plants use this reaction to generate 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 radioactive byproducts. Fusion reactions occur in the sun, using Hydrogen as fuel and producing Helium as waste. This reaction has not been commercially developed and is a serious research interest worldwide, due to its promise of limitless, pollution-free, and non-proliferation features.

Click here to see animations of fission and fusion reactions.

This site focuses on nuclear fission. In order to harness fusion, many daunting engineering and physics problems must be solved. The timeline for solving these problems is undefined, so we as a society must turn to other solutions to solve the energy problems.

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 materials. 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 10% of the fuel that is put into them and the other 90% or so is thrown away. 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 nearly infinite supply of uranium dissolved in seawater at very low concentration. No one has found a cheap way to extract it yet. Nuclear reactors can also run on Thorium fuel, which is 4x more abundant than Uranium in the crust.


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. Coal contains about 4 ppm thorium and uranium, and the radioactive dose given to the public by coal-fired plants is about 100 times the dose given by nuclear plants1. Now that's something to think about.

See the nuclear waste article for more info.


With nuclear power, the USA (and other countries!) can attain true energy independence. Being "addicted to oil" is a major national security concern for various reasons. Using plug-in hybrid electric vehicles (PHEVs) powered by nuclear reactors, we could reduce our oil demands by orders of magnitude. Additionally, nuclear reactor fuel is usually in a ceramic form, capable of reaching temperatures of 2000 degrees C and higher. At these temperatures, water can be thermo-chemically separated into Hydrogen and Oxygen from the waste heat of the electric power plant! The hydrogen could be put in fuel cells for vehicles, eliminating our need for oil altogether. Granted, practical hydrogen fuel cell technology is still a few years down the road. The technology is steadily advancing and a emissions-free distributed and transportation energy solution is on the radar with nuclear power. The waste heat could also be used for district heating or, conceivably, to power chemical processes that capture methane and carbon dioxide from the atmosphere to convert it back to hydrocarbon fuels.

Most of the world supply of uranium is in Australia and Canada. With fuel recycling, we wouldn’t need to mine any more uranium.

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. These dangerous materials 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.

Safety issues

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 30 people, requiring a mass evacuation of hundreds of thousands of people, and causing over 2000 cancer cases. Three Mile Island was a partial-core meltdown, where coolant levels dropped below the fuel in a reactor and allowed some fuel 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.

The next stop

Go on to our nuclear reactor page to find out about how nuclear energy is turned into electricity.


Sodium-cooled fast reactor
  1. http://www.epa.gov/ttn/caaa/t3/reports/eurtc1.pdf
  2. http://www.ornl.gov/info/ornlreview/rev26-34/text/colmain.html