The main use of nuclear energy is for electricity generation, with limited use on board marine vessels  and spacecraft. In 2011, there was 369 GW of installed nuclear power generation  capacity in 31 countries. This generated 11.7 percent of total global electricity  without direct greenhouse gas emissions. In 2013, nuclear energy generated 19.4 percent of all U.S. electricity.
Figure 1: Installed Nuclear Capacity
Source: International Energy Agency 
Nuclear power is generated using nuclear fission, which occurs when a neutron strikes the nucleus of a very heavy atom of an element that can sustain a chain reaction, splitting the atom and releasing energy in the form of heat and radiation. The energy released in the fission of one gram of uranium is equivalent to the energy released by combusting three tons of coal .
Figure 2: Nuclear Fission
Source: Visual Dictionary Online 
Under precise, controlled conditions, the nuclear fission process occurs as a continuous chain reaction that releases heat in useful amounts to create steam, which turns a turbine to produce electricity.
All U.S. nuclear power plants are light water reactors —so called because they use ordinary water to produce electricity, as opposed to heavy water (known as deuterium oxide) reactors used in Canada . There are two types of light water reactors, the pressurized water reactor and the boiling water reactor. In a pressurized water reactor, the water passing through the reactor core is kept under pressure so that it does not turn to steam but rather is used to exchange heat with a separate water loop to create steam and power a turbine-generator. With this design , most of the radioactivity stays in the reactor area. Whereas in a boiling water reactor, the water heated in the reactor vessel turns directly into steam to power the turbine-generator .
Figure 3: Pressurized Water Reactor
Source: U.S. Nuclear Regulatory Commission 
Figure 4: Boiling Water Reactor
Source: U.S. Nuclear Regulatory Commission 
The United States has 65 pressurized water reactors  and 35 boiling water reactors. Nuclear reactors are often classified in terms of their reactor generation, or stage of reactor technology development:
Uranium is a naturally occurring heavy metal whose most common isotope is the U-238, which is not capable of sustaining a nuclear chain reaction. Nuclear power plants predominantly use U-235, an isotope of uranium that can sustain such a reaction, as their fuel. To get the natural uranium into a useable fuel, it undergoes several processes.
Figure 5: Nuclear Fuel Cycle
Source: Congressional Research Service, 2011 
Uranium is extracted  from the earth through various mining methods, such as open-pit or underground mines. Due to its low capital costs , most uranium mines in the United States  use in situ leaching (ISL), where dissolved uranium is pumped to the surface with the groundwater. After mining, uranium ore is milled and leached with acid, resulting in a concentrated uranium oxide powder referred to as yellowcake.
Power plant operators usually purchase yellowcake and then contract it to be enriched by a third party. There are five commercial conversion (see fig 5) companies worldwide — in the United States, Canada, France, the United Kingdom, and Russia. Nearly all uranium enrichment  is supplied by facilities in the United States, France, Great Britain, Germany, the Netherlands, and Russia.
After enrichment , the uranium is fabricated into ceramic fuel pellets and loaded into tubes called fuel rods. A fuel assembly used in the reactor core of a nuclear power plant consists of a square or circular bundle of fuel rods. For more information, see NRC: Stages of the Nuclear Fuel Cycle  and World Nuclear Organization .
When a nuclear reactor’s fuel is spent, or no longer capable of supporting an adequate chain reaction, the fuel must be replaced. Typical reactor refueling intervals  vary from 12 to 24 months, after which the reactor is shut down for a few weeks for refueling and maintenance.
Spent nuclear fuel consists mostly of depleted uranium (up to 96 percent) mixed with certain highly radioactive elements—namely, fission products (e.g., cesium and strontium) and elements such as plutonium and americium. The decay heat and radiotoxicity of spent nuclear fuel is dominated by the fission products for roughly the first hundred years and then by the other elements for thousands of years afterward.
The conventional, once-through fuel cycle (the current U.S. approach) involves nuclear reactors that use enriched uranium as fuel and then discharge spent nuclear fuel for disposal. There are two alternative fuel cycles—a single-pass recycle option and a fully closed fuel cycle that would use anticipated advanced technology. The single-pass recycle option (currently used in France) involves reprocessing spent nuclear fuel to separate fissile uranium and plutonium from other nuclear waste. This material can then be recycled for future fuel fabrication, which reduces the volume of nuclear waste requiring disposal but not necessarily the decay heat and radiotoxicity of the waste.
A Massachusetts Institute of Technology study concluded that this single-pass recycle option costs more than a once-through cycle, and that the waste management benefits from a closed fuel cycle do not outweigh the attendant safety, environmental, and security considerations and economic costs . In a proposed fully closed fuel cycle, spent nuclear fuel could be reprocessed with the separated uranium, plutonium, and other long-lived radioisotopes recycled as fuel. This could reduce the long-term burden on the final nuclear waste repositories by reducing long-term decay heat and radioactivity. However, it would not eliminate the need for long-term disposal because there are long-lived fission products and wastes from processing operations that will still require permanent geological disposal. A fully closed fuel cycle, however, requires advanced fast burner reactors that are not yet commercially available. In theory, spent nuclear fuel from these fast reactors could be repeatedly reprocessed  until all the useable fuel was fissioned while also converting nearly all the uranium in the fuel cycle to useful fuel.
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While nuclear power emits no greenhouse gases or other air pollutants  and has a lower overall lifecycle greenhouse gas emissions profile than fossil fuels , nuclear power carries a number of unique challenges and risks. Paramount among these risks is the (albeit low) threat of a nuclear meltdown. Since the dawn of the nuclear age in the 1950s, the global nuclear power industry has experienced three serious nuclear reactor accidents – most recently in 2011 at Fukushima Daiichi in Japan – and several fuel cycle facility incidents . Nuclear reactor damage is a threat to public health as it can lead to release of radioactivity to the air and groundwater. To date , the United States has had no immediate radiological injuries or deaths among the public attributable to accidents involving commercial nuclear power reactors.
There is also concern in the U.S. about the storage of radioactive waste in the form of spent fuel rods. Countries such as France reprocess the plutonium from spent fuel rods with uranium , which can be used again in nuclear reactors. The United States currently does not engage in spent fuel processing, due to a number of factors such as concerns about nuclear weapons proliferation from the plutonium waste, and the cost of reprocessing compared to the once-through fuel cycle. Currently, spent fuel rods are stored onsite at nuclear power plants, initially in specially designed pools which cool the waste for several years and are later housed onsite in dry storage containers. The long-term storage of these wastes is currently a major public policy issue  in the United States.
Globally, there is concern about the spread of nuclear weapons, also known as nuclear proliferation. Many technologies and materials associated with civilian nuclear power can be adapted and used to make nuclear weapons. Organizations like the International Atomic Energy Agency  monitor nuclear programs around the world to ensure safety, security and transparency, among other things. Additionally, six reactor technologies have been chosen for future development by the Generation IV International Forum  for being resistant to diversion of materials for weapons proliferation.
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Three distinct categories characterize different levels of confidence regarding the existence of uranium resources. Reasonably assured resources are estimated quantities of uranium that could be economically recovered with current technologies and mining practices. In the United States, the term “reserve” is used interchangeably with reasonably assured resources. Estimated additional resources build on reasonably assured resources, and include likely uranium resources based mostly on geological evidence in little-explored deposits and in undiscovered deposits along well-defined deposits. In addition to estimated additional resources, speculative resources include uranium thought to exist mainly based on direct evidence and geological extrapolation.
Forward costs , which include the operating and capital costs that will be incurred in any future production, are used to estimate the total amount of uranium ore  (reserves) that can be extracted from different geological settings, and are independent of market price for the uranium produced from these reserves. Since 1975, estimates of world reserves  have increased nearly threefold due to higher expenditures for uranium exploration. Organization for Economic Co-operation and Development’s Nuclear Energy Agency reports  that in 2009, global uranium reserves for concentrated uranium oxide are approximately 7.7 billion pounds at a forward cost of $59 per pound. At this forward cost, Australia has the largest known recoverable resource at 2.6 billion pounds, accounting for nearly a third of the world’s total. The United States is ranked sixth globally at 457 million pounds . In comparison, the U.S. Energy Information Administration estimates 539 million pounds  of yellowcake uranium are recoverable in the United States at $50 per pound.
Figure 6: Known Recoverable Resources of Uranium
Source: World Nuclear Organization , 2014
Most of U.S. uranium reserves  are located in the western part of the country. In 2008, Wyoming led the nation in total uranium reserves, with New Mexico second. Taken together, these two states constituted about two-thirds of the estimated reserves available at up to $100 per pound of yellowcake uranium, and three-quarters of the reserves available at less than $50 per pound of yellowcake uranium.
Figure 7: Location of Major U.S. Uranium Reserves
Source: U.S. Energy Information Administration 
Just shy of 129 million pounds of uranium concentrate was produced from uranium resources worldwide in 2012. Around 37 percent of this came from Kazakhstan, 15 percent from Canada and 12 percent from Australia.
Figure 8: Uranium Produced from Mines
Source: World Nuclear Association , 2014
About 45 percent of this production was from In situ leach mining , 28 percent from conventional underground mining, 17 percent from open pit mining, and the rest was sourced as a byproduct of other mining endeavors. Overall world production has been increasing since the 1990s.
United States production and mining of uranium has changed significantly over the last 60 years. Uranium production in the 1950s  was initially stimulated by government incentives, which, along with key reserve discoveries, led to uranium prospecting in the 1950s. Uranium concentrate production decreased in the 1960s and increased again during the energy crises in the 1970s, peaking in 1980. In 2011, production was 4 million pounds of concentrated uranium oxide from eight mines .
Figure 9: U.S. Uranium Concentrate Production
Source: U.S. Energy Information Agency, Table 9.3 , 2014
Other sources of fissionable material include: stocks of uranium from civilian and military inventories and nuclear fuel from spent fuel reprocessing and military plutonium. The “Megatons to Megawatts ” program has converted about 17,698 Russian nuclear warheads  to fuel U.S. power plants. This 20-year program concluded in 2013.
The electricity sector is the principal consumer of nuclear energy; a small amount is used for ship propulsion and in spacecraft. From 1980 to 2011, global nuclear electricity installed capacity grew at an annual rate of 3.3 percent to 369 GW. Over the same period, U.S. installed capacity grew at an annual rate of 2.2 percent to 101 GW.
Figure 10: Nuclear Electricity Installed Capacity (GW)
Source: U.S. Energy Information Agency, 201 4
The United States’ nuclear generation capacity makes up about 27 percent of the world’s total nuclear generation capacity. In 2012, 100 nuclear reactors at 65 plants in 31 states provided about 19 percent of total U.S. electricity (around 769 billion kilowatt-hours).
In 2012, owners and operators of U.S. civilian nuclear power reactors purchased the equivalent of 58 million pounds  (29,000 tons) of Uranium. 83 percent of the uranium  purchased by U.S. nuclear plants is imported. In 2012, uranium from Australia and Canada, combined, accounted for 36 percent of uranium purchases, while uranium from Kazakhstan, Russia and Uzbekistan accounted for 29 percent.
It is widely believed that if demand were to increase significantly beyond current and projected levels, uranium resources would still be sufficient  for several decades. In the first three decades of civilian nuclear power, production of uranium was higher than worldwide demand. This resulted in large military and civilian stockpiles of uranium, which were used in subsequent years.
Processing costs make up 88 percent of the final cost of uranium , while they make up a smaller share of the final costs for other electricity fuels like coal (42 percent) and natural gas (33 percent). Yet unlike fossil-fuel electric generation, the cost of uranium is a small percentage of the total cost of nuclear power. As such, if fuel costs were to double , the cost of producing electricity from coal would increase by more than 50 percent, natural gas by more than 90 percent, and nuclear power by only 10 to 15 percent.
The uranium market has a number of characteristics that differentiate it from other energy and commodity markets. Since the application of uranium is very restricted (i.e. nuclear weapons, and fuel for nuclear power plants), there are a limited number of producers and consumers. Moreover, the uranium is traded globally in two distinct markets  based on Cold War divisions – the Americas, Europe, and Australia in one market and Russia, former USSR nations, Eastern Europe, and China in the other. During the Cold War, uranium use in nuclear weapons was the primary driver of demand. After the Cold War though, the uranium market started to behave more like a traditional commodity market.
The world spot price for uranium increased in the early 1970s due to growing reactor orders, but fell sharply following the Three Mile Island  accident in 1979. Specific industry events (e.g., incidents at nuclear plants or nuclear processing facilities) or the influx of secondary supply sources (from nuclear weapons – Megatons to Megawatts) tend to have a large impact on uranium prices. Beginning in the late 2003, global uranium prices began to increase after a fairly long sustained period of low prices. This was due to the expansion of nuclear power generation in growing economies like China and India, and a recognition that nuclear power can provide reliable, baseload generation with almost no greenhouse gas emissions. Note that uranium does not trade on an open market  like other commodities; buyers and sellers negotiate contracts privately. Prices are published by independent market consultants Ux Consulting and TradeTech.
Figure 11: Uranium Spot Price
Source: Cameco, 2014 
In the United States, the market for uranium concentrate is differentiated by country of origin and contract type . The weighted average price for all purchases was $54.99 per pound in 2012. Spot contracts made up 14 percent of sales with a price of $51.04 per pound, while the remaining was sold under longer-term contracts with a price of $55.65 per pound. U.S.-origin uranium made up 17 percent of sales in 2012. Average prices have increased more than four-fold  since 2004.
Figure 12: Annual Weighted Average Price of Uranium Purchased by Owners and Operators of U.S. Civilian Nuclear Power Reactors
Source: U.S.Energy Information Administration 
By 2040, global nuclear generation capacity is expected to increase by an average annual rate of 2.1 percent  to 717 GW. Led by China and India, growth in developing countries is projected to be more than seven times  higher than in developed countries.
Figure 13: Projected World Nuclear Generation
Source: U.S. Energy Information Administration, International Energy Outlook 201 3 
Generation from U.S. nuclear power plants is projected to grow at an average annual rate of 0.2 percent  to nearly 811 billion kWh by 2040, but its share of total generation is projected to fall to 16 percent. By the end of this period, China is projected to surpass the United States as the nation with the largest generating capacity - 160 GW.
Although five plants were retired in 1997 and 1998 , U.S. nuclear capacity today is slightly higher than in 1996 due to modifications that increased capacity at existing plants. Exelon, the largest operator of nuclear power generation in the United States expects to add an additional 1,300 MW  of nuclear capacity by 2017 by modifying some of its existing plants. It also plans to retire its Oyster Creek unit (636 MW) at the end of 2019. TVA is expected to add capacity  in 2015 when Watts Bar 2 is brought online. The United States Nuclear Regulatory Commission approved a construction and operating license for Southern Company’s two-reactor Plant Vogtle expansion in February 2012, and for Scana’s two-reactor Plant Summer expansion in March 2012. From 2007 to 2009, several companies submitted applications for new reactor licenses, and in 2012, there have been 3 applications for 8 new nuclear reactors , including an innovative new modular reactor . The renewed interest in U.S. nuclear power is being aided by government loan guarantees  intended to support clean energy technologies.
While nuclear energy is widely used throughout the world, the recent nuclear accident in Japan has many countries reevaluating their nuclear plans. In light of the disaster, Japan has decided to discontinue  a plan to build 14 new nuclear reactors by 2030, but still plans to restart reactors in the future. German policymakers are pushing ahead  with a plan to shut down all nuclear reactors by 2022, and Switzerland has also decided to not replace its five existing reactors. Overall, with an increased focus on nuclear safety, the use of nuclear power is expected to increase , driven by developing countries, especially China and India.
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