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Nuclear power is the process of harvesting heat from nuclear fission and fusion to produce electricity (usually by heating water to spin a turbine). All currently operating nuclear power plants use the energy from fission of heavy elements (usually uranium and plutonium) as their source of nuclear energy. Nuclear power been controversial since the 1970s and there have been three high profile accidents, but it has re-emerged in the debate about the future of energy production among concerns about global warming and the need to move towards zero carbon emissions methods of producing electricity.
Modern nuclear power is more or less a spin-off of the technology developed to power nuclear submarines, itself an offshoot of Project Manhattan, the United States effort to build the first nuclear bomb. The first nuclear bomb, known as "the gadget", exploded in 1945 in the Trinity test. The first nuclear submarine, USS Nautilus, was launched in 1954. It used a pressurized water reactor, the most popular design used in power stations today. The first power station was started in 1954 in Obninsk in the USSR. It was a 5 MW prototype. Some do not consider it a "real" power plant. The first commercial power plant, Calder Hall, was put into operation in 1956 in Windscale (now Sellafield) in the United Kingdom. It had four Magnox reactors for plutonium production with cooling systems modified to include steam turbines to generate 50 MW of electricity each, for a total of 200 MW. The first electricity-only power station was started in 1957 in Shippingport, Pennsylvania in the United States. It was essentially a modified beached submarine reactor, generating 60 MW of electricity.
A typical modern station is much more powerful than those early designs. A single reactor generates from 400 to 1400 MW, and a power station is virtually unlimited in output. The largest one, Kashiwazaki-Kariwa in Japan, has a total capacity of 8212 MW.
 Types of reactors
There are very many ways to convert the heat from nuclear fission to create electricity with a turbine. Here are some parameters that can be varied.
- Coolant - Substance that transfers the heat generated in the core to the electricity generation system. This can be ordinary water, heavy water, helium, carbon dioxide, even molten metals such as sodium, lead and lead-bismuth alloy, and even liquid salts of fluorine and chlorine.
- Moderator - Substance used to slow down neutrons, making them more likely to react with the fuel (usually uranium-235). The most popular choices are ordinary water, heavy water and graphite. Molten lithium-beryllium fluoride could also be used. There is also a group of designs, known as fast reactors, which do not use a moderator.
- Fuel material - Chemical form of fuel. Most reactors use uranium dioxide, but metallic fuel and ceramics such as uranium nitride have been used. It is also possible to use molten uranium or thorium salt.
- Fuel cladding - Material used to hold ceramic fuel pellets together. The material used is an alloy of zirconium (typically >95%) with tin and other metals, also known as zircalloy.
- Core type - The core can reside in single large tank, or a series of small tubes. CANDU and RBMK are examples of a tubular reactor.
 Light water reactor
Ordinary water can act both as a moderator and as a coolant, so the roles can be combined. This leads to a group of designs known as light water reactors. The drawback is that water absorbs some neutrons, which means it cannot achieve criticality on natural uranium - it requires uranium enrichment. There are two types of light water reactors. The more popular one is the pressurized water reactor. In a PWR, water circulating through the core is kept under high pressure and undergoes only localized boiling. Its heat is then passed to a second loop of coolant, typically also water, which drives the turbines. The second type is a boiling water reactor. In a BWR, water boils in the reactor core, which acts as a steam generator, and is passed directly into turbines.
LWRs have some degree of inherent safety. If the core overheats, the coolant, which is simultaneously the moderator, evaporates. As a result, the chain reaction slows down and power is automatically reduced. However, if the core ceases to be submerged in water, for example when the control equipment fails to detect a steam leak, the radioactive decay of fission products can generate enough heat to melt the fuel elements. This is known as a core meltdown. The two most well known cases of a meltdown are the Three Mile Island accident and the Fukushima Daiichi disaster.
Heavy water, aka deuterium oxide, has physical properties similar to ordinary water, but is a very weak neutron absorber. This means a reactor cooled and moderated with heavy water can run on natural uranium and doesn't require enrichment. In practice, low enriched uranium is used to achieve a higher power density. This technology was pursued by Canada, leading to heavy water-cooled, heavy water-moderated reactors known as CANDU (CANada Deuterium Uranium). They are in use in 7 countries.
Compared with light water reactors, CANDU has three advantages. Firstly, the horizontal orientation of fuel tubes allows refueling without turning off the reactor, which improves availability. Secondly, this design does not require large steel forgings for the pressure vessel, which can be made in only a few facilities in the world. Thirdly, its fuel can be of a lower grade than is required in most light water reactors — in fact, it can even use some of the spent fuel from a light water reactor as its own fuel.
CANDU's main disadvantage is that heavy water is expensive and requires dedicated infrastructure to produce. Improvements in the original CANDU design, known as Advanced CANDU, could get by with only a quarter of the heavy water needed by current CANDU reactors.
 Breeder reactors
Breeder reactors are reactors capable of transmuting low-quality fissionable material, such as thorium or depleted uranium, into more highly fissionable materials such as plutonium. Breeders usually require an initial amount of high grade fissile fuel to start the reaction. After this initial startup, most breeders can run without additional high grade fissile fuel.
 Other reactor technologies
This is not an exhaustive list of reactor technologies. Take for example the lead cooled and lead-bismuth cooled reactors of the Soviet Union. There are a plethora of plausible designs in various stages of development (usually in the stage "not being developed"), such as sodium cooled fast reactors, and liquid fluoride reactors. Each kind of reactor has its own particular unique advantages and disadvantages, the most obvious disadvantage of some is that they have not yet been demonstrated.
 Pros and cons of nuclear power
- Emissions-free electricity. Lots of it.
- Can be built almost anywhere.
- Low environmental impacts.
- Very high power density, more than 1000 W/m2.
- Dispatchable - generation is not subject to whims of the weather.
- Creates high income jobs.
- High up-front capital costs.
- Long lead times due to heavy regulation of the industry and the complexity of construction.
- Generates nuclear waste.
- Possibility of severe accidents.
- Nuclear weapon proliferation concerns.
- Some reactors types "burn" enriched uranium, which is a limited resource.
 Neither pros nor cons
You might note some things often cited as drawbacks of nuclear power are absent above. This includes:
- Production of plutonium for nuclear bombs in a power reactor. This would cost much more than a dedicated reactor.
- Radiation releases. The routine releases from nuclear power plants are at least three orders of magnitude lower than the sum of other variations in background radiation. One LNT-based estimate suggests that nuclear power might reduce the number of radiation-related deaths due to removing uranium from the ground, which reduces future radiation exposures. For the discussion of the most common model of radiation effects, see linear no-threshold.
 Nuclear accidents
The only severe nuclear accidents so far can't be used to make meaningful predictions about future issues. The kind and severity of the Chernobyl accident is not physically possible in any reactor built by the west in 40 years, and furthermore it was also caused by criminal stupidity. Three Mile Island had no off-site impact. Windscale was in a plutonium production reactor, and also failed to kill anyone. Fukushima has not thus far killed anyone off-site from radiation exposure (and at most a handful from on-site radiation exposure). Furthermore, most estimates put the estimated future death toll from radiation exposure around a few hundred to a few thousand, far less than the actual catastrophe: the tsunami. All the nuclear accidents in the history of the world have killed far less people than coal power kills in one year in the United States alone (estimated at 13,200 people per year). While exact numbers are hard to pin down, it looks like global coal power plant pollution kills as many people as the Chernobyl disaster every few weeks.
 Public opinion
An opinion poll conducted by YouGov in March 2011 on UK attitudes to nuclear power found several factors linked to the level of support.
- Men support continued use of nuclear power by 54% to 37%, but women oppose it by 57% to 25%.
- Nuclear power is unpopular with all age groups, but especially so in the 18-24 range (45% oppose to 31% support) and least so in the 60+ group (47% oppose to 43% support).
- London was the one area to express a slight preference for nuclear power (42%-40%), while Scotland was the area with the strongest dislike (52% oppose, 34% support).
- Conservative voters strongly support nuclear (54% to 37%), Labour voters strongly oppose it (56% to 33%) with Lib Dems being pretty much split (49% oppose to 45% support).
 Anti-nuclear movement
There is a very strong anti-nuclear movement. Many people think that the use of nuclear energy is unwise, dangerous and/or unethical; opposition also arises from its association with nuclear weaponry, though the two do not always go hand in hand. Though defenders of the industry would claim that many of the arguments against nuclear power are of a pseudoscientific nature, there is public concern worldwide about the use of the technology. In the U.S. successive governments have resisted the development of new nuclear power plants until recently, although there's indication that this development has been aided by a lot of lobbying and regulatory capture. Whether the public concern is appropriate or properly informed is a matter for debate.
 See also
- ↑ Even large bodies of water for cooling are not always necessary. See w:Palo Verde Nuclear Generating Station.
- ↑ David McKay, Sustainable energy - without the hot air, page 167
- ↑ Depleted Cranium: Why You Can’t Build a Bomb From Spent Fuel
- ↑ http://www.physics.isu.edu/radinf/np-risk.htm
- ↑ http://washingtonindependent.com/97196/study-predicts-13200-deaths-from-coal-pollutants-this-year
- ↑ coal kills 100,000 people per year worldwide: http://www.guardian.co.uk/science/blog/2011/apr/04/fear-nuclear-power-fukushima-risks Chernobyl killed 4,000 people total, so https://www.wolframalpha.com/input/?i=4000%2F100000+*+year+to+weeks
- ↑ YouGov/The Sunday Times Survey Results, YouGov, 20 March 2011
- ↑ w:Nuclear renaissance#United States
- ↑ http://motherjones.com/politics/2011/03/japan-nuclear-regulatory-commission