Nuclear power

Nuclear power is the process of extracting the binding energy of atomic nuclei – whether by fission, fusion, or radioactive decay – and using it to produce electricity, usually by heating water to spin a turbine. All currently operating nuclear commercial power plants use the energy from fission of heavy elements (usually uranium and plutonium) as their source of nuclear energy.

Nuclear power has been controversial since the 1970s and there have been three high-profile accidents and numerous lesser-known incidents, but it has re-emerged in the debate about the future of energy production among concerns about global warming and the need to reduce carbon emissions. All power plants emit carbon, but nuclear is one of the lowest. What the world really needs is a mature conversation about the role, if any, of nuclear power in society today rather than appeasing to the scientifically illiterate whose knee-jerk reactions to this topic are anything but productive.

Beginnings


Natural nuclear fission reactors can form in uranium deposits where self-sustaining nuclear chain reactions have occurred. Such a reactor was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda. However, those conditions were more likely in the Proterozoic than they are today as the percentage of fissile U235 in naturally occurring Uranium has fallen below the amount needed for a water moderated reactor to work.

Modern nuclear power is more or less a spin-off of the technology developed to power nuclear submarines, itself an offshoot of the Manhattan Project, the US 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, the USS Nautilus (SSN-571), 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. 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. 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 can contain multiple reactors. The largest one by power-output, Kashiwazaki-Kariwa in Japan, has a gross total capacity of 8212 MW (Two 1,356 MW reactor cores with five 1,100 MW cores). The largest by count of operational reactors is Bruce Nuclear Generating Station in Kincardine, Ontario Lake Huron's northeast shore, with eight CANDU reactors each generating roughly 750 MW of electricity.

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 - The fluid that transfers the heat generated in the core to the electricity generation system. This can be ordinary water, heavy water, helium, carbon dioxide, molten metals such as sodium, lead and lead-bismuth alloy, and even liquid salts of fluorine and chlorine.
 * Moderator - The 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 - The 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 - The 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 a 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. Despite this naming convention, both types of light-water reactor pressurize the water. The difference is that the coolant in a BWR goes directly to the turbine, forcing it and the generator to be within the containment building. Because BWRs pressurize the water less than PWRs do, a BWR's containment structure can be made in the form of a cube. Containment buildings for PWRs, however, usually need to be either spherical or cylindrical, and often with a domed roof.

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.

Latest models of a light water reactor include the and the.

CANDU
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 theoretically 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 (CANadian Deuterium Uranium). They are in use in 7 countries.

Compared with light water reactors, CANDU has four advantages.
 * Online refueling
 * The horizontal orientation of fuel tubes allows refueling without turning off the reactor, which improves availability.


 * Smaller fuel tubes
 * The CANDU rejects the long-standing convention of building the entire reactor chamber as one big pressure-vessel, requiring enormous steel castings that can only be made at one facility, the Japan Heavy Steel Works. (Fortunately, this was not in the region that had to be evacuated after Fukushima Daiichi.) Instead it puts each fuel-bundle into its own pressurized tube that can be made with thinner walls thanks to much smaller diameter, allowing Canada to make the tubes in their own factories, and more importantly, allowing each tube to be depressurized and opened independently of all the others to allow for the above-mentioned capability of refueling without shutting down, and allowing the moderator water to remain unpressurized.


 * Cheaper fuel
 * Its fuel can be of a lower grade than is required in most light water reactors &mdash; in fact, it can even use either natural (un-enriched) uranium or some of the spent fuel from a light water reactor as its own fuel.


 * Earthquake safety
 * It would have greatly benefitted Japan if the reactors at Fukushima Daiichi had been CANDU reactors, which offer greater safety in the event of earthquakes. If something caused one of the fuel-bundles to overheat and melt, it would soften its pressure-tube which would sag, shifting the reactor's fuel-geometry to a subcritical configuration, such that the nuclear reaction wouldn't be self-sustaining, making fission in the reactor stop.

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, at the cost of sacrificing the ability to use natural (un-enriched) uranium as fuel.

There was one reactor built at in Switzerland which was an odd combination: heavy-water moderator and carbon-dioxide coolant. It suffered a meltdown, but the Swiss were so worried about the potential dangers of a meltdown that they built the thing in a sealed chamber deep underground, and when the meltdown happened, they simply evacuated the cavern, sealed it up, and waited for the worst of the radioactive material to decay enough for cleanup to be safely achievable.

Graphite Moderated Reactors
Most reactors that use graphite as a moderator are gas-cooled, either with helium or with carbon dioxide. Two such nuclear plants were built in the United States, both of which are now shut down (one was Unit 1 at Peach Bottom Nuclear Plant, the other was Fort St. Vrain in Colorado, the latter of which was converted to a natural gas powered facility). These two American designs used helium as coolant and ran it through heat exchangers to boil water into steam that would run a turbine. Unfortunately, helium is a byproduct of oil refining, and thus helium-cooled reactors could never fully de-carbonize the electric grid. Designs are being researched that would use a gas-to-gas heat exchanger and a gas turbine, potentially allowing higher temperatures needed in order to achieve greater thermodynamic efficiency.

All of the nuclear power plants in Great Britain are graphite-moderated except for their newest nuclear power station which uses a French water-cooled-and-moderated design. Unlike the two American prototypes, the British gas-cooled reactors were cooled using carbon dioxide. Britain's first nuclear plant to generate electricity on the commercial grid, Calder Hall, was located at Windscale. These early designs used not carbon dioxide, but plain air as coolant. Nuclear physicist Sir John Cockcroft was extremely vocal in his protests that the design was too risky and demanded the addition of filters to the chimneys that would exhaust air that had passed through the reactors. The design never specified for filters, nor did it provide a place to easily equip filtration, so this was a massive headache for the engineers who complained quite loudly about the inconvenience. Despite these complaints, Cockcroft's demand was heeded. Filters were fitted, at great expense, and many referred to the filters as "Cockcroft's Folly", until when they prevented a then-unprecedented disaster from worsening into an otherwise-certain catastrophe.

A few of France's early reactors were also graphite-moderated and carbon-dioxide-cooled but those have all since been shut down and their newer reactors all use water as both moderator and coolant. Only two graphite-moderated reactor designs ever used water as their coolant, Russia's reactor (the letters stand for "Reaktor Bolshoi Moshchnosti Kanalnyi" which approximately translates to "High Power Channel-type Reactor"), an elaborate, complicated monstrosity that was designed to allow (like the CANDU) for refueling to take place without a shutdown, both to replenish reactor fuel and to extract plutonium for making bombs, but because its pressure tubes were vertical, this required enormous gantry cranes to be integrated into the plant design, in an open hall whose floor was the roof of the reactor core's "biological shield". This reactor was so enormous (the cylindrical reactor core was 14 meters in diameter and 8 meters tall) that a full-fledged containment building was considered infeasible to build as it would have doubled both the construction cost and time. Its control rods, the component which one inserts into the reactor to shut it down, had graphite tips, causing power output to initially go up when first putting control rods in. If it sounds like these design elements invited catastrophe, that would be because they did, which is why all reactors of this type have been shut down…except for those on Russian soil, and why all RBMKs that had been under construction at that time were immediately cancelled. There does exist one other design that uses a graphite moderator with water cooling, and that is the EGP-6 reactor, of which only four exist, all of them at the, the world's smallest nuclear power station (by watts of electricity) and the furthest north. This plant runs a quartet of 12 MWe EGP-6 reactors.

Breeder reactors
Breeder reactors are reactors capable of transmuting low-quality fertile or fissionable material, such as thorium or depleted uranium, respectively, into more highly fissionable materials such as uranium-233 or 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 and only need to replenish the fertile breedstock which is then transmuted into fissile fuel in situ.

Traveling Wave Reactor
Similar to a breeder reactor, this sort of reactor runs on depleted and spent materials, with thorium and natural uranium also being an option. It must be triggered by lightly enriched uranium (about 10%). While still a conceptual piece of hardware, a prototype for the grid reactor is expected to be built by 2020. With an estimated 700,000 metric tons of fuel available in the US alone (not to mention the amount of material generated every day), it has been theorized that the technology has the potential to power the planet up to 10 billion people at US consumption levels for 1,000,000 years. Its other theoretical advantage over other reactor designs is that it would (hypothetically) require fewer moving-parts.

Fusion Reactors
There are two main fusion reactor technologies pursued currently: inertial confinement and magnetic confinement. While the magnetic confinement line of pursuit has met with more success than inertial confinement, both face considerable problems which need to be overcome before fusion power can become economical. The earliest estimate for a commercial fusion power plant is 2033, although that seems overly optimistic given the long history of delays and the considerable technological breakthroughs which have not yet materialized. Nuclear fusion power has been pursued since the 1960s. Notably, the most common experimental fusion reactor seems to be the Toroidal Camera with Magnetic Coils or "Tokamak" reactor. The main problem seems to be that no experiment has consistently reached a power quotient above 1 (other than bombs). That is, all fusion experiments have failed to produce more energy than the energy used to create the fusion reaction. The record so far is .68, set in August of 2021 by NIF. However, until we reach 1, fusion power isn't even possible. Even above 1, the plant still needs to produce enough energy to commercially viable. A facility that costs $250m/yr needs to produce at least $250m/yr of electricity, after all.

A further issue arises in the fusion reaction is if the Deuterium-Tritium reaction is the only method feasible. Hydrogen naturally comes in 3 isotopes, Protium, Deuterium and Tritium, so called for the number of hadrons in the atom's core. Deuterium or H-2 isn't particularly abundant, with just less than 2 in 10,000 atoms, but that's more than enough for hydrolyzation of seawater to produce an inexhaustible supply of H-2. To get a sense of how little tritium is on Earth naturally, picture all the gold dissolved in the oceans, and then divide by a billion. Virtually all Tritium used in labs (and bombs) is produced as decay products from other nuclear reactions. This usually involves Lithium, which is already in extremely high demand for its use in renewable batteries.

Generation IV reactors
Most reactors in operation today belong either to Generation II, III, or III+, with each being an evolutionary improvement over its predecessor. Generation IV reactors, on the other hand, are revolutionary. They are not only much safer, but also more efficient. Some can consume multiple different types of fuel, including radioactive wastes.


 * Supercritical water-cooled reactors use water as coolant but at much greater temperatures and pressures than a typical Generation II or III pressurized-water reactor. Higher temperatures allow for much higher energy efficiency – about 45 percent as compared to 33 percent in PWR. Because the water is in a supercritical phase and can be directed onto the turbine, a steam generator and a secondary loop are not needed. These features reduce costs, and with fewer moving parts, there's fewer points of failure. However, the higher temperatures and pressures require more reinforcement of remaining parts.


 * Very high temperature gas reactor use graphite to moderate neutrons and helium gas as a coolant. One advantage over the current designs is that the helium is an inert gas. Its very high operational temperature of about 950 degrees Celsius, as compared to 315 degrees Celsius for a Generation II reactor, means a more efficient reactor. The very high temperatures could provide large amounts of heat for industrial applications and for the production of hydrogen fuel cells.


 * Gas-cooled fast reactors employ fast-moving neutrons to drive the chain reaction and helium gas to transfer heat from the reactor core. Temperatures would be high enough for the efficient production of both electricity and hydrogen. This type of reactor could be operated in a burner mode to consume long-lived fissionable materials that are radioactive wastes or in a breeder mode to produce more plutonium for fuel, which, in principle, could be used to make weapons-grade fissionable materials.


 * Lead-cooled fast reactors also use fast neutrons but instead would employ liquid lead or lead-bismuth to transfer heat from the core. Besides the ability to operate at sufficiently high temperatures to produce hydrogen, another benefit of this design is its fuel flexibility. It can consume uranium, plutonium, or thorium-based fuels, as well as other fissionable materials. A wide range of power ratings is possible, from smaller 300-megawatts of electrical power (MWe) units to large 1,400-MWe reactors. The former would offer the potential for connection to electrical grids in many densely populated developing countries.


 * Sodium-cooled fast reactors transfer heat with liquid sodium. This design has already been used in a few countries and is not generally considered as revolutionary as the other Generation IV designs. The sodium cooling can pose a hazard in the event of a leak, as sodium is chemically volatile. Introduced in Japan and France, these were found to suffer from poor safety and performance records and were quickly shut down. Russia is presently operating one sodium-cooled fast reactor. India appears interested this type of reactor, but the incentive to do so may change because of India’s newly acquired access to the commercial uranium market for refuelling thermal reactors.


 * Molten-salt fast reactors are in the fast neutron family of designs but employ liquid fluoride salts as coolant with the uranium fuel in the salt mixture. As before, the reactor core temperature would be high enough for hydrogen production. A variant of this design uses graphite for neutron moderation. The uranium fluoride salt fuel offers the advantage of producing no spent fuel assemblies. In theory, they can't melt down; as the fuel is also the coolant, a loss of coolant would result in the reaction immediately halting.  This does not mean an accident is impossible of course; pipes can crack and leak, but a Chernobyl-type event of a massive steam explosion resulting in the entire countryside becoming irradiated is unlikely.  Moreover, they are able to consume long-lived nuclear waste storage waste, thereby reducing the high-level waste storage requirements.  The uranium byproducts useful for nuclear bombs, such as U-233, are "poisoned" with U-232.  U-232 decays to Th-208, which emits enough gamma radiation to make the U-233 extremely difficult to handle.  Producing bombs would not be impossible, but so much extra processing and personnel would be required that it'd be much less practical and much more difficult to hide than an actual dedicated reactor.  China and the United States are actively pursuing this technology, but so slowly that it will never reach fruition.

Carbon emissions
It's widely agreed nuclear power produces much lower carbon emissions than other non-renewables (carbon emissions occur when mining and refining uranium ore to make reactor fuel, concrete for the power plant as well as for waste storage, transportation of fuel and waste, etc). However, there is disagreement as to whether nuclear produces more or less carbon than renewable energies; wind power still requires diesel trucks to transport the turbines, after all. According to a 2010 study, "nuclear energy results in 9-25 times more carbon emissions than wind energy, in part due to emissions from uranium refining and transport and reactor construction (e.g., Lenzen, 2008; Sovacool, 2008), in part due to the longer time required to site, permit, and construct a nuclear plant compared with a wind farm (resulting in greater emissions from the fossil-fuel electricity sector during this period; Jacobson, 2009)". On the other hand the 2014 IPCC calculates nuclear power has a lower median gCO2/kWh value than most renewable energies, excluding wind (see graph). These figures are though medians, not means; according to a separate study, all renewable technologies came in below mean 50 gCO2/kWh, but the average carbon footprint for nuclear power is 66 gCO2/kWh. Nuclear power has therefore been questioned and doubted as a solution to global warming.

Pros

 * Cures cancer. No, really, nuclear power literally does.  Radiopharmacology is the process of using transuranic elements, produced as byproducts from nuclear power plants,, to bind radioactive particles to tumors, destroying the tumors without incredibly invasive surgery nor collateral damage from radiotherapy.  Yes, this involves literally injecting people with nuclear waste.  We don't really have a good source for the various particles outside of actual reactors, so if the nuclear reactors get shut this form of treatment is halted.
 * Detects fires. Another application of nuclear waste products,  is used in household smoke detectors.  Yes, your home contains nuclear waste.
 * High efficiency yet low greenhouse emissions
 * Little land required, due to very high power density, more than 1000 W/m2.
 * Low environmental impacts, which mainly come from mining radioactive ores.
 * The actual operating costs are much cheaper than fossil fuels, roughly 1/3 less. And that's including the high security costs and all those employees with PhD's!
 * Can be built almost anywhere, unlike geothermal, hydro, wind, solar, or other forms of renewable energy.
 * Ability to operate in all conditions short of a hurricane.
 * Water mobile with a fuel density on the order of millions of times greater than diesel fuel, making it ideal for ships and submarines.
 * Creates high income jobs.
 * Low human cost: even including Chernobyl, similar safety as solar and wind in terms of deaths per unit of energy generated, and much safer than coal/oil/gas/biomass (largely due to the dangers of air pollution)
 * Can be used for spaceflight. Chemical rockets are heavy, and anything that makes a spacecraft lighter makes space travel exponentially faster.  Nuclear engines work a bit differently than nuclear power on Earth, but further research and refinement could potentially allow interstellar travel on the order of generations rather than millenia. However, space travel is inherently unsafe.  No matter how good your nuclear engine, the rocket itself has a very real possibility of having an rapid unintentional disassembly (explode) or execute an unplanned lithobraking maneuver (crash), releasing radioactive material over a wide area.  This is less of a problem in deep space itself which is already radioactive, but the nuclear engine and fuel has to get to space first.
 * Doesn't need an external power source to generate energy, making it useful for powering things like pacemakers, spacecraft, and underwater equipment.

Cons

 * Generates nuclear waste — which is a huge political and maybe technological problem. This problem is not unique to nuclear power, however; industrial waste such as arsenic pose a similar problem (though unlike nuclear, it stays hazardous forever), which we also discard through burial.
 * Possibility of severe accidents; again this isn't unique to nuclear power plants, as deadly explosions from traditional industrial buildings do happen. Also, see imaginary cons on nuclear explosions.
 * Nuclear weapon proliferation concerns; there's a reason Iran is framing their nuclear weapons debate as simply wanting more power plants. A civilian power plant doesn't produce the right kinds of nuclear materials that a nuclear bomb would need, and would require much more infrastructure to turn the waste products into nuclear weaponry than a dedicated weapons-reactor would require, but it's not impossible.  The real concern would be the potential creation of "dirty bombs" using highly radioactive nuclear waste that would be all but impossible to clean up, such as, e.g., powdered graphite from used moderators, assuming a delivery system could be made for it.
 * Some reactor types "burn" enriched uranium, which is a limited resource. Breeder reactors, Thorium reactors and spent fuel reprocessing all mitigate this problem but are thus far are either not economically viable or are unproven and still theoretical at best.
 * Mining is pretty much the "dirtiest" industry in existence and mining Uranium is no different. However, to get an equal amount of energy from Fossil Fuels or Uranium, Uranium requires much less disruption; being over a million times more energy dense than fossil fuels, to be equivalent to mining pure coal the uranium ores would need to be on the order of few hundred per million (reduced to roughly just a SINGLE ppm if a breeder reactor uses all that delicious U238), whereas they tend to be on the order of a thousand or more per million, so this is really only a con compared to renewable energies. Even then, wind and solar also rely upon mining and other heavy industries to produce the materials required.
 * High up-front capital costs — so high in fact that private investment for new power plants has not been forthcoming in the west for many years, with very few exceptions.
 * Long lead times due to heavy regulation of the industry and the complexity of construction.
 * Nuclear power plants produce a constant amount of energy for as long as they are turned on, which means they can't be as responsive to factors such as supply and demand.

Imaginary 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 weapon-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 could reduce the number of radiation-related deaths due to removing uranium from the ground, limiting future radiation exposures. Even if that reduction is inconsequentially small, it could still reduce the number of radiation related deaths because coal-burning power-plants release roughly 3.3 times as much radiation into the surrounding environment as a nuclear plant generating the same number of watts of electricity. For the discussion of the most common model of radiation effects, see linear no-threshold.
 * Nuclear explosions. Despite what you might see in trashy movies or Doctor Who, a nuclear reactor cannot explode in the manner of an atomic bomb, even if you try to make it explode deliberately.  In the case of Chernobyl, a reactor core was overloaded due to incompetent management, and because of poor reactor design, the emergency control rods became stuck (with their moderator tips in place).  This resulted in something known to physicists and chemists as a, more colloquially known as a "steam explosion".  This was really, really bad, but not the "giant mushroom cloud and entire city leveled" type of bad.  An analogy could be helpful.  Were you able to cause all the petrol in your car to oxidize simultaneously (or any organic material, for that matter), you'd have a bomb.  To do so you would need to mix the fuel with a strong oxidizer such as liquid oxygen.  Instead, your car uses atmospheric oxygen, which can only burn so much at a time.  Your engine turns a tiny amount of petrol into a mist which is ignited, causing a small explosion that gives your car power to move, but when a severe accident occurs, the surrounding air doesn't have a high enough concentration of oxygen to cause all the fuel to oxidize simultaneously.  Instead, the fuel can oxidize the normal way, i.e., it will burn.  Just as a crashed car doesn't have the concentrated oxidizer needed to cause the fuel tank to oxidize all at once, fuel for nuclear reactors also don't have the concentration or type of radioactive materials needed to cause the fission reactions seen in nuclear bombs.

Nuclear accidents
The only severe nuclear accidents so far really 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 — doing a safety test during the night shift that had never even been briefed on how to do the test just one of them. Three Mile Island had no off-site impact. Windscale was in a plutonium production reactor, and also thankfully 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. The United States Navy, whose entire aircraft carrier and submarine fleet are nuclear powered, has not had a single reactor-related accident in its 40+ years of nuclear power usage... that we know of, of course. However, there have been nuclear powered vessels that have been lost at sea, and a couple of subs with serious maintenance issues that needed their reactors replaced. A more serious incident would, of course, be difficult to cover up and would've been used as serious propaganda by the USSR.

All the nuclear accidents in the history of the world have killed far fewer 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. In comparison, the failure of a series of hydroelectric dams in China in 1975 as the result of Typhoon Nina is estimated to have killed 150,000-220,000 people.

Core meltdowns
A core meltdown (commonly known as a meltdown) is an accident scenario in nuclear reactors, and is one of the possible modes of failure for light water reactors, during which the reactor pile turns into a pile of reactor.

What a meltdown is
A meltdown happens when insufficient cooling of the core causes the fuel elements to melt from the heat of nuclear reactions. During normal operation of a Light water reactor, meltdown is not possible, because overheating of the core causes water to boil, which removes the moderator and the chain reaction automatically slows down, reducing the power. However, in a reactor that has been operating for some time, completely removing water from the core does not reduce power to zero. Some heat is still generated by the decay of fission products. This heat is sufficient to melt the fuel elements, release radioactive fission products into the cooling water, and cause the zirconium cladding to react with water, generating explosive hydrogen (Which tends not to end well, considering the heat still present in the core).

Even if the fuel elements completely melt, and escape from their containment vessel and pour into the ground, they are not going to burrow all the way through the Earth and come out in China, (though they might accomplish the latter if the reactor is already in China). They can, however, form a variety of minerals rarely or never found in nature, the best-known of which is a silicate of uranium and zirconium.

What a meltdown is not

 * Big explosion. A meltdown is not an explosion of any kind. The reaction of overheated fuel cladding (zirconium) with water generates hydrogen, which can explode if not vented properly. This hydrogen is what caused explosions in Fukushima. Hydrogen explosions have nothing to do with nuclear explosions and are millions of times less powerful.
 * Chernobyl disaster. While a meltdown of the reactor core did occur at Chernobyl, the main catastrophe was caused by a steam explosion.  The radiation was spread over large areas by a graphite fire; light water reactor cores do not contain graphite.
 * An event that makes large areas of land inhospitable. This would only happen if the radioactive fuel is dispersed as fine particles into the environment from a secondary event (see above), or in the case of the Fukushima Daiichi disaster, by flood waters spreading radioactive material into surrounding areas and out to sea.

Meltdowns in modern and future reactors
Modern reactor designs have several mechanisms that prevent a meltdown from occurring. This includes having multiple independent cooling systems, each capable of keeping the reactor core submerged on its own; not having any welds in the pressure vessel below the top of the core; elaborate instrumentation and control equipment; and "core catchers" under the pressure vessel — concrete structures that are designed to capture and redirect the flow of corium (the lava-like molten core material) when the meltdown does happen.

The maximum core damage frequency (which means any damage of the fuel elements, not an all-out meltdown) of currently operating Generation II reactors is lower than once per 10000 years. Generation III designs, of which a few are already deployed and will make up the majority of reactors built over the next decade, reduce the probability even further. However, since these numbers are usually provided by reactor manufacturers, they should be taken with a grain of salt.

CDF means "core damage frequency", MTBCDE means "mean time between core damage events". The total operating experience of the world's nuclear power plants is 14713 reactor years (as of January 2013). This does not include research or experimental reactors.

Core meltdown is a physical impossibility in some future designs of reactors. This includes the Liquid Fluoride Thorium Reactor (LFTR), where the fuel is in liquid form, and the Integral Fast Reactor (IFR), where fission stops due to thermal expansion of the fuel and decay heat is removed by convection of sodium. Passive shutdown of the IFR was successfully tested in 1986 using the EBR-II prototype at Argonne National Laboratory.

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).
 * Tory voters support nuclear (54% to 37%), Labour voters 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 are completely separate topics. This association is as sensible as that between chemistry and "conventional" explosives. 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.