|Splitting more than hairs|
Nuclear power is
a fictional form of energy production in the sci-fi series The Simpsons the process of harvesting heat from nuclear fission or 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 has been controversial since the 1970s and there have been three high-profile accidents and numerous less well-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.)
- 1 Beginnings
- 2 Types of reactors
- 3 Pros and cons of nuclear power
- 4 Nuclear accidents
- 5 Public opinion
- 6 Anti-nuclear movement
- 7 See also
- 8 External links
- 9 Notes
- 10 References
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, 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. 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[note 1] — 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.
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 — in fact, it can even use either natural (un-enriched) 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 Lucens 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. 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 (stop snickering. That's his name.) 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 10 October 1957 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 RBMK 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 Bilibino Nuclear Power Plant, 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 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 ONE MILLION (1,000,000) years. Its other theoretical advantage over other reactor designs is that it would (hypothetically) require fewer moving-parts.
There are two main fusion reactor technologies pursued currently: inertial confinement and magnetic confinement.
Inertial confinement involves heating and pressurizing a small piece of fusion fuel enough to cause a fusion reaction. The idea is to implode many pellets per second to obtain energy (estimated 20 pellets per second). This is currently done using lasers. The best result yet obtained by ICF is less than 1% of input energy.  The major problems with ICF include laser inefficiency (current lasers are extremely inefficient in that they can only deliver about 1% or less of the electrical energy used to power them into IR or Xray energy in the beams that are used to heat the target) and the cost of the pellet. Some research is being done to make more efficient lasers (10-20% efficiency) and the HiPER device was proposed to reduce the amount of energy needed to cause fusion. Even if somehow breakeven is achieved, it remains to be seen how an ICF reactor can implode 20 pellets per second.
Magnetic confinement involves confining ions and electrons using a magnetic field so as to maintain the pressure and temperature. The main magnetic confinement design being pursued currently is the tokamak. The idea is to sustain fusion for prolonged periods of time. In 1997 JET achieved a Q (fusion energy gain factor) of about 0.7. The major problems with magnetic confinement include plasma instability and material issues. The fusion plasma tends to form instabilities and escape from the magnetic confinement. Materials problems include damage from fast neutrons, tritium breeding, and plasma-material interaction (PMI). In the process of slowing down, the fast neutrons cause damage to the materials they pass through, including the wall lining, coils, breeding materials and so on. This creates voids and nuclear transmutations, causing problems like swelling, embrittlement, creep, fatigue and induced radioactivity. PMIs cause impurities to enter the plasma which causes heat to be lost and also wears away the wall lining. There is also the problem with the density limit - when plasma pressures exceed 1% of the magnetic pressure, the plasma becomes unstable. In an ideal device there would be no PMI, but in reality even in the machines with the best confinement the average particle is only confined for 1 second, thus every second the entire plasma content strikes the walls. 
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 powerplant is 2033, although that seems overly optimistic given the long history of delays and the considerable technological breakthroughs which have not yet materialized.
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
- Low emissions electricity. Lots of it.
- Once built, nuclear reactors are dirt cheap. 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, etc.
- Low environmental impacts — if you ignore the mining, that is.
- Very high power density, more than 1000 W/m2.
- Dispatchable — generation is not subject to whims of the weather.
- Water mobile, making it ideal for ships
- Creates high income jobs.
- Low human cost: similar safety as solar and wind in terms of deaths per unit of energy generated, and much safer than coal/oil/gas/biomass
- 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.
- Generates nuclear waste — which is a huge political and maybe technological problem.
- Possibility of severe accidents.
- Nuclear weapon proliferation concerns. (There's a reason why Iran is framing their nuclear weapons debate as simply wanting more power plants.)
- Some reactor types "burn" enriched uranium, which is a limited resource. Breeding reactors, Thorium reactors and spent fuel reprocessing all mitigate this problem but are thus far not economically viable.
- Mining is a decidedly "dirty" business and mining Uranium is no different. However, to get an equal amount of energy from Fossil Fuels or Uranium, Uranium requires much less disruption,. Although it occurs more rarely, it has a higher energy density, so this is really only a con compared to renewable energies
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 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 the movies, a nuclear reactor cannot explode like an atomic bomb, even if you try to make it explode deliberately. Explosions at nuclear plants are almost always something known to physicists and chemists as a BLEVE, more colloquially known as a "steam-explosion".
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 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.
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. In comparison, the failure of a series of hydro-electric dams in China in 1975 as the result of Typhoon Nina is estimated to have killed 150,000-220,000 people.
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 chernobylite, 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.
|Reactor||CDF/plant year ||MTBCDE|
|BWR/4 (Gen II)||1 × 10–5||10 000 years|
|BWR/6 (Gen II)||1 × 10–6||100 000 years|
|ABWR||2 × 10–7||5 000 000 years|
|ESBWR||3 × 10–8||33 000 000 years|
|AP1000||2.4 × 10–7||4 100 000 years|
|EPR||5.3 × 10–7||1 900 000 years|
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.
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).
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.
- That is why something like the natural reactor at Oklo couldn't happen today - U235 decays faster than U238 and thus there is too little U235 in modern Uranium ore to achieve criticality with light water as a moderator
- Kuroda, P. K. (1956). "On the Nuclear Physical Stability of the Uranium Minerals". Journal of Chemical Physics 25 (4): 781–782; 1295–1296.
- Some do not consider it a "real" power plant.
- Average Power Plant expenses
- Even large bodies of water for cooling are not always necessary. See Palo Verde Nuclear Generating Station.
- David McKay, Sustainable energy - without the hot air, page 167
- How Deadly Is Your Kilowatt? We Rank The Killer Energy Sources, James Conca, JUN 10, 2012
- Imagine having to "sell" a nuclear waste storage facility in your district
- Thus far no long term storage facility for highly active nuclear waste exists. Some proposals, like storing it in salt are patently absurd (salt is extremely mobile under tectonic pressure and highly corrosive), while others may have merit
- Depleted Cranium: Why You Can’t Build a Bomb From Spent Fuel
- 'No link' between nuclear plants and child cancer, NHS Choices
- Coal kills 100,000 people per year worldwide  (Chernobyl killed 4,000 people total, so calculate...)
- Typhoon Nina–Banqiao dam failure, Encyclopedia Britannica
- aka China Syndrome
- Believe it or not, the Soviets actually wound up obtaining their samples by shooting chunks off of the hardened lava formed in the meltdown (pass the vodka!), as remotely-controlled vehicles could not be controlled close to so highly radioactive a rock (see R.F. Mould's Chernobyl Record, p. 130-131).
- These numbers for internal events only, which roughly means "core damage not caused by natural disasters". CDF for all events is considered to be less than twice that of internal events, but the impact of natural disasters is considerably harder to model.
- The current number is available from the main page of World Nuclear Association.
- YouGov/The Sunday Times Survey Results, YouGov, 20 March 2011
- NIMBY factors may have something to do with this: Londoners can be pretty certain not to have nuclear plants located in their backyard, while the Scots would be highly likely to get more of them
- See the Wikipedia article on Nuclear renaissance#United States.