| Splitting more than hairs|
Nuclear energy is energy derived from the forces binding the nuclei of atoms. The primary earthly uses of nuclear energy are electricity generation, known as nuclear power, and spectacularly powerful explosives which can be used for
obliterating entire nations deterring military action, known as a nuclear bomb.
Different nuclei have different binding energies. Because they are generally very large, a reaction that changes one type of nucleus into another releases tremendous amounts of energy, millions of times more than the most energetic chemical reactions. The only problem is that to react, the nuclei need to come very close to each other. This requires either extremely high temperature and density, acceleration to very high speeds, or the use of neutrons.
Types of nuclear energy
The curve of binding energy (see picture below) has a maximum for elements with medium atomic weights. It reaches the maximum for iron-56, which is the most stable nuclide in the universe and forms the cores of massive burnt out stars. This means we can theoretically extract nuclear energy from any element other than iron. In practice, it is feasible for three cases:
- Heavier elements can be synthesized out of lighter ones. Specifically by:
- Nuclear fusion -- The process at the heart of stars. Its most practical variant on Earth involves reacting two hydrogen isotopes, deuterium and tritium, to yield helium and a neutron. This process is used in practice in thermonuclear bombs, an extremely powerful type of nuclear bomb, where the immense temperature and pressure required to create the reaction is usually provided through the detonation of a fission "primary" bomb which is used to compress and initiate a fusion "secondary": most bombs then use this reaction to bombard an outer "tamper" of enriched uranium (slightly enriched 238 or ideally 235) with fast neutrons to produce another fission reaction, which in at least two designs then ignites another fusion stage with a fission tamper, with theoretical designs having up to seven such stages. To use it to generate electricity presents massive engineering challenges, but research is progressing at a reasonable rate. If they succeed, it would produce unprecedented amounts of clean energy, particularly if helium-3 (which could be mined on the moon) were used instead of tritium.
- Heavy elements can break up or be broken up into lighter fragments:
- Nuclear fission -- The process of bombarding certain elements, such as uranium and thorium, with neutrons. This causes their nuclei to become very unstable and break up into fragments of varying size. Some new neutrons are released in the reaction, so it can be carried out in a self-sustaining fashion (a chain reaction). Fission of uranium-235 is the source of energy for almost all nuclear power stations, as well as nuclear-powered submarines and aircraft carriers. Fission of plutonium-239 (bred by irradiating uranium-238 with neutrons) is the source of energy for most nuclear bombs. Another potential nuclear energy method is irradiating thorium-232 (the only thorium isotope to occur naturally in earth's crust in appreciable quantities) with neutrons to produce uranium-233 which could then be used as core-fuel in a nuclear reactor, particularly one that uses molten salts as the fuel carrier.
- Radioactive decay -- Some heavy elements will also break down into smaller fragments spontaneously over time. For some artificial isotopes this process yields enough power to be practically useful. However, the power of a radioactive decay-powered device cannot be regulated, and decreases logarithmically over time. The most common use of this variant is the radioisotope thermoelectric generator (RTG), a type of "nuclear battery". It is used to provide a power source for the entire lifetime of a device which would be prohibitively expensive or impossible to refuel, such as deep space probes or navigation beacons in remote locations.
What it can do
Here are some things nuclear energy has done in the past, and remains capable of doing in the present. To a person living a century ago, this list would look entirely magical. This is an example of Clarke's third law.
- Provide concentrated, carbon-dioxide emissions-free electricity on demand, with little regard to geography and weather. Just don't build on a fault line.
- Propel a ship for many years without the need for refueling.
- Power a spacecraft far from the Sun so we can learn more about the outer reaches of the Solar System.
- Raze a large city to the ground in a few seconds.
- Fundamentally change the geopolitics of the 20th century.
- Seal blown-out gas wells.
- Cause earthquakes.
- Achieve transmutation - the "holy grail" of alchemy.
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.)
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.
Please note this list is by no means exhaustive.
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.
Why some people do not like it
Nuclear energy was the central instrument of geopolitics during the Cold War. Both rival and openly hostile superpowers, the United States and the Soviet Union, had a large arsenal of nuclear weapons and held each other in a stalemate. Neither could strike first, because the other side would quickly launch a retaliation, resulting in global destruction. This situation is known as mutually assured destruction. While the resulting standoff turned out to be surprisingly stable and prevented any open armed conflict from occurring between the superpowers, the threat of man-made Armageddon was something completely unprecedented in history. A powerful anti-nuclear movement intent on achieving nuclear disarmament was formed. It was successful in effectively banning nuclear explosions for any purposes.
Unfortunately, the fuss with nuclear weapons caused the very word "nuclear" to carry a connotation of danger, evil and death. To this day it causes a lot of animosity towards peaceful uses of nuclear energy, especially nuclear power.
Nuclear power (most especially fission) has been the subject of much controversy over the nearly 70 years that it has been studied and used; while the process of using nuclear-fired electricity generators is fairly clean, the technology of fission reactors is sometimes prone to problems. Although waste is small in volume (compared to most industries), safe waste disposal is a tremendous problem, demanding answers that will allow the waste to stay stored for tens of thousands of years - substantially longer than all of current recorded history. A proposed waste-storage facility at Yucca Mountain has yet to be built, and few new reactors have been built in the US in recent decades.
Spent fuel can be reprocessed to extract fissionable material, but this raises security and proliferation concerns; much reprocessed uranium fuel is plutonium-239 created during the fission process, which is far more readily useful for building small nuclear weapons than uranium (it is also harder to fission plutonium-239 in a thermal-spectrum reactor feasibly than it is to fission uranium-235 or uranium-233 in a thermal-spectrum reactor; for this reason, most reactors that consume plutonium-239 are fast-spectrum reactors, which tend more towards having a "twitchy" control response than most thermal-spectrum reactors). Even with fuel reprocessing, however, the problem of disposal of support materials (contaminated gear and the like) remains.
In the United States, new construction licenses were not granted for 33 years after the Three Mile Island accident wiped out half the power generation capacity of a Pennsylvania plant in 1979. However, some plants which obtained construction licenses earlier or were mothballed in a partially constructed state for a long time were completed in this period, notably Seabrook-I in New Hampshire - Seabrook-II was abandoned. The first new construction permit since then was granted at the beginning of 2012. The sentiment towards nuclear energy has a lot of regional variation. Several European nations, such as Austria and Germany, have taken radical anti-nuclear positions, with Austria completely banning civilian nuclear power. Meanwhile, France gets more than three quarters of its electricity from nuclear reactors.
In a nutshell
- Low and medium-mass stars like our Sun cannot fuse beyond carbon and oxygen
- When the Cassini probe was launched, there were protests over the fact that it contained some 68 pounds of plutonium. Some even worried that the plutonium could be stolen and used to make atomic bombs. Such fears were groundless; RTGs use plutonium-238, an isotope that's useless for making nuclear weapons.
- According to this old Soviet film, anyhow
- Specifically, it can turn an isotope of mercury into gold, but this is not economical. Extracting precious metals from nuclear waste is a more promising idea. See the Wikipedia article on Synthesis of precious metals.
- For example, a medical procedure known as "nuclear magnetic resonance" was renamed to "magnetic resonance imaging" specifically to avoid the use of the word "nuclear". The same technique used in analytical chemistry retains the original name, as it doesn't need to be sold to the general public.
- Windscale, Three Mile Island, Chernobyl, Fukushima, and many rather less severe incidents that don't get a lot of play.
- World Nuclear News: Approval for first nuclear new build in America
- http://www.world-nuclear.org/information-library/country-profiles/countries-a-f/france.aspx Nuclear Power in France
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