* Nuclear power is the most controversial quantum technology. The understanding of nuclear physics has led to the development of munitions as powerful as almost anyone would want, as well as troublesome efforts to try to restrain the spread of such weapons. Controlled nuclear power has become a widespread power source, if one that has been confronted with considerable public resistance over its production of radioactive wastes.
* After the demonstration of the atomic chain reaction in 1942, the major practical challenge was to obtain the appropriate "weapons grade" materials by separating the traces of fissile uranium-235 from the much more common uranium-238, or by synthesizing fissile plutonium-239 from uranium-238 in a "breeder reactor".
The US approach to separating uranium-235 from uranium-238 was based on a two-stage process, with the first stage consisting of a "thermal diffusion" process and the second consisting of an "electromagnetic separation" process. In thermal diffusion, the uranium was chemically converted into uranium hexafluoride or "hex", a nasty, toxic, corrosive chemical; when the liquid was placed in a "thermal gradient", meaning one side of a container of the liquid was hotter than the other, heavy isotopes tended to migrate to the hot side, while light isotopes tended to migrate to the cool side. The industrial scale separation process was based on assemblies of three concentric pipes stood vertically, with steam pumped up through the center pipe, liquid hex pumped through the middle pipe, and cold water pumped through the outer pipe. The temperature gradient set up a convection current in the liquid hex, with the enriched material drawn off the top.
The enrichment had to be performed many times to achieve significant levels of separation, and the thermal diffusion process was limited in the enrichment levels it could achieve. That was why electromagnetic separation was used for the final stages of enrichment. Electromagnetic separation used a scheme similar to that employed by a mass spectrograph: ions of the uranium to be separated were accelerated in an electric field and then sorted by a magnetic field into trajectories that differed according to mass. The devices that performed this feat were called "calutrons". Interestingly, due to wartime shortages of copper, the electric wiring of the calutrons was made of silver, loaned from the US Mint for the duration of the war. It was scrupulously returned to the Mint later.
Other schemes considered for enrichment included "gaseous diffusion" and "centrifugal enrichment". In both cases, the separation involved chemical conversion of uranium into hex gas. In gaseous diffusion, the gas was run through membranes or very fine screens, with the lighter isotopes seeping through more quickly; in centrifugal separation, the gas was run through sets of centrifuges, with the heavier isotopes being driven to the outside of the centrifuge. In both cases, the process had to be performed many times to achieve significant levels of enrichment. Neither approach was judged suitable for rapid production of enriched materials at the time, though ironically gaseous diffusion would become the norm after the war, to be replaced in turn by centrifugal diffusion. A modern variation on the centrifuge separation process, known as the "Zippe centrifuge", also uses heating to add a degree of thermal separation to the process.
Uranium enrichment still remains a difficult and expensive procedure, though advocates of a new technique, "laser enrichment", believe it may cut costs drastically. In laser enrichment, the uranium is stimulated by laser light, which allows the different isotopes of uranium to then be identified for sorting by their spectral response. The idea is not new, having been tinkered with since the 1960s, but it wasn't until the 1990s that a process, named SILEX for "separation of isotopes by laser excitation", was developed that could be scaled up into a practical production process. No SILEX production plant has been developed yet, though one is in planning.
A breeder reactor uses the chain reaction of natural uranium to bombard uranium-238 and produce plutonium. This is not a trivial process either, not merely because of the labor of building the reactor but because the proportion of plutonium in the end product is small. Since plutonium is a distinct element, it can be separated by chemical processes, but it's a big chore. Breeder reactors were set up during the war at Hanford, Washington, and near Aiken, South Carolina. All schemes to make weapons-grade materials are difficult and expensive, and amount to a barrier that keeps most poorer countries from obtaining the Bomb.
* The first nuclear weapon ever used in combat, the "Little Boy" bomb that destroyed Hiroshima on 6 August 1945, was a "gun-type" uranium-235-based weapon. Essentially it consisted of a gun tube with a stack of rings of uranium-235 at one end, with this "core" surrounded by a shield known as a "tamper" that kept neutrons from escaping and diluting the chain reaction. A matching plug of uranium-235 was placed at the other end of the tube. A pellet of radioactive material such as polonium was placed in the gun between the core and the wedge as an "initiator" to kick-start the fission chain reaction.
When the bomb was detonated, an explosive charge shot the plug down the tube, into the initiator and then into the core, forming a "supercritical" mass; the initiator set off a chain reaction that cascaded out of control to completion in a microsecond. The result was an explosion with a yield of about 12 kilotons of TNT. A simple redundant radar system was used to detonate the weapon at altitude to ensure greater blast effect. The weapon achieved destruction with a flash of incandescent energy, including high-energy radiation, and an explosive blast wave. It also left behind residual radioactive residues, or "fallout", that caused long-term environmental and health damage.
The "Fat Man" device that destroyed Nagasaki on 9 August 1945 was of completely different design, being an "implosion-type" plutonium-239 bomb. The gun scheme cannot be used with plutonium-239, since it achieves a chain reaction faster than uranium-235 and would "predetonate", greatly reducing explosive yield. The fast detonation is partly due to the presence of traces of the plutonium-240 isotope in plutonium-239. Plutonium-240 achieves fission more easily than plutonium-239, and it is difficult to eliminate plutonium-240 from it.
In the implosion scheme, the core of the weapon was arranged as segments of a sphere of plutonium-239, separated and focused towards each other by explosive "lens" structures that blasted them inward. The lenses were all detonated simultaneously to drive the plutonium-239 segments together around an initiator in the core, forming a supercritical mass and a massive explosion. The implosion has to be performed in a very precise fashion to prevent predetonation; when one critic in the bomb program was told about the scheme, he described it with only some exaggeration as like trying to crush a beer can while keeping all the beer inside. As with Little Boy, Fat Man was detonated at altitude. Fat Man had a yield of about 20 kilotons of TNT.
The first Soviet atomic bomb, detonated in 1949, was a copy of this weapon, using plans obtained by well-placed spies at Los Alamos. The Soviets had excellent physicists and were working on their own designs, but building a copy of an existing design gave the USSR a weapons capability more quickly.
The shot was performed in secret -- only to be quickly detected by aircraft carrying filters that acquired particulates from the air. Analysis of the fallout from the weapon actually gave US designers a good idea of the basic design principles of the Soviet weapon. This sort of surveillance would go on through the Cold War, with air-sampling aircraft complemented by ground stations to pick up seismic shock waves propagating through the Earth, and then by orbiting "Vela" satellites that could locate the gamma-ray flash of an aboveground nuclear detonation. The Velas were eventually replaced by nuclear detonation sensors carried as "piggyback" payloads on the US Global Positioning System (GPS) navigation satellites.
* A hydrogen fusion bomb uses a fission bomb to initiate the fusion of hydrogen into helium. The reaction is actually initiated using tritium, which has the lowest fusion temperature, which then promotes the fusion of deuterium. The first test device, codenamed MIKE and detonated in the Pacific on 1 November 1952, used liquid hydrogen, meaning it required cooling systems and was very big and bulky. Production weapons are based on lithium deuteride, a crystalline material that looks like table salt. The lithium deuteride for a fusion bomb has to be made using the lithium-6 isotope, which is only about 7.6% of the lithium found in nature, the rest being the common lithium-7 isotope. Neutron bombardment of the lithium-6 from the fission reaction converts it into tritium, which then initiates the fusion reaction.
As far as is publicly known, the full details still being secret, a fusion bomb consists of two sections. One section consists of a fission bomb in an empty chamber; the other section consists of a block of plastic foam in which a thermonuclear core is embedded. The thermonuclear core consists of a plutonium-239 rod, surrounded by lithium deuteride, encased in a uranium-238 jacket. The fission bomb converts the plastic foam into a hot plasma that promotes fusion reactions in the lithium deuteride, promoted by the fission reactions in the plutonium core and the uranium-238 jacket. The jacket helps promote radioactive fallout.
There is no real theoretical limit to the size of a fusion weapon. Fusion bombs have been built with yields in the range of hundreds of kilotons to hundreds of megatons. The larger weapons were effectively stunts, built mostly for propaganda purposes and intimidation; they were too big to be easily delivered, and were much more powerful than needed for any reasonable target. Indeed, hydrogen bombs were arguably overkill to begin with, since it was perfectly practical to develop fission weapons four times more powerful than the Little Boy munition that flattened Hiroshima.
Today, most fusion weapons have yields in the range of a hundred kilotons to a few megatons. Given the accuracy of modern delivery systems, the yield of the "smaller" warheads is all that is necessary to destroy most conceivable targets.
There was talk during the Cold War of deliberately designing nuclear weapons to maximize production of fallout, primarily by using a jacket of cobalt-60 as a means of building a "doomsday weapon", but there was never much interest in putting them into service. Work was done on developing "cleaner" fusion weapons, in which the uranium-238 jacket around the lithium deuteride was replaced with a tungsten jacket, greatly reducing the fallout from a detonation. The ultimate development of "clean" weapons was the "neutron bomb", which emphasized the production of high-energy radiation, with the intent of causing casualties but minimizing property damage. There was apparently never much interest in fielding such weapons, either.
BACK_TO_TOP* Of course, from the outset it was obvious that a nuclear reactor could provide heat to drive steam turbines to provide electric power. The reactors set up in the US during the war were strictly breeder reactors; they did not produce any electrical power. The first true nuclear power plant was put into operation in the Soviet Union in 1954 at Obninsk. In the 1950s, when Americans often heard the motto "our friend the Atom", atomic power was seen as the way of the future, and not without some good reason. Nuclear power plants didn't belch out plumes of smoke, didn't require strip mines, and the technology seemed to promise limitless power, "too cheap to meter". Nuclear power plants were set up around the world.
There are a number of different power reactor designs. The US focused on the "light water reactor (LWR)". In an LWR, the reactor core uses nuclear fuel enriched to about 4% uranium-235, with highly purified ordinary water flooding the core to act as a moderator and coolant. The fuel in the core is in the form of uranium oxide powder cast into ceramic pellets and loaded into fuel rods made of corrosion-resistant metals. A typical large reactor will have about 200 fuel rods, with a third of them replaced every year. The reactor is controlled by pulling out sets of control rods made of neutron-damping material, generally cadmium or boron steel.
There are two variations on this scheme. In the "pressurized water reactor (PWR)", water is pumped under high pressure into the reactor core, where it is superheated to about 325 degrees Celsius. It goes through a loop into a heat exchanger system where it heats an independent water loop system into steam, which drives a turbine. The steam is then condensed in another heat exchanger that uses water from a lake or river as the coolant. The hot water from the reactor core and the steam that drives the power turbine don't mix, and so the steam is not radioactive.
In another variation, the "boiling water reactor (BWR)", the water pumped through the core is kept at lower pressure to allow the water to boil into steam, which is then driven directly into the power turbine. The BWR is more efficient than the PWR, at the cost of some degree of safety. In any case, nuclear reactors are generally set up inside several levels of containment structures, most visibly a huge steel and concrete external containment structure, with extensive instrumentation systems to monitor reactor operations. In an emergency the control rods are in principle dropped into the reactor to halt the chain reaction, and a water supply is available to dump into the reactor to cool it if that proves necessary.
* During the initial rush of enthusiasm for nuclear power, when many countries were jumping on the bandwagon, enriched uranium was only available from the US and the USSR. As a result, the nuclear power programs of Canada, Britain, and France developed reactors that used natural unenriched uranium as fuel. As unenriched uranium does not support a chain reaction as easily as enriched uranium, ordinary water can't be used as the moderator, since it damps out the chain reaction.
There were several approaches to the solution of this problem. Canada developed a reactor named "CANDU" that used heavy water, or deuterium oxide (D2O), and similar heavy-water reactors were later built all over the world. The British and French developed reactors fueled with unenriched uranium that used graphite as a moderator and high-pressure carbon-dioxide gas as a coolant, with the British bringing the first gas-cooled reactor, named the "Dragon", online in 1965.
Both the British and the French would eventually obtain sources of enriched uranium and build reactors using the material, with the French focusing on PWR technology. The Soviet Union built both PWR and graphite-moderated reactors. The gas-cooled scheme pioneered by the British Dragon was not widely adopted; it was simpler than liquid-cooled reactors, but it was harder to scale up to larger power outputs.
* Uranium is a relatively scarce resource, and so during the high tide of nuclear power technology, there was a push towards development of breeder reactors that could provide both electricity and plutonium to fuel other reactors. The main focus of research was on the "liquid metal fast breeder reactor (LMFBR)", conceptually like a pressurized water reactor, except that the coolant liquid looped through the reactor core is hot liquid sodium, not pressurized water. Sodium is not a strong absorber of neutrons, allowing the core to get to its business of breeding plutonium. In addition, sodium has a relatively low melting temperature of about 100 degrees Celsius (212 degrees Fahrenheit), a relatively high boiling temperature of about 900 degrees Celsius (1,650 degrees Fahrenheit), plus good heat transfer characteristics. The problem with sodium is that it burns spontaneously on any contact with moisture, and so a coolant leak would emit burning radioactive sodium.
The US set up the first LMFBR, the "EBR-1", in the early 1950s, but never went on to a production reactor. However, LMFBRs have been built in the Britain, France, and the Soviet Union.
* Nuclear power also seemed to be an excellent source of power for submarines, allowing them to cruise underwater almost indefinitely, and the US, USSR, Britain, and France all developed nuclear-powered submarines. Atomic submarines generally use a PWR scheme with highly enriched uranium to provide a more compact core; a core using natural uranium would be too big and bulky to be a reasonable fit on a ship. The Americans also built surface warfare vessels, most prominently aircraft carriers, with nuclear power plants. The Soviet Union built a number of nuclear-powered icebreakers, and the US, Germany, and Japan each built an experimental nuclear-powered merchantman. Civilian vessels using nuclear powerplants have been a nonstarter, due to questionable economics and port restrictions.
Both the US and the USSR tinkered with the idea of nuclear-powered aircraft for a time. However, nuclear-powered aircraft were too heavy and definitely too unsafe to ever be built -- and with aerial refueling, there was really no great need for such vehicles anyway. Both superpowers did put small nuclear reactors in existing aircraft as test and engineering exercises, but the reactors were strictly payloads and were not used for propulsion or electric power.
The Soviets did develop nuclear reactors, as opposed to RTGs, to power thermocouple electric generators for their "radar ocean reconnaissance satellites (RORSATS)", which used high-power radars to track Western fleets, but these spacecraft were abandoned after a number of accidents and close calls. However, there has long been research into nuclear thermal rocket (NTR) propulsion for deep-space missions, in which a fluid is heated to high temperatures by running it through a reactor, to be ejected as exhaust. An NTR generates low but highly-efficient thrust. There's also been research on using nuclear reactors as power sources on the Moon or in deep space -- but there are no plans for flying NTR spacecraft or space nuclear reactors at present.
* Doubts began to grow in the US concerning nuclear power in the 1960s, reflecting increased public concern over the environment. There were worries about the possible breakdown of a nuclear power station, with the worst-case scenario being an out-of-control chain reaction that melted down the core, causing it to burn through the bottom of the containment vessel and continue on its way to the center of the Earth. Such an accident, popularly known as the "China syndrome", was at the least unlikely, but as with any complicated technology there were a lot of different things that could go wrong, some of them that couldn't even be imagined before they happened. There was also the completely unavoidable problem of what to do with the highly radioactive wastes produced by a nuclear reactor.
The argument remained more or less emotional and partisan until 1978, when a nuclear reactor at Three Mile Island (TMI), near Harrisburg, Pennsylvania, suffered a catastrophic breakdown. There were no fatalities and no major release of radiation, but the powerplant was effectively wrecked and the power utility suffered major financial losses. Since that time, the US has not approved any new reactors, and very few have come on line. In 1996, the US produced about 22% of its electricity from nuclear power plants.
Although nations like France and Japan are still heavily reliant on nuclear power, it isn't seen as the future any longer. Other nations, such as Sweden and Germany, have been phasing out nuclear power. The former Soviet Union still remains heavily dependent on nuclear power, despite the fact that the worst nuclear accident in history took place at Chernobyl in Ukraine on 26 April 1986, when one of the older and more poorly designed reactors exploded, spewing core contamination into the environment. There were a number of casualties among emergency-response crews; the region around the reactor site was badly contaminated, affecting the long-term health of the residents; and the radioactive plume stretched into Western Europe. Containment of the damaged reactor was expensive and is an ongoing chore.
However, the modern Russian state remains enthusiastic about nuclear power, with new stations coming on line, and neighboring China is even more enthusiastic, with dozens of stations in planning. Japan was also building new plants, but the country's nuclear power program suffered a major setback on 11 March 2011, when an earthquake along with an associated tidal wave caused widespread damage in northern Japan -- most significantly, crippling a nuclear reactor at Fukushima. The reactor accident proved troublesome to deal with, and Japanese enthusiasm for nuclear power went on the decline as a result.
* Although the Chinese are currently building PWR designs, they are also interested in "pebble bed" reactors, which are simpler, cheaper, and safer than traditional power reactor designs. The idea is not new, having been invented by American physical chemist Farrington Daniels (1889:1972) of the University of Madison, Wisconsin, just after World War II. Daniels, incidentally, was one of the early and influential advocates of solar power.
A pebble bed reactor does not use fuel rods. Flecks of lowgrade enriched uranium are formed with carbon and silicon carbide into sand-sized grains, which are then mixed with graphite powder and pressed into "pellets" the size of tennis balls, which are then coated and hardened. The pellets are only about 4% uranium. Hundreds of thousands of pellets are dumped into a ring-shaped reactor core, which has a carbon reflector wall around the outside, with cavities for control rods. Helium is pumped through the pebble bed and heated to directly drive a turbine, or drive an independent steam loop through a heat exchanger to drive a turbine.
The scheme is not only simple and relatively clean -- the graphite pebbles seal in radioactive isotopes -- it also does not need to be shut down to be refueled. Every day, a few pellets are pulled off the bottom of the pebble bed and weighed to see if they are still usable. If they are, they are fed back on the top of the pile. As pebbles become inactive, they are yanked and replaced with new pebbles. A pebble will last three years, and will be circulated six times.
The pebble bed reactor is also much safer than traditional reactor designs. The low concentrations of uranium in the pebbles limits the level of heat that can be produced, and the nonfissile U238 that makes up most of the uranium in the fuel pebbles also has the interesting property of becoming a better neutron absorber when it gets hot -- meaning that a hot core will be self-limiting, damping its own reaction.
A demonstrator pebble-bed reactor went online in Germany in 1968, followed by a commercial-scale reactor in 1985, but they were both shut down after the Chernobyl disaster soured the Germans on nuclear power. The Chinese brought a 10 MW "High Temperature Reactor" online in 2003 to test pebble bed technology, and went so far as to yank out the control rods completely several times to see what would happen. The reactor core rose in temperature and then fell back again; even if all the helium coolant is lost, the reactor would not melt down. The Chinese are now working towards construction of commercial pebble bed reactors. South Africa is also working on a demonstrator pebble bed reactor.
There are other, similar approaches to such simple, safe gas-cooled reactors. One variation presses uranium pellets into hexagonal blocks about the size of two big paint cans stacked on top of each other, with the blocks stacked into a core. A demonstrator "prismatic" gas-cooled reactor, the "High Temperature Test Reactor", went online in Japan in 1998.
Pebble bed and prismatic helium-cooled reactors work best at power levels of a few hundred megawatts, in contrast to the gigawatt-plus power output of modern liquid-cooled plants, but advocates believe there are benefits to distributing power generation over a number of smaller plants instead of consolidating power generation in one big plant. Large projects can suffer from "diseconomies of scale", with cost escalating sharply as the complexity of the project rises, and can be difficult to fund even if the project goes according to plan. A smaller plant is cheaper and easier to build, and with experience, each successor plant is just that much cheaper and easier. Building multiple plants also gives some insurance in case one of them goes down. Critics reply that distributing power generation over multiple plants instead of one big plant makes for expensive power.
* India is also enthusiastic about nuclear power, and is developing a unique reactor technology based on thorium-232. This is attractive because India does not have much uranium but has the world's biggest known deposits of thorium. The difficulty is that thorium-232 is not fissionable in itself; to be used as fuel, it must be irradiated by neutrons from U235 or P238, transmuting it into fissionable U233. However, advocates say a thorium reactor produces much less waste than a uranium reactor.
India began the country's nuclear program in 1958, beginning construction of what would become a dozen heavy-water natural uranium reactors. The second stage, now in implementation, is to build fast breeder reactors using plutonium obtained from the spent cores of the original reactors to breed thorium-232 into U233. This fuel, a mix of thorium-232 and U233, will then be used to power a third generation of heavy-water reactors.
* Along with big power stations, a good number of small reactors have been built for research, education and training, and synthesis of radioisotopes for use and tracers and the like. Many of these small reactors are of the "swimming pool" configuration, with a core made of partially or fully enriched uranium embedded in aluminum alloy plates, sitting at the bottom of a pool of water acting as coolant and moderator.
One of the interesting footnotes to fission power is that in the mid-1970s French researchers found ore from a uranium mine in the African country of Gabon that had a composition like that of waste from a fission reactor, about two billion years on. In the distant past, natural uranium had reached a level of concentration that allowed the deposit to "go critical".
BACK_TO_TOP* Nuclear weapons tests in the 1950s were generally performed aboveground, resulting in an increase in the environmental radiation background. This led to the 1963 Test-Ban Treaty, in which the US and the USSR agreed to cease aboveground testing.
Production of weapons and nuclear power stations of course produces a good deal of radioactive waste, and nuclear power stations have to be refueled on a continuous basis, leaving the problem of what to do with the spent fuel. Some of the incidental wastes can simply stored in a secure area until they "cool off", but that isn't possible for the highly radioactive spent fuel.
The spent fuel can be "reprocessed" in a breeder reactor to be used as fuel, but of course this process also produces incidental wastes. Work is being done, particularly in France, on reactors that can actually reduce the level of troublesome persistent high-level wastes by transmuting them through neutron bombardment into materials with shorter half-lives that decay away rapidly. Research is also being done on a similar scheme to use a beam of high-powered neutrons to bombard the worst high-level wastes, transmuting them into isotopes with shorter half-lives. The heat released can be used to generate power, in fact more power than is needed to perform the neutron bombardment.
High-level wastes have to be "vitrified" or turned into glass, and then stored in deep underground repositories. The search for underground storage sites has been a frustrating one, since very few communities are eager to have a nuclear waste site, an attitude referred to as "Not In My Back-Yard (NIMBY)". Although the problem is getting worse, solutions don't seem to be around the corner by any means.
* Nuclear power advocates were feeling comparatively optimistic in the new millennium, believing that the threat of climate change provides an opportunity for nuclear power, and that renewables can't do the job over the short run. The accident at Fukushima did slow down the push for nuclear power, though as nuclear advocates are quick to point out, it was a very old and not very safe design. Modern LWR designs are much simpler and safer than their ancestors, and advocates of pebble bed and prismatic helium-cooled reactors feel such designs reduce the probability of catastrophic failure to the noise level.
Still, nuclear power has clear liabilities that the critics have been quick to point out. Even if the assurances that new reactor technology will be safe in itself, to actually put a dent in climate change, over a dozen new nuclear power plants would have to come online each year into mid-century. That would not only create a massive waste disposal problem, but maintaining control over nuclear materials would be much more difficult, making it more possible that terrorists could not only perform attacks with dirty bombs, but even obtain nuclear weapons themselves.
Worse, a major investment in nuclear power would come at the expense of investment in renewables -- with the case for renewables getting better every year as costs drop and the technology improves. The track record of nuclear power, in contrast, does not inspire confidence; during the 1980s,s attempts to build nuclear power plants in the USA were persistently expensive failures. Investors are becoming ever more enthusiastic about renewables, but they remain suspicious of nuclear power.
* Nuclear power of course remains almost irreplaceable for submarines, but there have been a number of accidents in which nuclear submarines, mostly Soviet vessels, have been lost at sea. The only good thing about these accidents is that the deep sea is likely the best place for a nuclear accident to happen, since there is very little life in the abyss, and observations of the accident sites have shown little dispersal of radioactive material.
After the fall of the Soviet Union, the Russian Navy was scaled down considerably, and many old submarines were mothballed and scrapped. Disposing of their radioactive cores and other radioactive materials has proven very expensive and troublesome. The Soviets built a few submarines that used a liquid sodium reactor cooling system, and trying to figure out how to dispose of these vessels has proven extremely difficult. Western nations and Japan have provided funding and expertise to help get rid of the old submarines.
BACK_TO_TOP* Nuclear power advocates believe that there is another option, controlled fusion, that offers better long-term prospects for nuclear power technology. The big problem is that nobody's figured out how to do it.
The fusion of hydrogen into helium must overcome a considerable energy barrier to take place, and the fact that in principle it releases more energy than it takes to make it happen doesn't make the barrier any lower. In nature, it takes the pressures and temperatures at the core of a star to make fusion take place, and even then at a surprisingly low rate, and a hydrogen bomb achieves such pressures and temperatures using a fission bomb. The high pressure and temperature makes controlled fusion very difficult to achieve, though on the other hand the process is much more "fail-safe" than fission: it's so hard to initiate fusion that if something goes wrong, the fusion process will simply fizzle out instead of going into runaway.
There have been two main approaches to controlled fusion. "Magnetic confinement" squeezes a plasma to high pressure and temperature in a magnetic field, the usual device being a "tokamak", a toroidal (donut-shaped) magnetic loop. The scheme was suggested by two of the giants of Soviet nuclear research, Igor Y. Tamm (1895:1971) and Andrei D. Sakharov (1921:1989), and in the early 1980s tokamaks built in the US and the USSR both achieved controlled fusion. Unfortunately, it wasn't for very long, and it took more energy to perform the fusion than was released.
The other approach is "inertial confinement", in which a fuel pellet, a tiny glass sphere containing tritium or deuterium, is hit with a high-power laser or ion beam, pumping so much energy into it so rapidly that fusion should take place before the pellet dissipates. Progress has been made on this front as well -- as discussed in more detail later -- but as critics point out fusion experiments of all sorts have been working from a very low level of efficiency.
A group of seven nations is now building the "International Thermonuclear Energy Reactor (ITER)" at Cadarache in France, with the facility to be completed in 2025. It will be based on a magnetic confinement scheme, using "superconducting" magnets, a concept discussed in a later chapter. ITER is strictly a demonstrator, nothing close to a practical power plant. Nobody expects to see a prototype operational fusion plant for decades.
Advocates insist that fusion power will be clean, but even though the fusion process doesn't yield long-lived radioactive waste, neutron bombardment from fusion will irradiate the containment vessel, making it radioactive. The advocates claim that fusion produces much less waste, and the waste it does produce will decay in a few centuries -- which admittedly is better than a few millennia, but still not encouraging.
* As a footnote to fusion, in the 1980s two American chemical researchers claimed they had observed low-level fusion in what amounted to a fairly conventional chem-lab apparatus. There was considerable excitement about "cold fusion" at the time, along with a great deal of skepticism. When nobody else could get the same results, the exercise was generally discounted. Various other "cold fusion"-like concepts have popped up since then, but none of them have panned out either, and the phrase "cold fusion" has become closely linked to the phrase "quack science".
BACK_TO_TOP* This document started life as a component of a general document on quantum physics that I originally released in 2006. In 2017, I decided to break that document down into four smaller ones focused on different aspects of quantum physics, this document being one of the results.
* Sources include:
Some materials were obtained from the Microsoft Encarta encyclopedia and the Wikipedia online encyclopedia -- the Wikipedia was particularly handy for figuring out birth and death dates of even relatively obscure scientists.
* Revision history:
v1.0.0 / 01 aug 17 v1.0.1 / 01 jul 19 / Review & polish. v1.1.0 / 01 jun 21 / Cleanup, review, & polish. v1.1.1 / 01 may 23 / Review & polish.BACK_TO_TOP
