Ticking Time Bomb

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If nuclear energy were to replace gasoline and fossil fuels used for generation of electricity, then the U.S. would require at least an eightfold increase in nuclear power production, increasing from about 10% of all energy supplied to about 90%.
Conventional Fission reactors

Nuclear engineers estimate that the world could derive 400,000 quads (quadrillion, 1015,British thermal units), or about 420,000 EJ (exajoules = 1018 joules), of energy (1000years at current levels of consumption, assuming new technology) from uranium isotope 235, if reprocessing is not employed.

As uranium ore supplies are limited, a majority of this uranium would have to somehow be cost-effectively extracted from seawater. But this technology does not exist. However, at the current technology and consumption, the reserves will last 50 years. .......

Fast breeder reactors are another possibility. As opposed to current LWR (light water reactors), which burn the rare isotope of uranium U-235 (producing and burning about an equal amount of plutonium in the process), fast breeder reactors produce much larger amounts of plutonium from common U-238, then fission that to produce electricity and thermal heat.

Because there is about 139 times more U-238 than U-235 on Earth, it has been estimated that there is anywhere from 10,000 to 5,000,000,000 years' worth(sustainable but not renewable, depending on future technology) of U-238 for use in these power plants, and that they can return a high ratio of energy returned on energy invested (EROEI), and avoid some of the problems of current reactors by being automated, passively safe, and reaching economies of scale via mass production.

In addition, wastes produced by these plants are less toxic than those of conventional reactors. There are a few such research projects working on fast breeders. Lawrence Livermore National Laboratory is currently working on the small, sealed, transportable, autonomous reactor (SSTAR). Problems arise from the higher levels of heat and radiation produced by this reactor. There are other, more exotic nuclear projects (such as pebble bed reactors), each with their own technical problems.

The long-term radioactive waste storage problems of nuclear power have not been solved, although on-site spent fuel storage in casks has allowed power plants to make room in their spent fuel pools. Today, the only industrial solution lies with storage in underground repositories.Since automobiles and trucks consume a great deal of the total energy budget of developed countries, some means would be required to deliver the energy generated from nuclear power to these vehicles.

The most direct solution is to use electric vehicles. Mass transit will be an important aspect of this solution, as it is readily electrified. Some think that hydrogen may play a role. If so, it could be produced by electrolysis, either conventionally or at high-temperatures supplied by reactor heat.

Another possibility for producing hydrogen by nuclear power is the heat-driven sulfur-iodine cycle.Hydrogen need not be used directly in transportation. A hybrid chemical-energy storage process might use such hydrogen to produce methanol from CO2 , which would then feed into the present internal-combustion-engine transportation infrastructure with far less modification than would be needed for hydrogen.

See methanol economy. To reduce the amount of CO2 in the atmosphere, hydrogen can be combined with nitrogen from air to produce ammonia which can then be used as fuel for internal combustion engines.

Fusion reactors

Fusion power refers to power generated by nuclear fusion reactions. In this kind of reaction, two light atomic nuclei fuse together to form a heavier nucleus and in doing so, release energy. In a more general sense, the term can also refer to the production of net usable power from a fusion source, similar to the usage of the term "steam power."

Most design studies for fusion power plants involve using the fusion reactions to create heat, which is then used to operate a steam turbine, similar to most coal-fired power stations as well as fission-driven nuclear power stations.

The largest current experiment is the Joint European Torus [JET]. In 1997, JET produced a peak of 16.1 MW of fusion power (65% of input power), with fusion power of over 10 MW sustained for over 0.5 sec. In June 2005, the construction of the experimental reactor ITER, designed to produce several times more fusion power than the power put into the plasma over many minutes, was announced. The production of net electrical power from fusion is planned for DEMO, the next generation experiment after ITER.

Fuel cycle

The Sun is a natural fusion reactor.The basic concept behind any fusion reaction is to bring two or more atoms very close together, close enough that the strong nuclear force in their nuclei will pull them together into one larger atom. If two light nuclei fuse, they will generally form a single nucleus with a slightly smaller mass than the sum of their original masses.

The difference in mass is released as energy according to Einstein's mass-energy equivalence formula E = mc². If the input atoms are sufficiently massive, the resulting fusion product will be heavier than the reactants, in which case the reaction requires an external source of energy.

 The dividing line between "light" and "heavy" is iron. Above this atomic mass, energy will generally be released in nuclear fission reactions, below it, in fusion.Fusion between the atoms is opposed by their shared electrical charge, specifically the net positive charge of the nuclei.

In order to overcome this electrostatic force, or "Coulomb barrier", some external source of energy must be supplied. The easiest way to do this is to heat the atoms, which has the side effect of stripping the electrons from the atoms and leaving them as bare nuclei. In most experiments the nuclei and electrons are left in a fluid known as a plasma.

The temperatures required to provide the nuclei with enough energy to overcome their repulsion is a function of the total charge, so hydrogen, which has the smallest nuclear charge therefore reacts at the lowest temperature. Helium has an extremely low mass per nucleon and therefore is energetically favoured as a fusion product.

As a consequence, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (³He or 4He).Perhaps the three most widely considered fuel cycles are based on the D-T, D-D, and p-11B reactions. Other fuel cycles (D-³He and ³He-³He) would require a supply of ³He, either from other nuclear reactions or from extraterrestrial sources, such as the surface of the moon or the atmospheres of the gas giant planets.

D-T fuel cycle

Diagram of the D-T reactionThe easiest (according to the Lawson criterion) and most immediately promising nuclear reaction to be used for fusion power is:D + T4He + n

Deuterium is a naturally occurring isotope of hydrogen and as such is universally available. The large mass ratio of the hydrogen isotopes makes the separation rather easy compared to the difficult uranium enrichment process.

Tritium is also an isotope of hydrogen, but it occurs naturally in only negligible amounts due to its radioactive half-life of 12.32 years. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium using one of the following reactions:n + 6Li → T + 4He n + 7Li → T + 4He + n

The reactant neutron is supplied by the D-T fusion reaction shown above, the one which also produces the useful energy. The reaction with 6Li is exothermic, providing a small energy gain for the reactor. The reaction with 7Li is endothermic but does not consume the neutron.

At least some 7Li reactions are required to replace the neutrons lost by reactions with other elements. Most reactor designs use the naturally occurring mix of lithium isotopes. The supply of lithium is more limited than that of deuterium, but still large enough to supply the world's energy demand for thousands of years.Several drawbacks are commonly attributed to D-T fusion power:

  1. It produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure.
  2. Only about 20% of the fusion energy yield appears in the form of charged particles (the rest neutrons), which limits the extent to which direct energy conversion techniques might be applied.
  3. The use of D-T fusion power depends on lithium resources, which are less abundant than deuterium resources.
  4. It requires the handling of the radioisotope tritium. Similar to hydrogen, tritium is extremely difficult to contain and is expected to leak from reactors in some quantity. Estimates suggest that this would represent a fairly large environmental release of radioactivity.

The neutron flux expected in a commercial D-T fusion reactor is about 100 times that of current fission power reactors, posing problems for material design. Design of suitable materials is underway but their actual use in a reactor is not proposed until the generation after ITER.

After a single series of D-T tests at JET, the largest fusion reactor yet to use this fuel, the vacuum vessel was sufficiently radioactive that remote handling needed to be used for the year following the tests.On the other hand, the volumetric deposition of neutron power can also be seen as an advantage.

If all the power of a fusion reactor had to be transported by conduction through the surface enclosing the plasma, it would be very difficult to find materials and a construction that would survive, and it would probably entail a relatively poor efficiency.

D-D fuel cycle

Though more difficult to facilitate than the deuterium-tritium reaction, fusion can also be achieved through the reaction of deuterium with itself. This reaction has two branches that occur with nearly equal probability:

The optimum temperature for this reaction is 15 keV, only slightly higher than the optimum for the D-T reaction. The first branch does not produce neutrons, but it does produce tritium, so that a D-D reactor will not be completely tritium-free, even though it does not require an input of tritium or lithium.

Most of the tritium produced will be burned before leaving the reactor, which reduces the tritium handling required, but also means that more neutrons are produced and that some of these are very energetic. The neutron from the second branch has an energy of only 2.45 MeV, whereas the neutron from the D-T reaction has an energy of 14.1 MeV, resulting in a wider range of isotope production and material damage.

Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons is only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding is not required. Other advantages are independence from limitations of lithium resources and a somewhat softer neutron spectrum. The price to pay compared to D-T is that the energy confinement (at a given pressure) must be 30 times better and the power produced (at a given pressure and volume) is 68 times less.

p-11B fuel cycle

If aneutronic fusion is the goal, then the most promising candidate may be the proton-boron reaction:p + 11B → 3 4He Under reasonable assumptions, side reactions will result in about 0.1% of the fusion power being carried by neutrons.

At 123 keV, the optimum temperature for this reaction is nearly ten times higher than that for the pure hydrogen reactions, the energy confinement must be 500 times better than that required for the D-T reaction, and the power density will be 2500 times lower than for D-T.

Since the confinement properties of conventional approaches to fusion such as the tokamak and laser pellet fusion are marginal, most proposals for aneutronic fusion are based on radically different confinement concepts.

History of research

The idea of using human-initiated fusion reactions was first made practical for military purposes, in nuclear weapons. In a hydrogen bomb, the energy released by a fission weapon is used to compress and heat fusion fuel, beginning a fusion reaction which can release a very large amount of energy.

The first fusion-based weapons released some 500 times more energy than early fission weapons.Civilian applications, in which explosive energy production must be replaced by a controlled production, are still being developed. Although it took less than ten years to go from military applications to civilian fission energy production, it was very different in the fusion energy field, more than fifty years having already passed without any energy production plant being started up.

Magnetic approach

Registration of the first patent related to a fusion reactor by the United Kingdom

Atomic Energy Authority, the inventors being Sir George Paget Thomson and Moses Blackman, dates back to 1946. Some basic principles used in the ITER experiment are described in this patent: toroidal vacuum chamber, magnetic confinement, and radio frequency plasma heating.

The U.S. fusion program began in 1951 when Lyman Spitzer began work on a stellarator under the code name Project Matterhorn. His work led to the creation of the Princeton Plasma Physics Laboratory, where magnetically confined plasmas are still studied. The stellarator concept fell out of favor for several decades afterwards, plagued by poor confinement issues, but recent advances in computer technology have led to a significant resurgence in interest in these devices.

A wide variety of other magnetic geometries were also experimented with, notably with the magnetic mirror. These systems also suffered from similar problems when higher performance versions were constructed.A new approach was outlined in the theoretical works fulfilled in 1950–1951 by I.E. Tamm and A.D. Sakharov in Soviet Union, laid the foundations of the tokamak.

Experimental research of these systems started in 1956 in Kurchatov Institute, Moscow by a group of Soviet scientists lead by Lev Artsimovich. The group constructed the first tokamaks, the most successful of them being T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, conducting the first quasistationary thermonuclear fusion reaction ever. The tokamak was dramatically more efficient than the other approaches of the same

 era, and most research after the 1970s concentrated on variations of this theme.The same is true today, where very large tokamaks like ITER are hoping to demonstrate several milestones on the way to commercial power production, including a burning plasma with long burn times, high power output and online fueling.

 There are no guarantees that the project will be successful, as previous generations of machines have faced formerly unseen problems on many occasions. But the entire field of high temperature plasmas is much better understood now due to the earlier research, and there is considerable optimism that ITER will meet its goals.

If successful, ITER would be followed by a "commercial demonstrator" system, similar to the very earliest power-producing fission reactors built in the era before wide-scale commercial deployment of larger machines started in the 1960s and 1970s.

Even with these goals met, there are a number of major engineering problems remaining, notably finding suitable "low activity" materials for reactor construction, demonstrating secondary systems including practical tritium extraction, and building reactor designs that allow their reactor core to be removed when it becomes embrittled due to the neutron flux. Practical generators based on the tokamak concept remain far in the future.

The public at large has been somewhat disappointed, as the initial outlook for practical fusion power plants was much rosier than is now realized; a pamphlet from the 1970s printed by General Atomic stated that "Several commercial fusion reactors are expected to be online by the year 2000."

Pinch devices

 A "wires array" used in Z-pinch confinement, during the building process.The Z-pinch phenomenon has been known since the end of the 18th century. Its use in the fusion field comes from research made on toroidal devices, initially in the Los Alamos National Laboratory right from 1952 (Perhapsatron), and in the United Kingdom from 1954 (ZETA), but its physical principles remained for a long time poorly understood and controlled.

Pinch devices were studied as potential development paths to practical fusion devices through the 1950s, but studies of the data generated by these devices suggested that instabilities in the collapse mechanism would doom any pinch-type device to power levels that were far too low to suggest continuing along these lines would be practical. Most work on pinch-type devices ended by the 1960s.

Recent work on the basic concept started as a result of the appearance of the "wires array" concept in the 1980s, which allowed a more efficient use of this technique. The Sandia National Laboratory runs a continuing wire-array research program with the Zpinch machine. In addition, the University of Washington's ZaP Lab have shown quiescent periods of stability hundreds of times longer than expected for plasma in a Z-pinch configuration, giving promise to the confinement technique.

Laser inertial devices

The technique of implosion of a microcapsule irradiated by laser beams, the basis of laser inertial confinement, was first suggested in 1962 by scientists at Lawrence Livermore National Laboratory, shortly after the invention of the laser itself in 1960.

Lasers of the era were very low powered, but low-level research using them nevertheless started as early as 1965. More serious research started in the early 1970s when new types of lasers offered a path to dramatically higher power levels, levels that made inertial-confinement fusion devices appear practical for the first time.

By the late 1970s great strides had been made in laser power, but with each increase new problems were found in the implosion technique that suggested even more power would be required. By the 1980s these increases were so large that using the concept for generating net energy seemed remote. Most research in this field turned to weapons research, always a second line of research, as the implosion concept is somewhat similar to hydrogen bomb operation.

Work on very large versions continued as a result, with the very large National Ignition Facility in the US and Laser Mégajoule in France supporting these research programs.More recent work had demonstrated that significant savings in the required laser energy are possible using a technique known as "fast ignition".

The savings are so dramatic that the concept appears to be a useful technique for energy production again, so much so that it is a serious contender for pre-commercial development once again. There are proposals to build an experimental facility dedicated to the fast ignition approach, known as HiPER.

At the same time, advances in solid state lasers appear to improve the "driver" systems' efficiency by about ten times (to 10- 20%), savings that make even the large "traditional" machines almost practical, and might make the fast ignition concept outpace the magnetic approaches in further development.

The laser-based concept has other advantages as well. The reactor core is mostly exposed, as opposed to being wrapped in a huge magnet as in the tokamak. This makes the problem of removing energy from the system somewhat simpler, and should mean that a laser-based device would be much easier to perform maintenance on, such as core replacement.

Additionally, the lack of strong magnetic fields allows for a wider variety of low-activation materials, including carbon fiber, which would both reduce the frequency of such swaps, as well as reducing the radioactivity of the discarded core. In other ways the program has many of the same problems as the tokamak; practical methods of energy removal and tritium recycling need to be demonstrated, and in addition there is always the possibility that a new previously unseen collapse problem will arise.

Other systems

Throughout the history of fusion power research there have been a number of devices that have produced fusion at a much smaller level, not being suitable for energy production, but nevertheless starting to fill other roles.

Inventor of the Cathode Ray Tube Television Philo T. Farnsworth patented his first Fusor design in 1968, a device which uses inertial electrostatic confinement.

Towards the end of the 1960s, Robert Hirsch designed a variant of the Farnsworth Fusor known as the Hirsch-Meeks fusor. This variant is a considerable improvement over the Farnsworth design, and is able to generate neutron flux in the order of one billion neutrons per second. Although the efficiency was very low at first, there were hopes the device could be scaled up, but continued development demonstrated that this approach would be impractical for large machines.

Nevertheless, fusion could be achieved using a "lab bench top" type set up for the first time, at minimal cost. This type of fusor found its first application as a portable neutron generator in the late 1990s. An automated sealed reaction chamber version of this device, commercially named Fusionstar was developed by EADS but abandoned in 2001.

Its successor is the NSD-Fusion neutron generator.Robert W. Bussard's Polywell concept is roughly similar to the Fusor design, but replaces the problematic grid with a magnetically contained electron cloud which holds the ions in position and gives an accelerating potential. Bussard claimed that a scaled up version would be capable of generating net power.In April 2005, a team from UCLA

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