Public interest in cold fusion began in dramatic fashion in 1989 when chemists Stanley Pons of the University of Utah and Martin Fleischmann of the University of Southampton reported in a press conference that they had conducted low-cost experiments that led to the production of excess heat in an electrolytic cell, apparently by a nuclear fusion process.
The fusion of nuclei is an energy source, as is witnessed by the hydrogen bomb. A number of nuclei fuse together in an exploding hydrogen bomb releasing enormous amounts of energy, but, unfortunately, this happens in an uncontrolled manner. Since the early 1950's worldwide research has been underway to control the fusion process because it would give an unlimited source of energy. This research focuses on very high temperatures (on the order of a billion degrees Celsius) to achieve the fusion process. Cold fusion, on the other hand, seems to proceed in the laboratory at room temperature, hence its name.
The report of the results of Pons and Fleischmann briefly raised hopes that a cheap and abundant source of energy had been found. These and similar claims for unexpected nuclear reactions were not replicated with consistency by other laboratories. In 1989 and 1990, 20 groups at major U.S. laboratories, with 135 researchers, published papers describing attempted replications that failed. Consequently the interest of mainstream science has waned.
Two review panels organized by the United States Department of Energy, in 1989 and in 2004, concluded that the evidence was unconvincing. Many mainstream scientists have since cited cold fusion as an example of either irreproducible science or pseudoscience. However some researchers are continuing to investigate phenomena in which more heat is apparently produced in experimental conditions than can be explained conventionally. To avoid the negative connotations of the initial, largely discredited approach, research in the field now presents itself as the study of "low-energy nuclear reactions" 
When a very hot (on the order of 108 to 109 °C) plasma is used to produce fusion between two deuterons, the process is called "plasma fusion" (or sometimes "hot fusion"). This reaction emits neutrons and produces tritium in equal amounts. The established theory is that nuclear fusion reactions cannot be initiated without the input of significant energy to overcome the charge barrier between nuclei, called the Coulomb barrier.
Reactions involving neutrons (or muons, or other neutral particles) can occur because these particles do not have a charge and can pass through the barrier. However, neutrons are not observed to form under conditions that produce the cold fusion reactions and they are not known to exist as free particles in ordinary materials.
Cold fusion generated widespread publicity since it seemed to defy these theoretical considerations and it represented a potentially cheap and clean source of energy.
Fleischmann and Pons propose that nuclear reactions can be initiated without extra energy or application of neutrons by creating a special solid material: highly loaded palladium deuteride (i.e. palladium which has absorbed nearly as many atoms of deuterium as the number of palladium atoms in the sample). When fusion of deuterium takes place in this environment, they claim the main product is ordinary helium and heat, which are produced in the same ratio as they are with plasma fusion, and also tritium, neutrons, and mild radiation, which are detected at levels much lower than plasma fusion produces. In addition, subsequent studies claim that more complex nuclear reactions can occur that convert one element into another in a process called transmutation for which the Coulomb barrier is even greater than between deuterium nuclei.
Conventional theory cannot explain such claims, and the observations have been difficult to reproduce. Some claims can be explained as being caused by error or unrecognized prosaic processes. Despite these objections, study of the effect continued, and by 2000, over 200 groups had published replications.
Evidence for a variety of nuclear processes has been presented including transmutation, fusion, and fission. The terms "Low Energy Nuclear Reactions" (LENR), “Chemically Assisted Nuclear Reactions” (CANR), and "Condensed Matter Nuclear Science" (CMNS) are now used to describe work in this area of study. Many theories have been explored, but none have gained acceptance by conventional science. 
Excess heat production is an important characteristic of the effect and has attracted the most criticism. This is because calorimetry can be a difficult measurement. In addition, the original measurements, as well as a few other studies, were based on complex methods of isoperibolic calorimetry. Subsequently, evidence based on more readily understandable methods such as flow and Seebeck calorimetry have been published. For example, McKubre et al. at SRI developed a state of the art flow calorimeter (Fig. 1), which was used to study many samples that showed production of significant anomalous energy. Over 36 similar studies have observed the same general behavior as was reported by these workers. Of course, all of the positive results could be caused by various errors.  but no single error has been identified that can explain all of the positive results.
To show that the source of the energy is a nuclear reaction, it is necessary to show that the amount of energy is related to the amount of a nuclear product. Until the work of Miles et al.  various unexpected nuclear products had been detected, but never in sufficient amounts. Miles et al. showed that the helium was generated when anomalous heat was measured and that the relationship between the two measurements was consistent with the amount of energy known to result from a d-d fusion reaction. Since then, five other studies have observed the same relationship. Some of the detected helium could have resulted from helium known to be in normal air. It is unlikely that the heat and helium measurements were wrong by just the right amount every time the measurements were made. When helium leaks into a cell with air, it is found in much larger amounts than Miles and others observed, and it leaks in along with other gases that were not present in the sample. Thus, heat and helium appear to be correlated, but the nuclear process producing helium is still to be determined.
Nuclear products other than helium are detected in much smaller quantities. Early in the history, great effort was made to detect neutrons, an expected nuclear product from the d-d fusion reaction. Except for occasional bursts, the emission rate was near the limit of detection or completely absent. This fact was used to reject the initial claim. Tritium is another expected product of d-d fusion, which was sought. Again, tritium was detected but only in small amounts that were inconsistent with expectations.
Finally, the presence of heavy elements having unnatural isotopic ratios and in unexpectedly large amounts are detected under some conditions. These are the so called transmutation products. Work in Japan  has opened a new aspect to the phenomenon by showing that impurity elements in palladium, through which D2 is caused to pass, are converted to heavier elements to which 2D, 4D or 6D have been added.
Explanations for the phenomena
Many theories are being explored, a few examples of which are:
- Reduction of the Coulomb barrier by electrons being concentrated between the nuclei;
- Conversion of deuterium into a wave structure that ignores the Coulomb barrier,
- Creation or release of neutrons within the structure, which add to nuclei that are present,
- Creation of clusters of deuterons that interact as units,
- Involvement of phonons to concentrate energy at the reaction site and carry away the released energy.
- Models showing that the Coulomb barrier is not as high as previously thought if certain conditions are present.
All of these mechanisms are only possible because a regular lattice of atoms and electrons is available and because the normally applied large energy does not hide these subtle processes. Models based on experience using high energy and/or a plasma, in which this regular array of atoms is not present, are not applicable.
A clean, cheap source of energy?
Like plasma fusion -- which is produced in a Tokamak reactor such as the upcoming ITER -- cold fusion is also proposed to produce energy from the fusion reaction. Unlike plasma fusion, cold fusion produces only helium without a significant amount of radioactive products. The main source of energy proposed for plasma and cold fusion is deuterium, which is present in all water.
The hopes of nuclear fusion as an energy source are fuelled by the fact that enough deuterium is available on earth to produce energy at present rates for billions of years. The cost of refining deuterium from water is far cheaper per unit of energy than for chemical, wind or solar energy. While plasma fusion requires huge installations to be practical, the attraction of cold fusion is that it could be practical on a small scale. If it could be made to work, mankind could expect to produce pollution-free, low cost power without the risk posed by radioactive products, far into the future.
The history of cold fusion before 1989
Before 1989, other researchers had reported some tentative evidence for cold fusion. In the 1920's Paneth and Peters thought they had measured helium from a metal hydride room temperature fusion reaction, but they later retracted the claim. Y. E. Kim believes that P. I. Dee may have seen evidence for cold fusion in 1934.
In 1929, A. Coehn showed anomalies in the electromigration of protons. In 1949, Fleischmann became aware of this work, and speculated that the observations might open up "a slim chance of inducing nuclear processes if his methodology were combined with such explosions (which one would now describe under the heading of inertial confinement) . . ." He also cited work by Oliphant, Harteck and Rutherford, and the work by Dee later cited by Kim. Fleischmann revisited the topic in the late 1960s, and in the 1980s he began experimental studies in collaboration with Pons.
In the 1980s, some theoreticians speculated that extremely high pressure can be developed in microscopic areas on the surface of a palladium hydride. In 1981, around the time Fleischmann and Pons were beginning their experiments, Mizuno was working with palladium deuterides for reasons unrelated to cold fusion. He observed evidence of excess heat and charged particles, but after puzzling over them for some time, he dismissed them as instrument error. He later wrote that most electrochemists who worked with palladium deuterides were aware of reports of possible nuclear anomalies.
Unlike these early researchers, Fleischmann and Pons observed a clear signal, which they repeated many times, and developed fairly reliable techniques to reproduce the effect. One of their experiments produced a gigantic heat release during the night that destroyed the apparatus and burned a hole in the table underneath it. The energy required to do this is far greater than the chemical energy available in the cell. (Six similar explosions have been reported since 1989.)
Shortly before Fleischmann and Pons announced their work, they became embroiled in an acrimonious dispute over academic priority with S. Jones. The University of Utah, which held intellectual property rights to their discovery, was forced to hold a press conference and release the results several years before they originally planned to. Jones claimed priority in the discovery of neutrons from palladium deuterides. This dispute is largely moot because, soon after the announcement, it was shown that the Fleischmann and Pons neutron results were in error, and Jones later expressed doubts about his own neutron studies, which have proved difficult to reproduce. He does not believe any excess heat reports, so does not dispute this priority.
Soon after the announcement, researchers in many laboratories attempted to replicate the experiment. In 1989 and 1990, 20 groups at major U.S. laboratories, with 135 researchers, published papers describing attempted replications that failed. Storms later wrote: ". . . the many failures and the serious errors found in the Fleischmann and Pons paper fueled a growing doubt about the original claims. Too many people had spent too much time to get so little. They were beginning to feel they had been had." Skeptics pointed in particular to the work of three internationally leading laboratories (in Caltech, Harwell and MIT) as proof that the reported evidence for cold fusion was unsound. Experts in calorimetry and electrochemistry later reviewed these results and disputed the conclusions. 
In May 1989 the US Energy Research Advisory Board (ERAB) formed a special panel to investigate cold fusion. The scientists in the panel found the evidence to be unconvincing.
In 1989, the state of Utah funded a National Cold Fusion Institute. Researchers there published several papers describing production of tritium, at rates of 7 x 1010 to 2.1 x 1011. The institute was soon closed down in response to harsh opposition to cold fusion research and ridicule in the national press.
Many scientists concluded that the reported evidence for cold fusion must be incorrect, and some declared that it is fraudulent. In 1991, Robert Park denounced cold fusion in the Washington Post as the result of "foolishness or mendacity." . . . "What began as wishful interpretations of sloppy and incomplete experiments ended with altered data, suppression of contradictory evidence and deliberate obfuscations." Francis Slakey, the Science Policy Administrator of the American Physical Society, said that cold fusion scientists are "a cult of fervent half-wits...While every result and conclusion they publish meets with overwhelming scientific evidence to the contrary, they resolutely pursue their illusion of fusing hydrogen in a mason jar. . . . And a few scientists, captivated by [Fleischmann and Pons'] fantasy . . . pursue cold fusion with Branch Davidian intensity."
Cold fusion researchers feel they have been persecuted. At least three were forced to retire early or fired outright. Full professors were locked out of laboratories and reassigned to menial jobs such as stockroom clerks. They were repeatedly investigated by university committees, and their lives disrupted by attacks in newspapers and magazines. Most Federal researchers have been ordered not to publish their results or attend conferences. No research has been funded by the Department of Energy (except some discretionary funding at a few National Laboratories). The U.S. Department of Defense has funded some programs.
In 1990 the American Physical Society (APS) told Nobel laureate Julian Schwinger he would not be allowed to publish papers or even letters on cold fusion in APS journals. Schwinger resigned in protest, saying:
- "The pressure for conformity is enormous. I have experienced it in editors’ rejection of submitted papers, based on venomous criticism of anonymous referees. The replacement of impartial reviewing by censorship will be the death of science."
In January 2011, Andrea Rossi and Sergio Focardi of the University of Bologna announced that they developed a device that they called the 'Energy Catalyzer' that they said could produce 12,400 W of heat power with an input of just 400 W. At a private invitation press conference in Bologna, they demonstrated what they claim is a nickel-hydrogen fusion reactor, and declared that they planned to start shipping commercial devices within the next three months and start mass production by the end of 2011. Rossi and Focardi have failed to get their work accepted for publication in peer-reviewed journals, and their claims have been criticised as unsupported by objective evidence; "For example, many people have suggested to Rossi that one of the best ways to verify his claim is to start an experiment with 30 gallons of cold water and end up with 30 gallons of hot water by condensing the device's output into a second tank while monitoring the temperature change of the water. Rossi's refusal to perform such a simple and potentially unambiguous test significantly undermines the credibility of his claim." 
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