|
|
| Fig. 1: Schematic diagram of the experimental setup of Fleischmann and Pons. The cathode is palladium metal. (Source: Wikimedia Commons) |
Mainstream research into nuclear fusion as a alternative energy source is focused on hot fusion - the containment and control of hot plasma at high enough temperatures such that light nuclei have enough kinetic energy to overcome Coulomb energy barriers and undergo nuclear fusion. Less noted is research into so-called "cold fusion", the conjectured realization of fusion at temperatures close to room temperature. The most notorious claims of this are those made by Fleischmann and Pons in 1989. [1] Today, cold fusion is considered as a fringe field by mainstream science. In this review article, we outline the reasons why cold fusion is unlikely to be possible based on an understanding of nuclear physics.
Claims of the realization of cold fusion involve the presence of a solid state host, oftentimes palladium metal, in which deuterium nuclei can undergo fusion. [1,2] Palladium is a very good at absorbing hydrogen within its lattice and the claim is that deuterium nuclei, when enmeshed within a lattice, can somehow be induced to fuse. Experiments such as those of Fleischmann and Pons claim to measure heat of the order of 10 W per cubic centimeter of palladium, far exceeding what can be accounted for by chemical reactions, when electrolysis of heavy water is performed using a palladium cathode. [2] They therefore attribute their observed extra enthalpy to fusion of deuterium. A schematic of the experimental setup is shown in Fig. 1.
Here, we outline three reasons for why reports of cold fusion cannot be true.
The energetics - for two deuterium nuclei to fuse, they must come within nuclear distances of each other. The Coulomb energy associated with repulsion is then of the order of MeV. Thermal energy scales near room temperature are in contrast on the order of meV. It simply requires a miracle for the Coulomb barrier to breached at room temperatures. Even proponents of cold fusion admit this is a difficult aspect to explain and resort to referring to unknown effects caused by the solid state host.
Lack of neutrons produced - The two nuclear fusion reactions with the largest cross sections for deuterium are [2]:
| 2D + 2D → 3T + 1H |
| 2D + 2D → 3He + n |
If fusion does occur, then by the second reaction, neutrons should be produced in large numbers. However, this is not observed. [1]
Lack of gamma rays produced - The final reason also pertains to the products of the reaction. If deuterium nuclei were to fuse, it is expected that most of the energy released should be in the form of gamma rays rather than as heat. There is also no evidence for gamma rays in claims of observations of cold fusion. [1]
The three reasons above are known as the "three miracles" that are required to happen in order to explain the early experiments claiming to observe cold fusion. The only way to get past these problems is to argue that just because we cannot understand something, it does not mean it is not real. Yet, the problem with this is that a real result should be reproducible. Currently, the reproducibility rate of such solid state cold fusion experiments is at a meager 3%. [2] Indeed, while the prospect of cold fusion is undoubtedly attractive, based on all of what we know about nuclear physics, it is likely a fruitless pursuit.
© Alfred Cheung. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Fleischmann and S. Pons, "Electrochemically Induced Nuclear Fusion of Deuterium," J. Electroanal. Chem. 261, 301 (1989).
[2] M. Fleischmann et al., "Calorimetry of the Palladium-Deuterium-Heavy Water System," J. Electroanal. Chem. 287, 293 (1990).
