Nuclear Fusion The Physics behind Nuclear Fusion The binding energy curve shows that energy can be released if two light nuclei combine to form a single larger nucleus. This process is called nuclear fusion. The process is hindered by the electrical repulsion that acts to prevent the two particles from getting close enough to each other to be within range and fusing. To generate useful amounts of power, nuclear fusion must occur in bulk matter. That is, many atoms need to fuse in order create a significant amount of energy. The best hope for bringing this about is to raise the temperature of the material so that the particles have enough energy--due to their thermal motions alone--to penetrate the electrical repulsion barrier. This process is known as thermonuclear fusion. Calculations show that these temperatures need to be close to the suns temperature of 1.5 X 107K. Thermonuclear Fusion in the Sun and other Stars The sun radiates energy at the rate of 3.9 X 1026 W (watts) and has been doing so for several billion years. The sun burns hydrogen in a nuclear furnace. The fusion reaction in the sun is a multistep process in which hydrogen is burned into helium, hydrogen being the fuel and helium the ashes. The figure below shows the cycle. Fusion cycle of the Sun The cycle starts with the thermal collision of two protons (1H + 1H) to form a deuteron (2H), with the simultaneous creation of a positron (e+) and a neutrino (v). The positron very quickly encounters a free electron (e-) in the sun and both particles annihilate, their mass energy appearing as two gamma-ray photons. Once the deuteron has been produced, it quickly collides with another proton and forms a 3He nucleus and a gamma ray. Two such 3He nuclei may eventually (within ten thousand years) find each other, as the bottom row shows. Overall, this amounts to the combination of four protons and two electrons to form an alpha particle (4He), two neutrinos, and six gamma rays. Thus, the overall equation is . The energy release in this reaction is where 1.007825u is the mass of a hydrogen atom and 4.002603u is the mass of a helium atom; neutrinos and gamma-ray photons have no mass and thus do not enter into the calculation of disintegration energy. The burning of hydrogen in the suns core is alchemy on a grand scale in the sense that one element is turned into another. The medieval alchemists, however, were more interested in changing lead into gold than in changing hydrogen into helium! Hydrogen burning has been going on in the sun for about 5 billion years and calculations show that there is enough hydrogen left to keep the sun going for about the same length of time into the future. If the core temperature increases to about 108K then energy can be produced by burning helium to make carbon. As a star evolves and becomes still hotter, other elements can be formed by other fusion reactions. However, elements more massive than those with atomic number equal to 56 (iron) cannot be manufactured by further fusion processes as atomic number equal to 56 makes the peak of the binding energy curve. If nuclides were to fuse after that, then energy would be consumed as opposed to produced. Fusion here on Earth The first thermonuclear fusion reactions to take place on Earth occurred at Eniwetok Atoll on October 31, 1952, when the United States exploded a fusion device, generating an energy release equivalent to 10 million tons of TNT. The high temperatures needed to initiate the reaction were triggered by a fission bomb. A sustained and controllable source of fusion power, a fusion reactor, is considerably harder to achieve. The goal, however, is being pursued vigorously in many countries around the world because many look to the fusion reactor as the power source of the future, at least as far as the generation of electricity is concerned. The scheme for fusion on the sun is not suitable for an Earth-bound fusion reactor because the scheme is hopelessly slow. The reaction succeeds in the sun only because of the enormous density of protons in the
Posted on: Mon, 28 Oct 2013 04:13:24 +0000
Trending Topics
Recently Viewed Topics
© 2015