test - 1 NUCLEAR FISSION AND FUSION Nuclear energy is...

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1 NUCLEAR FISSION AND FUSION Nuclear energy is energy that is released either by splitting the nuclei of large atoms [fission] or the merging together of very small nuclei [fusion]. This is energy that is contained within the nucleus of the atom. Both of these processes obey the laws that matter and energy can neither be created or destroyed, but they can be changed in form. Einstein postulated that mass and energy are the same but exist in different forms, and that mass can be converted into energy and vice versa. Spontaneous and Induced Nuclear Fission Nuclear fission is the process in which a very heavy nucleus splits into two lighter nuclei, releasing energy in the process. Of real significance is that fission processes are able to produce more energy than any other chemical or nuclear reaction. Very heavy nuclei are highly unstable because the attractive nucleon force only acts over short distances. In large nuclei the nucleon force is unable to overcome the repulsive electrostatic force between protons. In experiments involving the bombardment of uranium atoms with neutrons it was found that barium nuclei were produced. That is, the large uranium nucleus had in some way split up to produce a nucleus (of barium) that was about half its size. In order to produce more stable nuclei, a nucleus may spontaneously split (in the absence of a stimulus) into two smaller, more stable nuclei. Gamma rays and several neutrons are also emitted in this process. This is called spontaneous nuclear fission. Energy in the MeV range results. This is usually seen as kinetic energy of the recoiling fragments and as emitted gamma photons. Nuclear fission can also be induced. This is done by causing the nucleus to capture a neutron. The result is an excited (unstable) nucleus, which will then undergo spontaneous fission. This is called induced nuclear fission.
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2 Atoms such as U-235, U-233 [uranium] and Pu-239 [plutonium] can be forced to undergo nuclear fission when they absorb (capture) slow moving neutrons. Essentially, only very large nuclei are capable of undergoing fission, i.e. splitting. Slow neutrons can be captured by nuclei. These neutrons have energies values of about 0.03 eV or 5 x10 -20 Joules. With this kinetic energy they have speeds of approximately 20000 ms -1 . Because the neutrons have no charge, they can enter a nucleus without any Coulombic repulsion and once in the nucleus fission occurs. A typical induced (forced) fission of U-235 by a neutron is shown below where two nuclei of lighter mass than the parent U-235 nucleus and a number of neutrons are emitted (commonly 2 or 3) along with gamma photons. The energy given out by a fission process is evident in the gamma photons and the kinetic energy of the neutrons and the smaller nuclei. Before we discuss the specifics of nuclear fission we will look at the source of the emitted energies. A possible nuclear equation for the above reaction would be written as: 3n Kr Ba U 1 0 92 36 141 56 235 92 1 0 n
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