49 FISSION Three steps involved for any chain reaction Initiation reaction of a

49 fission three steps involved for any chain

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49 FISSION Three steps involved (for any chain reaction) Initiation- reaction of a single atom needed to start the chain Propagation- This part repeats itself over and over again producing more product Termination- The chain will end, either because the reactants are used up or neutrons no longer available.
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FIGURE 21.14 When a slow neutron hits a fissionable U-235 nucleus, it is absorbed and forms an unstable U-236 nucleus. The U-236 nucleus then rapidly breaks apart into two smaller nuclei (in this case, Ba-141 and Kr-92) along with several neutrons (usually two or three), and releases a very large amount of energy. 50
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FIGURE 21.16 The fission of a large nucleus, such as U-235, produces two or three neutrons, each of which is capable of causing fission of another nucleus by the reactions shown. If this process continues, a nuclear chain reaction occurs. 51
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52
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FIGURE 21.18 (a) The nuclear fission bomb that destroyed Hiroshima on August 6, 1945, consisted of two subcritical masses of U-235, where conventional explosives were used to fire one of the subcritical masses into the other, creating the critical mass for the nuclear explosion. (b) The plutonium bomb that destroyed Nagasaki on August 12, 1945, consisted of a hollow sphere of plutonium that was rapidly compressed by conventional explosives. This led to a concentration of plutonium in the center that was greater than the critical mass necessary for the nuclear explosion. 53
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54 FISSIONABLE MATERIAL fissionable isotopes include U-235, Pu-239, and Pu-240 natural uranium is less than 1% U-235 Is mostly U-238 not enough U-235 to sustain chain reaction to produce fissionable uranium, the natural uranium must be enriched in U-235 to about 7% for “weapons grade” to about 3% for reactor grade
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PLUTONIUM-239 Half Life of 24,000 years Used in weapons Used in some reactors Produced as by product in some 55
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56 NUCLEAR POWER Nuclear reactors use fission to generate electricity About 19.2% of U.S. electricity 36% of electricity in Virginia The fission of U-235 produces heat The heat boils water, turning it to steam The steam turns a turbine, generating electricity
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57 NUCLEAR POWER PLANTS VS. COAL-BURNING POWER PLANTS Nuclear Plant Use about 50 kg of fuel to generate enough electricity for 1 million people No air pollution Coal Burning Plant Use about 2 million kg of fuel to generate enough electricity for 1 million people Produces NO 2 and SO x that add to acid rain Produces CO 2 that adds to the greenhouse effect
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58 NUCLEAR POWER PLANTS - CORE the fissionable material is stored in long tubes, called fuel rods , arranged in a matrix subcritical between the fuel rods are control rods made of neutron absorbing material B and/or Cd neutrons needed to sustain the chain reaction the rods are placed in a material to slow down the ejected neutrons, called a moderator allows chain reaction to occur below critical mass
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59 Power Plant
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60 REACTOR Cold Water Fuel Rods Hot Water Control Rods
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61 CONCERNS ABOUT NUCLEAR POWER core melt-down/ natural disaster water loss from core, heat melts core Chernobyl Japan
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