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Nuclear Chemistry. The energy of nuclear reactions. Energy in Nuclear Reactions. There is a tremendous amount of energy stored in nuclei. Einstein’s famous equation, E = mc 2 , relates directly to the calculation of this energy.
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Nuclear Chemistry The energy of nuclear reactions
Energy in Nuclear Reactions • There is a tremendous amount of energy stored in nuclei. • Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy. • In nuclear reactions the amount of energy released can be enormous.
For example: • The energy released from the nuclear reaction of one kilogram of uranium is equivalent to the energy released during the combustion of 4 billion kilograms of coal.
Energy in Nuclear Reactions It’s that c2 term that makes the energy released in nuclear reactions so large: The c represents the speed of light: 3.0 x 10 8 m/s. The change in energy, E, is then E = (m) c2 E= (m)(3.00 108 m/s)2 E
Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is 0.0046 g. The change in energy, E, is then E = (m) c2 E= (4.6 10−6 kg)(3.00 108 m/s)2 E= 4.1 1011 J
Nuclear Fission • How can we tap all that energy? • Nuclear fission is the type of reaction carried out in nuclear reactors (and nuclear bombs). • The splitting of a heavy nucleus into smaller, more stable fragments is known as nuclear fission. • Know that definition!
Nuclear Fission • Can happen spontaneously in nature, or can be induced (man-made) • Induced nuclear fission is what happens in nuclear power plants.
Isotopes to use in a nuclear reactor • Not all nuclei are capable of absorbing a neutron and then undergoing a fission reaction (induced fission) U-235 Pu-239 U-238 YES NO
Uranium - 235 Uranium-235 has an interesting property that makes it handy for the production of both nuclear power and nuclear bombs. U-235 decays naturally, just as U-238 does, by alpha radiation. (Why alpha radiation?) U-235 also undergoes spontaneous fission a small percentage of the time. However, U-235 is one of the few materials that can undergo induced (man-made) fission.
Here’s the problem: Uranium-238 (U-238) has an extremely long half-life (the time it takes for half its atoms to decay) of 4.5 billion years. Therefore, it's still present in fairly large quantities. U-238 makes up 99 percent of the uranium on Earth, while uranium-235 (U-235) makes up about 0.7 percent of the remaining uranium found naturally.
Enriching Uranium(a term from the news) The uranium oxide from the mine is about 99 percent U-238. So you need to somehow separate the U-235 from the U-238 and increase the amount of U-235. The process of concentrating the U-235 is called enrichment.
Enrichment of Uranium • Be careful! Enrichment is not converting U-238 into U-235. • Centrifuges are used to remove some of the U-238, thereby increasing the percentage of U-235.
Nuclear Fission When a U-235 nucleus is struck with a neutron, it undergoes fission.
Nuclear Chain Reaction # fissions double every generation 10 generations 1024 fissions 80 generations 6 x 1023 fissions
Nuclear Chain Reactions • An uncontrolled chain reaction is used in nuclear weapons • A controlled chain reaction can be used for nuclear power generation
How is it controlled? To maintain a sustained controlled nuclear reaction, for every 2 or 3 neutrons released, only one must be allowed to strike another uranium nucleus. If this ratio is less than one then the reaction will die out; if it is greater than one it will grow uncontrolled (an atomic explosion). A neutron absorbing element must be present to control the amount of free neutrons in the reaction space. Most reactors are controlled by means of control rods that are made of a strongly neutron-absorbent material such as boron or cadmium.
