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Where nuclear energy comes from & How a nuclear power plant works

Science Education for the Early Grades Forums Volume Three Learning Progression A. Nature of Matter Lesson A-25. Nuclear Energy Where nuclear energy comes from & How a nuclear power plant works

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      Bernard Nebel

      Uranium is an element, atomic number 92 (92 protons) widespread in the earth’s crust. Uranium ores (minerals rich enough in uranium to make mining/refining worth while) occur in certain locations (Google: uranium ore image). Refined uranium is a metal denser than lead. 

      The element, uranium, exists as two intermixed isotopes, 99%  U238 and 0.72% is U235. Only the tiny fraction of U235 is significant for nuclear energy. The radioactive half-life of U235 is 0.7 billion years, but, the decay of U235 atoms is unique. Upon spontaneous decay or being struck by a neutron the nucleus of an U235 atom actually breaks apart and forms: two lower-atomic-number atoms, plus two or three “stray” neutrons, and a comparatively large amount of energy is released.

       The energy derives from the fact that the products of this fission (breaking apart) don’t quite add up to the mass of original. There has been a conversion of some of the mass to energy. (E = MC2, energy equals mass times the speed of light in meters per second squared. That’s a very large amount of energy from a small amount of mass.)

      Beyond this, the key factor of nuclear power lies in those “stray” neutrons. When one of these neutrons strikes another U235 atoms, it causes it to fission in turn. With two or three neutrons from each fission, have kids describe what happens—one triggers two more, triggers 4, 8, 16, …

      It sets off what is called a chain reaction. (See video below.) With sufficiently concentrated U235 it leads to the nuclear explosion.  


      Note that if U235 is not sufficiently concentrated, the neutrons resulting from one fission simply miss the next U235 atom and the chain reaction does not occur. This is why nuclear explosions don’t occur in nature. 

      Therefore, obtaining nuclear power begins with the need to enrich U235, that is, separate U235 from U238 and make it more concentrated. How this is done is described in the following video. 

      Emphasize that the enrichment needed for atomic bombs is 90% plus. For nuclear power plants it is 3 – 5%. With this much more modest enrichment, nuclear power plants cannot explode like bombs. They can get hot enough to melt the reactor (see later). 

      U235 enrichment—How is U235 separated from U238 and concentrated, i.e., enriched?

      In order for a chain reaction to occur, the concentration of U235 must be increased from its 0.7% in natural ores to 3 – 5% for nuclear power and to over 90% for nuclear bombs. (Note the pronounced difference.)

      Recall that chemically speaking, isotopes of an element behave identically. Therefore, to obtain enriched U235, the isotopes must be separated on the basis of their slight difference in mass (238 vs. 235). The following video describes methods for doing this. 


      How a nuclear power plant works

      The challenge of brining nuclear power to fruition was in designing a system in which the chain reaction of U235 fissioning will go on but at a controlled steady rate. To slow and the reaction “dies”; to fast and the reaction amplifies into a meltdown. (Limiting the enrichment of U235 to 3 – 5% precludes a nuclear explosion but it could get hot enough to melt.) The basic design that meets these criteria is described in the following video. 

      Emphasize the key points shown in the video.

      The 3 – 5% U235 is fashioned into pellets placed into long thin rods (fuel rods) which are mounted in clusters. 

      Control rods, rods of material that will absorb neutrons are positioned between and among the fuel rods. Removal of control rods and the chain reaction amplifies; insertion of control rods and the chain reaction is quelled.

      Water circulating though the reactor rods is heated by nuclear reaction and used to generate stem which drives a conventional turbogenerator. (The separate loops of water are a safety precaution to prevent radioactivity escaping into the environment. 

      Have kids note that nuclear power is not a new technology for producing electricity. It is a new technology for boiling water. 






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