Chapter 20 Nuclear Fusion/Fission

20.1: Nuclear Fission Uranium-235 can be hit with a free neutron, which elongates the nucleus, which weakens the strong nuclear force (due to the distance) If the nucleus continues to stretch, the nucleus can split into fragments and several smaller ones (see page 339) This splitting is called nuclear fission.

20.1: Nuclear Fission (cont.) The energy released from the fission of one U-235 nucleus is about 7 million times the energy released by the explosion of one TNT molecule- mostly kinetic energy, but some gamma radiation.

20.1: Nuclear Fission (cont.) In this fission process, three additional neutrons are released, which can go on to initiate the fission of 3 other uranium-235 nuclei, which releases 9 additional U-235 nuclei, and so on... This is called a chain reaction. (See figure 20.2 on page 340)

20.1: Nuclear Fission (cont.) Fission usually occurs in U- 235, which is relatively rare in nature (making up only 0.7% of the uranium in pure uranium metal.) Uranium-235 is found with much larger quantities of U-238, which can absorb these loose neutrons, preventing naturally occurring fission.

20.1: Nuclear Fission (cont.) In order for an explosion to occur, a large quantity of U- 235 needs to be used, which is called critical mass. In a critical mass-sized quantity of U-235, the neutrons cannot escape the surface, due to the decreased surface area.

20.2: Nuclear Reactors About 20% of electrical energy in the United States is generated by nuclear fission reactors. The purpose of a nuclear reactor is to boil water to propel a turbine.

20.2: Nuclear Reactors (cont.) The benefit of using a nuclear reactor over fossil- fuel fired plant is that the nuclear plant does not pollute the environment, and 1kg of uranium yields more energy than 30 cars of coal.

20.2: Nuclear Reactors (cont.) The fuel for a nuclear plant is Uranium-238 and about 3% Uranium-235. Due to this low percentage of U- 235, an explosion is not possible.

20.2: Nuclear Reactors (cont.) Water surrounding the nuclear fuel is maintained at a high pressure, which keeps the water at a high temperature without boiling. The fission-heated water transfers heat to a lower-pressure water system, which drives a turbine and an electric generator. Two separate water systems are used to prevent cross-contamination of water. Water acts to absorb most of the energy produced in the reactor

20.2: Nuclear Reactors (cont.) A big drawback of using nuclear power is the disposal of nuclear waste.

20.3: Nuclear Energy Any time a nucleus splits into two smaller nuclei, the combined mass of all the nucleons in the smaller nuclei is less than the combined mass of the nucleons in the original nucleus. (There is less mass after the split than before) The mass “missing” is converted to energy and transferred to surroundings.

20.4: Nuclear Fusion Nuclear Fusion- opposite of fission. Uses lighter elements, rather than heavy. When 2 nuclei fuse together, the mass of the new nucleus weighs less than the 2 individual nuclei- the mass lost by the nucleons is converted to energy.

20.4: Nuclear Fusion (cont.) Before fusing, nuclei must be traveling at high speeds to overcome mutual electrical repulsion. Fusion brought about by high temperatures is called thermonuclear fusion.

20.5: Nuclear Research & Control Fusion reactions require temperatures of millions of degrees. Such a reaction would need to be confined in a non- material container, such as a magnetic field. A magnetic field could contain plasmas. Once in a magnetic nonmaterial container, magnetic compression could heat to fusion temperatures.

20.5: Nuclear Research & Control At this point in fusion research, more energy is being used to start and continue the reaction than there is energy being produced by the system.