Phy107Lect33_6per

Phy107Lect33_6per - Exam 3: Wed. Dec. 1 Covers Chap. 13-16,...

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1 Wed. Nov 24 Phy107 Lecture 33 Exam 3: Wed. Dec. 1 Covers Chap. 13-16, part of 17 No HW assignment over Thanksgiving Last time: • Radioactive decay: alpha, beta, gamma • Radioactive half-life • Decay type understood in terms of number neutrons, protons. • Understand in terms of weak interaction, Quark internal structure. Wed. Nov 24 Phy107 Lecture 33 Energy production •Coal-burning plant –10,000 tons coal/day to produce 1,000 MW •Fission reactor –Uses 100 tons uranium/yr to produce 1,000 MW •Hydroelectric plant –Uses 60,000 tons/sec water to produce 1,000 MW Wed. Nov 24 Phy107 Lecture 33 Another comparison Wed. Nov 24 Phy107 Lecture 33 Stored energy • Indicates that stored energy density can vary dramatically. • How can so much energy be stored in the nucleus? • Nuclear binding energy is very large • This binding energy can be turned into other forms of energy using fission or fusion . Wed. Nov 24 Phy107 Lecture 33 Fission and Fusion • Fission: – Heavy nucleus is broken apart – Total mass of pieces less than original nucleus – Missing mass appears as energy E=mc 2 • Fusion – Light nuclei are fused together into heavier nuclei – Total mass of original nuclei greater than resulting nucleus – Missing mass appears as energy. Wed. Nov 24 Phy107 Lecture 33 Fission • Fission occurs when a heavy nucleus breaks apart into smaller pieces. • Does not occur spontaneously, but is induced by capture of a neutron
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2 Wed. Nov 24 Phy107 Lecture 33 Neutron capture • When neutron is captured, 235 U becomes 236 U – Only neutron # changes, same number of protons. Nucleus distorts and oscillate, eventually breaking apart (fissioning) Wed. Nov 24 Phy107 Lecture 33 Neutron production • Fission fragments have too many neutrons to be stable. • So free neutrons are produced in addition to the large fission fragments. • These neutrons can initiate more fission events # protons # neutrons Wed. Nov 24 Phy107 Lecture 33 Chain reaction • If neutrons produced by fission can be captured by other nuclei, fission chain reaction can proceed. Wed. Nov 24 Phy107 Lecture 33 Neutrons • Neutrons may be captured by nuclei that do not undergo fission – Most commonly, neutrons are captured by 238 U – The possibility of neutron capture by 238 U is lower for slow neutrons. • The moderator helps minimize the capture of neutrons by 238 U by slowing them down, making more available to initiate fission in 235 U. Wed. Nov 24 Phy107 Lecture 33 The critical mass • An important detail is the probability of neutron capture by the 235 U. • If the neutrons escape before being captured, the reaction will not be self-sustaining. • Neutrons need to be slowed down to encourage capture by U nucleus • The mass of fissionable material must be large enough, and the 235 U fraction high enough, to capture the neutrons before they escape. Wed. Nov 24
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This note was uploaded on 01/05/2012 for the course PHYS 107 taught by Professor N/a during the Spring '08 term at Wisconsin.

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Phy107Lect33_6per - Exam 3: Wed. Dec. 1 Covers Chap. 13-16,...

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