Lecture21NuclearPower

Lecture21NuclearPower - Lecture 20 Nuclear Power I....

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Lecture 20 Nuclear Power I. Principles of Operation A. Review of Neutron Induced Fission  Z A 0 1 Z A+1 Z A 1 + Z A 2 X n X s X X n + Q 1 1 2 2 1    * 10 18 Bookkeeping: A + 1 = A 1 + A 2 + Z = Z 1 + Z 2 Q 180 MeV 2 10 10 kJ/mole E kinetic + E * 1 + E * 2 ~ 160 MeV ~ 20 MeV 1. IF < > 1, chain reaction possible HEAT ACTIVITY 2. Controlled conditions: power reactor 90% 10% 3. Maximum efficiency: nuclear explosion B. Fuels Only U and Th of nature's elements undergo fission readily Reaction cross sections largest for thermal neutrons (0.03 eV @ 300 K) 1 . 90 232 Th (100%): (n,f) 2 10 4 b (e-e) (n, ) = 7.36 b 232 Th   92 233 U ( f =530 b) 2 . 92 238 U (99.27%): (n,f) 5 10 4 b (e-e) (n, ) = 2.71 b 239 U   94 239 Pu ( =742 b) 3 . 235 U (0.72%): (n,f) = 577 b (n, ) = 101 b 4. Conclusions 235 U good fuel 232 Th and 238 U – poor fuels but good feed stocks (Breeder reactors) ; v = 2.44 v = 2.89 v = 2.51
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C. Chain Reaction Conditions: 235 U 1. Minimum Conditions Since neutrons have a long range, can escape volume of uranium. QUESTION: What fraction of emitted neutrons ( ) must be captured to sustain a fission chain reaction? i.e., capture efficiency? 2. Efficiency Example: start with 1000 neutrons ( n , ) 236 U:   (,) n b b R 101 678 1000 = 150 236 U NO NEUTRONS (n,f): n f b b R 577 678 1000 = 850 fissions 2.44 n 2074 n's capture efficiency must be 1000/2074 100% 48% 3. Stated Mathematically Minimum Efficiency for chain reaction a. IF Efficiency > 48%, explosion b. IF Efficiency < 48%, no chain reaction c. IF Efficiency = 48%, steady-state energy production: REACTOR (can operate with lower abundance 235 U if efficiency > 48%) 4. Critical Mass: M crit a. M crit is the minimum mass of fissionable material necessary to exceed the minimum efficiency criterion; Geometry problem: b. Depends on surface to volume ratio of sphere (optimum shape), since this determines the escape probability for neutrons. c. Critical conditions: M M crit ~ 10 6 s; M crit ~ 2 kg for 235 U; R baseball (10 15 dps) GO 1000 n + U fission 1000 needed  R f
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II. Reactor Construction A. Components 1. Fuel Rods: Energy Source 3% enriched 235 U uranium in Zr-stainless steel rods rod design ; 3% abundance make explosion impossible in reactor fission neutron energy spectrum 0-5 MeV; need to slow down n's 2. Moderator: Enhance Efficiency Low A materials thermalize neutrons best; must have low (n, ) to keep neutrons alive and minimize activation Materials: H 2 O – light water, 1 H: (n, ) = 0.333 b D 2 O – heavy water, 2 H: (n, ) = 5.2 10 4 b Graphite – Fermi's first reactor, Chernobyl 3. Control Rods: Thermostat Maintain the P(fission)/ P(n, ) 1 Control with neutron sponges 10
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Lecture21NuclearPower - Lecture 20 Nuclear Power I....

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