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Unformatted text preview: 1 Introduction to Nuclear Engineering & Engineering Physics Fission,Fusion and NEEP Facilities Don Steiner Fission Energy Release 2 Fission Cross Sections Low E n fission of 235 U is clearly the more favorable route for practical fission power  so let us consider this fission reaction further ~1 b ~2 b (~1MeV) High E n ~106 b ~580 b (~1eV) Low E n 238 U 235 U ( 29 * 235 1 236 93 141 92 143 1 37 56 55 86 U U Rb Cs 2 n n + + + Typical Fission Reaction in U235 When 235 U + n fission occurs we observe : (1)a distribution of fission fragment masses; (2)the emission of ~ 2.5 n / fission event (these observations reflect statisically based observations  a single event has unique fission fragments & an integer # of n s) 3 Neutron balances & the fission chain reaction consider idealized reactor neutron leakage (through all faces) scattered neutrons escape system prior to absorption moderator (E.G. ) 12 6 C fuel ( 235 U + 238 U) Possible Fates of Fission Neutrons x leakage x absorption/capture in moderator 1 2 3 (absorption in 238 U or 235 U of low E n ) 4 Consider Neutron Fate Between Generations of Low E Fission (Gen) i 100 Low E n neutrons absorbed in 235 U ( 29 ( 29 c c c / ~ fission probability 580 100 ~ 85 fissions 680 ~ / ~ capture probability 100 100 ~ 15 captures 680 f f f + + Neutron Fate Between Generations of Low E Fission 85 fissions x 2.5 n / fission ~ 210 fission ns As they slow down ~20% leakout ~40 ~10% captured in 238 ~20 ~60 lost 150 survive to low E n ~20% captured in 238 & graphite @ low E n therefore ~30 lost and 120 remain The remaining 120 at low E n can be absorbed in 235 U in next time sequence / generation. (Gen) i+1 5 k neutron multiplication factor k n i+1 / n i 1.2(for our case) using n rather than n...
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This note was uploaded on 04/07/2008 for the course ENGR 1961 taught by Professor Carracappa during the Spring '08 term at Rensselaer Polytechnic Institute.
 Spring '08
 Carracappa

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