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In Thermal Reactors , the absorption rate in a “medium” of thermal ( Maxwellian ) neutrons

Neutron Flux and Reaction Rate. In Thermal Reactors , the absorption rate in a “medium” of thermal ( Maxwellian ) neutrons Usually 1/v cross section, thus then The reference energy is chosen at 0.0253 eV . Look for Thermal Cross Sections. Actually, look for evaluated nuclear data.

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In Thermal Reactors , the absorption rate in a “medium” of thermal ( Maxwellian ) neutrons

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  1. Neutron Flux and Reaction Rate • In Thermal Reactors, the absorption rate in a “medium” of thermal (Maxwellian) neutrons • Usually 1/v cross section, thus • then • The reference energy is chosen at 0.0253 eV. • Look for Thermal Cross Sections. • Actually, look for evaluated nuclear data. Reference What if not? Introduce non-1/v Factor Independent of n(E). 2200 m/s flux ENDF Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  2. Neutron Moderation HW 6 Show that, after oneelastic scattering the ratio between the final neutron energy E\ and its initial energy E is given by: For a head-on collision: After ns-wave collisions: where the average change in lethargy is 1H ? Collision Parameter Reference Average decrease in ln(E) after one collision. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  3. Neutron Moderation HW 6 (continued) On 12C. • Reproduce the plot. • Discuss the effect of the thermal motion of the moderator atoms. First collision. Most probable and average energies? Second collision. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  4. Neutron Moderation HW 6 (continued) • Neutron scattering by light nuclei • then the average energy loss • and the average fractional energy loss • How many collisions are needed to thermalize a 2 MeV neutron if the moderator was: • 1H 2H 4He graphite 238U ? • What is special about 1H? • Why we considered elastic scattering? • When does inelastic scattering become important? Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  5. Nuclear Fission Surface effect Coulomb effect ~200 MeV  Fission Fusion  Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  6. Nuclear Fission • B.E. per nucleon for 238U (BEU) and 119Pd (BEPd) ? • 2x119xBEPd – 238xBEU = ?? K.E. of the fragments  1011J/g • Burning coal  105J/g • Why not spontaneous? • Two 119Pd fragments just touching •  The Coulomb “barrier” is: • Crude …! What if 79Zn and 159Sm? Large neutron excess, released neutrons, sharp potential edge, spherical U…! Crude Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  7. Nuclear Fission • 238U (t½ = 4.5x109 y) for -decay. • 238U (t½ 1016 y) for spontaneous fission. • Heavier nuclei?? • Energy absorption from a neutron (for example) could form an intermediate state  probably above barrier  induced fission. • Height of barrier is called activation energy. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  8. Nuclear Fission Liquid Drop Shell Activation Energy (MeV) Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  9. Nuclear Fission = Volume Term (the same) Surface Term Bs = - as A⅔ Coulomb Term BC = - aC Z(Z-1) / A⅓  fission  Crude: QM and original shape could be different from spherical. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  10. Nuclear Fission Consistent with activation energy curve for A = 300. Extrapolation to 47  10-20 s. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  11. Nuclear Fission 235U + n  93Rb + 141Cs + 2n Not unique. Low-energy fission processes. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  12. Nuclear Fission Z1 + Z2 = 92 Z1  37, Z2  55 A1 95, A2  140 Large neutron excess Most stable: Z=45 Z=58  Prompt neutrons within 10-16 s. Number depends on nature of fragments and on incident neutron energy. The average number is characteristic of the process. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  13. Nuclear Fission The average number of neutrons is different, but the distribution is Gaussian. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  14. Why only left side of the mass parabola? Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  15. Higher than Sn? Delayed neutrons ~ 1 delayed neutron per 100 fissions, but essential for control of the reactor. In general,  decay favors high energy. • Waste. • Poison. Follow -decay and find the most long-lived isotope (waste) in this case. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  16. Nuclear Fission Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

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