I0 I Probability Neutron Attenuation X Recall t = N t

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I0 I Probability Neutron Attenuation (revisited) X Recall t = N t

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Presentation transcript:

I0 I Probability Neutron Attenuation X Recall t = N t mfp for scattering ls = 1/Ss mfp for absorption la = 1/Sa …………. total mfp lt = 1/St Probability per unit path length. Probability Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

Neutron Moderation (revisited) Show that, after elastic scattering the ratio between the final neutron energy E\ and its initial energy E is given by: For a head-on collision: After n s-wave collisions: where the average change in lethargy is HW 6 1H ? Collision Parameter Reference Average decrease in ln(E) after one collision. Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

Most probable and average energies? Neutron Moderation HW 6 (continued) Reproduce the plot. Discuss the effect of the thermal motion of the moderator atoms. On 12C. Most probable and average energies? Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

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, Second Semester, 2008-2009 (Saed Dababneh).

Nuclear Fission Surface effect Coulomb effect ~200 MeV  Fission Fusion  Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

Nuclear Fission B.E. per nucleon for 238U (BEU) and 119Pd (BEPd) ? 2x119xBEPd – 238xBEU = ??  K.E. of the fragments   1011 J/g Burning coal  105 J/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, Second Semester, 2008-2009 (Saed Dababneh).

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, Second Semester, 2008-2009 (Saed Dababneh).

Nuclear Fission Liquid Drop Shell Activation Energy (MeV) Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

Nuclear Fission =  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, Second Semester, 2008-2009 (Saed Dababneh).

Nuclear Fission Consistent with activation energy curve for A = 300. Extrapolation to 47   10-20 s. Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).