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Nuclear Fission

Nuclear Fission. 235 U + n  93 Rb + 141 Cs + 2 n Not unique. Low-energy fission processes. Nuclear Fission. Z 1 + Z 2 = 92 Z 1  37, Z 2  55 A 1  95, A 2  140 Large neutron excess Most stable: Z=45 Z=58  Prompt neutrons within 10 -16 s.

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Nuclear Fission

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  1. Nuclear Fission 235U + n  93Rb + 141Cs + 2n Not unique. Low-energy fission processes. Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

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

  3. Nuclear Fission The average number of neutrons is different, but the distribution is Gaussian. Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

  4. Why only left side of the mass parabola? Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

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

  6. Nuclear Fission Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

  7. Nuclear Fission 1/v Fast neutrons should be moderated. 235U thermal cross sections fission  584 b. scattering  9 b. radiative capture  97 b. Fission Barriers Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

  8. Nuclear Fission Fissile • Q for 235U + n 236U is 6.54478 MeV. • Table 13.1 in Krane: Activation energy EAfor 236U 6.2 MeV (Liquid drop + shell)  235U can be fissioned with zero-energy neutrons. • Q for 238U + n 239U is 4.??? MeV. • EA for 239U  6.6 MeV  MeV neutrons are needed. • Pairing term:  = ??? (Fig. 13.11 in Krane). • What about 232Pa and 231Pa? (odd Z). • Odd-N nuclei have in general much larger thermal fission cross sections than even-N nuclei (Table 13.1 in Krane). Fissionable Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

  9. Nuclear Fission Why not use it? f,Th584 2.7x10-6 700 0.019 b Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).

  10. Nuclear Fission • Thermal neutron fission of 235U forms compound nucleus that splits up in more than 40 different ways, yielding over 80 primary fission fragments (products). 23592U + 10n ► 9037Rb + 14455Cs + 210n 23592U + 10n ► 8735Br + 14657La + 310n 23592U + 10n ► 7230Zn + 16062Sm + 410n ! • The fission yield is defined as the proportion (percentage) of the total nuclear fissions that form products of a given mass number. Revisit thermal and look for fast. Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh). Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh). 10

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