Nuclear Fission 235 U + n 93 Rb + 141 Cs + 2n Not unique. Low-energy fission processes. 1 Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
Mar 26, 2015
Nuclear Fission
235U + n93Rb + 141Cs + 2nNot unique.
Low-energy fission processes.
1Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
Nuclear FissionZ1 + Z2 = 92Z1 37, Z2 55A1 95, A2 140Large neutron excess
Most stable:Z=45 Z=58 Prompt neutronsPrompt neutrons within 10-16 s.Number depends on nature of fragments and on incident neutron energy.The average number is characteristic of the process.
2Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
The average number of neutrons is different, but the distribution is Gaussian.
3Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
4
Why only left side of the
mass parabola?
Delayed neutronsDelayed neutrons
Higher than Sn?
~ 1 delayed neutron per 100 fissions, but essential for control of the reactor.
Follow -decay and find the most
long-lived isotope (waste) in this
case.
5Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
• Waste.• Poison.
In general, decay favors high
energy.
Nuclear Fission
6Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
Nuclear Fission
1/v
235U thermal cross sectionsfission 584 b.scattering 9 b.radiative capture 97 b.
Fast neutrons should be moderated.
Fission Barriers 7Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
8
Nuclear Fission
• Q for 235U + n 236U is 6.54478 MeV.• Table 13.1 in Krane: Activation energy EA for 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).
Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).
9
Nuclear Fission
f,Th 584 2.7x10-6 700 0.019 b
Why not use it?Why not use it?
10
Nuclear Fission
Nuclear Reactors, BAU, 1st Semester, 2007-2008 (Saed Dababneh).
• 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 + 1
0n ► 9037Rb + 144
55Cs + 210n
23592U + 1
0n ► 8735Br + 146
57La + 310n
23592U + 1
0n ► 7230Zn + 160
62Sm + 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.
10Nuclear Reactor Theory, JU, Second Semester, 2008-2009 (Saed Dababneh).