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).