7. Fission
• Sequence of events – using the reaction with
U-235:
A. The neutron approaches the nucleus of U-235
B. U-236 nucleus has been formed, in an excited
state.
C. The excess energy in some interactions may
be released as a gamma ray, but more
frequently, the energy causes distortions of
the nucleus into a dumbbell shape.
D. The parts of the nucleus oscillate in a manner
analogous to the motion of a drop of liquid.
Because of the dominance of electrostatic
repulsion over nuclear attraction, the two
parts can separate.
They r then called fission fragments, bearing
most of the mass-energy released.
They fly apart at high speeds, carrying some 166
MeV of kinetic energy out of the total of
around 200 MeV released in the whole
process.
The resultant thermal energy is recoverable if
the fission takes place in a nuc reactor.
235 + n1  ( U236)*
U
92
0
92
• ‘*’  excited state
• Mass of (U-236)*
= 235.043925 + 1.008665 AMU
= 236.052590 Atomic mass unit
• Mass of U-236 = 236.045563 AMU
[in its ground state]
- it is lower by 0.007027 amu or 6.5 MeV
- This amount of excess energy is sufficient
to cause fission.
• Above calc did not include any kinetic energy
brought to the reaction by the neutron
• For very slow neutrons – by absorption, e.g.,
– Only 1 natural isotope – U-235 &
– 94Pu239 & 92U233 – main artificial isotopes
Recall – fast neutron vs. slow neutron
• Most other heavy isotopes – need significantly
larger excitation energy to bring the
compound nucleus to the required energy
level – for fission to occur.
• This extra energy must be provided by the
motion of the incoming neutron.
E.g.,
• neutrons of at least 0.9 MeV r required to
cause fission from U-238.
• Other isotopes need more energy.
• Fissile materials are those giving rise to fission
with slow neutrons;
• many isotopes are fissionable, if enough
energy is supplied.
• It is advantageous to use fast neutrons – of
the order of 1 MeV energy – to cause fission.
Byproducts of fission
- Some [ν] neutrons [good for chain reaction!]
- ν ranges from 1-7, with an avg in the range of 2-3
– depending on the isotope n the bombarding
neutron energy.
E.g.,
- U-235 with slow neutrons  avg ν = 2.43
- Prompt neutrons – released instantly
- delayed neutrons – [0.65% for U-235] as the
result of radioactive decay of certain fission
fragments
 36
90
Kr
235 + n1
U
92
0
144
1
+ 56Ba + 20n + Energy
• Fission fragments or products are – Krypton &
Barium
• These r usually more unstable – as more
neutrons than natural main isotope [e.g.,
barium is mainly – 56Ba137 and a prominent
element of mass 144 is – 60Nd144 ] – so decay…
… decay down to stable forms.
90  33sec
Kr
36
90 2.91min
Rb
37
90 27.7yr
Sr
38
90 64hr
Y
39
90
Zr
40
• With fission – a part is the absorptions of
neutrons in U – merely results in radioactive
capture,
235 + n1  U236 + γ
U
92
0
92
- U-236 is relatively stable
- About 14% of the absorptions r of this type
- Rest 86% r fission
η [eta] = no. of neutrons produced per absorption
- η in U-235 is lower than ν, the no. per fission.
- avg ν = 2.43
- η = (0.86)(2.42) = 2.07
 The effectiveness of any nuclear fuel is essentially
dependent on the value of η.
So if fast neutrons – fission has larger η.