Nuclear Fission
UCSD
UCSD
2
Lecture 14
1
Nuclear Fission
UCSD
UCSD
.
Barium and Krypton represent just one of many potential outcomes
3
Lecture 14
4
2
Nuclear Fission
UCSD
UCSD
5
Lecture 14
6
3
Nuclear Fission
UCSD
UCSD
4 protons:
mass = 4.029
2 neutrinos, photons (light)
4He
nucleus:
mass = 4.0015
7
Lecture 14
26 MeV of energy
THIS FUSION IS THE SOURCE OF THE SUN’S ENERGY,
(and of energy in thermonuclear hydrogen bombs)
8
4
Nuclear Fission
UCSD
UCSD
Question
Nuclear energy in nuclear reactors comes from:
A. Uranium 238
B. Uranium 235
C. Plutonium 239
D. Hydrogen
E. Both B and C
9
Lecture 14
10
5
Nuclear Fission
UCSD
UCSD
11
Lecture 14
12
6
Nuclear Fission
UCSD
UCSD
Little boy:Hiroshima
Plutonium bomb is a little harder; need high
explosives to squeeze Pu close. But critical
mass is only 10 kg. In fact with better design
and neutron reflecting materials critical masses
can be made much smaller (e.g. about 15 kg for
235U and less than 5 kg 239Pu)
Fat boy: Nagasaki (Pu)
Lecture 14
13
14
7
Nuclear Fission
UCSD
UCSD
15
Lecture 14
16
8
Nuclear Fission
UCSD
UCSD
17
Lecture 14
18
9
Nuclear Fission
UCSD
UCSD
• 1.2GW BWR
• 46000 fuel rods (170 tons)
• 71 atmospheres pressure
• 2.8% enriched U-235
• Emergency core cooling system
if control rods fail (adds boron to
water); back-up water system
19
Lecture 14
not shown are
the control rods
that absorb
neutrons and
thereby keep the
process from
running away
20
10
Nuclear Fission
UCSD
UCSD
21
Lecture 14
22
11
Nuclear Fission
UCSD
UCSD
A
Z
XN
12
€6 C6
14
6 C8
235
92 143
238
92 146
239
94
145
U
U
Pu
€
€
23
Lecture 14
€
€
€
24
12
Nuclear Fission
UCSD
UCSD
25
Lecture 14
26
13
Nuclear Fission
UCSD
UCSD
27
Lecture 14
28
14
Nuclear Fission
UCSD
UCSD
29
Lecture 14
30
15
Nuclear Fission
UCSD
UCSD
31
Lecture 14
32
16
Nuclear Fission
UCSD
UCSD
33
Lecture 14
34
17
Nuclear Fission
UCSD
UCSD
35
Lecture 14
36
18
Nuclear Fission
UCSD
UCSD
•  Iron (Fe) is at the peak
•  On the heavy side of iron, fission delivers energy
•  On the lighter side of iron, fusion delivers energy
•  This is why normal stars stop fusion after iron
•  Huge energy step to be gained in going from
hydrogen (H) to helium-4 via fusion
37
Lecture 14
19
Nuclear Fission
UCSD
UCSD
39
Lecture 14
40
20
Nuclear Fission
UCSD
UCSD
41
Lecture 14
42
21
Nuclear Fission
UCSD
UCSD
43
Lecture 14
44
22
Nuclear Fission
UCSD
UCSD
Participation Question
Participation Question
Nuclear energy in nuclear reactors and atomic bombs comes
from:
A. Uranium 235
B. Uranium 238
C. Plutonium 239
D. Either A or C
E. Either B or C
Breeder reactors A.  Make more plutonium from U238
B.  Greatly increase the amount of Uranium useful for
nuclear reactors
C. Increase the risk of nuclear proliferation
D. Are being considered throughout the world
E. All of the above
45
Lecture 14
46
23
Nuclear Fission
UCSD
UCSD
Participation Question
Class Participation Question
One GW nuclear reactor makes enough Plutonium:
A. For one atomic bomb per decade
B. For one atomic bomb per year
C. For one atomic bomb per month
D. For one atomic bomb per week
E. Reactors don't make weapons grade material
San Diego has a chronic water shortage. Are you paying
attention to water waste, the water you use, or trying to use
less water? If so, describe in one or two sentences what you
are noticing or doing. If you aren’t paying attention to the
amount of water you use say why.
47
Lecture 14
48
24
Nuclear Fission
UCSD
UCSD
49
Lecture 14
50
25
Nuclear Fission
UCSD
UCSD
51
Lecture 14
52
26
Nuclear Fission
UCSD
UCSD
53
Lecture 14
54
27
Nuclear Fission
UCSD
UCSD
55
Lecture 14
56
28
Nuclear Fission
UCSD
UCSD
57
Lecture 14
58
29
Nuclear Fission
UCSD
UCSD
59
Lecture 14
60
30
Nuclear Fission
UCSD
UCSD
How long does nuclear waste have to be stored?
Current plan
61
Lecture 14
62
31
Nuclear Fission
UCSD
UCSD
63
Lecture 14
64
32
Nuclear Fission
UCSD
UCSD
alpha (4He)
tritium
•  Iron is most tightly bound nucleus
•  Can take loosely bound light nuclei
and build them into more tightly bound
nuclei, releasing energy
•  Huge gain in energy going from protons
(1H) to helium (4He).
•  It’s how our sun gets its energy
•  Much higher energy content than fission
dueterium
proton
65
Lecture 14
66
33
Nuclear Fission
UCSD
UCSD
4 protons:
mass = 4.029
2 neutrinos, photons (light)
4He
nucleus:
mass = 4.0015
67
Lecture 14
68
34
Nuclear Fission
UCSD
UCSD
69
Lecture 14
70
35
Nuclear Fission
UCSD
UCSD
71
Lecture 14
72
36
Nuclear Fission
UCSD
UCSD
73
Lecture 14
74
37
Nuclear Fission
UCSD
UCSD
Bottom line: at thermal energies (arrow), 235U is 1000 times more likely
to undergo fission than 238U even when smacked hard
75
Lecture 14
76
38
Nuclear Fission
UCSD
UCSD
235U
daughter 1
daughter 2
stable nuclide
Lecture 14
radioactive (unstable) nuclide
77
source: www.epa.gov/radiation/students/calculate.html
78
39