10. All you need to know is..
1. Nuclear Physics is about
probabilities (interaction).
2. Particle interactions can be
understood in terms of simple
billiard balls.
3. You have to think like a neutron
24. Final Method of Decay
n n
+ +
+ ++ ++ ++ ++ +
++ ++ +
+ + ++
+ + ++ +
+
++ ++ +
+ +++++++ +
+ +++ ++ ++
+ ++ +
+
+ +++ +++
+++ +
+ ++ +
++ +
+ +
+ ++ + + +
++ +
+
++ +
++ + +
+
+ ++++
+ ++
+
n
SPONTANEOUS FISSION
A VERY RARE FORM OF DECAY
FOR URANIUM-235
25. FINAL TYPE OF DECAY FISSION
ALL FISSILE
MATERIALS
UNDERGO FISSION “Fission”
Used with Permission: Benoît Kloeckner
26. Plutonium
Uranium
Pu-239
U-235 0.711%
Pu-240
U-238 99.289%
Pu-238
Actinides
Minor Actinides = All except U and Pu Transuranium = TRU
27. FISSION PROCESS
Fissile
Material
Fission Fission
Product Product
Spontaneous Fission Induced Fission
28.
29. MODULE 2 , CHAPTER 2
• Actinides fission and some isotopes can
sustain a chain reaction
• 2 types of fission – spontaneous and induced
fission
• Can look up isotope characteristics
• A sustained chain reaction can be used for
peaceful and non-peaceful purposes
32. Uncontrolled Nuclear Chain Reaction
10-8 s Fission
=10 ns Number
= Shake Gen # neutrons
1 20=1
2 31=3
3 32=3x3=9
4 33=3x3x3=27
5 24=16
6 25=32
64 263=9x1018
80 279=6x1023
81 280=1.2x1024
In U-235 Metal / Pu-239 Metal 82 281=2.4x1024
This happens very fast!
33. Energy/Fission 0.94 kg
Of U-235
= 200 MeV
= 3.2e-11 J
82 generations
Knee Bend = 100 J = 2^81 = 2.4x1024 fissions
< 1x10-6 s = 1 millionth of a second
UNCONTROLLED Total Energy = (2.4e24 fis)(3.2x10-11 J/fis)
CHAIN REACTION = 7.68e13 J
= 18.3,000 t of TNT!
= 18.3 kT TNT = 1.2 X Little Boy Bomb!
34. Fissile Materials: Can sustain an explosive fission chain
reaction – notably plutonium of almost any isotopic
composition and highly-enriched uranium (Def’n, IPFM).
Neutrons
U-235 Density
HIGH Mouse Trap Unclamping
FAST Simulates fission
U-235 35
37. Critical Mass=Sustain Chain
Reaction
GUN TYPE WEAPON
SELF SUSTAINING = CRITICAL MASS
ESCAPE
ABSORBED
FISSIONS STOPS
AFTER SECOND FISSIONS SELF WITH REFLECTOR
GENERATION SUSTAINING (CONFINING)
INCREASING FISSILE MATERIAL MASS
Prevent neutrons from escaping is the goal! 38
Figure: Courtesy Wikipedia
38. Crowded Room Analogy: 2
Ways to Sustain a Chain
REACHING CM BY Reaction INCREASING SIZE OF ROOM
INCREASING DENSITY Fissile Material
Neutron
IMPLOSION WEAPON
GOAL: INCREASE
Bring nuclei PROBABILITY OF
closer together GUN-TYPE WEAPON
FISSION TO OCCUR
40. Theodore Taylor (1925-2004)
1 Significant Quantity
=25 kg U-235 HEU
HEU= >20% U-235
See comment:
http://goo.gl/b04i0
“Amounts of U-235 as small as 1 kg are significant quantities. He did not state that
anyone can build a bomb with 1 kg of U-235, but did suggest that this is roughly
the amount that good designer would need” Consistent with 82 gen’ns
NRC 2004 Testimony, “Nuclear Arms Race”, Craig & Jungerman
41. Because of the high
density of uranium
metal a SQ will
occupy a small volume!
42. 1) Prevent neutrons from escaping
2) Increase the probability of fissions
3) Prevent neutron absorption
GUN TYPE BOMB
Two pieces of subcritical HEU
brought together for an instant!
RECALL: Crowded room analogy
make the room bigger so the
probability of fission increases
before neutrons escape!
43. With modern weapons-grade uranium, the
background neutron rate is so low that
terrorists, if they had such material, would
have a good chance of setting off a high-
yield explosion simply by dropping one half
of the material onto the other half.Most
people seem unaware that if separate HEU
is at hand it's a trivial job to set off a
nuclear explosion . . . even a high school
kid could make a bomb in short order.
Luis Alvarez, Adventures of a Physicist (Basic Books, 1987), p. 125.
44. • Not very safe – 2 pieces that are
subcritical but if inadvertently combined
could cause an explosion.
• Must keep away from moderators
• Not very efficient – not very high yield
• Weapon of choice for non-state actors
(terrorists)
• Barrier is getting the HEU in the first
place!
45. • Uranium is mined but is only 0.711% U-235 and
99.289% U-238.
• Need to remove the U-238 to increase the
proportion of U-235 to U-238.
• Start with 7 U-235 and 993 U-238 marbles
46. • Start with 7 U-235 and 993 U-238 marbles
• To get to 5% HEU it means that 7/140=0.05
• I have to go from 993 to 133!
• I need to pull out 860 U-238 marbles!
• A large part of the work is done to get from
0.711% to 5% HEU!
47. • 7 U-235 /133 U-238 = 5%
• How do I increase to 90%?
• 90% ~ 7 U-235/ (7 U-235 + 1 U-238)
• I go from 133 to 1! Need to subtract 132 more
which is a big improvement from 853!
• BOTTOM LINE: Enriching from 0.711% to 5%
is a large part of the work.
48. It has to do with “pre-detonation” or triggering the bomb
to explode before the optimum conditions have been
Established.
Nuclide SF /kg per Neutrons/fiss CM (kg) SF/CM
100 μsec ion per 100
μsec
U-235 5.627e-7 2.637 45 0.0000
25
U-238 6.776e-4 2.1
Pu-239 6.916e-4 3.172 17 0.0011
Pu-240 48.33 2.257 822
99 % Pu-239 + 1% Pu-240 8
BOTTOM LINE: Gun Type Will Not Work for Plutonium!
49.
50. 1) Prevent neutrons from escaping
2) Increase probability of fissions
3) Prevent neutron absorption
IMPLOSION TYPE BOMB Done with explosive
Lenses causing implosion
RECALL: Crowded room
analogy
Increase the density of
nuclei so the probability
of fission increases
before neutrons escape!
51. For Plutonium need
to use implosion
technique!
Increase
Density!
2 3 6 HEU or Pu is
4
1
7
5
compressed
to Critical Mass
(bring atoms
Closer together)
53. We can get HEU by enriching
natural U what about Pu?
54. Pu Production in Reactors
U-238
For Gun Type Bomb
U-235
For Implosion
n + U-238 U-239 Np-239 Pu-239 Type Bomb
24 min 2.4 days
Fissile Material
Fertile Material (can sustain chain rxn)
(can’t sustain chain rxn
but can become fertile) U-238
U-235 Pu-239 is Produced
55. • Fissionable = Nuclide undergoes
fission after neutron capture
• Fissile = Nuclide can sustain a chain
reaction (U-235, Pu-239)
• Fertile = Nuclide can become fissile
after irradiation in a reactor (ex: U-
238)
56. Trick: Pu-239 Absorption of neutron
Pu-239
94 p
145 n + = Pu-240
94 p
146 n
HIGH
To produce WG Pu SF RATE
need to remove Pu fuel (not good for
(generally U fuel is 1% Pu) NW’s)
WG = <7% Pu-240 so that SF rate is relatively low!
57. Plutonium Production in Nuclear Reactors
Plutonium is produced in Nuclear Reactors
whereas uranium is found in nature
Neutron activation on Pu-239 produces Pu-240 (bad for NW)
% Pu-239/Pu-240
58. A Policy Analysts Rule of Thumb
1 megawatt-day of operation produces
~ 0.9 gram of plutonium in any
reactor using 20-percent or lower
enriched uranium
Power = 2650 MW
Mass Pu-239 = 0.9*2650MW *45 days /1000 g/kg
= 106 kg not quite 80 kg
59. EXAMPLE
• A company is making a proposal to sell one turn-key
LWR’s (2700 MW) to Burma. You are the one responsible
for deciding whether a reactor project should proceed
from the point of view of proliferation of the nuclear
technology to that country. The official in a condescending
way claimed that this particular reactor will in one month
not produce any Plutonium so that there are absolutely
“no risks or worries that should concern you”. What would
you say (in a polite way) in response to the official about
how much Pu is produced in this reactor/month in terms
of SQ?
• 1kg = 1000 g, And the rule of thumb is: 1 megawatt-day
of operation produces ~ 1 gram of plutonium in any
reactor using 20-percent or lower enriched uranium 60
60. Answer:
• Since the rule of thumb is 1 g produced per MW days then in 30
days the reactor produces 2700 MW * 30 days =80,000 MW days
• Applying the rule of thumb = 80,000 MW days = 80,000* MW days *
(1 g Pu/MW days)
=80,000 g Pu and 1 kg = 1000 g
=80,000 g Pu * (1 kg / 1000 g)
• =80 kg Pu
• In terms of SQ this is 10 SQ which is enough roughly for 10 bombs!
“Excuse me but your assertion that a 2700 MW reactor will produce only 100 g
Pu/month is simply not correct. In fact, if you go through the calculation which I
just did while you were talking, you will find that the reactor you propose
produces 10 IAEA Significant Quantities per month!” (And the whole room will be
silent because they will be impressed at your knowledge)
61
61. QUESTION: In the satellite pictures below
why is the lack of steam significant?
62. “I remembered the line from the Hindu scripture, the
Bhagavad-Gita: Vishnu is trying to persuade the
Prince that he should do his duty and to impress
him he takes on his multi-armed form and says,
“Now I am become death, the destroyer of worlds”
I suppose we all thought that, one way or another”
R. Oppenheimer, pg 676, R. Rhoades, “The Making of the Atomic Bomb”
On the Trinity Shot : first test of an atomic weapon.
63. MODULE 2 , CHAPTER 3
Material Advantage Disadvantag
e
U (HEU) Testing not Difficult to
Necessary get HEU
Pu Easy to Testing
Produce Necessary
66. Power Reactors: NW: HEU,Pu
LEU
Large Amount of Energy Large Amount of Energy
Slow, Controlled
Release of Energy FAST , Uncontrolled
Release of Energy
72. • Neutron moving slow will easily fission
U-235 but not U-238
• Neutron moving fast will easily fission
U-238 but also U-235
• Neutron are produced through fission
at high energy
• It is almost as if the neutron at low
energy is completely different from a
neutron travelling at high energies
73. • Like people meeting..
• When neutron encounters an isotope, it
must make a decision how it interacts
when it meets
1) Fission
2) Bounce off
3) Be absorbed
(give off a gamma)
4) Not interact at all
Probabilities described by
Size of the target (cross-section)
74. U-238 Neutron
(Very small target neutron will miss)
U-235
Neutron
(Large target
neutron will hit)
75. • Neutrons can also do other things
besides fission (bounce off, be
absorbed, not interact)
• It all has to do with the size of the
target which changes for different
neutron energies
• You have to think like a neutron
76. • Graph of ‘Energy’ or speed of the Neutron vs
‘Cross-section’ or probability of interaction or
size of the target
CROSS-SECTION
PROBABILITY
NEUTRON SPEED OR ENERGY
77. Target size of U-235 1000X bigger than at 1 MeV
Energy Region
of neutrons
produced by
FISSION
Size of the target
For U-238
Energy Region
of neutrons
slowed down
Thermal
Neutrons
78.
79. Gamma
Ray MeV
Incoming Neutron
2-3 Neutrons
Fission free to fission
U-235
Slow Neutron (Thermal)
MeV
Incoming Neutron
Gamma
U-238 Ray
Tend to absorb
No Fission Not fission!
Radiative Capture
80. Self-Sustaining Chain Reaction
90% U-238, 10% U-235:
NOT SELF SUSTAINING Gamma
Particle
Absorbed!
Not enough U-235
Around to continue
process
Fission! Absorbed!
Gamma
Particle
81. INCREASE U-235 (Red!)
Will
Fission!
50% U-238, 50% U-235 :
PROBABLY SELF SUSTAINING Will
Fission!
Fission!
Enough U-235 around
to continue process
Fission!
Will
Absorb!
82. Natural Uranium = 0.711 % U-235!
Need to get to 50%
Need to enrich $
83. Neutrons from fission (MeV)
Appear to require 50% U-235
and 50% U-238 – this is expensive!
THE TRICK: Slow the neutrons
down! Moderate them!
84. Essentially turn Into
From a Neutron
point of view
(Neutron is travelling slowly and feels the
attraction of the neutron for a longer time)
85.
86. • Possible to have self-sustaining chain reaction
by using a moderator.
87. Need less U-235 because
they appear bigger for
Slow/thermal neutrons
Instead of 50% U-235
with trick can use 3-5% U-235
88. U-235 atoms have a higher fission
interaction probability than U-238 capture
Need only 3-5% to Start a
sustained Chain Reaction
89. Go from
here to here
In a few bounces
Moderation TRICK! FISSION
ENERGIES
90. FUEL (assembly/rods)
3% 3% 3%
LEU LEU LEU
UOxide UOxide UOxide Nuclear Reactor has
Fissile Material (LEU)
just below a Critical Mass
(Controlled Chain Reaction)
Fission Energy
=Heat Produced!
Moderator: Slows down neutrons This is the TRICK!
so fission with U-235 can occur
91. Moderator Characteristic
Water (H2O) Good moderator but can also capture
neutrons. Can use 3-5% U-235. Most reactors
around the world use H2O as moderator
Heavy Water (D2O) Excellent moderator. Low probability of
neutron capture. Can start with natural (0.7%)
uranium.
Carbon (Graphite) Not a great moderator (mass A is high) but
very small capture probability. Can use
natural (0.7%) uranium as long as very pure
and no neutrons present.
95. • Isotopes that absorb neutrons. Why would it
be useful?
Gamma
Incoming
Neutron Gd-155 Ray
No Fission
Looks HUGE for incoming
Radiative Capture
neutron!
102. Partial Batch Refueling
A = Removed after 1 cycle
B A C B= Removed after 2 cycles
C= Removed after 3 cycles
Neutron Flux
Less at Outside
LIKE A BARBECUE!
107. MODULE 2 , CHAPTER 4
• Basics of how a nuclear reactor works at the core
level
• The trick was Think like
a neutron
• Moderators and the moderation trick
• Neutron poisons and control rods
• Nuclear fuel
• Fission products
115. Removal of
500 t Yellowcake
from Iraq (2008)
Natural U contains 99% U-238 which is not very radioactive
You could not make a dirty bomb out of natural uranium
116. HF+fluorine gas mixed with yellowcake produces UF6 crystals
(at room temperature it is a solid)
At this point: Proliferation Risk
117. 3-5% for LWR
85-95% for
Weapon
Utilizing 1% mass difference between
isotopes to separate them
Remove U-238 until desired ratio
U-235/U-238 is reached (93% is WG)
118. Gas is “spun” in a supersonic
centrifuge, forcing lighter U-235
to the top, where it can be
“scooped off”
This demands high strength
materials and precision
engineering.
To reach speeds of 100,000 rpm,
centrifuges need:
Light, strong rotors
Well-balanced rotors
High-speed bearings (usually
magnetic) to reduce friction
119. Separation < 1 kg SWU/yr
Separation much
better 300kg
SWU/yr
126. 1.E+07
1.E+06
PYRO Product
1.E+05
NOT IN PYRO
1.E+04 Pu-Odd
Pu-Even
1.E+03
Ci/MTHM
Np-237
Cs-137 + Sr-90
1.E+02
Fission Products Excluding
1.E+01 Tc-99 + I-129
TOTAL
1.E+00 TOTAL LOW
1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Am
1.E-01
1.E-02
1.E-03
Years After Discharge
: The radioactivity profile of SNF throughout time calculated by the author with ORNL’s Scale6 code system [SCALE6] for a Westinghouse 50 MWd/kg
HM and 4.5% enriched PWR fuel assembly. The dotted line indicates the time when the SNF cooled in reactor cooling ponds are moved to interim
storages such as dry casks. The actinides are generally represented by the thicker lines and thinner lines correspond to the actinides. Notice that
after the fission products (especially Cs-137 and Sr-90) decay the actinides, and Tc-99 and I-129 will start to dominate the profile. Notice also that
the pyroprocessing products will after several decades be at the level of hundreds of Ci and will be below the level that the IAEA considers self-
protective [Kang and von Hippel] since the fission products that would produce a self-protective dose are removed in pyroprocessing (see NOT IN
PYRO label).
127. Spent Fuel Problem
• Spent fuel remain radioactive for many years
• Very hot needs to be cooled down
• Cooled in a spent fuel pond for 3-5 yrs
• Further cooling in dry casks
• Future: emplaced in geological repository or
other measures
• So far no solution while we have 250k MT
waste
130. • Closed fuel cycle – recycle plutonium produced in
other fuel
• “Russian policy is to close the fuel cycle as far as
possible and utilize recycled uranium, and
eventually also to use plutonium in MOX fuel.
However, its achievements in doing this have
been limited - in 2011 only about 16% of used
fuel was reprocessed.”
(WNA see: http://goo.gl/Zrtvn)
• The United States does not reprocess fuel
although does advocate research in this area
• Controversial because of use of plutonium
132. MODULE 2 , CHAPTER 5
• Fuel cycle is complex
• Open cycle – no reprocessing
• Closed cycle – reprocess
• Vulnerabilities – After enrichment step if cycle
is closed and plutonium is used
• Nations differ on policy for reprocessing (US
does not reprocess, Russian Federation does)
• More detail on all of this in the next Module!
Editor's Notes
Explain fast and slow fission Know how fast and slow fission are appliedDemonstrate critical massDemonstrate power production by nuclear fissionDemonstrate the difference between fission and fusion reactionsRecognize that a mass greater than the critical mass is needed to produce an uncontrollable chain reaction.
Explain fast and slow fission Know how fast and slow fission are appliedDemonstrate critical massDemonstrate power production by nuclear fissionDemonstrate the difference between fission and fusion reactionsRecognize that a mass greater than the critical mass is needed to produce an uncontrollable chain reaction.
