Nuclear Chemistry/Physics
Recommended Problems
Learning Objectives: 1, 3, 4, 6, 7
Self Assess & Review: 2, 3, 4, 10, 11, 12, 18,
19, 20, 21
2009
1
What is Radioactivity?
•
•
•
Bequerel accidentally discovered in 1896
Rutherford found two types, a third was added later.
Radioactive Emission Reactions – 6 types
1. Alpha decay: spontaneous nuclear fission
 24 = 24He2+ - a helium nucleus emitted
 the identity of the emitting nucleus changes
2. Beta decay:
 -10 = -10e - an electron emitted
 the identity of the emitting nucleus changes
3. Electron capture:
 A proton gains an electron becoming a neutron
 the identity of the emitting nucleus changes
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2
Rutherford’s experiment
2009
3
1
Nuclear decay reactions (natural)
226
88
1. Alpha decay:
Ra

Rn 
222
86
4
2
He
Note: Mass # decreases by 4, proton count by 2
1
0
2. Beta decay:
n  11 p   10 e
14
6
Carbon 14 decay
3. Electron capture:
1
1
A neutron splits !!
C
14
7
N 
0
1

p  1 e 0 n
0
1
N   10  
14
7
Formation of carbon 14
In fact a neutron (cosmic ray product) collides with a
electron is captured and a proton emitted.
14 N
7
14
6
C
and the
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4
Nuclear decay reactions (natural)
4. Positron, +10e /+10, emission:
 A proton emits a positron becoming a neutron
 the identity of the emitting nucleus changes
5. Gamma, 00 , emission:
 An unstable nucleus (i.e. higher in energy) releases energy
as a gamma photon ( = 10-12 m or less)
 Geometric re-arrangement
re arrangement of neutrons and protons in
nucleus to lower energy state.
 Identity of emitting nucleus is unchanged
6. Spontaneous fission: (not involving alpha particle)
 Similar to alpha emission
 Relatively uncommon
 Identity of emitting nucleus is changed – two new
“daughter” nuclei are formed.
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5
Nuclear decay reactions (natural)
4. Positron emission:
95
p 0n 1e
1
1
1
0
43
99 m
5. Gamma emission:
6. Spontaneous Fission:
95
0
Tc  42 Mo  1 e
43
236
92
U
99
0
Tc  43 Tc  0 
96
39
Y
136
53
1
I  40 n
• Why does radioactive decay occur?
• The existence of stable nuclei with more than one proton
is due to the nuclear force.
– The nuclear force is the strongest force of attraction between
nucleons acting only at very short distances (about 10-15 m).
– This force more than compensates for the electrostatic
repulsion resulting in a stable nucleus.
2009
6
2
Nuclear decay – Why?
• Factors affecting/effecting nuclear stability
– The shell model of the nucleus: a model in which
protons and neutrons are in energy shells, analogous
to the shell structure in electron configurations.
– Empirical evidence that nuclei with certain numbers
off protons
t
andd neutrons
t
appear to
t be
b very stable
t bl
– These numbers, called magic numbers, are the
numbers of nuclear particles in a completed shell of
protons or neutrons.
• Because the strong nuclear force differs from the
electrostatic force, these numbers are not the same as those
for electrons in atoms.
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7
Magic Numbers and the Band of Stability
• For protons, the magic numbers are
– 2, 8, 20, 28, 50, and 82
• For neutrons, the magic numbers are
– 2, 8, 20, 28, 50, 82, and 126
– No stable nuclides are known with atomic numbers
ggreater than 83.
• Evidence also points to the special stability of
pairs of protons and pairs of neutrons.
Table 21.1
Number of Stable Isotopes
157
52
50
5
Number of protons
Even
Even
Odd
Odd
Number of neutrons
Even
Odd
Even
Odd 8
• Alpha emission:
Band of Stability
 occurs for those nuclei
with Atomic # > 83.
 This is the release of 2
protons and 2 neutrons –
He nucleus.
• Beta emission:
 For nuclei with N/Z too
l
large
:np
• Electron capture or
positron emission:
 For nuclei with N/Z too
small.
 p  n electron capture
 p  n positron emission
2009
9
3
2009
10
Spontaneous Nuclear Decay Processes
•
Uranium 238 decay sequence
– 14 step process ending in the formation of Pb-206
1. 23892U  23490Th + 42He (alpha emission)
2. 23490Th  23491Pa + 0-1e (beta emission)
3. 23491Pa  23492U + 0-1e (beta emission)
4 23492U  23090Th + 42He
4.
H (alpha emission)
–
–
This process continues with several ,  following
Radioactive nuclei continue to decay until a stable
non-radioactive state is reached.
14.
14.
210
84Po
206 Tl
81
 20682Pb +
 20682Pb +
4
2He (alpha emission)
0
-1e
(beta emission)
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11
U-238 14 step decay process
2009
12
4
Radioactive Half-Life
• HalfHalf-life:
life the length of time required for one half
of the radioactive material to decay.
• Radioactive decay is a First order process
– Exponential decay curve, ½ life is constant
• Half-life time, t1/2 is strictly dependent upon the
exact material:
– 238U t1/2 = 4.5 billion years!
– 214Po t1/2 = 0.00016 second
– 31H, primary coolant product, t1/2 = 12.3 yr
– 14C  14N + 0-1e
5730 yr.
• 14 14C decay every second in your body/kg mass
because humans are 18% C by weight.
2009
13
Neutron Induced Nuclear Fission
• The Process (U-235)
– a high energy/speed neutron slams into the nucleus of a
235U atom.
– The 235U nucleus splits into two ‘daughter’ nuclei and
releases more high speed neutrons.
– The new neutrons can collide with more 235U.
– This is initiation of a chain reaction (one that feeds
itself)
• Where does the energy come from?
– In all exothermic reactions a small quantity of matter is
converted to energy.
– The total mass of the ‘daughter’ particles is less than
the mass of the parent particles.
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14
Neutron
induced
fission of
235U
2009
15
5
Nuclear Fission
• Mass defect
– If any system loses energy, it must also lose mass.
– Though mass loss in chemical reactions is small
(10-12 kg), the mass changes in nuclear reactions
are approximately a million times larger.
• Where does the energy come from?
E = mc2
Binding energy
in joules
Speed of light
squared
mass
in kg
E = 1 kg x (3.0 x 108m/s)2 = 9 x 1016 joules
1 kg matter  energy = 2.5 years power plant operation!
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Mass Defect
238
92
U
234
Th
90
238.05078
(234.04359 + 4.00260)
E = mc2
g/
mol
234.04359
-
 42 He
4.00260
238.05078g/mol
= -0.00459 g/mol
E = -0.00459 g/mol x ((2.9979x108 m/s)2
E = -0.00459 g/mol x 1 Kg/1000 g x 8.9874 x 1016 (m/s)2
E = -4.1252 x 10
11
J/mol
E = -4.13 x 108
kJ/mol
NOTE: Kg/m2/s2 = J
2009
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2009
18
6
Nuclear Fusion
• Energy from Fusion?
– If splitting releases energy, then from a reaction
standpoint, Fusion should take energy!
• Nuclear fusion:
– a nuclear reaction in which light nuclei combine to
give a stabler heavy nucleus, possibly neutrons, and
energy is released.
– Fusion again results in a mass defect
– The resultant particles are less massive than the
original particles.
• Binding Energy
– The energy needed to break a nucleus into its
2009individual protons and neutrons.
20_16
19
Figure 21.16 Plot of binding energy per nucleon
versus mass number
9
n
Fission
Fu
sio
Binding energy p
per nucleon (MeV)
Fe-56
8
Mass # below 56, Fusion more
stabilizing.
Mass # above 56
56. Fission more
He-4 stabilizing.
The larger the difference in energy
the more energy released.
7
2
1
6
0
2009
U-235
H  31 H  42 He  01 n
50 2.01400 + 3.01605
100
--
150 + 1.008665 =200
4.00260
0.018785
250
number
E = .018785/1000 x C2 xMass
1kJ/1000
J = 1.69 x 109 kJ/mol
20
4 times the energy per mole of U-235 fission
Figure 21.20: Plasma confinement in a tokamak reactor.
Courtesy of Princeton Plasma Physics Laboratory.
2009
21
7
Fissionable vs. non-fissionable Isotopes
•
235U
is a fissionable isotope of U
– only 0.7% of all U is 235U
– 99.3% is 238U a non-fissionable isotope
• Nuclear reactors cannot ‘run’ on normal U
– Special chemical processes needed to ‘enhance’
enhance the
235U % and make a “fuel” grade U.
– Very difficult and expensive process
– Result: 93% 238U and 7% 235U = fuel grade
• Nuclear Reactor - how does it work?
2009
22
Nuclear Power: the Seabrook Saga
• Benefits of Seabrook (nuclear power)
–
–
–
–
No CO2 No SOx’s, No NOx’s
1.15 gigawatts of power. (1.15 x 109 watts)
Equivalent to: 1.84 million gallons of oil/day
OR 10,000 tons of coal/day
• New Hampshire plant - power for Boston
– 1972 plans announced for 2 plants, 1st to come on line in 1979,
2nd in 1981.
– Original projected cost, 2 plants: $973 million
– Construction delayed until 1976
– 1984 second plant cancelled
– 1986 first plant finally completed - 7 years late
– 1989 testing completed
– 1990 first plant licensed - 11 years behind schedule
–2009Final cost for 1 plant only:
$6.45 billion
23
Nuclear Reactors
• Fuel Elements
– UO2 pellets the size of pencil eraser, stacked end to end
in Cd alloy tubes.
– Cd tubes bundled into stainless steel clad bundles called
fuel assemblies.
– Cd Control Rods adjust neutron flux.
• Primary coolant, H3BO3, aqueous boric acid
– B is another neutron absorber,
– the solution transfers heat energy of nuclear reaction to
a heat exchanger (secondary coolant), which flashes to
steam.
• Seabrook: 28,000 gal vaporized per minute!!!!
• Steam turns the electricity producing turbine.
• Tertiary coolant cools secondary coolant for return
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24
to the system.
8
Uranium fuel pellets
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25
Cadmium
fuel rod
Stainless
steel
bundles of
fuel rods
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26
Reactor
pressure vessel
Boric acid
primary
coolant
Fuel
assemblies
2009
Control rods:
for neutron flux
27
9
Schematic of Seabrook plant
2009
H3BO3(aq) - boric acid
28
Cooling tower: for condensing the secondary
2009
coolant before return to the system 29
Nuclear Reactors
• Can a nuclear reactor have a nuclear
explosion?
– NO!
• Chernobyl was a chemical explosion caused by a reactor
meltdown.
– WHY not?
– 93% 238U and 7% 235U = fuel grade
– Weapons grade U must be nearly pure 235U
• Can nuclear reactor fuel be used for making
bombs?
– Qualified NO.
– Refinement to weapons grade very very difficult
and expensive.
2009
30
10
2009
Reactor building at Chernobyl
31
Nuclear Breeder Reactors
• Can nuclear reactor fuel be used for making
bombs?
– Spent fuel contains 239Pu that is fissionable, so it
can be used for more power or refined chemically
for weapons grade material.
238
92U
+ 10n 
239
P
94Pu
– 239 nucleons but protons  by 2!!!
1
0n

1
+1p
+
0
+ 20-1e
-1e
– Fortunately, Plutonium is one of the most toxic
substances known, so that chemical refinement is
especially dangerous.
2009
32
Military grade
Pu-239
recovered in
Germany
2009
33
11
Nuclear Power
Benefits and Risks
• Benefits:
– No CO2 or SOx or NOx produced.
– Much power produced for a power hungry
world.
• ~17% of total world power usage.
• Risks
Ri k
– from operating plants - minimal, less than that
of conventional power plants.
– Radioactive waste: LLW & HLW
• Safe level is considered to be after 10 half-lives, thus
~250,000 years for current Pu waste to be “safe”.
• Storage facilities must contain waste for this long!
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34
% Total Power from Nuclear Plants
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35
Total Operating Reactors
2009
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12
2009
37
Low-level nuclear waste storage …?
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38
Cooling Pool for Spent Fuel Rods
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39
13
2009
40
HLW heated with finely
ground glass and melted
together.
Poured into stainless steel
canisters and sealed for longterm storage.
Methods of sequestering HLW
2009
2009
41
A geologically stable
salt cavern 1400 ft
below the mountain
42
14
2009
43
Measurement of Radiation and
Radiation Effects
• RAD - Radiation Absorbed Dose
– 1 RAD = 0.01 joule of energy absorbed /kg
mass of material absorbing the radiation.
– A measure of the raw energy in the radiation.
– All radiation is not equal
q in effect!
• REM - Roentgen Equivalent Man
– includes physiological damage caused as a
factor in rating radiation absorbed.
 particles are 10 x and neutrons 5 x more damaging
than  particles ,  and x-rays. This factor is
multiplied times the RAD to give the REM value.
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45
15
Dose response curves for radiation
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46
Your annual radiation dose
NOTE:
1 mrem = 10 Sv
0
300
100
1
2009
1361
47
Your Sources
of Radiation
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16