Transcript Nuclear Chemistry Chapter 25

Nuclear Chemistry
Chapter 25
Characteristics of Chemical &
Nuclear Reactions

Chemical Reactions
1. Occur when bonds are
broken and formed
2. Atoms remain unchanged,
though they may be
rearranged
3. Involve only valence
electrons
4. Associated with small
energy changes
5. Reaction rate is influenced
by temperature, pressure,
concentration, and
catalysts

Nuclear Reactions
1. Occur when nuclei emit
particles and/or rays
2. Atoms are often converted
into atoms of another
element
3. May involve protons,
neutrons, and low-orbit
electrons
4. Associated with large
energy changes
5. Reaction rate is not
normally affected by
temperature, pressure, or
catalysts
Balancing Nuclear Equations
Rubidium undergoes electron capture to
form krypton. Show the balanced
equation.
 Reactant: 81Rb + 0e

37

Product:
-1
81
0
36
0
Kr + g (x-ray)
Balancing Nuclear Equations
Oxygen-15 undergoes positron emission.
Show the balanced equation.
 Reactant: 15O

8

Product:
15
7
0
N+ b
1
Balancing Nuclear Equations
Thorium-231 becomes Protactinium-231.
Show the balanced equation and identify
the type of radioactive decay.
 Reactant: 231Th

90

Product:
231
0
91
-1
Pa + b
Uranium
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Uranium is a naturally radioactive element that can be found in the
crust of the Earth.
This element, quite abundant in many areas of the world, is naturally
radioactive.
Certain isotopes of uranium can be used as fuel in a nuclear power
plant.
The uranium is formed into ceramic pellets about the size of the end
of your finger.
By bombarding uranium with neutrons, neptunium can be
synthesized, which decays into plutonium:
1
n
U
+
92
0
238
Np 94

93
239
239
239
U
92
0
Pu +
b
-1
0
Np
+
b
93
-1
239
Conservation of Mass
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Matter is neither created nor destroyed.
This is true, with the caveat that matter can be
converted into energy (and vice versa) according to the
equation:
–
–
–
–
DE= Dmc2
DE= change in energy,
Dm=change in mass,
c=speed of light (3.00x108 m/s)
Thus, ANY reaction that has a consumes or produces
energy will also consume or produce an accompanying
quantity of mass.
 Thus, the total conversion of 1kg of matter yields an
equivalent of 1 x (3x108)2 = 9x 1016 joules - this is
approximately the energy output of a 200 MW power
station running for 14 years!

Binding Energy & The Mass Defect

Recall: for nuclei to be stable there must exist a strong nuclear force
between the nucleons that is short range, attractive, and can
overcome the coulomb repulsion of the protons.
– Now suppose we assemble a nucleus of N neutrons and Z protons.
– There will be an increase in the electric potential energy due to the
electrostatic forces between the protons trying to push the nucleus
apart
– but there is a greater decrease of potential energy due to the strong
nuclear force acting between the nucleons and attracting them to one
another.
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As a consequence, the nucleus has an overall net decrease in its
potential energy.
This decrease in potential energy is called the nuclear binding
energy
The decrease per nucleon is called the binding energy per nucleon.
The loss of this energy is, by the mass-energy relation, equivalent to
a loss of mass called the mass defect.
The variation of binding energy per
nucleon with atomic mass number
So how is energy released in stars? This can be explained by a graph
of the binding energy per nucleon against atomic mass number A
Releasing Nuclear Energy
The curve reaches a maximum at iron, which,
because of its high binding energy per nucleon,
indicates that the protons and neutrons are very
tightly bound and iron is a very stable nucleus.
 Beyond iron, the binding energy per nucleon
falls slightly as A increases towards the more
massive nuclei.
 Two processes can release energy from the
nucleus of an atom. They are nuclear fission and
nuclear fusion.

Nuclear Fission
In nuclear fission a massive nucleus such as
uranium splits in two to form two lighter nuclei
of approximately equal mass.
 This happens on the falling part of the curve so
that mass is lost and binding energy released
when very heavy elements fission to nuclei of
smaller mass number. Nuclear fission is
responsible for the release of energy in nuclear
reactors and atomic bombs.
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Fission Inside Nuclear Reactors
235
1
n
U
+
92
0

236
U
92
92
Kr + 1n + 141Ba + 1n
36
0
56
0
Each fission of Uranium-235 releases additional
nuetrons. If 1 fission reaction produces 2
neutrons, these 2 neutrons can create 2
additional fission reactions each.
– This is a self-sustaining process called a chain
reaction!
– Both the # of fissions and amt of energy release
increase extremely rapidly.
– The explosion from an atomic bomb represents the
results of an uncontrolled chain reaction.
Critical Mass
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It isn’t enough just to have a sample of fissionable
material, like uranium-235.
You must also have a critical mass of your material.
– If there is not a sufficient amount of mass, the released
neutrons will dissipate before finding another unstable nucleus
with which to react.
– No chain reaction will form and the reaction will be
unsustainable.
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The amount of mass necessary to sustain a chain
reaction is called the critical mass.
– Below this amount is called the subcritical mass.
– Above this amount is called the supercritical mass.
 Supercritical masses cause rapid acceleration of the reaction and
can lead to a violent explosion.
Pressurized Water Reactor
Components of a Nuclear Reactor
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Fuel Elements: Usually pellets of uranium oxide (UO2)
arranged in corrosion-resistant tubes to form fuel rods.
The rods, enriched with 3% uranium-235, are arranged
into fuel assemblies in the reactor core.
Control Rod: cadmium, hafnium, or boron rods absorb
excess neutrons, controlling the reaction within the
reactor. (Secondary shutdown systems involve adding
other neutron absorbers, usually as a fluid, to the
system.)
– If the reaction isn’t properly controlled, disaster results
– Cf. Three Mile Island (U.S. 1979), Chernobyl (Ukraine, 1986)
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Moderator: This is material which slows down the
neutrons released from fission so that they cause more
fission. It may be water, heavy water (deuterated), or
graphite (carbon).
Coolant: fluid circulating in the reactor core, serving to
lower the reaction temperature; usually water
Producing Electricity from Nuclear
Reactors
In America today, nuclear energy plants are the
second largest source of electricity after coal -producing approximately 21% of our electricity.
 With the exception of solar, wind, and
hydroelectric plants, all others including nuclear
plants:

–
–
–
–
Convert water to steam
The steam spins the propeller-like blades of a turbine
The turbine blades spin the shaft of a generator.
Inside the generator, coils of wire and magnetic fields
interact to create electricity
Turbine & Generator
Converting Water to Steam

The energy needed to boil water into steam is
produced in one of two ways:
– by burning coal, oil, or gas (fossil fuels) in a furnace
– by splitting certain atoms of uranium in a nuclear
energy plant.
Nothing is burned or exploded in a nuclear
energy plant.
 Rather, the uranium fuel generates heat through
fission.
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Fast Breeder Reactors
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Under appropriate operating conditions, the neutrons
given off by fission reactions can "breed" more fuel from
otherwise non-fissionable isotopes.
The most common breeding reaction is that of
plutonium-239 from non-fissionable uranium-238.
The term "fast breeder" refers to the types of
configurations which can actually produce more
fissionable fuel than they use, such as the LMFBR.
This scenario is possible because the non-fissionable
uranium-238 is 140 times more abundant than the
fissionable U-235 and can be efficiently converted into
Pu-239 by the neutrons from a fission chain reaction.
France has made the largest implementation of breeder
reactors with its large Super-Phenix reactor and an
intermediate scale reactor (BN-600) on the Caspian Sea
for electric power and desalinization.
Breeding Plutonium-239
Fissionable plutonium-239 can be
produced from non-fissionable
uranium-238 by the reaction
illustrated.
 The bombardment of uranium238 with neutrons triggers two
successive beta decays with the
production of plutonium. The
amount of plutonium produced
depends on the breeding ratio.
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Plutonium Breeding Ratio
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In the breeding of plutonium fuel in breeder reactors, an important
concept is the breeding ratio, the amount of fissile plutonium-239
produced compared to the amount of fissionable fuel (like U-235)
used to produced it.
In the liquid-metal, fast-breeder reactor (LMFBR), the target
breeding ratio is 1.4 but the results achieved have been about 1.2 .
This is based on 2.4 neutrons produced per U-235 fission, with one
neutron used to sustain the reaction.
The time required for a breeder reactor to produce enough material
to fuel a second reactor is called its doubling time, and present
design plans target about ten years as a doubling time.
A reactor could use the heat of the reaction to produce energy for
10 years, and at the end of that time have enough fuel to fuel
another reactor for 10 years.
Liquid-Metal, Fast-Breeder
Reactor
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The plutonium-239 breeder reactor is commonly called a fast breeder
reactor, and the cooling and heat transfer is done by a liquid metal.
– The metals which can accomplish this are sodium and lithium, with sodium being
the most abundant and most commonly used.
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The construction of the fast breeder requires a higher enrichment of U-235
than a light-water reactor, typically 15 to 30%.
The reactor fuel is surrounded by a "blanket" of non-fissionable U-238.
No moderator is used in the breeder reactor since fast neutrons are more
efficient in transmuting U-238 to Pu-239.
At this concentration of U-235, the cross-section for fission with fast
neutrons is sufficient to sustain the chain-reaction.
Using water as coolant would slow down the neutrons, but the use of liquid
sodium avoids that moderation and provides a very efficient heat transfer
medium.
LMFB Reactor Diagram
Liquid Sodium Coolant
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Liquid sodium is used as the coolant and heat-transfer medium in
the LMFBR reactor.
– That immediately raised the question of safety since sodium metal is an
extremely reactive chemical and burns on contact with air or water
(sometimes explosively on contact with water).
– It is true that the liquid sodium must be protected from contact with air
or water at all times, kept in a sealed system.
– However, it has been found that the safety issues are not significantly
greater than those with high-pressure water and steam in the lightwater reactors.
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Sodium is a solid at room temperature but liquifies at 98°C.
It has a wide working temperature since it does not boil until 892°C.
– That brackets the range of operating temperatures for the reactor so
that it does not need to be pressurized as does a water-steam coolant
system.
– It has a large specific heat so that it is an efficient heat-transfer fluid.
The Super-Phenix
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The Super-Phenix was the first large-scale breeder reactor. It was
put into service in France in 1984.
The reactor core consists of thousands of stainless steel tubes
containing a mixture of uranium and plutonium oxides, about 1520% fissionable plutonium-239. Surrounding the core is a region
called the breeder blanket consisting of tubes filled only with
uranium oxide. The entire assembly is about 3x5 meters and is
supported in a reactor vessel in molten sodium. The energy from
the nuclear fission heats the sodium to about 500°C and it transfers
that energy to a second sodium loop which in turn heats water to
produce steam for electricity production.
Such a reactor can produce about 20% more fuel than it consumes
by the breeding reaction. Enough excess fuel is produced over about
20 years to fuel another such reactor. Optimum breeding allows
about 75% of the energy of the natural uranium to be used
compared to 1% in the standard light water reactor.
Nuclear Fusion
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In nuclear fusion, energy is released when two light nuclei are fused together to form
a heavier nucleus.
This happens on the rising part of the graph.
Nuclear fusion is the principal source of energy in stars and fusion can happen if each
nucleus has sufficient kinetic energy to enable them to overcome their mutual
repulsion, be captured by the strong nuclear force and stick together.
The minimum temperature required to initiate a fusion reaction is 4.0 x108 K.
In star formation, the kinetic energy to do this comes from the conversion of
gravitational energy into thermal energy by the Kelvin Helmholtz contraction.
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–
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In the case of stars like the sun, fusion can occur when the temperature of the contracting
cloud reaches about 8 x 106 K.
It is because of the high temperatures which are needed to give the protons sufficient
kinetic energy, that these nuclear reactions are also known as thermonuclear fusion
reactions.
It is fusion of hydrogen nuclei by thermonuclear fusion reactions with a
release of binding energy that is the primary source of energy generation
in stars.
The Tokamak Reactor
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To satisfy the conditions of thermonuclear fusion, using deuteriumtritium fuel,
– the plasma temperature T must be in the range 1~3×108 K,
– the energy confinement time tE must be about 1~3 s and
– the density n must be around 1~3×1020 particles/m3.
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To startup a reactor some means of auxiliary heating must be used
to attain the minimum initial temperature of about 108 K.
– After the ignition of the fuel mixture the plasma will be heated by the
alpha-particles released in the reaction and the source of auxiliary
heating may be turned off.
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The rate of fusion reactions increases with the square of the plasma
density.
– However, the density cannot increase above certain limits without
spoiling the plasma stability.
– On the other hand, the energy confinement time increases with the
density, with the degree of plasma stability, and with the plasma
volume.
– Balancing these requirements, it is possible to determine the minimum
size for a reactor, which depends on the magnetic configuration
adopted.
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http://w3.pppl.gov/~dstotler/SSFD/
How much energy is released
during thermonuclear reactions?
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4H  He + energy released

mass of 4 H atoms = 4 x 1.008 = 4.032 amu
- mass of 1 He atom = 4.003 amu
therefore... mass defect = 4.032 - 4.003 = 0.029 amu
Using the mass-energy relation, the mass converted into
energy is
= (0.029 amu x 1.66 x 10-27 kg/amu) x (3 x 108 m/s)2
= 4.33 x 10-12 J or, equivalently, 27 MeV.
Trinity 1945
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On July 16, 1945, at 5:29:45 a.m., the
first atomic explosion in history took place
at the Jornado del Muerto (Journey of
Death) trail on the Alamagordo Bombing
Range in New Mexico. An extremely tense
group of scientists looked on as the bomb,
named "Gadget," released its 18.6 kiloton
yield, vaporizing the 100-foot steel tower
it had been hoisted atop.
A-Bomb: The Nevada Test
Test Able:
An Air Drop in the Bikini Island
Test Baker:
An Underwater Detonation at the
Bikini Atoll
Nuclear Fallout
The National Cancer Institute recently estimated
that 10,000-75,000 cases of thyroid cancer in
the United States were caused by the radioactive
isotope iodine-131 from Nevada A-bomb fallout.
 In addition to the military personnel exposed to
high levels of radiation in the vicinity of the
tests, thousands of U.S. citizens downwind may
have paid a lethal price for the atomic ambitions
of their own government.

Project Ivy:
Hydrogen Test Bomb