Download 25.3 Fission and Fusion of Atomic Nuclei

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25.3 Fission and Fusion of Atomic Nuclei
25.3
1
FOCUS
Objectives
Guide for Reading
25.3.1 Describe what happens in a
nuclear chain reaction.
25.3.2 Explain the role of water in the
storage of spent fuel rods.
25.3.3 Distinguish fission reactions
from fusion reactions.
Key Concepts
• What happens in a nuclear
chain reaction?
• Why are spent fuel rods from a
nuclear reaction stored in
water?
• How do fission reactions and
fusion reactions differ?
Vocabulary
Guide for Reading
Build Vocabulary
L2
Reading Strategy
Graphic Organizers Have students
draw a concept map that correctly
relates the vocabulary terms for this
section.
Reading Strategy
L2
Preview Before students read the
section in detail, have them preview
the section headings, visuals, and
boldfaced material.
2
fission
neutron moderation
neutron absorption
fusion
Identifying Details In the three
paragraphs following the heading
Nuclear Fission, the sentence
describing a chain reaction states
the main idea. List the details
about energy changes in fission
and chain reactions that are
related to the main idea.
INSTRUCT
The sun appears as a fiery ball in
the sky—so bright it should never be viewed with unprotected eyes.
Although its surface temperature is about 5800 K,
the sun is not actually burning. If the
energy given off by the sun were the
product of a combustion reaction, the sun
would have burned out approximately
2000 years after it was formed, long
before today. In this section, you will learn
how energy is produced in the sun.
Nuclear Fission
When the nuclei of certain isotopes are bombarded with neutrons, they
undergo fission, the splitting of a nucleus into smaller fragments. Uranium235 and plutonium-239 are the only fissionable isotopes. Figure 25.10
shows how a fissionable atom, such as uranium-235, breaks into two fragments of roughly the same size when struck by a slow-moving neutron. At
the same time, more neutrons are released by the fission. These neutrons
strike the nuclei of other uranium-235 atoms, continuing the fission by a
chain reaction.
In a chain reaction, some of the neutrons produced react
with other fissionable atoms, producing more neutrons which react with still
more fissionable atoms.
Nuclear fission can release enormous amounts of energy. The fission
of 1 kg of uranium-235, for example, yields an amount of energy equal to
that generated in the explosion of 20,000 tons of dynamite. In an uncontrolled nuclear chain reaction, the total energy release takes only fractions
of a second. Atomic bombs are devices that start uncontrolled nuclear
chain reactions.
91
36 Kr
Krypton-91
Neutron
Have students study the photograph
and read the text that opens the section. Explain that the sun contains
hydrogen nuclei. Ask, In which state
of matter do these hydrogen nuclei
exist? (plasma) Describe a hydrogen
nucleus. (a proton) Explain that protons can combine to produce helium
nuclei. Ask, How is the energy of the
sun produced? (Energy is released
during the formation of helium nuclei.)
Energy
Figure 25.10 In nuclear fission,
a uranium-235 nucleus breaks
into two smaller nuclei and
releases neutrons. Predicting
What happens when the
released neutrons strike other
uranium-235 nuclei?
235
92 U
Uranium-235
(fissionable)
236
92 U
Uranium-236
(very unstable)
142
56 Ba
Barium-142
810 Chapter 25
Nuclear Fission
Discuss
L2
Ask, What is the general meaning of
fission? (An object splits into smaller
parts.) What is the general meaning
of fusion? (Objects combine into a
larger whole.) Explain that these terms
have more precise meanings when
applied to nuclear reactions but still
include the concepts of fragmentation
and merging.
810 Chapter 25
Section Resources
Print
• Guided Reading and Study Workbook,
Section 25.3
• Core Teaching Resources,
Section 25.3 Review
• Transparency, T293
Technology
• Interactive Textbook with ChemASAP,
Animation 30, Assessment 25.3
chem_TE_ch25.fm Page 811 Thursday, April 20, 2006 11:23 AM
L1
Use Visuals
Containment
shell
Steam
Electrical
output
Hot
coolant
Control
rods
Condenser
(steam from
turbine is
condensed)
Fuel
rods
Carbon
moderator
Reactor
Cool
coolant
38oC
Pump
Pump
Steam generator
Water
source
27oC
Fission can be controlled so energy is released more slowly. Nuclear
reactors, such as the one illustrated in Figure 25.11, use controlled fission
to produce useful energy. In the controlled fission reaction within a nuclear
reactor, much of the energy generated is in the form of heat. A coolant
fluid, usually liquid sodium or water, removes the heat from the reactor
core. The heat is used to generate steam, which drives a turbine that in turn
generates electricity. The control of fission in a nuclear reactor involves two
steps, neutron moderation and neutron absorption.
Figure 25.11 A nuclear reactor
uses controlled fission to
produce useful energy. The
illustration shows the basic
components of a nuclear reactor.
Energy from the fission process
heats the circulating coolant.
The heated coolant is used to
produce steam that turns a
steam-driven turbine. The
turbine drives a generator to
produce electrical energy.
Figure 25.10 Display the figure on an
overhead projector and explain that
uranium-235 does not spontaneously
fission. Uranium-235 is called a fissionable material because its nucleus
becomes unstable when it is struck by
a neutron. Note that two additional
neutrons (for a total of three) are produced by each fission event. Fission
reactions also produce radioactive
waste products such as krypton-91 and
barium-142. Ask, Which types of radiation do these two products emit?
(Barium-142 and krypton-91 both emit
beta particles.) Have students draw a
diagram showing how the neutrons
from the single fission process in Figure 25.10 can be used to induce subsequent fissions in other uranium-235
nuclei. The diagrams should illustrate
the exponential growth of fission processes. Have students use their diagrams to discuss why fission reactors
must be carefully monitored and
controlled.
Neutron Moderation Neutron moderation is a process that slows down
neutrons so the reactor fuel (uranium-235 or plutonium-239) captures
them to continue the chain reaction. Moderation is necessary because
most of the neutrons produced move so fast that they will pass right
through a nucleus without being absorbed. Water and carbon in the form
of graphite are good moderators.
TEACHER
Checkpoint Why must neutrons in a reactor be slowed down?
Animation 30 Take a close
look at a nuclear fission chain
reaction.
with ChemASAP
Section 25.3 Fission and Fusion of Atomic Nuclei 811
Differentiated Instruction
L2
Have students write the definitions of fission
and fusion in their notebooks in their native
language and in English. Have them prepare
a table, complete with sample reactions,
describing the characteristics of fission and
fusion processes in nuclear chemistry.
English Learners
L2
Model a Chain Reaction
Neutron Absorption To prevent the chain reaction from going too fast,
some of the slowed neutrons must be trapped before they hit fissionable
atoms. Neutron absorption is a process that decreases the number of slowmoving neutrons. Control rods, made of a material such as cadmium, are
used to absorb neutrons. When the control rods extend almost all the way
into the reactor core, they absorb many neutrons, and fission occurs slowly.
As the rods are pulled out, they absorb fewer neutrons and the fission process speeds up. If the chain reaction were to go too fast, heat might be produced faster than the coolant could remove it. In this case, the reactor core
would overheat, which could lead to mechanical failure and release of
radioactive materials into the atmosphere. Ultimately, a meltdown of the
reactor core might occur.
Demo
Purpose Students observe a chain
reaction.
Materials wooden matchsticks
Safety Keep all flammable materials
away from the demonstration area.
Have a fire extinguisher ready.
Procedure Cut some wooden matchsticks in half and arrange the heads in
branching chains on a noncombustible
surface. Construct the chain so that the
end of each matchstick is touching the
heads of two other matchsticks, forming a “Y.” Ignite the first matchstick
head.
Expected Outcome The matches
ignite in a chain reaction.
Differentiated Instruction
L3
The difference between the mass of a
nucleus and the masses of the separated
protons and neutrons is called the mass
defect. Challenge students to use the mass
defect and Albert Einstein’s famous equation,
E mc2 , to calculate the energy released
during a chemical reaction and during a
nuclear fission reaction. Have students
explain why so much more energy is
released during a nuclear fission reaction.
Gifted and Talented
Answers to...
Figure 25.10 They react and cause
the production of more neutrons.
Checkpoint
so the reactor
fuel can capture them to continue
the chain reaction
Nuclear Chemistry
811
chem_TE_ch25.fm Page 812 Wednesday, April 26, 2006 1:45 PM
Section 25.3 (continued)
Nuclear Waste
Nuclear Waste
L2
Discuss
Have students compare and contrast
nuclear fission and nuclear fusion. Ask
students to describe the advantages
and disadvantages of each type of process as a means to meet future energy
needs. Ask, What types of technical
hurdles remain to be solved in order
to utilize the energy produced by
nuclear fusion? (Accept any reasonable
answer, such as containment of the
reaction.)
L2
Relate
Explain that the wastes produced in fission reactors contain isotopes with
half-lives measured in the thousands
or hundreds of thousands of years.
Many proposals for storing or disposing of these wastes involve methods
and materials that may be highly
unsuitable. For example, the placement of wastes in thick drums that
would then be sunk in the oceans—
one suggested method—may contain
the waste for only decades or a few
centuries. The containers would begin
to leak long before the contents were
safe. The disposal problem has led
some people to propose permanent
abandonment of nuclear reactors as
sources of energy. After students read
the text, lead a discussion on the pros
and cons of using nuclear energy from
fission reactors.
CLASS
Figure 25.12 Racks at the
bottom of this pool contain
spent fuel rods. The blue glow is
from beta particles that the rods
emit into the water.
Fuel rods from nuclear power plants are one major source of nuclear waste.
The fuel rods are made from a fissionable isotope, either uranium-235 or
plutonium-239. The fuel rods are long and narrow—typically 3 meters long
with a 0.5-cm diameter. Three hundred fuel rods are bundled together to
form an assembly, and one hundred assemblies are arranged to form the
reactor core. During fission, the amount of fissionable isotope in each fuel
rod decreases. Eventually there is no longer enough fuel in the rods to ensure
that the output of the power station remains constant. The isotope-depleted,
or spent, fuel rods must be removed and replaced with new fuel rods.
Spent fuel rods are classified as high-level nuclear waste. They contain
a mixture of highly radioactive isotopes, including both the fission products and what remains of the nuclear fuel. Some of these fission products
have very short half-lives, on the order of fractions of seconds. Others have
half-lives of hundreds or thousands of years. All nuclear power plants have
holding tanks, or “swimming pools,” for spent fuel rods.
Water cools
the spent rods, and also acts as a radiation shield to reduce the radiation levels. The pools, like the one shown in Figure 25.12, are typically 12 meters
deep, and are filled with water. Storage racks at the bottom of these pools
are designed to hold the spent fuel assemblies. The rods continue to produce heat for years after their removal from the core.
The assemblies of spent fuel rods may spend a decade or more in a
holding tank. In the past, plant operators expected used fuel rods to be
reprocessed to recover the remaining fissionable isotope, which would be
recycled in the manufacture of new fuel rods. However, with large deposits
of uranium ore available—many in the United States—it is less expensive
to mine new fuel than to reprocess depleted fuel. At some nuclear plants,
there is no space left in the storage pool. In order to keep these plants open,
their fuel rods must be moved to off-site storage facilities.
Activity
L2
Nuclear Fission
After the neutron was discovered in
the 1930’s, scientists set out to create
elements heavier than uranium in the
laboratory. The culmination of this
work was the discovery of nuclear fission. Have students research the contributions of Lise Meitner and Enrico
Fermi.
812 Chapter 25
Facts and Figures
Nuclear Waste and Nuclear Power
Concerns about nuclear wastes are magnified by the timeframe during which the
wastes will be hazardous. As of 1997, 440
nuclear power plants were in operation
worldwide and about 30 countries got some
of their electricity from nuclear power stations. In France, for example, 75% of the electricity is generated by nuclear power. By
contrast, only about 20% of electricity
generated in the United States comes from
nuclear power plants. The DOE is responsible
for cleaning up 130 nuclear sites and safely
managing their waste. The sites include locations where uranium was milled, research
labs, and former nuclear weapons production facilities. Have students research how
the cleanups are progressing.
chem_TE_ch25_PPL.fm Page 813 Monday, August 9, 2004 5:04 AM
ⴙ
ⴙ
4 11 H
hydrogen nuclei
4
2 He
helium nucleus
Figure 25.13 In solar fusion,
hydrogen nuclei fuse to produce
helium nuclei. Interpreting
Diagrams What other
particles are produced by
this reaction?
2 +10 e
positrons
Nuclear Fusion
Word Origins
L2
A fusionist might bring together
different factions into one cohesive
group.
Nuclear Fusion
The sun, directly and indirectly, is the source of most energy used on Earth.
The energy released by the sun results from nuclear fusion. Fusion occurs
when nuclei combine to produce a nucleus of greater mass. In solar fusion,
hydrogen nuclei (protons) fuse to make helium nuclei. Figure 25.13 shows
that the reaction also produces two positrons.
Fusion reactions, in
which small nuclei combine, release much more energy than fission reactions, in which large nuclei split. However, fusion reactions occur only at
very high temperatures—in excess of 40,000,000°C.
The use of controlled nuclear fusion as an energy source on Earth is
appealing. The potential fuels are inexpensive and readily available. One
reaction that scientists are studying is the combination of a deuterium
(hydrogen-2) nucleus and a tritium (hydrogen-3) nucleus to form a helium
nucleus.
2
1H
31H ¡ 42He 10n energy
The problems with fusion lie in achieving the high temperatures necessary to start the reaction and in containing the reaction once it has started.
The high temperatures required to initiate fusion reactions have been
achieved by using a fission bomb. Such a bomb is the triggering device
used for setting off a hydrogen bomb, which is an uncontrolled-fusion
device. Such a process is clearly of no use, however, as a controlled generator of power.
3
Evaluate Understanding
Word Origins
Fusion comes from the Latin
word fusus meaning “melt
together.” Fusion is the combination of two low-mass nuclei
to form a nucleus of larger
mass, accompanied by the
release of a large amount of
energy. In politics, what
might be the goal of a
fusionist?
25.3 Section Assessment
15.
Key Concept Explain what happens in a
nuclear chain reaction.
16.
Key Concept Why are spent fuel rods from a
nuclear reaction stored in water?
17.
Key Concept How are fusion reactions
different from fission reactions?
18. What does nuclear moderation accomplish in a
Handbook
Heavy Water Reactors Read the paragraph on
Heavy Water Reactors on page R39 of the Elements
Handbook. Use what you have learned about nuclear
reactors to explain what a neutron moderator does,
and then explain the advantages of using heavy water
instead of ordinary water as a neutron moderator.
nuclear reactor?
19. What is the source of the radioactive nuclei
present in spent fuel rods?
20. Assuming technical problems could be over-
come, what are some advantages to using a
fusion reactor to produce electricity?
ASSESS
Assessment 25.3 Test yourself
on the concepts in Section 25.3.
with ChemASAP
L2
Ask students to write nuclear equations describing fission and fusion. Ask,
How might a meltdown of a nuclear
reactor occur? (Rapid removal of the
control rods could allow too many neutrons to react with fissionable nuclei.)
Describe the conditions under which
nuclear fusion will occur. (temperatures in excess of 4.00 × 107°C, methods
to confine and control the dense
plasmas are needed)
L1
Reteach
Explain that both nuclear fission and
nuclear fusion produce energy by the
conversion of matter to energy. In fission, heavy nuclei are split into lighter
nuclei. In fusion, light nuclei combine to
form heavier nuclei. Fission reactions
are relatively easy to control but produce radioactive wastes. Fusion reactions are difficult to initiate and control
but produce little radioactive waste.
Elements
Handbook
A neutron moderator slows neutrons
so that reactor-fuel nuclei may
capture them and sustain the chain
reaction. D2O is a more efficient
moderator than H2O because D2O
absorbs fewer neutrons.
Section 25.3 Fission and Fusion of Atomic Nuclei 813
Section 25.3 Assessment
15. Neutrons produced by fissionable atoms
react with other fissionable atoms, producing more neutrons that react with
other fissionable atoms.
16. Water cools spent fuel rods and provides
a radiation shield.
17. Fission reactions involve splitting nuclei.
In fusion reactions, small nuclei combine
and release much more energy.
18. slows down neutrons
19. unused nuclear fuel and fission products
20. Potential fuels are inexpensive and
readily available.
If your class subscribes to the
Interactive Textbook, use it to
review key concepts in Section 25.3.
with ChemASAP
Answers to...
Figure 25.13 positrons
Nuclear Chemistry
813
chem_TE_ch25_PPL.fm Page 814 Monday, August 9, 2004 5:04 AM
Dating a Fossil
Discuss
L2
Discuss with students the limits of
using carbon-14 to date fossils. First,
some organic material must be
present, as the material tested must be
from an organism that once lived. If
only an imprint remains and all organic
material is gone, therefore, carbon-14
cannot be used in dating the fossil.
Second, after a certain time, so little
carbon-14 remains that it cannot be
detected. Experts disagree as to what
this age is, but it is commonly accepted
that carbon-14 dating cannot be used
reliably on objects that are more than
75,000 years old. Ask, To the nearest
whole number, how many half-lives
of carbon-14 is 75,000 years? (75,000
years/5730 years per half-life = 13 halflives. ) What percent of the carbon-14
remains after this number of half
lives? (0.0012%)
Dating a Fossil
Direct information about earlier life on Earth comes
from fossils and artifacts. The age of an artifact of
biological origin, such as bone, wood, cloth, and
plant fibers, from 200 to about 50,000 years old can
be determined by carbon-14 dating. All living
organisms contain carbon-12 and carbon-14 in a
fixed ratio. After an organism dies, the ratio of
carbon-12 to carbon-14 changes. Carbon-14 is
radioactive with a half-life of 5730 years and decays
to nitrogen-14, while carbon-12 remains constant.
Calculating An unearthed wooden tool was found to
have only 50% of the carbon-14 content of a sample of
living wood. How old is the wooden tool?
Archaeology Archaeologists
study the life and culture of
the past, especially ancient
peoples, by excavating ancient
cities, relics, and artifacts.
The Wooly Mammoth The fossil
remains of a woolly mammoth are
found in Pleistocene deposits (the
Pleistocene epoch was from
2,500,000 to 10,000 years ago). The
abundance of well-preserved
carcasses in the permanently
frozen ground of Siberia have
provided much information about
these extinct animals.
C. 23,000 BC
C. 17,300 BC
C. 11,600 BC
C. 5,900 BC
At the time of death, the
carbon-12 to carbon-14
ratio of the woolly
mammoth was very
similar to that of an
animal living today.
The carbon-14 in the dead
mammoth was one-half
of its value in 23,000 BC.
The carbon-14 was 25% of
its initial value.
After another 5,730 years,
the carbon-14 is now
12.5% of its 23,000 BC
value.
814 Chapter 25
Facts and Figures
Relative Dating
Fossils can also be dated by relating them to
known geological events. For example, during a large volcanic eruption, vast amounts
of volcanic ash are spread over large areas of
the surrounding area. This ash will settle and
814 Chapter 25
eventually become a distinctive clay layer.
Any fossil in or near this layer can be dated
according to the date of the event that
formed the layer.
chem_TE_ch25_PPL.fm Page 815 Monday, August 9, 2004 5:04 AM
Use Visuals
L1
Note in the large photo that the fish
fossil is in a distinct rock layer. Ask,
What methods other than carbon-14
dating might be used to date the
fossil? (Answers will vary but might
include looking for other fossils in the
layer and dating them.) Discuss how
organisms found in the same rock layer
probably formed at the same time.
Some of these fossils might be dated by
using carbon-14, and others might be
from organisms that existed only during a known, short period of time.
Mummies This
mummy from Peru was
dated between the late
13th and early 14th
century by carbon-14
dating of the
decorative cloth
around the remains.
L2
Relate
Ask students to relate the half-lives in
Table 25.3 on page 805 to the usefulness of isotopes in dating materials.
For example, uranium-238 is used to
date rocks that are quite old. If U-238
were used to date a sample that is only
50,000 years old, not enough of the
U-238 would have decayed for the
change to be noticed. Ask, Which of
the radioisotopes listed in the table
might be used to date a rock that is
20 million years old? (Answers will
vary, but students might suggest
potassium-40.)
Today,
AD 2005
C. 200 BC
The carbon-14 is 6.25% of
its initial value.
Woolly Mammoth This well-preserved
frozen skin of a baby mammoth was
discovered in frozen ground in
Siberia in 1900. It had been
completely refrigerated for about
25,000 years.
In what year will the
carbon-14 level have
decreased to 3.125% of
its 23,000 BC value?
Technology and Society 815
Differentiated Instruction
Gifted and Talented
L3
Have students write down the exact procedure used to work problems involving wholenumber half-lives. Then have them use scientific calculators to expand this procedure to
include partial half-lives. Provide students
with problems to work that involve partial
half-lives. Examples might include problems
such as the following: A fossil was found to
contain 60% of its original carbon-14. How old
is the fossil? (k = ln 2/t1/2 = 0.693/5,730 yr =
1.21 × 10–4yr–1; t = 1/k ln Ao/At =
(1/1.21 × 10–4yr–1)ln(1/0.60) = 4,200 yr)
Answers to...
Calculating 5,730 years.
Nuclear Chemistry
815