ChemCh25.3.ppt

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25.3 Fission and Fusion >

Chapter 25

Nuclear Chemistry

25.1 Nuclear Radiation

25.2 Nuclear Transformations

25.3 Fission and Fusion

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Where does the sun’s energy come from?

CHEMISTRY & YOU CHEMISTRY & YOU

The sun is about

halfway through its life cycle. It has been

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25.3 Fission and Fusion > Nuclear FissionNuclear Fission

Nuclear Fission

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When the nuclei of certain isotopes are

bombarded with neutrons, the nuclei

split into smaller fragments.

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The figure below shows how uranium-235 breaks into two smaller fragments of roughly the same size when struck by a slow-moving neutron.

U

Uranium-235 (fissionable)

235

92 U

Uranium-236 (very unstable) 236 92 Ba Barium-142 142 56 Kr Krypton-91 91 36

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In a chain reaction, some of the emitted

neutrons react with other fissionable

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Nuclear fission can release enormous

amounts of energy.

• The fission of 1 kg of uranium-235 yields an amount of energy equal to that

produced when 20,000 tons of dynamite explode.

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Nuclear reactors use controlled fission

to produce useful energy.

• The reaction takes place within uranium-235 or plutonium-239 fuel rods.

• A coolant absorbs heat produced by the controlled fission reaction and transfers the heat to water, which changes to

steam.

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Neutron Moderation

Nuclear moderation

is a process that

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Neutron Moderation

Nuclear moderation

is a process that

slows down neutrons so the reactor fuel can

capture them to continue the chain reaction.

• Moderation is necessary because most of the neutrons produced move so fast that they

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Neutron Moderation

Nuclear moderation

is a process that

slows down neutrons so the reactor fuel can

capture them to continue the chain reaction.

• Moderation is necessary because most of the neutrons produced move so fast that they

would pass right through a nucleus without being captured.

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Neutron Absorption

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Neutron Absorption

To prevent the chain reaction from going too

fast, some of the slowed neutrons must be

trapped before they hit fissionable atoms.

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Neutron Absorption

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Neutron Absorption

Control rods, made of materials such as

cadmium, are used to absorb neutrons.

• When control rods extend almost all the way into the reactor core, they absorb many

neutrons and fission occurs slowly.

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Neutron Absorption

Control rods, made of materials such as

cadmium, are used to absorb neutrons.

• If the chain reaction were to go too fast, heat might be produced faster than the coolant

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Nuclear Waste

Fuel rods from nuclear power plants are one

major source of nuclear waste.

• The fuel rods are made from a fissionable

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Nuclear Waste

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. • During fission, the amount of fissionable

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Nuclear Waste

Spent fuel rods are classified as high-level

nuclear waste.

• 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

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Nuclear Waste

Spent fuel rods are classified as high-level

nuclear waste.

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Nuclear Waste

Spent fuel rods are classified as high-level

nuclear waste.

• At some nuclear power plants, the storage pool has no space left.

• Finding appropriate storage sites is difficult because high-level waste may need to be stored for a long time.

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How does the fission of a uranium-235

nucleus cause a chain reaction?

When slow-moving neutrons bombard uranium-235, the atom splits and releases more

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25.3 Fission and Fusion > Nuclear FusionNuclear Fusion

Nuclear Fusion

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The energy emitted by the sun results

from nuclear fusion.

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The energy emitted by the sun results

from nuclear fusion.

• _Fusion_{occurs when nuclei combine to} produce a nucleus of greater mass.

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The energy emitted 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.

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Fusion reactions, in which small nuclei

combine, release much more energy

than fission reactions, in which large

nuclei split apart and form smaller

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Fusion reactions, in which small nuclei

combine, release much more energy

than fission reactions, in which large

nuclei split apart and form smaller

nuclei.

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The use of controlled nuclear fusion as

an energy source on Earth is appealing.

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The use of controlled nuclear fusion as

an energy source on Earth is appealing.

• The potential fuels are inexpensive and readily available.

• The problems with fusion lie in achieving the high temperatures needed to start the reaction and in containing the reaction

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The high temperatures required to start

fusion reactions have been achieved by

using a fission bomb.

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The high temperatures needed to

support fusion exist within the sun’s

core. Late in the sun’s life cycle, other

fusion reactions will occur. What

element would form when two helium

nuclei fuse?

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The high temperatures needed to

support fusion exist within the sun’s

core. Late in the sun’s life cycle, other

fusion reactions will occur. What

element would form when two helium

nuclei fuse?

CHEMISTRY & YOU CHEMISTRY & YOU

Be

8

He + He

4 4

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Choose the correct words for the

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Choose the correct words for the

spaces. In solar fusion, _______ nuclei

fuse to form _______ nuclei.

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25.3 Fission and Fusion > Key ConceptsKey Concepts

In a chain reaction, some of the emitted neutrons react with other fissionable

atoms, which emit neutrons that react with still more fissionable atoms.

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25.3 Fission and Fusion > Glossary TermsGlossary Terms

• fission: the splitting of a nucleus into smaller fragments, accompanied by the release of

neutrons and a large amount of energy

• _{neutron moderation:}_{a process used in}

nuclear reactors to slow down neutrons so the reactor fuel captures them to continue the

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25.3 Fission and Fusion > Glossary TermsGlossary Terms

• neutron absorption: a process that decreases the number of slow-moving neutrons in a nuclear reactor; this is

accomplished by using control rods made of a material such as cadmium, which absorbs

neutrons

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Electrons and the Structure of Atoms

• During fission and fusion, atoms change their chemical identity as the number of protons in their nuclei change.

• In fission, large nuclei split into two or more smaller nuclei.

• In fusion, smaller nuclei combine to form larger

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