Bellwork- Half-life

A patient is administered 20 mg of iodine-131.

How much of the isotope will remain in the body after 40 days if the half- life for iodine-131 is 8 days?

Fission and Fusion of Atomic Nuclei

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. You will learn how energy is produced in the sun.

When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments.

This often produces more neutrons.

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In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms.

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Nuclear Fission

25.3A Nuclear Power Plant uses fission to produce energy

Neutron Moderation

Neutron moderation slows down neutrons so the reactor fuel (uranium-235 or plutonium-239) can capture them to continue the chain reaction.

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Neutron absorption is a process that decreases the number of slow-moving neutrons.

Control rods, made of a material such a cadmium, are used to absorb neutrons.

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Animation 30

Take a close look at a nuclear fission chain reaction.

Fission reactor

Used fuel rods are kept under water in holding tanks.

Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels.

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Fusion occurs when nuclei combine to produce a nucleus of greater mass.

In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons.

Fusion reactions, in which small nuclei combine, release much more energy than fission reactions, in which large nuclei split.

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The use of controlled fusion as an energy source on Earth is appealing.

•The potential fuels are inexpensive and readily available.

•There is no radioactive waste product

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•The problems with fusion lie in achieving the high temperatures necessary to start the reaction and in containing the reaction once it has started.

• VIDEO

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25.3 Section Quiz.

1. One of the control mechanisms for a sustainable nuclear chain reactor involves slowing down the released neutrons so they may be captured by other nuclei. This is done using

a. moderators.

b. shielding.

c. absorbers.

d. control rods.

25.3 Section Quiz.

2. Spent fuel rods are stored in

a. lead-lined containers.

b. deep pools of water.

c. thick concrete bunkers.

d. cadmium or graphite containers.

25.3 Section Quiz.

3. Choose the correct words for the spaces. In solar fusion, _______ nuclei fuse to form _______ nuclei.

a. helium, hydrogen

b. hydrogen-1, hydrogen-2

c. hydrogen, helium

d. hydrogen-1, hydrogen-3