Nuclear Fission and Fusion - PowerPoint PPT Presentation

Nuclear Fission and Fusion

• Power can be obtained two ways.
• Fission Splitting atoms
• Get energy if the nucleus is big.
• The smaller ones are more stable.
• What we do in nuclear reactors.
• Fusion Joining atoms
• Get energy if the nuclei are small.
• The larger one is more stable.
• This is how the sun works.

Fission is the splitting of atoms

These are usually very large, so that they are not as stable

Fission chain has three general steps:

1. Initiation. Reaction of a single atom starts the chain (e.g., 235U + neutron)

2. Propagation. 236U fission releases neutrons that initiate other fissions

3. Termination.

• A very heavy nucleus splits into more stable nuclei of intermediate mass.
• The mass of the products is less than the mass of the reactants.
• Missing mass is converted to energy

Fission

• In 1939 Lise Meitner an Austrian chemist in Sweden also working on neutron bombardment experiments proposed that the barium formed as a result of the neutrons splitting the Uranium atoms into two fragments
• She proposed the term “fission” to describe this process because of its similarity to the reproductive “fission” of a biological cell

Uranium Isotopes

• Naturally occurring Uranium contains two major isotopes
• Uranium-238 (99.3%)
• Uranium-235 (0.7%)
• As it turns out the only isotope of Uranium that undergoes fission is Uranium-235

235U Fission

• 23592U + 10n 23692U*
• and 10-14 seconds later...
• 23692U*9236Kr + 14156Ba + 3 10n + ENERGY
• 50 possible sets of fission products (sum of atomic numbers = 92)
• 3 neutrons released for ONE 23592U
• each neutron can split another 23592U
• CHAIN REACTION POSSIBLE
• If amount of 23592U is sufficient (CRITICAL MASS) then the number of neutrons generated is high enough to result in a nuclear explosion )

Where does all this energy come from?

E = mc2E = Energy (joules)m = mass (kg)c = speed of light = 3 x 108 m/s

• Currently about 103 nuclear power plants in the U.S. and about 435 worldwide.
• 17% of the world’s energy comes fromnuclear.

• Light-mass nuclei combine to form a heavier, more stable nucleus.
• More energetic than fission reactions
• Source of energy for the H-bomb
• Origin of the elements

FUSION

411H 42He + 2 ? + + energy

The most destructive force on the planet

H-bombs 1000s of times more powerful than A-bombs

Answers42He + 84Be 126C + energy42He + 126C168O + energy42He + 168O 2010Ne + energy42He + 2010Ne 2412Mg + energy