Module 8

Module 8—Nuclear Decay, Energy, and the Standard Model of the Atom

 

Lesson Summary

 

In this lesson you focused on the following questions:

• Why do nuclear reactions release so much energy?
  
• What is nuclear fission?
  
• What is nuclear fusion?

In this lesson you learned that both fission and fusion nuclear reactions are processes that lead to an increase in the binding energy per nucleon, increasing the stability of the resulting nuclei. In the process of fission, a nucleus with more than 120 nucleons splits into smaller nuclei with greater binding energy per nucleon. In the process of fusion, a nucleus with fewer than 60 nucleons combines with another to form a larger nucleus with greater binding energy per nucleon. Both of these nuclear reactions release large amounts of energy.

 

For both fission and fusion reactions, the energy released is equal to the difference between the total binding energy of the original nucleus or nuclei and the final binding energy of the nucleus or nuclei. The energy released can also be found by calculating the total mass defect during the reaction. This mass defect was transformed into energy according to Einstein’s mass-energy equation E = mc2.

 

The product of fusion (helium) is stable and safe, which is in contrast to the unstable, dangerous daughter nuclei produced in a fission reaction.

 

In fission, a free neutron is absorbed by a large nucleus, such as uranium-235, causing it to become unstable and break apart into two smaller nuclei, accompanied by the release of several more neutrons. If the free neutrons encounter more uranium atoms they will again be absorbed, causing further fission and the production of more neutrons capable of continuing the process. Left unchecked with sufficient amounts of uranium, this chain reaction could produce a nuclear explosion. Controlling the rate of the chain reaction allows the energy to be released slowly, a strategy employed by current fission reactors.

 

In fusion, temperatures greater than 100 million Kelvin are needed to cause small nuclei, such as tritium and deuterium, to combine and form larger, more stable helium atoms while releasing an amount of energy equal to the change in the mass or difference in binding energies before and after the reaction. Advancing fusion reactor technology holds the promise of a seemingly infinite supply of clean energy.

 

Lesson Glossary

 

binding energy: the net energy required to liberate all of the protons and neutrons in a nucleus (overcome the strong nuclear force)

 

fission: when a nucleus with more than 120 nucleons splits into smaller nuclei with greater binding energy per nucleon

 

fusion: when a nucleus with fewer than 60 nucleons combines with another to form a larger nucleus with greater binding energy per nucleon

 

plasma: ionized gas in which the electrons have been separated from the nucleus

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