The Canadian CANDU nuclear reactor uses the fission of uranium-235 as an energy source.  In this reactor a free neutron is absorbed by a uranium nucleus 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 be absorbed again, causing further fission and the production of more neutrons capable of continuing the process.  If enough uranium is present in a small enough area (critical mass), the probability of a neutron causing another uranium atom to split is very high and a chain reaction will occur.  A chain reaction would cause the release of a massive amount of energy in a very short period of time, producing a nuclear explosion.  In a nuclear reactor, by contrast, the uranium atoms are spread out in fuel rods and some of the released neutrons are blocked by control rods in order to slow the chain reaction.  When the reaction rate is slow, a smaller amount of energy is released over a long period.  In essence, a nuclear reactor is a nuclear bomb going off in a controlled manner over a prolonged period.  When all or most of the uranium-235 atoms in the fuel have been spent, the reactor cools, at which point new fuel would have to be inserted to ensure continued energy production.

 

 

Example Problem How much energy is released by the fission of one U-235 atom?

 

Given

The atomic masses come from "Table 7.5" and "7.6" on page 881 of your physics textbook.

 

Required

the amount of energy released

 

Analysis and Solution

Determine the mass defect.

NOTE: Technically, the above mass change should be negative, but do not be concerned about it.  The energy released is still the same.

Change the mass defect into kilograms.

Determine the amount of energy released.

 

Paraphrase

The energy released by the fission of one uranium-235 atom is 2.78 × 10-11 J.

This value may seem small by comparison, but in a single kilogram of uranium there are enough fissionable uranium atoms to produce 7.09 × 1013 J of nuclear energy, which is approximately 1.6 million times greater than the chemical energy within one kilogram (≈1.4 litres) of gasoline.


Read
Read "Comparing Chemical Energy with Nuclear Energy" on page 820 of the textbook.


Do This
Read the information below and search the Internet for the specified animations.

In nuclear reactors, the rate of the reaction must be controlled to prevent a chain reaction.  In the fission of uranium, each uranium nucleus decays spontaneously, which is a very slow reaction.  This decay also occurs when a uranium nucleus absorbs a neutron in a nuclear reactor, which can be slow or fast depending on the position of the control rods.  The reaction can be slowed when the ejected neutrons are absorbed by inserting control rods.  The reaction can also be "sped-up" by removing the control rods which lets the ejected neutrons strike other uranium nuclei, thus continuing the chain reaction.  

Conduct an Internet search using the term "atomic archive."  You should be directed to a website that explores the history, science, and invention of the atomic bomb.  Use the website's search function to locate "Nuclear Chain Reaction Animations".  Your search should display animations explaining nuclear fission and fusion reactions.  If you're having trouble finding any of the animations, click on "Media" in the top navigation bar on the website, and then on "Animations" on the page that appears.


Try This
Complete "Practice Problems" 1 to 3 on page 819 of the textbook.