Nov. 21 – Halflife and Fission

Have a look at this simulation:

http://phet.colorado.edu/en/simulation/alpha-decay

What happens?  One particle can decay at any moment, but when you have hundreds or thousands (in real life, billions and billions) or particles we can find a characteristic time when half of the particles have decayed.  This time is called the half-life.  Once the time of the half-life has elapse, half of the sample remains.  After another half-life, the sample is once again halved.  A graph of the amount remaining looks like this:

 

Every substance has a different half-life.  For example:

• U-238 has a half life of 4.5 billion years. 
• U-239 has a half life of 24 minutes. 
• C-14 has a half life of 5700 years.  (Used for carbon dating)

Here's the formula for calculating amount of material remaining:

Here are some examples:

Next we talked about fission:

Fission

A large nucleus breaks apart into smaller nuclei.  This can be spontaneous (happens on its own) or induced (something “kicks off” the reaction).

Spontaneous.

Induced.

Induced fission can cause a chain reaction. 

Nuclear Reactors

   - Use enriched uranium to carry out a chain reaction producing thermal energy.

   - Natural uranium has 0.7 % U-235

      the rest is U-238

    - Enriched uranium has 3-4% U-235

    - Weapons grade: 90% U-235

How a nuclear power plant works.

A simplified version of the image above.

Moderator

    - A substance used to slow down neutrons to control the chain reaction.  graphite or heavy water.

Control Rods

    - Cadmium or Boron is used to absorb neutrons to control the reaction.

    - Nuclear bomb uses uncontrolled reactions to release massive amounts of energy.