Source: fusion, Justin Matis, Lawrence Berkeley National Laboratory
The decay discussed in the previous section will occur in nature under normal everyday conditions. There are two other important processes that require special conditions to occur. These are fusion and fission.
Fusion is a nuclear reaction where two smaller nuclei are brought together with enough energy and force to overcome the electric repulsion from the protons. As soon as they are close, the strong force takes over, and they come together with a big release of energy. If there are enough nuclei and they are under enough pressure, the energy that is released during fusion can sustain the reaction. This is the reaction that occurs in the core of the sun; there is so much gravitational pressure and so much heat generated by this reaction that it keeps itself going.
Source: fusion, Justin Matis, Lawrence Berkeley National Laboratory
Fission is the breakdown of an unstable nucleus into two smaller nuclei, some left over neutrons, and a release of energy. Fission rarely occurs in nature because the nucleus needs a kick-start like being struck by a high-energy neutron.
In the following simulation, click the red button on the neutron gun.
| Click to Run |
Source: Nuclear Fission, University of Colorado – Physics
What happens to the uranium-235?
Interactive popup. Assistance may be required.
It becomes uranium 236.
What happens to the energy of the nucleus when the neutron hits it?
Interactive popup. Assistance may be required.
The energy level of the nucleus is raised enough for it to break apart.
What are the end products of this reaction?
Interactive popup. Assistance may be required.
The end products are two smaller nuclei and three neutrons.
Now click the chain reaction tab at the top of the simulation. Use the sliders in the bottom right corner of the simulation to fill the screen with 50 U-235 nuclei and 25 U-238 nuclei. Fire the neutron gun.
What happens when a neutron hits a nucleus of U-238?
Interactive popup. Assistance may be required.
It becomes uranium 239.
What happens when a neutron hits a nucleus of U-235?
Interactive popup. Assistance may be required.
It undergoes fission and creates three more neutrons that can strike other nuclei.
Increase the number of U-235 and decrease the number of U-238. Fire the neutron gun again.
What happens?
Interactive popup. Assistance may be required.
A chain reaction occurs where all (or most) of the U-235 reacts.