3.65 Nuclear Fission
Q:What causes the nucleus of uranium-235 atom to fission?
A:Another particle collides with it.
How Nuclear Fission Occurs
TheFigure3.117 shows how nuclear fission of uranium-235 occurs. It begins when a uranium nucleus gains a
neutron. This can happen naturally when a free neutron strikes it, or it can occur deliberately when a neutron is
crashed into it in a nuclear power plant. In either case, the nucleus of uranium-235 becomes extremely unstable
with the extra neutron. As a result, it splits into two smaller nuclei, krypton-92 and barium-141. The reaction also
releases three neutrons and a great deal of energy. It can be represented by this nuclear equation:
235
92 U + 1 neutron→92
36 Kr +141
56 Ba + 3 neutrons + energyNote that the subscripts of the element symbols represent numbers of protons and the superscripts represent numbers
of protons plus neutrons.
FIGURE 3.117
To see an animated version of uranium-235 fission, go to this URL: http://www.classzone.com/books/earth_science
/terc/content/visualizations/es0702/es0702page01.cfm?chapter_no=visualizationNuclear_Fission_of_Uranium-235
.
Nuclear Chain Reaction
The neutrons released when uranium-235 fissions may crash into other uranium nuclei and cause them to fission as
well. This can start a nuclear chain reaction. You can see how this happens in theFigure3.118. In a chain reaction,
one fission reaction leads to others, which lead to others, and so on. A nuclear chain reaction is similar to a pile of
wood burning. If you start one piece of wood burning, enough heat is produced by the burning wood to start the rest
of the pile burning without any further help from you. You can see another example of a chain reaction at this URL:
http://www.youtube.com/watch?v=0v8i4v1mieU
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5018