nuclear fission


Inside the nucleus, there are neutrons and protons which are held together by strong attractive forces. At the same time, the electrical repulsion among the protons tends to split the nucleus apart. In most nuclei, especially small nuclei, the attractive forces predominate the repulsive forces so the nuclei are stable. But in some nuclei, the attractive forces just barely predominate.

If an object hits such a nucleus, it may split apart.



There are two questions to consider :"what object is needed?" and "what nuclei are suitable?".

Obviously, the object we use must be submicroscopic particles such as protons and neutrons.

<IMG SRC="electron.gif" WIDTH=400 HEIGHT=250 BORDER=0>


Result: The interaction is not strong enough. The nucleus does not split.

<IMG SRC="proton.gif" WIDTH=400 HEIGHT=250 BORDER=0>

Result: Protons cannot approach the nucleus because they are electrically repelled before they can hit the nucleus.

<IMG SRC="neu.gif" WIDTH=400 HEIGHT=250 BORDER=0>

Result: BINGO! Neutron is exactly what we need. It is not electrically repelled and it interacts strongly with the nucleus.

 

Fissle Nucldies
Nuclides in which fission can be induced by neutrons are said to be fissile. They include U-235 and Pu-239. However, plutonium-239 is almost non-existent in nature and must itself be made in a nuclear reactor.

Naturally occurring uranium consists of two isotopes, uranium-238 (99.3%) and uranium-235(0.7%). Uranium-235 is the rare [isotope] which undergoes fission.

Nuclear fission ~ Step by Step
When a nucleus of U absorbs a low energy neutron, it will form U* which is not stable.
n + U --> U*


84% of the unstable nucleus quickly splits up, usually into two fragments with mass numbers around 150 and 90.


One typical example is : U --> La + Br
The resulting nuclei are still unstable because the number of neutrons become too large for stability. Therefore some neutrons have to be ejected. For example, one neutron from each fragment, followed by a series of ( decay.


e. g. La --> La + n ,
La --> Ce --> Pr --> Nd
Br --> Br + n ,
Br --> Kr


The final products are stable. At the same time, many neutrons are emitted. There is an appreciable mass decrease which contributes to the vast amount of energy released. The overall equation can be represented as:


U + n --> Nd + Kr + 2 n + 4( + energy


Please remember that the above series of reactions refers to only one possibility. There may also be some ( decays in the process. Also, all the reactions actually take place in extremely quick succession. We just separate them for explanation purpose.

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