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Q & A: fission chain reaction

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Most recent answer: 10/22/2007
Q:
When there is a nuclear fission, and an atom releases radiation, there is a chain reaction. N=Neutron N E E E=Electron NP__________N P=Proton PN _=Collision course N E E When the neutron collides with another atoms nucleus, and that atom which was just hit, sends neutrons, it creates a chain reaction. How does that chain reaction stop after sometime, instead of continuing forever, and if the neutrons fall to the ground, howcome the ground doesn’t start exploding?
- Socrates (age 12)
SFS (Seoul Foreign school), South Korea
A:
A neutron striking a nucleus won't make it fall apart and release more neutrons unless the nucleus is one of the types that does that. Most don't. The ground contains very few nuclei of the types (e.g. U235) which sustain a chain reaction, so on the average each neutron in the ground will release much less than one new neutron, and the reaction quickly stops.

There are several ways a nuclear chain reaction in concentrated fissile material can stop. One is that the material may heat up and blow apart, more or less violently, so that the neutrons from one part no longer are likely to hit the nuclei of the other parts. Another is that enough of the material is used up that the remainder no longer generates more than one new neutron for each neutron released.

In a more relaxed and commonplace application, nuclear chain reactions are taken advantage of to generate power. In this case, the intention is to keep a chain reaction going at a slow, controlled rate. The fissile material is kept mixed in with other atoms which do not support the chain reaction -- materials which slow down neutrons, or which absorb them. Water, heavy water, and graphite are good materials for slowing down neutrons, and atoms with heavy nuclei have good neutron capture cross-sections and can absorb neutrons quickly. These materials are used in control rods.

The time dependence of the reaction rate is an exponential with an adjustable sign in the exponent -- it can either react more and more, exponentially getting bigger, or the reaction rate can gradually slow down. Another piece of the puzzle is the constant production of new neutrons as the atoms spontaneously decay. That way, if you put enough neutron-slowing materials and enough neutron-absorbing materials around so that chain reactions never exponentiate off in the increasing direction, they will still be kept going at a lower level because of the natural decay processes.

Mike W. and Tom J

(published on 10/22/2007)

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