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Q.As I have heard the neuclear fission reaction can undergo only if the heavy nuclues is bombarded with thermal neutron i.e.slow moving neutron,but not by fast moving neutron.So,in reactor moderator is used to slow down the fast moving neutron emitted from the reaction.Is there any moderatorin atom bomb?But the reaction continues for many years. From Kanghujam Sunindra Singh

Q.As I have heard the neuclear fission reaction can undergo only if the heavy nuclues is bombarded with thermal neutron i.e.slow moving neutron,but not by fast moving neutron.So,in reactor moderator is used to slow down the fast moving neutron emitted from the reaction.Is there any moderatorin atom bomb?But the reaction continues for many years.


   From


          Kanghujam Sunindra Singh

Grade:12

1 Answers

Chetan Mandayam Nayakar
312 Points
9 years ago

The Atomic BombThe energy source is a mass of radioactive material such as uranium or plutonium. This material is very unstable; every atom's nucleus is ready to fall apart ('decay') at the slightest nudge, releasing unneeded energy and extra neutrons. In the diagram, the plutonium (B) is given that nudge by the outer casing of TNT (A), which explodes all around it.

Here's what happens; the process is called 'Nuclear Fission':


The plutonium is unstable, or radioactive. Its atoms are constantly 'falling apart', breaking up into smaller elements that are more stable. Every time one nucleus does this, it releases the extra energy it no longer needs to hold it together, as well as a few left-over neutrons. This energy, and the escaping neutrons, is what we describe as the radiation being emitted from the radioactive plutonium. This energy and flow of escaping neutrons can damage human cells, so radioactivity is dangerous.
Enough atoms in the chunk of plutonium are breaking down at any one time to make the chunk of plutonium warm up, but not enough to be considered an explosion.

What happens in the bomb, however, changes that! The force of the TNT explosion causes the plutonium to be squashed, or compressed in size, and become very dense. This is called its 'critical mass'; the plutonium is now so densely packed together that the neutrons escaping from the decaying nuclei of plutonium cannot escape from the plutonium without bumping into another plutonium atom!

 

When they hit another atom, they cause that nucleus to break down too, whether it was ready to or not. That second nucleus releases more energy, and more neutrons, which in turn go on to hit and break up further nuclei. The decaying nuclei cause more decaying nuclei, and so on, in a rapidly escalating chain reaction ... and all because the plutonium has been squeezed into such a dense state (by the TNT) that the escaping neutrons that normally would fly out of the material now can't, without hitting other nuclei!

Within a very tiny fraction of a second, all the nuclei in the chunk of plutonium have been hit by escaping neutrons, and have broken down. The extra energy in trillions of atomic nuclei is all released at once! This energy is considerable; the atomic bomb dropped on Hiroshima in WWII was an example of this process.

Thankfully, more peaceful uses for this process have been found. Critical mass can also be achieved by just collecting together enough plutonium in one place; if it's thick enough, the neutrons can't escape without hitting another nucleus, and the chain reaction will start. By inserting special neutron-absorbing material in between portions of the plutonium, the rate at which the chain reaction proceeds can be controlled, resulting in a 'slow burn' instead of an explosion. This is the process that takes place inside a nuclear power plant. The heat generated by the nuclear fission is used to heat water into steam that turns a generator.

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