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What do you normally do when you want to start a reaction? Well you might say that you are not a scientist or do not work in a lab so have never started a reaction. But just think about the last time you lighted your gas stove with an electric lighter. What you did that time was to supply a spark of heat to start off the exothermic reaction which leads to the self sustaining flame in your stove. How I wish that starting a nuclear reaction could be as simple as switching on a gas stove but then it is actually not so.
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Initiating a Nuclear Reaction
The bonds between atomic level particles are sometimes stronger than those of our human bonds which could give way with slight heat. Atomic bonds require a significant amount of energy to force them to part ways. Here are some of the common methods by which a nuclear reaction could be initiated
Acceleration - an accelerator is used to energize hydrogen or Helium nuclei which are them bombarded on the target nuclei with a high velocity. These materials are preferred because they possess a smaller amount of repulsive force and hence are suited for the purpose. Unfortunately this type of method has very little practical use as far as a nuclear reactor is concerned because of the lack of capability on their part to sustain a chain reaction.
Temperature - this method utilizes and imitates the principle used by nature inside the biggest ball of fire as we know it. Yes I am talking about our very own "Sun" where hydrogen nuclei are constantly undergoing fusion under the influence of millions of degrees of temperature which gives enough energy to the neutrons to overcome the electrostatic forces. Obviously this is not a very practical method either to be used inside a nuclear reactor because of the practical difficulties involves with such high temperature ranges.
Neutron Bombardment - when neutrons are used to bombard the target nuclei they need not be of great energy because they are neutral and have no charge. Hence no energy is wasted in overcoming the attractive and repulsive forces and they can simply strike silently at the nucleus. This is the only feasible method which is useful in nuclear reactors.
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The Importance of Cross Section
Speed is not everything and size does really matter when it comes to the question of whether a bombarding nucleus will cause fission in the target nucleus or not. The greater the cross section area of the target nucleus, the greater the chance of it getting absorbed and causing fission. Also there are two energy levels which need to be clearly understood in this context. The nuclear binding energy is the energy required to overcome the repulsive forces between two fragments while the critical energy refers to the excitation energy required for the two fragments to part ways. If the critical energy is greater than nuclear binding energy only fast moving neutrons can cause fission, but if it is vice versa thermal neutrons can do the job.