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7. The most important feature of the assembly times calculated in subsection (6) above is that they are more than 10 times the total fission chain reaction time. THis means that it is essential for the fissile material to be neutron free during the assembly or the reaction will take place prematurely. If the fissile material is neutron free, then it is necessary to "turn on" neutrons to initiate the chain reaction at the desired time. This is the role of the so called initiator
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