A conventional reactor that use slightly enriched or natural uranium as fuel, needs slowing neutrons to run. Neutrons when they are produced by fission, are too fast to trigger enough further fissions and maintain the chain reaction. The neutrons are slowed down or cooled by a series of collisions with nuclei. In order to avoid too many losses of neutrons by collisions leading to neutron captures, the slow down must be fast to minimize the number of collisions.
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