Distribution of slow and fast neutrons in a PWR:
This repartition of the neutrons in a pressurised water reactor shows the different roles played by slow and fast neutrons. Slow neutrons are responsible for most of nuclear fission and therefore help sustain the chain reactions. Fast neutrons, on the other hand, play a small role in fission but can transform nuclei of uranium 238 into fissile plutonium 239. Other neutrons are lost when they are captured by 'sterile' nuclei or when they escape from the reactor as they slow down. Among these, some can poison the performance of the reactor, others are used for the control of the chain reaction (control rods).