If an least one neutron from U-235 fission<span> strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be "critical", and the mass of U-235 required to produced the critical condition is said to be a "</span>critical mass<span>". A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are </span>moderated<span> to lower their speed, since the probability for fission with slow neutrons is greater. Hope that helps ya.</span>
