Neutrons Starting Fission Chain Reactions

What is the source of neutrons to begin reactions in a nuclear reactor? I understand that U-235 undergoes fission readily on absorbing a slow-moving neutron and releases neutrons as part of the process, hence creating the possibility of a chain reaction, but how is the reaction started, that is, what is the source of the neutrons that initialise the reactions?
- Joe (age 24)
Melbourne, Australia

There's some spontaneous decay of the radioactive isotopes. That's enough to trigger the chain reaction. Only bombs, which require a rapid growing chain reaction, require a separate burst of neutrons to get them started quickly. There are several different ways of obtaining those bursts. Perhaps we should not describe them, in case there are any over-enthusiastic do-it-yourself types among our readers.

Mike W.

(published on 05/06/2012)