How is a nuclear chain reaction started?

Answer 1

Uranium produces spontaneous fissions in small numbers, and each fission releases two or three free neutrons, so there is always a source of neutrons present in any assembly of uranium fuel. If enough fuel is assembled, in a geometrical array with a moderator, the conditions are there to start a chain reaction, and this will happen whenever the reactor approaches what is called criticality, which is when the number of free neutrons present starts to reach a high level. The approach to criticality is controlled by slowly withdrawing the control rods, and if at stages the rods are held steady, the neutron flux also steadies out. As criticality is approached, the flux gets higher and when the rods are pulled a bit more beyond the critical point it will start to increase exponentially, the reactor is then said to be supercritical and the neutron flux will go on increasing with a certain doubling time. So with a nuclear reactor there is no need to do anything to cause "ignition" as in a coal furnace, it is just a matter of getting the amount of neutron absorber reduced to the point where the reactor is critical. Note that often a neutron source is loaded into the reactor as a permanent feature, this is done to enable the flux measuring instruments to see a reading during the approach to critical, but even without this the reactor will still start itself, if the rods are withdrawn.