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Fission is started in nuclear power plants by withdrawing the control rods. The rods are pulled in groups beginning around the perimeter of the reactor. These are extracted and pulled all the way out. The reactor design permits this to happen without starting the reactor up. Then the middle rods are pulled. These actually permit the reactor to start up. The rods being pulled last are the "control group" because they are going to set up the operating conditions. Rods pulled to control the reactor, those in the middle, permit a more uniform burn of the fuel. If it was done differently, the fuel in the middle would burn more quickly. And the fuel around the perimeter would not be used as efficiently. Here's the scoop. The control rods are a neutron absorbing material. They have to be because they must be able to absorb neutrons to control or shut down the reactor. Boron works really well because 1) boron has a fairly high neutron absorption cross section (it is a good or "big" target for a neutron), and 2) as boron is transmuted by neutron absorption, it (usually) becomes another boron isotope, and boron's isotopes all have good neutron absorption cross sections so they all continue to be pretty good neutron absorbers. The rods are pulled to a point where there is not enough of them in the reactor to absorb the neutrons that are spontaneously being generated by the fuel. (It always generates a few neutrons. Always. And that's the hinge for critical mass.) So the rods are pulled and the effective critical mass is reached and the chain begins. Monitoring instruments pick up the increase in neutron flux. Operators know the chain has begun and is building. Then by gradually heating things up and incrementally pulling the rods a bit more, the plant is brought to operating temperature and is able to provide heat to generate steam in a secondary system. The secondary system is gradually heated by bleeding steam when things in the primary are heated up. That way the secondary system can be brought on line efficiently and power production can begin. There are more subtle aspects to reactor operations, but this is a good start on a path to understand the workings of the reactor.

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