number of control rods
No place, we have not yet determined how to make a fusion reactor. Only fusion bombs.
chain reactions
The fission happens in the fuel, which is usually in fuel rods inside the reactor. The rods are spaced at a particular distance apart and fill the reactor.
At the center, where the gravitational pressure is highest.
These are not chemical reactions but thermonuclear reactions.
In the reactor core, which is the volume filled with the fuel assemblies
The dangerous condition that can occur in a nuclear reactor and due to overheating is a nuclear meltdown.
There are many ways a nuclear accident may occur, however most nuclear power plant accident occur due to problem in an reactor.
One type of nuclear reactions is fusion. This usually occur under extreme conditions - extreme temperatures and pressures. This most commonly occurs in the center of stars (including our Sun); in some cases also on the surface of a star (when matter falls onto the star, temporarily producing the high temperatures required). So far, it it not economically practical to do this artificially (search for "fusion reactor") - except for the destructive H-bomb. Fusion reactions usually occur if sufficient amounts of Uranium or some other heavy elements come together. This is the case with a nuclear reactor, and an atomic bomb.
No place, we have not yet determined how to make a fusion reactor. Only fusion bombs.
chain reactions
The fission happens in the fuel, which is usually in fuel rods inside the reactor. The rods are spaced at a particular distance apart and fill the reactor.
That's where the nuclear reactions occur that keep it lit.
Natural nuclear fusion reactions occur in all stars
At the center, where the gravitational pressure is highest.
These are not chemical reactions but thermonuclear reactions.
1986, the same year as the Challenger Space Shuttle explosion.