Lucy Padelford, age 12, of South St. Paul, Minnesota, for her question:

Why is graphite used in a nuclear reactor?

An ordinary coal furnace requires flues and dampers to control its energy. Nuclear fuel has two million times more energy than coal and extra special devices are needed to control the output of an atomic furnace. Graphite is one of the substances used to keep its seething energy within bounds.
The nuclear reactor, alias the atomic pile, is the power plant of the future. True, it is expensive to build and its operation must be supervised by highly trained experts.  But after a year of producing 1,000 kilowatts of heat, it may consume less than one pound of its fuel. If it happens to be a breeder pile, it may produce more usable atomic fuel than it consumes. This stupendous energy plant has tremendous advantages. But it requires watching. It operates by nuclear fission, the dynamic atom splitting that exploded the devastating A bomb.
The atomic pile controls this energy at a steady rate. Its fuel is uranium or some other highly radioactive material. The atom of such fuels split apart into smaller atoms, releasing heat energy and fast flying neutrons. Neutrons, of course, are those heavy, noncharged particles in the atomic nucleus. When released by the fission, or splitting, of a nucleus, they fly off and smash into nearby unstable atoms in the radioactive fuel. This bombardment splits more atoms and more speeding neutrons are released to split more atoms.
This causes a chain reaction of nuclear fission. In the A bomb, the chain reaction is allowed to explode with a tremendous bang. In a nuclear reactor, the chain reaction of fission is slowed down and controlled. This is done by packing the fissionable fuel with special substances that absorb scads of the flying neutrons. These substances are called "moderators." They are made of small, light atoms with the kind of nuclei that can adopt extra neutrons without going to pieces..
One useful moderator is heavy water. Its atoms of the hydrogen isotope deuterium are eager to soak up neutron bullets flying from the fission reaction in the fuel. Some reactors use graphite as a moderator. Its atoms also slow down the chain reaction by confiscating some of the neutron bullets. Thick layers of the graphite are packed between the bars or plates of a fissionable fuel such as uranium. Together they form the core of  nuclear reactors It is encased in a thick, sturdy shield and kept reasonably cool, usually with rivers of flowing water.
Something more than a moderator such as graphite, however, is needed to keep the atomic pile under control. There are long control rods to push through slots in the shielding, right into the core of the pile. The control rods are made of cadmium or some other metal that absorbs large quantities of flying neutrons. If the pile slows down, the control rods are pulled out and more neutrons are left to split more atoms. If the pile gets too active, the control rods are pushed in deep to soak up more neutron bullets or the dynamic reactor plant might heat up and explode with an A or the dynamic reactor plant might heat up and explode with an A
Uranium 235 is a favorite reactor fuel. One pound of it can yield as much energy as 2 1/2 million pounds of coal. It can be made to produce 1,000 kilowatts by the fission of 10 million billion atoms per second. In a breeder reactor, surplus neutrons are captured by atoms of a uranium isotope which then becomes uranium 239. The uranium 239 quickly changes itself into plutonium 239 and this radioactive substance happens to be another type of fuel suitable to run a nuclear reactor.