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That’s really not quite right.

Radioactivity is the spontaneous decay of a nucleus, usually by alpha (helium nucleus), beta (electron), or gamma (electro-magnetic) emission, although several other forms can occur. Neutrons are rarely emitted by radioactive decay (although the emission of neutrons by radioactive decay is important in controlling a reactor).

Nuclear fission is the splitting of an atom using a neutron typically, resulting in two (or rarely three) atoms, roughly half the mass of the original atom and several neutrons (usually two or three). These released neutrons can then cause additional fissions, which would be a chain reaction.

A sustained chain reaction occurs when the splitting of one atom (usually uranium or plutonium) releases neutrons which cause the splitting of one additional atom.

An uncontrolled chain reaction may occur when the splitting of one atom releases neutrons which cause the splitting of more than one atom (which does not necessarily cause an uncontrolled chain reaction - when a reactor is started up, then the splitting of one atom causes the splitting of more than one additional atoms, but the process is easily controlled).

To control a chain reaction, one can employ several methods. Control rods in the reactor can be moved. Control rods are made of a neutron poison such as silver/indium/cadium or boron. A neutron poison such as boric acid can be manipulated. The coolant temperature or coolant flow can be changed. The leakage of neutrons out of the core (where they can no longer contribute to a chain reaction) can be changed. Carbon is definitely not a neutron sponge.

A problem is that those two atoms resulting from the splitting (fission) of the atom, mentioned above, are radioactive, meaning that they are giving off beta and gamma emissions and releasing energy. The amount of energy being released is about 7% of the reactor power, if the reactor were immediately shutdown. If the reactor was producing 3000 megawatts of energy (a typical amount) and was shutdown, then immediately after shutdown, it would still be producing 210 megawatts, even though the chain reaction has been stopped. The amount of this energy, known as decay heat, gradually diminishes after reactor shutdown.

However, it must be removed to avoid a core meltdown. Normally, the heat is removed using the reactor coolant system, at least initially after shutdown, although other cooling systems will be used as conditions allow following a shutdown.

The emergency core cooling system is used should the reactor cooling system for some reason not be available. The control rods have nothing to do with cooling the reactor. The emergency core cooling system is not a backup to the control rods.

A reactor scram or trip occurs when the reactor is abruptly shutdown. The control rods are inserted when a scram occurs. During a normal shutdown, the reactor power will be decreased in a planned manner, and the reactor may be scrammed when the power is sufficiently low.

Or a scram could occur at full power if conditions warrant.

The reactor at Hanford is a boiling water reactor, which is a common type in the United States and throughout the world. It is not a graphite reactor.