Question

why is nuclear energy able to be used for practical purposes?
the reactions are controlled to regulate energy output.
the reactions are uncontrolled for maximum energy output.
all of the products are fed back into the reaction to keep it going.
all of the products are immediately removed to inhibit more reactions.

Explanation:

Nuclear energy is used for practical purposes like power generation. In nuclear reactors, the nuclear fission reactions are controlled. Control rods (e.g., made of materials like cadmium) can absorb neutrons. By adjusting the position of control rods, the rate of the fission reaction (and thus energy output) can be regulated. Uncontrolled reactions (like in atomic bombs) are not for practical energy - producing purposes. If all products were fed back (without control), the reaction would be uncontrolled. Removing all products immediately would stop the reaction completely, which is not practical for continuous energy production.

Answer:

The reactions are controlled to regulate energy output.