Question

which statement describes the use of control rods?
control rods are lowered into the reactor to slow down the reaction by absorbing neutrons.
control rods are lowered into the reactor to speed up the reaction by releasing neutrons.
control rods are raised out of the reactor to slow down the reaction by absorbing neutrons.
control rods are raised out of the reactor to speed up the reaction by releasing neutrons.

Explanation:

In nuclear reactors, control rods (usually made of materials like boron or cadmium) work by absorbing neutrons. Neutrons are crucial for sustaining the nuclear fission chain reaction. When control rods are lowered into the reactor core, they absorb more neutrons, reducing the number of neutrons available to cause further fissions, thus slowing down the reaction. If they were raised, fewer neutrons would be absorbed, allowing the reaction to speed up (but the options about raising are incorrect in their descriptions here). The second option is wrong because control rods don't release neutrons. The third and fourth options have incorrect actions (raising to slow or speed with wrong mechanisms). So the first option correctly describes that lowering control rods absorbs neutrons to slow the reaction.

Answer:

A. Control rods are lowered into the reactor to slow down the reaction by absorbing neutrons.