In a controlled thermal fission reactor, the use of control rods will not affect the

Analyze the question: Control rods (made of boron or cadmium) are inserted into a reactor to absorb neutrons. The question asks which quantity they will not affect.

Concept: In a fission reactor, a chain reaction occurs. Each fission releases fast neutrons, which are slowed by a moderator (such as graphite or heavy water) into thermal neutrons that cause further fission. Control rods regulate the reaction by absorbing excess neutrons.

Checking each option:

• A. Energy generated: Control rods change the number of fissions, so they change the energy output. They do affect this.
• B. Rate of production of neutrons: Absorbing neutrons reduces the number available for further fission, so the rate at which new neutrons are produced changes. They do affect this.
• C. Speed of the neutrons released on fission: Neutrons are released at a speed determined by the fission event itself, and they are slowed by the moderator, not by the control rods. Control rods simply capture neutrons and remove them. They do not change the speed of the neutrons released. They do not affect this.
• D. Amount of radiation produced: Fewer fissions means fewer fission products and less radiation. They do affect this.

Answer: The speed of the neutrons released on fission is unaffected by control rods, so the answer is C.

Exam tip: Remember the two roles:

• Moderator changes the speed of neutrons (slows them down).
• Control rods change the number of neutrons (absorb them).

When a question asks about speed, think moderator. When it asks about rate, energy or quantity, think control rods.