What is the concept of control rod worth and how is it measured?

• A. The reactivity change per unit rod position; measured via critical experiments or rod worth curves.
• B. The physical weight of the rod in the core.
• C. The time required to insert the rod.
• D. The color code used for rod identification.

Answer: (A)

The concept being tested is control rod worth, which is the reactivity change produced by moving a control rod. It answers how much reactivity you gain or lose for each unit of rod position, and that’s why it’s the quantity operators use to quantify how effectively a rod can control the reactor.

To measure it, engineers perform critical experiments and construct rod worth curves. In practice, the reactor is brought to a known configuration, the rod is moved by a precise amount, and the resulting change in reactivity is recorded (often expressed in pcm). Repeating this over a range of positions builds a rod worth curve, where the slope gives the worth per unit position and the area under the curve gives the total worth over a movement range. This experimental approach ties the physical movement of the rod directly to the reactor’s reactivity response, providing a practical measure of how much control the rod can exert.