What are worth curves and how are they used in reactor operation?

• A. Graphs of reactivity versus control rod position used to calibrate control strategy and ensure shutdown margins.
• B. Graphs of coolant temperature over time used to set pressure.
• C. Curves of fuel enrichment versus burnup.
• D. Diagrams showing the physical path of rods.

Answer: (A)

Worth curves are graphs that show how much reactivity changes as a control rod moves, plotted against the rod’s position in the core. Reactivity measures how far the reactor is from criticality, and control rods remove reactivity to slow, control, or shut down the reactor. By mapping the change in reactivity (Δρ or Δk/k) to how far a rod is inserted, you get a curve that tells you how effective that rod is at different positions.

This information is crucial for operation: it lets operators know how far to insert or withdraw a rod to achieve a desired change in power, and it provides the data needed to guarantee a shutdown margin—the negative reactivity that can be provided by fully inserting the rods to drive the reactor subcritical under worst-case conditions. Worth curves also help validate and calibrate the control strategy, ensuring that planned rod movements reliably achieve target reactivity without violating safety limits. Keep in mind that the curves are not perfectly linear, since flux shape and other effects can alter a rod’s effectiveness at different positions, but they remain the practical tool for relating rod position to reactivity control.