What is a rod worth calibration method used in practice?

• A. Gamma spectroscopy
• B. Visual inspection
• C. Thermal-hydraulic tests
• D. Critical experiments and period measurements

Answer: (D)

Determining how much reactivity a control rod adds or removes is best done by using critical experiments and measuring the reactor period after changing the rod position. In practice, the reactor is brought to or near critical, the rod is moved by a known amount, and the resulting change in power level over time is recorded. The time constant of that power change—the reactor period—is directly related to reactivity through the reactor's kinetics equations. By applying the period data (often via the inhour relation) and the known delayed neutron parameters, you can compute the exact reactivity worth of the rod move. Repeating this for different positions builds up a calibration curve of rod worth versus rod insertion.

This approach is preferred because it uses actual reactor behavior and dynamics, capturing real spatial effects, feedback, and timing that model-based or indirect methods might miss. Other techniques, like gamma spectroscopy, visual inspection, or purely thermal-hydraulic tests, don’t measure the change in reactivity caused by moving a rod, so they aren’t used for rod-worth calibration.