Reactor Operations and Control
Core concepts
Understand normal plant evolution: criticality approach, power ascension, steady-state operation, and shutdown. Know rod worth, differential vs integral rod worth curves, and the S-curve shape from rod shadowing. Be comfortable with boron dilution/boration, xenon and samarium poisoning (peak xenon after shutdown), and the iodine-xenon kinetics that drive post-trip reactivity transients. Operators must know technical specification action statements, mode definitions, and the difference between a reactor trip, a scram, and an engineered safety feature actuation.
Common traps
Questions love the post-shutdown xenon peak: after a trip from high power, xenon builds for several hours because I-135 decay outpaces Xe-135 burnout, creating a window where restart is prohibited. Another trap is confusing rod withdrawal for power increase with rod insertion for power decrease in a startup - always ask whether the reactor is subcritical, critical, or at power. Beware of mixing automatic and manual actions, or assuming a trip always reduces reactivity fastest (it does not if xenon is building).
Worked example / rule of thumb
Rule of thumb: xenon-135 peak occurs roughly 8–10 hours after shutdown from full power and can add several dollars of negative reactivity. If a question asks "when is restart most restricted," the answer is during the xenon peak, not immediately after trip. Also remember: criticality approach uses inverse count rate (1/CR) plotting; a straight line on 1/CR vs rod position indicates a valid subcritical approach.
Study tip
Build a timeline chart for iodine/xenon and samarium after shutdown and after power changes. Many operational questions are answered by reading that timeline.