A reactivity control accident is defined as?

• A. Positive reactivity additions that typically cause rapid power increases and potentially damaging fuel temperatures.
• B. Any change causing a decrease in reactor power.
• C. Any routine maintenance of control rods.
• D. An accident unrelated to reactivity.

Answer: (A)

A reactivity control accident happens when there is a rapid, positive insertion of reactivity that drives the neutron population—and thus the reactor power—up quickly, potentially pushing fuel temperatures into damaging ranges. The key idea is the combination of a positive reactivity change and its fast, power-raising consequence. This is what makes such events particularly dangerous: the reactor can overpower its cooling and fuel may overheat before feedback mechanisms can counteract the rise.

Decreases in power are the opposite scenario (negative reactivity change) and are not accidents of this type. Routine maintenance of control rods is planned operation, not an accident. An event unrelated to reactivity misses the defining feature of reactivity-driven power excursions.