When operating the reactor for a great length of time WITHOUT refueling, there is a loss in reactivity. Why is that?

When operating the reactor for a great length of time WITHOUT refueling, there is a loss in reactivity. Why is that?

• A. Increased fuel temperature
• B. Neutron leakage increases
• C. Burnup
• D. Moderation decreases

Answer: (C)

Reactivity decreases during long operation without refueling because of burnup. As the reactor runs, fissile atoms like U-235 are consumed to fuel fission, and some fertile material is converted to other isotopes, so the total amount of material capable of sustaining the chain reaction declines. At the same time, fission products accumulate in the fuel, and many of them are neutron absorbers (poisons) that capture neutrons without causing fission. This buildup further reduces the number of neutrons available to sustain the reaction, lowering reactivity. Temperature effects (like Doppler feedback) also trend negative but are secondary over extended operation; the dominant cause of the long-term reactivity loss is burnup.

Reactivity decreases during long operation without refueling because of burnup. As the reactor runs, fissile atoms like U-235 are consumed to fuel fission, and some fertile material is converted to other isotopes, so the total amount of material capable of sustaining the chain reaction declines. At the same time, fission products accumulate in the fuel, and many of them are neutron absorbers (poisons) that capture neutrons without causing fission. This buildup further reduces the number of neutrons available to sustain the reaction, lowering reactivity. Temperature effects (like Doppler feedback) also trend negative but are secondary over extended operation; the dominant cause of the long-term reactivity loss is burnup.