Q75 — Axial Peaking Factor and Boron Dilution
T4 193009 K1.07 (3.3)
Given:
- Salem Unit 1 is operating at 80% power, middle of life of the fuel cycle.
- All control rods are withdrawn except D bank is at 190 steps and in manual control.
- Core axial power distribution is peaked below the core midplane.
Which of the following will INCREASE the core maximum axial peaking factor? (Assume no operator action is taken unless stated and that main turbine load and core xenon distribution do not change unless stated)
- Salem Unit 1 is operating at 80% power, middle of life of the fuel cycle.
- All control rods are withdrawn except D bank is at 190 steps and in manual control.
- Core axial power distribution is peaked below the core midplane.
Which of the following will INCREASE the core maximum axial peaking factor? (Assume no operator action is taken unless stated and that main turbine load and core xenon distribution do not change unless stated)
A. Turbine load/reactor power is reduced by 10 percent.
B. The control bank D is withdrawn 4 steps.
C. Reactor coolant system boron concentration is reduced by 15 ppm.
D. A fully withdrawn control rod located at the edge of the core drops to the bottom of the core.
▶ Show Answer & Explanation
✓ C. Correct. Reducing boron concentration (diluting) by 15 ppm will insert positive reactivity in the core, resulting on both heat balance (calorimetric) reactor power and average coolant temperature increasing. Also, diluting raises hot leg temperature, making the water in the upper regions of the core less dense (less neutron moderation), thereby shifting the flux to the already flux-dense portion below midplane. Another way to think about this is that the water is colder in the bottom of the core resulting in a greater change in the atoms per cubic centimeter being removed in the bottom of the core. This again adds positive reactivity in the bottom of the core and will shift the flux downward.
✗ A. Plausible because the operator may believe that the load reduction will lower hot leg temperatures causing the flux in this portion to increase and resulting in an increase in the axial peaking factor. Incorrect in that the lower hot leg temperature will result in a higher density causing the neutron flux to shift higher in the core due to the increased neutron moderation in these regions (delta-T gets smaller as power is reduced). This will reduce the axial peaking factor as the flux in the lower portions of the core lowers relative to the average core flux.
✗ B. Plausible because the operator may believe that withdrawing control rods 4 steps will increase the hot leg temperature resulting in a higher flux in this portion and higher peaking factors. Incorrect in that withdrawing one bank of control rods 4 steps will cause the flux to be depressed less (with no change in average flux). This results in a decrease in axial peaking factor.
✗ D. Plausible because the operator may believe that one control rod dropped in the core will increase the flux in that portion of the core adjacent to the dropped rod resulting in a higher axial peaking factor. Incorrect in that one control rod inserting will affect radial flux peaking, but will have no effect on axial flux peaking because the overall axial flux profile does not shift significantly since it is an axially uniform poison.
Ref: INPO GFES Core Thermal Limits (R3) | LO: NOS05CORTHR-02, ELO 1.3 Reactor Operation Effects on Peaking Factors | Source: Bank - INPO GFES P7650 | Cognitive: Comprehension/Analysis
Connections
- Related systems: Rx Vessel & Internals, CVCS
- Related exam: 2023 NRC Written Exam