Q71 — Natural Circulation Boron Mixing and Reactivity
T4 192007 K1.05 (3.2)
Given:
- Salem Unit 2 was operating at 100% power when a Loss of All Offsite Power occurs.
- Due to plant complications, the Shift Manager has directed the crew to cooldown the RCS to Cold Shutdown per 2-EOP-TRIP-4, Natural Circulation Cooldown.
- The crew is borating the RCS for the RCS cooldown to Cold Shutdown.
Which of the following completes both statements?
1. As compared to adding boron to the RCS during forced circulation, adding boron during natural circulation requires (1) time to achieve complete mixing in the RCS.
2. Once the RCS boron mixing is complete, a 1 ppm increase in RCS boron concentration during natural circulation will cause a/an (2) change in reactivity that it would during forced circulation operation.
- Salem Unit 2 was operating at 100% power when a Loss of All Offsite Power occurs.
- Due to plant complications, the Shift Manager has directed the crew to cooldown the RCS to Cold Shutdown per 2-EOP-TRIP-4, Natural Circulation Cooldown.
- The crew is borating the RCS for the RCS cooldown to Cold Shutdown.
Which of the following completes both statements?
1. As compared to adding boron to the RCS during forced circulation, adding boron during natural circulation requires (1) time to achieve complete mixing in the RCS.
2. Once the RCS boron mixing is complete, a 1 ppm increase in RCS boron concentration during natural circulation will cause a/an (2) change in reactivity that it would during forced circulation operation.
A. (1) less (2) smaller
B. (1) less (2) equal
C. (1) more (2) smaller
D. (1) more (2) equal
▶ Show Answer & Explanation
✓ D. Correct. Due to loss of offsite power, no RCPs will be running resulting in a significantly lower RCS flowrate (NC conditions). Therefore, more time will be required to achieve complete mixing in the RCS for a given boron concentration change. Once the RCS mixing is complete, a 1 ppm increase (or decrease) in RCS boron concentration during NC operation will cause the same change in core reactivity that it would during forced circulation operation.
✗ A. Part 1 is incorrect. Plausible if the operator misreads the question and therefore believes the question is asking about time to mix when forced circulation. Part 2 is incorrect. Plausible if the operator believes that reactivity effects will be smaller due to the lower RCS flowrates. Incorrect in that the change in reactivity will be the same when the boron is thoroughly mixed.
✗ B. Part 1 is incorrect. Part 2 is correct. Plausible if the operator misreads the question and therefore believes the question is asking about time to mix when forced circulation.
✗ C. Part 1 is correct. Part 2 is incorrect. Plausible if the operator believes that reactivity effects will be smaller due to the lower RCS flowrates. Incorrect in that the change in reactivity will be the same when the boron is thoroughly mixed.
Ref: INPO GFES Fuel Depletion and Burnable Poisons (R3) | LO: NOS05POISON-02, ELO 1.6 Boron and Natural Circulation | Source: Bank - NRC GFES Reactor Theory P3364 | Cognitive: Fundamental/Memory
Connections
- Related systems: RCS, CVCS
- Related EOPs: EOP-TRIP-4 — Natural Circulation
- Related exam: 2023 NRC Written Exam