Q70 — Xenon-135 / Reactor Theory Following Trip
T4 192006 K1.07 (3.4)
Given:
- Salem Unit 1 has been operating at 100% power for the past six months
Then:
- The reactor trips
- Shortly afterwards, a reactor startup is performed
- Twelve (12) hours after the trip, the startup has been paused with reactor power at 5%
Which of the following completes both statements?
1. Xenon-135 concentration will be (1) .
2. To maintain reactor power and T-avg stable over the next hour, the operator must add (2) reactivity.
- Salem Unit 1 has been operating at 100% power for the past six months
Then:
- The reactor trips
- Shortly afterwards, a reactor startup is performed
- Twelve (12) hours after the trip, the startup has been paused with reactor power at 5%
Which of the following completes both statements?
1. Xenon-135 concentration will be (1) .
2. To maintain reactor power and T-avg stable over the next hour, the operator must add (2) reactivity.
A. (1) rising (2) negative
B. (1) lowering (2) negative
C. (1) lowering (2) positive
D. (1) rising (2) positive
▶ Show Answer & Explanation
✓ B. Correct. Directly following a reactor trip, the neutron flux is reduced essentially to zero. Therefore, Xe-135 is no longer produced directly from fission, but is no longer removed by burnup. The only remaining production mechanism is beta-decay of the iodine-135; the only removal mechanism for xenon-135 is beta-decay. Because the decay rate of iodine-135 is faster than the decay rate of xenon-135, the concentration builds to a peak (about -4700 pcm if tripped at 100% power). The time to peak is slightly less than the square root of the power from the trip (8-9 hours from 100%). Following the peak, the Xe-135 concentration will decrease at a rate controlled by the decay of Xe-135. Therefore, an additional twelve (12) hours after the power change, Xe-135 concentration will be decreasing due to the decay of I-135 and Xe-135. Additionally, returning to 5% power will result in a small amount of the burnout term removing Xe-135. A reduction in Xe-135 inserts positive reactivity (Xe-135 is a fission product poison). Therefore, to maintain stable reactor coolant temperature, the operator must insert negative reactivity because xenon-135 concentration is decreasing.
✗ A. Part 1 is correct. Part 2 is incorrect. Plausible if the operator confuses the concept on how the fission product poison (xenon-135) effects reactivity and believes that a higher xenon concentration will require negative reactivity. Incorrect in that after 12 hours xenon has already peaked and xenon concentration will be decreasing adding positive reactivity, and therefore, require adding negative reactivity.
✗ C. Part 1 is incorrect. Part 2 is correct. Plausible if the operator confuses the concept on how the fission product poison (xenon-135) effects reactivity and believes that a lower xenon concentration will require adding positive reactivity. Incorrect in that after 12 hours xenon has already peaked and xenon concentration will be decreasing adding positive reactivity, and therefore, require adding negative reactivity.
✗ D. Part 1 is incorrect. Part 2 is incorrect. Plausible if the operator confuses the concept on how the fission product poison (xenon-135) effects reactivity and believes that a higher xenon concentration will require positive reactivity. Incorrect in that after 12 hours xenon has already peaked and xenon concentration will be decreasing adding positive reactivity, and therefore, require adding negative reactivity.
Ref: INPO GFES Fission Product Poisons (R3) | LO: NOS05POISON-02. INPO GFES Fission Product Poisons Rev. 3, ELO 1.4 Xenon Terms | Source: Bank - NRC GFES Bank P3563 | Cognitive: Fundamental/Memory
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