Q57 — PZR Level Channel I Fails High
016000K3.02 (3.4)
Given:
• Unit 1 is operating at 100% Power.
• All systems are operating in AUTO.
• PZR Level Control System selected to Channel I for Control.
• PZR Level Control System selected to Channel II for Alarm.
A failure causes the following sequential plant events. Assume NO operator actions are taken.
• Charging Flow reduces to minimum.
• Actual PZR level drops slowly.
• Letdown isolates and PZR heaters turn off.
• A Reactor Trip eventually occurs on high PZR level.
Which ONE of the following failures would cause the above sequential events?
• Unit 1 is operating at 100% Power.
• All systems are operating in AUTO.
• PZR Level Control System selected to Channel I for Control.
• PZR Level Control System selected to Channel II for Alarm.
A failure causes the following sequential plant events. Assume NO operator actions are taken.
• Charging Flow reduces to minimum.
• Actual PZR level drops slowly.
• Letdown isolates and PZR heaters turn off.
• A Reactor Trip eventually occurs on high PZR level.
Which ONE of the following failures would cause the above sequential events?
A. Auctioneered Tavg failed high.
B. PZR Level Channel I failed low.
C. PZR Level Channel I failed high.
D. PZR Level Channel II failed low.
▶ Show Answer & Explanation
✓ C. Correct. A high failure of the controlling channel of PZR Level will result in a lowering of charging flow to minimum, actual level slowly dropping until 17% actual level is seen by the alarm channel which then results in letdown isolation and PZR heaters off. Now minimum charging flow with no letdown will eventually lead to a Rx Trip on high PZR Level as seen by channels II & III (2/3 @92%).
✗ A. Incorrect. Plausible because the student may confuse this with the controlling channel failing high. Incorrect because PZR program level will only fail to approximately 59% and level will be maintained around that value.
✗ B. Incorrect. Plausible because a failure of the controlling channel low will result in immediate letdown isolation and an eventual Rx Trip on high level with NO operator action, but not in the SEQUENTIAL order discussed in the question stem. Charging flow will actually rise due to the low failure of the controlling channel.
✗ D. Incorrect. Plausible because the alarm channel failing low will result in immediate letdown isolation and an eventual Rx Trip on high level with NO operator action, but not in the SEQUENTIAL order discussed in the question stem.
Ref: NOS05PZRP&L-10, Pressurizer Pressure and Level Control | LO: ELOs 4b, 15, and 16 | Source: Bank, Salem Vision Database | Cognitive: Comprehension
Connections
- Related systems: Pressurizer Level & Press Control, CVCS
- Related exam: 2020 NRC Written Exam