Q53 — SAC Power Supply & Control Air Pressure
078 K2.01 (2.7)
Given:
• Both Units are at 100% Power.
• #1 Station Air Compressor (SAC) is running.
• #2 SAC is in AUTO and next to load.
• #3 SAC is C/T for maintenance.
• There is no air usage at either unit's condensate polisher system.
• Control Air headers A and B on both units are indicating 105 psig.
Subsequently, the following occurs;
• Unit 1 reactor trips and the 4KV Group Busses fail to automatically transfer resulting in ALL 4KV Group Busses being de-energized.
• Unit 2 is NOT affected.
15 minutes later, which ONE of the following would be an expected air pressure indication for both control air headers on Units 1 and 2?
• Both Units are at 100% Power.
• #1 Station Air Compressor (SAC) is running.
• #2 SAC is in AUTO and next to load.
• #3 SAC is C/T for maintenance.
• There is no air usage at either unit's condensate polisher system.
• Control Air headers A and B on both units are indicating 105 psig.
Subsequently, the following occurs;
• Unit 1 reactor trips and the 4KV Group Busses fail to automatically transfer resulting in ALL 4KV Group Busses being de-energized.
• Unit 2 is NOT affected.
15 minutes later, which ONE of the following would be an expected air pressure indication for both control air headers on Units 1 and 2?
A. 60 psig and lowering rapidly.
B. 80 psig and lowering slowly.
C. Stable at approximately 85-88 psig.
D. Stable at approximately 100 psig.
▶ Show Answer & Explanation
✓ D. Correct. Instrument air is supplied from one of three station air compressors. Unit 1 has two station air compressors (SAC), #1 SAC supplied from 1H 4KV group bus and #3 SAC supplied from 1G 4KV group buses and Unit 2 has one SAC (#2 SAC) supplied from the 2G 4KV group bus. Therefore, with a loss of the 4KV Group Buses on Unit 1 only, the #2 SAC on Unit 2 would start (if not already running) and supply station air to both units control air headers. One SAC is capable of supplying both unit's station air and control air headers to maintain around 100 psig (assuming no demands from the polisher).
✗ A. Incorrect. Plausible if you believe that the ECACs cannot maintain control air headers around 85-90 psig when all SACs are lost.
✗ B. Incorrect. Plausible if you believe that the ECACs cannot maintain control air headers around 85-90 psig when all SACs are lost.
✗ C. Incorrect. Plausible if you believe that no SACs are running and both units Emergency Air Compressors (ECACs) have started and are maintaining both CA headers around the auto start setpoint of 85 to 90 psig. The Unit 1 ECAC (B CA header) is supplied from 1C 460 V Vital Bus and the Unit 2 ECAC (A CA header) is supplied from the 2C 460 V Vital Bus.
Ref: NOS05STAAIR-18 & NOS05CONAIR-13 | LO: NOS05STAAIR-18, Obj 5 — State the power supply to the following Station Air/Service Air System components: Station Air Compressors (SACs) | Source: Bank - Diablo Canyon 2016 NRC RO26 | Cognitive: Fundamental/Memory
Connections
- Related systems: 4KV, Control Air
- Related exam: 2022 NRC Written Exam