Q2 — Small Break LOCA / RCS Heat Removal Methods
EPE 9 EK1.03 (4.0)
Given:
• A small break LOCA and a Loss of ALL Offsite Power occurred on Salem Unit 2
• SG pressures are 1015 psig and stable
• RCS pressure is 1600 psig and slowly lowering
• Highest CET temperature is 590°F and lowering
• RCS T-Hot temperatures are 554°F and lowering
• RCS T-Cold temperatures are 548°F and stable
• 2B 4KV Vital Bus is locked out on Bus Differential
What is the primary method for RCS heat removal for current plant conditions?
• A small break LOCA and a Loss of ALL Offsite Power occurred on Salem Unit 2
• SG pressures are 1015 psig and stable
• RCS pressure is 1600 psig and slowly lowering
• Highest CET temperature is 590°F and lowering
• RCS T-Hot temperatures are 554°F and lowering
• RCS T-Cold temperatures are 548°F and stable
• 2B 4KV Vital Bus is locked out on Bus Differential
What is the primary method for RCS heat removal for current plant conditions?
A. Forced RCS Cooling
B. Natural Circulation
C. Reflux Boiling
D. ECCS Injection and Break flow
▶ Show Answer & Explanation
✗ A. Incorrect. Forced cooling requires RCPs running, which are unavailable due to Loss of All Offsite Power.
✓ B. Correct. Natural circulation conditions are met based on the following parameters: (1) subcooling > 0°F — RCS pressure at 1600 psig has a saturation temperature of approx. 606°F, hottest CET is 590°F, therefore subcooling is approx. 606 - 590 = 16°F of subcooling, (2) SG pressures are stable and lowering, (3) RCS T-Hots are stable or dropping, (4) CETs are lowering, and (5) RCS T-colds are at saturation temp for SG pressure. SG pressure is 1015 psig (pre-set pressure for MS110s) which is a saturated temp from the outlet of the SGs of approx. 548°F. Therefore per 2-EOP-LOCA-2 step 19.1 conditions for Natural Circulation are met and will be the RCS heat removal method.
✗ C. Incorrect. Reflux boiling occurs when natural circulation has been lost and steam is condensed on SG tubes. Current conditions show T-hot > T-cold with stable/lowering parameters, indicating natural circulation is still established.
✗ D. Incorrect. Only one Charging Pump is running and no SI pumps based on RCS pressure above the shutoff head (1540 psig), therefore ECCS injection flow and break flow are insufficient for RCS heat removal.
Ref: 2-EOP-LOCA-2 (R42), ASME Steam Tables (year 2000) | LO: NOS05LOCA02-06, Obj 5 | Source: Modified - Callaway 2017 NRC RO8 | Cognitive: Comprehension/Analysis
Connections
- Related systems: RCS, ECCS, Steam Generator & Blowdown
- Related procedures: EOP-LOCA-2 — Post LOCA Cooldown and Depressurization
- Related exam: 2023 NRC Written Exam