Q40 — SSPS Train B 48 VDC Power Supply Loss
013 K4.07 (3.7)
Given:
• Unit 2 is at 100% Power.
• OHA A-42, SSPS TRN B TRBL, is LIT
• Investigation reveals that a 48 VDC power supply on SSPS Train B is tripped.
Which ONE of the following describes the impact, if any, of this failure?
• Unit 2 is at 100% Power.
• OHA A-42, SSPS TRN B TRBL, is LIT
• Investigation reveals that a 48 VDC power supply on SSPS Train B is tripped.
Which ONE of the following describes the impact, if any, of this failure?
A. The Train B Reactor Trip breaker shunt trip is unavailable.
B. NONE of the Train B SSPS components will automatically actuate when required.
C. ALL of Train B SSPS components will automatically actuate when required.
D. A portion of the Train B SSPS components will NOT automatically actuate when required.
▶ Show Answer & Explanation
✓ C. Correct. Two of four vital instrument buses (VIB) supply one set of 48V and 15V power supplies. SSPS Train A is supplied from A and D VIB and SSPS Train B is supplied from B and C VIB. B and D VIBs are supplied from the same "B" 230V VIB. The 48V and 15V power supplies are auctioneered such that a loss of one power supply will NOT cause or prevent an RPS/ESFAS actuation. Therefore, a loss of one 48 VDC power supply will have no adverse effects on the Train B SSPS to function as designed, with the exception of a loss of 48 VDC power supply redundancy.
✗ A. Incorrect. Plausible if the operator believes that the shunt coil is supplied from 48 VDC from SSPS. The 48 VDC power supplies feed the UV coils for each reactor trip breaker. A loss of both 48 VDC power supplies in one SSPS train will cause a reactor trip due to the UV coils de-energizing. The Shunt coils are supplied by 125 VDC and are energized to actuate.
✗ B. Incorrect. Plausible because the operator may believe that the loss of 48 VDC will prevent the actuation of the SSPS train by affecting either the logic or the output cabinet slave relays which are supplied from their respective 115 VAC train (A or B). Loss of one 48 or 15 VDC power supply will have no adverse effect on the function of SSPS due to the auctioneered design. However, a loss of the 115 VAC B Vital Instrument bus will prevent the slave relays from energizing and will prevent the Train B SSPS from actuating components and equipment which is not the case.
✗ D. Incorrect. Plausible if the operator believes that a loss of one power supply could affect the SSPS train from fully actuating components. The 48V and 15V power supplies are auctioneered so a loss of one has no adverse effect.
Ref: NOS05RXPROT | LO: NOS05RXPROT-15, Obj 26 — Given plant conditions, relate the RPS with 115V vital power | Source: Bank — DC Cook 2008 NRC RO46 | Cognitive: Comprehension/Analysis
Connections
- Related systems: RPS/SSPS, ESF & Design
- Related exam: 2022 NRC Written Exam