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Exams > 2020 Q88 — Loss of VIB / ESFAS Phase B Logic

2020 Q88 — Loss of VIB / ESFAS Phase B Logic

⚠️ DRAFT
Q88 — Loss of VIB / ESFAS Phase B Logic 013A2.04 (4.2)
Given:
• Unit 2 is at 100% Power.
• A Loss of 2B Vital Instrument Bus has occurred.
• No operator action has been taken.

Complete the following statements concerning an impact on Engineered Safety Feature Actuation System (ESFAS) instrumentation due to the loss of the Vital Instrument Bus.

(1) Prior to any operator action taken, the current Containment Pressure channel logic (coincidence) for the remaining channels to cause a Phase B actuation is __(1)__.
(2) In accordance with Technical Specification 3.3.2.1, ESFAS Instrumentation, the CRS will direct maintenance to remove the inoperable channel from service by placing the Containment Pressure HI-HI Bistable in the __(2)__ condition.
A. (1) 2/3
(2) Tripped
B. (1) 2/3
(2) Bypassed
C. (1) 1/3
(2) Tripped
D. (1) 1/3
(2) Bypassed
▶ Show Answer & Explanation
✓ B. Correct. Containment Spray bi-stables are energized to actuate. The loss of the 2B Vital Instrument Bus will result in the Hi-Hi Containment Pressure bi-stable associated with 2B Vital Instrument Bus remaining in the deenergized state, therefore the logic will go from 2/4 to 2/3. Technical Specifications do not place the Containment Spray bi-stables in the tripped condition, they are bypassed to reduce the possibility of a spurious actuation of Containment Spray.
✗ A. Incorrect. The first part is correct. The second part is plausible as the candidate may believe the one bi-stable for Hi-Hi Containment Pressure has deenergized with the loss of the 2B Vital Instrument Bus. Incorrect as Containment Spray bi-stables are energized to actuate. Technical Specifications do not place the Containment Spray bi-stables in the tripped condition, they are bypassed to reduce the possibility of a spurious actuation of Containment Spray.
✗ C. Incorrect. The first part is plausible because the candidate may remember that the High Containment Pressure SI logic is 2/3 channels and may believe the Hi-Hi Containment Pressure Containment Spray logic is 2/3 as well. Incorrect as the Hi-Hi logic is 2/4 channels. The second part is plausible in that most ESF actuation bi-stables are deenergized to trip and tech specs require those bi-stables to be placed in the tripped condition. Incorrect as Containment Spray bi-stables are energized to actuate. Technical Specifications do not place the Containment Spray bi-stables in the tripped condition, they are bypassed to reduce the possibility of a spurious actuation of Containment Spray.
✗ D. Incorrect. The first part is plausible because the candidate may remember that the High Containment Pressure SI logic is 2/3 channels and may believe the Hi-Hi Containment Pressure Containment Spray logic is 2/3 as well. Incorrect as the Hi-Hi logic is 2/4 channels. The second part is correct.
Ref: S2.OP-AB.115-0002(Q), Loss of 2B 115V Vital Instrument Bus and Bases; ESFAS Instrumentation TS 3.3.2.1 and Bases | LO: N/A | Source: Bank – Catawba 2017 NRC Exam Q87 | Cognitive: Comprehension

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