Q5 — Excess Letdown / SI Isolation
000022AA1.07 (2.8)
Given:
• Unit 2 is at 100% Power.
• Normal Letdown has been isolated due to a problem with the Letdown Pressure Control Valve, 2CV18.
• Excess Letdown has been established to the VCT in accordance with S2.OP-SO.CVC-0003, Excess Letdown Flow.
Immediately following a Safety Injection (SI) signal, Excess Letdown will __(1)__.
Note: 2CV278, Excess Letdown Isolation Valve
2CV131, Excess Letdown Isolation Valve
2CV284, Seal Return Isolation Valve
2CV116, Seal Return Isolation Valve
• Unit 2 is at 100% Power.
• Normal Letdown has been isolated due to a problem with the Letdown Pressure Control Valve, 2CV18.
• Excess Letdown has been established to the VCT in accordance with S2.OP-SO.CVC-0003, Excess Letdown Flow.
Immediately following a Safety Injection (SI) signal, Excess Letdown will __(1)__.
Note: 2CV278, Excess Letdown Isolation Valve
2CV131, Excess Letdown Isolation Valve
2CV284, Seal Return Isolation Valve
2CV116, Seal Return Isolation Valve
A. Continue to flow to the RCDT due to seal return relief valve CV115 cycling following the automatic closure of CV116 and CV284.
B. Continue to flow to the PRT due to seal return relief valve CV115 cycling following the automatic closure of CV278 and CV131.
C. Continue to flow to the RCDT due to seal return relief valve CV115 cycling following the automatic closure of CV278 and CV131.
D. Continue to flow to the PRT due to seal return relief valve CV115 cycling following the automatic closure of CV116 and CV284.
▶ Show Answer & Explanation
✓ D. Correct. Normal Excess Letdown flow is directed to the VCT (see stem) via the CV134 3-way valve and following a Safety Injection signal, Seal Return Isolation valves CV116 & 284 will close causing both seal return and excess letdown flow to continue to flow to the PRT due to the cycling of relief valve CV115.
✗ A. Incorrect. Plausible because the candidate may believe that the CV115 relief valve relieves to the RCDT. Candidate may also remember that Excess Letdown flow can be directed via the 3-way valve, CV134 to the RCDT instead of the VCT. Incorrect as CV115 relieves to the PRT and normal excess letdown flow is directed to the VCT (see stem). CV-134 also fails to the VCT on loss of power and air.
✗ B. Incorrect. The first part is correct as excess letdown flow will relieve to the PRT via the seal return relief valve CV115. Incorrect as the Excess Letdown Isolation valves do not receive any automatic closure signals. Incorrect as Seal Return Isolation Valves CV116 & 284 do receive automatic closure signals via Phase A Isolation.
✗ C. Incorrect. The first part is plausible because the candidate may believe that the CV115 relief valve relieves to the RCDT. Candidate may also remember that Excess Letdown flow can be directed via the 3-way valve, CV134 to the RCDT instead of the VCT. Incorrect as CV115 relieves to the PRT and normal excess letdown flow is directed to the VCT (see stem). CV-134 also fails to the VCT on loss of power and air. The second part is plausible because the candidate may remember that although the excess letdown valves themselves do not receive a Phase A signal, when the control air (CA-330s) valves are isolated on Phase A, control air pressure will bleed off in containment and the CV278 & CV131 will fail closed isolating excess letdown flow. Incorrect as the flow would then be terminated and not flow through the CV115 relief valve. Incorrect as Seal Return Isolation Valves CV116 & 284 do receive automatic closure signals via Phase A Isolation. It will take considerable time for the control air pressure to bleed down and cause the valves to fail closed.
Ref: S2.OP-SO.CVC-0003(Q), Excess Letdown Flow | LO: N/A | Source: New | Cognitive: Fundamental
Connections
- Related systems: CVCS, Pressurizer & PRT
- Related procedures: S2.OP-SO.CVC-0003 — Excess Letdown Flow
- Related exam: 2020 NRC Written Exam