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Exams > 2020 Q61 — Steam Dump PT-507 Fails High

2020 Q61 — Steam Dump PT-507 Fails High

⚠️ DRAFT
Q61 — Steam Dump PT-507 Fails High 041000K3.05 (2.9)
Given:
  • Unit 2 is operating at 100% Power.
  • The Steam Dumps are in Main Steam Pressure Control Mode - AUTO due to a previous failure of 2PT-506, Main Turbine Steam Line Inlet Pressure Channel II.

A Reactor Trip and a malfunction causing 2PT-507, Main Steam Header Pressure, to fail high have just occurred.

With 2PT-507 failed HIGH, which ONE of the following describes how the Steam Dump System and the plant will respond, assuming no operator action is taken?
A. Steam Dumps remain CLOSED. RCS temperature rises until the MS10s, Atmospheric Dump valves, open to control RCS temperature.
B. Steam Dumps remain CLOSED. RCS temperature rises until Main Steam Safety Valves open to control RCS temperature.
C. Steam Dumps OPEN. RCS temperature rapidly lowers until 543°F when Main Steam Line Isolation (MSLI) and Safety Injection (SI) actuate.
D. Steam Dumps OPEN. RCS temperature rapidly lowers until steam dumps close at 543°F. Steam Dumps will then cycle and maintain RCS temperature at 547°F.
▶ Show Answer & Explanation
✓ C. Correct. Following the Rx Trip, all steam dumps will fully open due to the failure of 2PT-507. This will result in high steam flows on each SG and rapidly lowering RCS temperatures. When Tavg is < 543°F, a Main Steam Line Isolation (MSLI) and Safety Injection (SI) signal will actuate based on high steam flow coincident with low-low Tavg signal. At the same time, all the steam dumps will close due to low-low Tavg of 543°F and Tavg will be below 543°F.
✗ A. Plausible because the candidate might believe that the steam dumps are not armed due to the previous failure of PT-506 or may confuse the response with that of a low failure of PT-507. Incorrect as the steam dumps are automatically armed in the pressure control mode and this is a failure high, resulting in the dumps opening.
✗ B. Plausible for the same reasons as A above, but the candidate may also believe the MS10s will not open or handle the loss of load by themselves.
✗ D. Plausible because the steam dumps will close on the P-12 Tavg Low-Low Tavg Block signal. Incorrect because when Tavg lowers to < 543°F, the steam dumps will all close and not maintain steam header pressure due to P-12 block signal. The steam dumps (Group 1 only cooldown valves) will not re-open until the operator manually selects BYPASS TAVG on both trains on 2CC3.
Ref: NOS05STDUMP-12, Steam Dump System | LO: ELO 8 | Source: Bank – Salem Vision Database | Cognitive: Comprehension

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