Q40 — Steam Dump Bypass Valve Failure
G2.1.28 (4.1)
Given:
• Unit 2 is at 60% Reactor Power and stable
At time 10:00:00
• 23TB40 (Turbine Bypass Valve) goes fully open
Which ONE of the following completes the statements below?
With 23TB40 open, Reactor power will rise _(1)_ 60% power.
Considering ONLY the Steam Dump system design, 23TB40 will FIRST close when depressing BEZEL "OFF & RESET BYPASS TAVG" Pushbutton(s) _(2)_.
• Unit 2 is at 60% Reactor Power and stable
At time 10:00:00
• 23TB40 (Turbine Bypass Valve) goes fully open
Which ONE of the following completes the statements below?
With 23TB40 open, Reactor power will rise _(1)_ 60% power.
Considering ONLY the Steam Dump system design, 23TB40 will FIRST close when depressing BEZEL "OFF & RESET BYPASS TAVG" Pushbutton(s) _(2)_.
| (1) | (2) |
A. (1) and stabilize above — (2) A and B
B. (1) and stabilize above — (2) A or B
C. (1) then stabilize back at — (2) A and B
D. (1) then stabilize back at — (2) A or B
▶ Show Answer & Explanation
✓ B. Correct. For Part 1, ALL TB40 Turbine Bypass Valves are rated for 5.33% flow each. Consequently, when 23TB40 fails open, steam flow from the SGs will increase by 5.33% and Reactor Power will follow SG steam flow. Reactor power will then rise to approximately 65.33%. For Part 2, since the Steam Dump Train A and Train B Arming/Blocking solenoids are in series, depressing BEZEL Pushbutton(s) A OR B will remove air pressure to 23TB40 which will cause the valve to close.
✗ A. Incorrect but plausible. Part 1 is correct. For Part 2, if the Steam Dump Train A and Train B Arming/Blocking solenoids were in parallel, depressing BEZEL Pushbutton(s) A AND B would be the minimum pushbutton manipulation(s) required to close 23TB40. Additionally, for safety systems, only 1 train is required to be operating for the safety system to perform its safety function. Consequently, the operator could apply this logic to the steam dumps and incorrectly conclude that BOTH Trains of Steam Dumps need to be taken to OFF for 23TB40 to close.
✗ C. Incorrect but plausible. For Part 1, the candidate could incorrectly determine that with the turbine controls initially set at 60% that the turbine control valves will automatically throttle to maintain reactor power/turbine load at 60%. Consequently, the candidate would then incorrectly conclude that when 23TB40 opens that the turbine control valves will automatically throttle more closed which would return reactor power back to the original setting of 60%. For Part 2, the solenoid logic error described in distractor A applies.
✗ D. Incorrect but plausible. For Part 1, the candidate could incorrectly determine that the turbine control valves will automatically compensate. Part 2 is correct.
Ref: NOS05STDUMP-12 (STEAM DUMP SYSTEM) | LO: NOS05STDUMP-12 (STEAM DUMP SYSTEM), Obj 10 — Identify and describe the Control Room controls, indications, and alarms associated with the Steam Dump System | Source: Bank — LOR 2018 BI-ANNUAL WRITTEN EXAM | Cognitive: Comprehension/Analysis
Connections
- Related systems: Steam Dumps
- Related exam: 2019 NRC Written Exam