Q2 — Mixed Bed Demineralizer Malfunction
004 CVCS-K6.20 (2.5)
Given:
• Unit 2 is at 95% Reactor Power and stable
• The Rod Bank Selector Switch is in MANUAL
• The 21 MIXED BED DEMINERALIZER is in service
At time 20:00:00
• 2CC71 (Letdown Heat Exchanger Component Cooling Water Outlet Control Valve) malfunctions causing the letdown temperature to rise and stabilize at 130 °F
Which ONE of the following completes the statements below?
When in service, the mixed bed demineralizer is used to _(1)_.
As the 21 MIXED BED DEMINERALIZER heats up, RCS TAVG will INITIALLY _(2)_.
• Unit 2 is at 95% Reactor Power and stable
• The Rod Bank Selector Switch is in MANUAL
• The 21 MIXED BED DEMINERALIZER is in service
At time 20:00:00
• 2CC71 (Letdown Heat Exchanger Component Cooling Water Outlet Control Valve) malfunctions causing the letdown temperature to rise and stabilize at 130 °F
Which ONE of the following completes the statements below?
When in service, the mixed bed demineralizer is used to _(1)_.
As the 21 MIXED BED DEMINERALIZER heats up, RCS TAVG will INITIALLY _(2)_.
A. (1) provide Lithium control and help maintain RCS pH — (2) rise
B. (1) provide Lithium control and help maintain RCS pH — (2) lower
C. (1) remove chemical impurities from the RCS — (2) rise
D. (1) remove chemical impurities from the RCS — (2) lower
▶ Show Answer & Explanation
✓ D. Correct. For Part 1, when in service, the mixed bed demineralizer is used to remove chemical impurities from the RCS. For Part 2, boron removal is more efficient at lower temperatures. Consequently, the demineralizer will release boron as temperature rises resulting in an unplanned reactivity change. With more boron in the RCS, TAVG will lower to offset the negative reactivity inserted into the core when the demineralizer heated up and released boron.
✗ A. Incorrect but plausible. For Part 1, the Cation Demineralizer is used to remove Lithium from the RCS and to maintain RCS pH. Consequently, the candidate could incorrectly conclude that mixed bed demineralizer is used to help maintain RCS pH. For Part 2, the candidate could incorrectly conclude that the demineralizer will actually absorb more boron as it heats up which would cause TAVG to rise.
✗ B. Incorrect but plausible. For Part 1, the Cation Demineralizer is used to remove Lithium from the RCS and to maintain RCS pH. Consequently, the candidate could incorrectly conclude that mixed bed demineralizer is used to help maintain RCS pH. Part 2 is correct.
✗ C. Incorrect but plausible. Part 1 is correct. For Part 2, the candidate could incorrectly conclude that the demineralizer will actually absorb more boron as it heats up which would cause TAVG to rise.
Ref: NOS05CVCS00-17 (Chemical and Volume Control System) | LO: NOS05CVCS00-17 — Describe demineralizer function and abnormal operations | Source: New | Cognitive: Higher
Connections
- Related systems: CVCS, CCW
- Related exam: 2018 NRC Written Exam