Q2 — PZR Safety Valve Tailpipe Temperature
APE08 AK2.02 (2.7)
Given:
• A manual reactor trip and SI have been actuated on Unit 1.
• A PZR safety valve has been stuck slightly open for ten minutes.
• RCS pressure is 1900 psig and slowly lowering.
• PZR vapor space temperature is 630 °F and stable.
• PRT level is 80% and slowly rising.
• PRT pressure is 35 psig and slowly rising.
• Containment pressure is 4 psig and slowly rising.
Currently, safety valve tailpipe temperature indication should be reading approximately:
• A manual reactor trip and SI have been actuated on Unit 1.
• A PZR safety valve has been stuck slightly open for ten minutes.
• RCS pressure is 1900 psig and slowly lowering.
• PZR vapor space temperature is 630 °F and stable.
• PRT level is 80% and slowly rising.
• PRT pressure is 35 psig and slowly rising.
• Containment pressure is 4 psig and slowly rising.
Currently, safety valve tailpipe temperature indication should be reading approximately:
A. 282 °F
B. 265 °F
C. 230 °F
D. 200 °F
▶ Show Answer & Explanation
✓ A. Correct. The safety valve leak is a throttling process that follows a constant enthalpy process across the valve. The RCS pressure of 1900 psig (1915 psia) equates to a saturated vapor enthalpy (hf) of 1140 BTU/lb. Using the Mollier diagram and following the 1140 BTU/lb enthalpy line (horizontal) until it intersects with the pressure line for the PRT of 35 psig (50 psia). Then follow up the 50 psia line to determine the temperature which will be approx. 282 °F.
✗ B. Incorrect. Plausible if the applicant misapplies the Mollier diagram by throttling to PRT pressure of 35 psia and not 50 psia (35 psig).
✗ C. Incorrect. Plausible if the applicant misapplies the Mollier diagram by throttling to containment pressure of 4 psig (19 psia). The current conditions support the PRT rupture disc remaining intact, therefore this is incorrect.
✗ D. Incorrect. Plausible if the applicant misapplies the Mollier diagram by throttling to the saturation curve.
Ref: GFES Fundamentals, Steam Table Mollier Diagram | LO: NOS05PZR&L-12, Obj 16 — Given any of the following and appropriate control room reference material, evaluate and determine the effect on the Pressurizer Pressure and Level Control System: Malfunctioning Valves | Source: Bank — Byron 2019 NRC RO1 (October exam) | Cognitive: Comprehension/Analysis
Connections
- Related systems: Pressurizer & PRT
- Related exam: 2022 NRC Written Exam