Pressurizer Pressure Channel Drift and Tech Spec Action
A PWR has three pressurizer pressure channels, A, B, and C, used in a 2-out-of-3 high-pressure reactor trip. The trip setpoint is 15.55 MPa. During a surveillance, channel A reads 15.25 MPa while B and C read 15.50 MPa. The allowable channel uncertainty is ±0.07 MPa. The as-found calibration shows channel A has drifted 0.25 MPa low since its last calibration. The crew proposes to leave channel A in service because B and C still provide two high-quality channels and the 2/3 logic will still trip if both B and C exceed the setpoint. The shift technical advisor asks whether channel A is operable, what action the Tech Specs require, and whether placing channel A in trip is a valid response.
1. Is channel A operable? Support your answer with the setpoint and uncertainty comparison.
Read the worked answer
No. The indicated difference is 15.50 − 15.25 = 0.25 MPa, and the as-found drift is also 0.25 MPa. The allowable channel uncertainty is ±0.07 MPa, so the drift is more than three times the allowed value. Channel A is reading low, which is non-conservative for a high-pressure trip because it delays or prevents that channel from reaching the trip setpoint. The channel cannot be considered operable for the high-pressure trip function. The correct conclusion is that channel A must be declared inoperable and the Tech Spec action statement for an inoperable pressurizer pressure channel must be entered.
2. The crew proposes to leave channel A in service. Evaluate that proposal and explain the treatment of a channel placed in trip.
Read the worked answer
Leaving channel A in service is not acceptable. With A reading low, the 2/3 logic still trips if both B and C reach the setpoint, but the plant has lost the redundancy that protects against a second failure. If B or C also fails low, only one high-quality channel remains and the trip function may not actuate. The Tech Spec action typically requires restoring the channel within a specified time or placing it in trip. Placing A in trip is conservative for a high-pressure trip because the channel then continuously supplies a trip signal; the coincidence becomes effectively 1-out-of-2 on the remaining channels, so any one of B or C reaching the setpoint will trip the reactor. The crew must verify that the Tech Specs permit placing the channel in trip, complete the action within the allowed time, and investigate the cause of the drift rather than simply recalibrating and returning the channel to service without an evaluation.