Failed Power-Range Channel and Rod Control Response
At 90% power, the plant is stable. One of four excore power-range channels, channel A, suddenly reads 105% while channels B, C, and D read 90%, 89%, and 91%. The control rod system uses the average of the four channels for power error and begins stepping rods in. The reactor power-range high-flux trip logic is 2-out-of-4 above 109%. The operator notices that channel A detector current is increasing but the other channels are steady. The plant technical specification requires that an inoperable power-range channel be placed in the tripped condition within 1 hour, and if the channel cannot be restored, power must be reduced to 75% within 4 hours. The shift supervisor must decide whether to place rod control in manual, trip the reactor, or attempt to recalibrate channel A.
1. Diagnose the most likely cause and explain why immediately recalibrating channel A is or is not appropriate.
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The most likely cause is a failed channel A power-range instrument, not a real reactor power increase. The three independent channels B, C, and D are steady at about 90%, and other plant indications such as Tavg and steam flow are not described as rising. Channel A detector current is increasing while the others are steady, which points to a detector, cable, or electronics fault rather than a core-wide flux change. Immediately recalibrating channel A to match the other channels would be inappropriate because it could mask the fault and leave a degraded channel in service. Calibration should not be used to force agreement during an unexplained transient. The correct approach is to verify the failure with independent indications, check the channel's electrical output and power supply, and follow the technical specification for an inoperable channel. If the channel is confirmed failed, it must be placed in the tripped condition within the required time, and maintenance should troubleshoot the actual cause.
2. Recommend immediate actions, including rod control mode and technical specification compliance. Calculate the average power signal seen by rod control and the resulting power error if the demand is 90%.
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The immediate action is to place rod control in manual so that the failed channel does not continue to drive automatic rod insertion. The operator should verify that rods stop moving, confirm reactor power with the remaining three channels and with secondary-side indications, and notify the shift supervisor and instrumentation personnel. The average of the four channels is (105 + 90 + 89 + 91)/4 = 93.75%. If the demand is 90%, the rod control power error is 93.75 - 90 = +3.75%, which explains why the system is inserting rods. In manual, the crew holds rods and evaluates the plant. The technical specification requires the inoperable channel to be placed in the tripped condition within 1 hour, and if it cannot be restored, power must be reduced to 75% within 4 hours. The crew should not bypass the safety trip logic or recalibrate the channel to hide the failure. If the channel cannot be restored and the technical specification cannot be met, the reactor should be shut down in an orderly manner.
3. Assess the risk of a spurious trip and explain how the 2-out-of-4 logic affects the decision.
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With 2-out-of-4 logic, a single failed channel reading 105% cannot cause a high-flux trip by itself because the trip setpoint is 109% and at least two channels must exceed it. However, the failed channel is an unreliable input. If it drifts above 109% while a second channel experiences a transient above 109%, the logic would trip the reactor even though the second channel might be a valid but temporary high signal. The failed channel also distorts the rod control average and causes automatic rod insertion, which changes reactor power and can create its own transient. The risk is therefore not only a spurious trip but also an unnecessary power reduction and a challenge to operators. The supervisor should remove the failed channel from automatic control by placing rod control in manual, follow the technical specification for the inoperable channel, and avoid any action that would leave the plant dependent on the failed channel. A reactor trip is not automatically required at 105% on one channel, but the crew must be prepared to trip if another channel rises or if plant stability cannot be maintained.