[en] Primary coolant flow monitoring margin for one of the plants Westinghouse services was approaching 0% due to Steam Generator tube plugging and design changes. The approved methodology requires calibrating the primary coolant flow to the calorimetrically measured flow. The calorimetric flow measurement has a high uncertainty and has errors due to process variations. The path identified to gain margin was to develop a new method to reduce the uncertainty of the reference flow measurement. The new method identified for the uncertainty reduction was to determine the reference flow based on diverse, independent indications of flow. Utilizing the variance weighted averaging technique, the method produces a more accurate best estimate reference flow. Our team used three alternate indications available in the plant and the simulation of the flow loop as the diverse indications of flow. The benefit to the plant was a 60% reduction of the uncertainty. Introduction: Standard monitoring method: based on reactor coolant pump pressure differentials, periodically calibrated to the calorimetrically measured flow. - RCP involvement: Difficulty with RCP DP based flow measurement: it needs to be calibrated to a reference flow since the conditions of performance testing, if it is done at all, differ from the operating conditions. - Calorimetric involvement: Difficulties with calorimetric flow measurement: high uncertainty due to process noise and the process variation based biasing. - Plant Problem: Cycle independent calibration, applied at various plants, would not be adequate at the plant without RSG due to high uncertainty and low monitored flow. Increased resistance could put plant operability at risk. - Identification of the problem: Identified reference flow uncertainty as dominant margin reduction - Proposed Solution: Use Diverse Methods to reduce uncertainty and get more accurate best estimate flow, calibrate pump DP data to best estimate flow to allow plant to not have to change procedures except for removing the periodic calibration to calorimetric flow. - Implementation of Solution: Simulation of true phenomenon based on true physical modeling as method one - Implementation of Solution: Utilizing DP signals for Core and Steam Generator and calibrating based on simulation. - Implementation of Solution: Utilizing Design Basis method for flow measurement - Data combination: Instrument results must be poolable to utilize data and confirm each instruments validity - Data combination: Utilizing variance weighting of poolable instruments to reduce uncertainty and obtain best estimate flow - Final calibration: Calibrate Pump DP result to best estimate flow, possibly gain margin. - Final uncertainty calculation: combine Pump DP uncertainty with variance weighting uncertainty to gain margin. (authors)