Development and validation of a PWR on-line power-distribution monitoring system NECP-ONION

A 3D on-line power-distribution monitoring system NECP-ONION has been developed and validated in this paper. It includes plant data processing, core calculation and on-line power-distribution monitoring. The diagnosis and treatment of the detector failure, core state-parameter calculation and monitoring method has been investigated to address the key issues of the on-line monitoring system. Detector failure is diagnosed by comparing with average measurements and monitored responses. The fuel-burnup calculation is performed based on the macro-depletion method due to its high efficiency. The harmonics expansion method is adopted as the major monitoring method due to its long history of maturity and success. In the investigation of the harmonics expansion method, the determination of expansion orders and choice of expansion basis functions have also been studied. The on-line monitoring system NECP-ONION was verified and validated by the diffusion calculation and real detector measurements for the BEAVRS benchmark. The verification shows that, for BEAVRS cycle 1 with a complex power history, the monitoring power distributions are almost identical to the reference power distributions once the detector “measurements” are derived from the diffusion calculation. The validation shows that differences between monitored and measured ones are small for most of the burnup steps from BOC to EOC, except for a few abnormal discrepancies at specific burnup steps. Investigation shows that those discrepancies are caused by the detector failure, complex operation history, unknown control rod positions, and unidentified operation events which cannot be modeled directly in the simulation. Two improvements have been studied to increase the monitoring accuracy of the NECP-ONION system. The numerical results indicate that these two improvements are useful to increase the monitoring accuracy for the BEAVRS benchmark.