ON-LINE ESTIMATION OF LOCAL AND TOTAL CORE FLOW RATES BY NEUTRON NOISE ANALYSIS IN BWR

The extensive work on total and local core flow estimation for measurement by analysing in-core neutron noise signals was carried out using measured data of commercial BWR plants. A large database of LPRM signals and process data measured in this project enabled us to investigate the physical interpretation of the transit time measured by the LPRM neutron noise signals. The improved core flow estimation algorithm has been developed based on the new findings of logical inconsistency in the axial transit time of the void propagation measured directly LPRM B through D against LPRM B through C adding to C through D. The newly-developed algorithm showed a good agreement and predictability in the measurement tests during the RIP (reactor internal pump)-trip testing and the start-up testing of the first 1350 MWe-ABWR. The measurement results exhibited the verification and validation of the present flow estimation method within about five per cent estimation error for high flow rates and about ten per cent estimation errors over a wide-range operating area. The error shall be reduced especially in the low flow rate by considering the average of longer sampling period. Further findings of the LPRM fluctuation signals on the NRMS (normalised root mean square) against the void fraction divided by square root of the void velocity showed the possibilities of the two-phase flow monitoring by the in-core neutron noise analyses. Introduction For a safe and efficient operation of BWR plants, monitoring of two-phase flow conditions is of the most importance. In commercial BWR plants, the two-phase flow behaviour is monitored through analytical thermal-hydraulic model prediction using the total core flow rate measured by differential pressure. The in-core neutron detector (LPRM) signals have promising possibilities, which provide low-cost and useful tools for two-phase flow monitoring not only for total core flow but also for local core flow or two-phase flow regimes. In the past two decades, these possibilities have been widely studied [1-8]. Through these studies, qualitative behaviour of neutron and two-phase flow fluctuations was well understood. Nonetheless, there remains ambiguity of the quantitative behaviour, which arises from the questions, ”What kind of velocity of two-phase flow de we really measure?” and “Can we measure two-phase flow regimes?”. In order to apply the neutron noise analysis technique to two-phase flow monitoring, it is necessary to make this ambiguity clear. In the present work, we evaluated the accuracy of the measurement of local and total core flow rates using in-core neutron noise signals, which were measured in operating BWR plants. In the first step, the discussion was introduced for the physical interpretation of the two-phase flow transit time, based on the measurements by LPRM neutron noise signals. The new evidence was found with deep understanding by the review of the transittime behaviour. Also, we evaluated the two-phase flow velocity detection process by sub-channel void distribution analyses and field-of-view analyses of a neutron detector. Based on these results, the core flow estimation algorithm was improved and optimised, and an on-line core flow measurement system using personal computers has been developed. The tests for verification of the algorithm and the system were carried out in BWR-5 and ABWR type plants. Interpretation of in-core neutron noise behaviour Model description of two-phase flow The present flow estimation algorithm is based on the drift-flux model [4], which leads to the radially-averaged steam void fraction, α, expressed as: