Abstract
The feed water mass flow is the key measured variable used to determine the thermal reactor output in a nuclear power plant. Usually this parameter is recorded via venturi nozzles or orifice plates. The problem with both principles of measurement, however, is that an accuracy of below 1 % cannot be reached. In the case of nuclear power plants and depending on the size of the plant, this corresponds to an electrical output of 4 MWel to 16 MWel. In order to make more accurate statements about the feed water amounts recirculated in the water-steam circuit, tracer measurements that offer an accuracy of up to 0.2 % are used. A drawback of this method is that this measuring principle is suitable only for providing an instantaneous picture but does not provide continuous operating information about the feed water mass flow. Process data reconciliation based on VDI 2048 is a mathematical-statistic process that makes use of redundant process information. The uncertainty of reconciled feed water flow rates and the thermal reactor output calculated on this basis can be reduced to 0.4 %. The overall process monitored continuously in this manner therefore provides hourly process information of a quality equal to that obtained with acceptance measurements. In the NPP Beznau both methods have been used in parallel to determine the feed water flow rates in 2004 (unit 1) and 2005 (unit 2). Comparison of the results shows that a high level of agreement is obtained between the results of the reconciliation and the results of the tracer measurements. For this reason it was decided that no future tracer measurements will be conducted anymore. A a result of the findings of this comparison, a high level of acceptance of process data reconciliation based on VDI 2048 was achieved.
