Recent advances in ultrasonic downcomer flow-measurement techniques for recirculating steam generators

Non-intrusive ultrasonic measurements of downcomer flow velocity have been successfully used in the past to determine recirculation ratios and water inventory in CANDU steam generators. Knowledge of these process conditions allows operators to assess the effectiveness of maintenance programs, monitor the effects of tube fouling, and observe flow conditions following component modifications. It also provides designers with a means to verify or improve code predictions. Ultrasonic measurement systems have recently been installed on sixteen steam generators at the Bruce B Nuclear Generating Station, as part of an investigation into the possible effects of long-term boiler degradation. The most recent version of AECL's downcomer-flow technology was used, which features high-temperature transducers that are attached magnetically and then welded to the steam-generator outer shell. This method eliminates the complications of precision surface preparation, high-temperature couplants and awkward mechanical attachments. The paper will outline the method and summarize flow velocities measured during normal operation, over extended periods of time. It will also describe how the information might be used, e.g., to assess thermalhydraulic conditions, verify design calculations and support the case for reactor uprating. Further improvements that may allow the reliable measurement of flow in steam generators with steam carry-under are suggested, and preliminary results are presented from a dual-purpose single- and two-phase flow-measurement system. (author)