Evaluation of low flow characteristics of the Vermont Yankee plant

Boiling water reactor (BWR) core flow instrumentation inaccuracies under low-flow conditions have been the subject of both reactor vendor and regulatory communications in response to incidents of the reported core flow being less than the flow corresponding to the natural-circulation line on the power flow map. During single recirculation loop operation, low-flow conditions exist in the idle recirculation loop, and these flow inaccuracies can affect the usefulness of the reported core flow. Accurate core flow indications are needed above 25% power to administer fuel thermal limits and comply with restrictions associated with the potential for thermal-hydraulic instability. While the natural-circulation line on the power flow map is recognized to be a nominal estimate of the flow expected at and near natural-circulation conditions, the boundaries of the stability regions are associated with conditions assumed in safety analyses performed to demonstrate compliance with general design criteria 10 and 12