Off-calibration effects on boiling water reactor water level instruments that tap into jet pump diffusers

A number of water level instruments are included in boiling water reactor (BWR) plant design. These instruments measure reactor pressure vessel (RPV) water level by measuring the differential pressure (DP) between a fixed-height water column (the reference leg) and the RPV (the variable leg) and converting the DP to a height of water. Thus, plant conditions that affect the DP without changing actual water level will affect the indicated water level monitored by the operator in the control room. These conditions include reactor pressure and drywell and reactor building temperatures near the instrument lines as they affect fluid densities and measured pressures in the reference and variable legs. The fuel zone water level instruments are unique in that their variable legs tap into jet pump diffusers. Therefore, these instruments are also affected by flow past the jet pump taps and jet pump developed head. The purpose of the study is to provide a method to determine actual water level using fuel zone water level instruments for various transient and accident conditions. The concept of developing a simple correction as function of jet pump flow became more complicated as the investigation progressed. The present course of action is to (1) continue the development of jet pump flow-related calibration curves that would be used to determine actual water level under off-normal conditions; and (2) evaluate relocation of the fuel zone water level instrument variable leg tap to a place where the effects of jet pump flow are minimized