DEVELOPMENT OF WATER-LEVEL GAUGE WITH BINARY CODING THERMOCOUPLES AND ITS APPLICATION TO MEASUREMENT OF IN-VESSEL REACTOR WATER-LEVEL

In response to requirements from the authorities after the TMI-II accident, the authors proposed a new reactor water level gauge named BICOTH to perform direct measurement of the in-vessel reactor water level. The BICOTH is based on a binary coding arrangement of differential thermocouples with heaters. The success of trial fabrication and out-pile tests of prototype BICOTHs was followed by in-pile tests of a reactor-grade BICOTH in the Dodewaard nuclear power plant in the Netherlands. The in-pile tests proved its excellent in-vessel performances and revealed some points to be improved and developed. Its absolute high measuring accuracy is assured by the geometrically-fixed positions of distributed hot-junctions of thermocouples in the BICOTH. The continuous measurement of water level change becomes possible with the additional utilization of analog information about the axial temperature profile in the BICOTH; i.e. the digital/analog hybrid processing of BICOTH output signals. The mechanical structure of BICOTH is as same as that of a metal-sheathed multi-wire thermocouple so that it is stout and heat- and pressure-resistant.