Development of a liquid level gauge with heated differential thermocouples

Accepting the demand of improvement presented on the basis of the investigation results after the TMI-2 accident, the heated differential thermocouple type liquid level gauge which can measure directly and accurately the water level in reactor vessels has been developed. The principle of detection of the developed liquid level gauge is the detection of liquid surface by heat dissipation, and it is the features that the basic structure was made into the metal sheath thermocouples with built-in heaters so as to withstand the severe environment in reactors of high temperature, high pressure and high radiation, and that the binary coding using differential thermocouple group was carried out so as to be applied to the continuous measurement of liquid level of wide span. Also the liquid surface switch using a single differential thermocouple was developed. The applicability to nuclear reactors of these liquid level gauges was confirmed by the basic characteristic test, and the practicality in reactors was verified by the various functioning tests by actually installing in power reactors. The principle and the structure of the liquid level gauge, the manufacture and the basic characteristic test of the liquid surface switch/sensor, and the manufacture and the reactor application test of BICOTH liquid level gauges and liquid surface switch assembly are reported. (K.I.)