In order to measure steam void fractions in operating Boiling Water Reactors, two types of void-meter, solenoid type and digital type, were developed. The former consist of a solenoid, a high frequency oscillator, a bridge and other suitable electric circuits and detects impedance change induced by voids passing through the solenoid. The latter uses a thin needlelike electrode as detecting head and detects the changes of capacitance and resistance induced when the electrode collides with and penetrates a bubble. Since the changes of impedances induced by void were very small, special electric circuits were also devised to detect the small signals.
The solenoid type void-meter responds promptly with high sensibility, and is suitable for measuring mean void fractions in the solenoid under transient conditions, while the digital type voidmeter is small in size and is applicable to measuring time-averaged local void fractions. These performances were examined through out-pile experiments. The detecting heads of the void-meters were designed and manufactured with special considerations so as to be used under the conditions of high temperature, high pressure and high irradiation.
These void-meters were used to measure void fractions in the chimnies and in the downcomer of operating reactor JPDR (Japan Power Demonstration Reactor; a boiling water reactor with natural circulation).