A Twin Self-Powered Neutron Detector for Steam Velocity Determination in a Boiling Water Reactor

A promptly responding self-powered detector was developed. It consists of two in-line mounted neutron sensitive elements, each containing one emitter of cadmium-magnesium alloy. It can be inserted into the core of an operating boiling water reactor to measure steam void velocity by cross correlating the two noise signals of the emitters. The short emitter length (2 cm) and distance (15 cm) provide sufficient coherence to determine void velocity and enable a good spatial resolution. The dc components of the signal currents appear to be affected by activation of the alumina insulator and a (possibly) thermal effect. From the noise measurements, the gamma sensitivity of the detector could be established.