MICROWAVE VOID FRACTION MONITOR FOR ORGANIC COOLANTS IN NUCLEAR REACTORS.

The use of microwave open-ended cavities in the field of nuclear instrumentation, specifically for determination of the void fraction in organic coolants used for cooling of nuclear reactors is discussed. The operating conditions for a sensing device are rather severe because of the relatively high temperature (400 degree C) and a very high oil pressure inside the piping. A diagram of the open-ended cavity used as a sensor is shown. The sensor consists of a section of circular waveguide (L) inserted in series with the circular piping system transporting the coolant. The diameter of the sensor (D) is larger than that of the coolant piping (d). For the selected frequency of operation, the end waveguides formed by the coolant piping operate below cut-off, while the length of the sensor section (L) is chosen to form a resonant cavity operating in the H//1//1//1 mode. Microwave energy is coupled to the cavity by a small electric probe at the end of a coaxial line, which protrudes through the cavity side wall and was designed to stand the high temperature and pressure of the coolant.