Reactor in-core void meter

Electrode plates are disposed between fuel rods of a reactor fuel assembly to form an electrode lattice. Each electrode plate is coated with an insulating material and connected to an electrostatic capacitance meter. An rf signal is applied between the electrode plate and a grounded electrode of a fuel rod metal cladding tube to measure the changes in the electrostatic capacitance due to void. Electrodes not provided with an insulating covering are attached to the upper and lower section of the lattice. These naked metal electrodes immersed in the reactor water bring the water and the tubular cladding of the fuel elements to the same electrical potential and serve to shield the lattice. The longitudinally sensed portion is thus restricted to an area located between the disposed uninsulated electrodes and voids appearing at any location in the fuel assembly can be detected in detail. This assures a high-precision measurement of the mean void even if a transversal distribution of void exists over the fuel assembly. (Ohno, Y.)