Patent Number: 
Section: claims

1. A sensor assembly for measuring the relative position of a control rod connected to a lead screw within a nuclear reactor from within a metallic probe tube housing the sensor assembly, the sensor assembly including:a primary electromagnetic coil arranged to generate a time varying magnetic field; anda secondary electromagnetic coil coaxial with the primary electromagnetic coil, the primary and secondary electromagnetic coils being wound around a core body coaxially, the secondary electromagnetic coil being arrangedto detect the time varying magnetic field as affected, directly or indirectly, by the lead screw, andto output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the lead screw;whereinthe core body being formed of a material having the same conductivity and/or magnetic permeability as the lead screw to which the control rod is connected in the nuclear reactor. 2. A sensor assembly according to claim 1 including a plurality of primary electromagnetic coils. 3. A sensor assembly according to claim 1 including a plurality of primary and secondary coils, wherein the primary coils are mutually arranged in electrical series; and/or wherein the secondary coils are separately mutually arranged in electrical series. 4. A sensor assembly according to claim 1 wherein the primary and/or secondary coils are formed of an alloy comprising 86% copper, 12% Manganese and 2% Nickel. 5. A sensor assembly according to claim 1 wherein the core body is formed of the same material as the lead screw. 6. A sensor assembly according to claim 1 including a plurality of secondary electromagnetic coils. 7. A sensor assembly according to claim 6 wherein the plurality of primary and secondary coils are arranged in a mutually alternating sequence of primary and secondary coils. 8. A method of optimising the output of a sensor assembly for measuring the relative position of a control rod connected to a lead screw within a nuclear reactor from within a metallic probe tube housing the sensor assembly, the sensor assembly including a primary electromagnetic coil arranged to generate a time varying magnetic field and a secondary electromagnetic coil coaxial with the primary electromagnetic coil, the primary and secondary electromagnetic coils being wound around a core body coaxially, the secondary electromagnetic coil being arranged to detect the time varying magnetic field as affected, directly or indirectly, by the lead screw, and to output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the lead screw, the core body being formed of a material having the same conductivity and/or magnetic permeability as the lead screw to which the control rod is connected in the nuclear reactor, the method including the steps of:supplying the primary coil with an alternating current to result in the generated time varying magnetic field;locating the lead screw in a first position and recording the signal output by the secondary electromagnetic coil for a range of respective frequencies of the supplied alternating current;locating the lead screw in a second position and recording the signal output by the secondary electromagnetic coil for the range of respective frequencies of the supplied alternating current;calculating, for each of the respective frequencies, a value for the span-to-offset ratio of the measured signals on the basis of the respective signals measured for the lead screw in the first and second positions; anddetermining the frequency of the supplied alternating current which provides the maximum span-to-offset ratio on the basis of the calculations. 9. A method according to claim 8, whereinwhen the lead screw is in the first position, the output from the secondary coil is a maximum; and/orwhen the lead screw is in the second position, the output from the secondary coil is a minimum. 10. A method according to claim 8, whereinthe calculation step includes, for each respective frequency:calculating the difference between the amplitudes of the signals measured for the lead screw in the first and second positions; anddividing the difference by the amplitude of the signal measured for the lead screw in the second position. 11. A combination of a lead screw connected to a control rod within a nuclear reactor and a sensor assembly for measuring the relative position of the control rod connected to the lead screw within the nuclear reactor from within a metallic probe tube housing the sensor assembly, the combination including:a lead screw connected to a control rod within a nuclear reactor; anda sensor assembly including:a primary electromagnetic coil arranged to generate a time varying magnetic field; anda secondary electromagnetic coil coaxial with the primary electromagnetic coil, the primary and secondary electromagnetic coils being wound around a core body coaxially, the secondary electromagnetic coil being arrangedto detect the time varying magnetic field as affected, directly or indirectly, by the lead screw, andto output, on the basis of the detected time varying magnetic field, a signal indicative of the relative location of the lead screw;whereinthe core body being formed of a material having the same conductivity and/or magnetic permeability as the lead screw to which the control rod is connected in the nuclear reactor.