Patent Number: 055442072
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of and an apparatus for measuring in a non-destructive manner the thickness of the overlay clad of the pressure vessel of a nuclear reactor. 2. Prior Art An overlay clad of stainless steel is provided on the inside of a vessel made of a low alloy steel in a pressure vessel of a nuclear reactor and it is necessary to measure in a non-destructive manner the thickness of the overlay clad in the course of the testing of the soundness of this pressure vessel of the nuclear reactor. According to a prior-art, in ultrasonic wave measurement method has been proposed which method utilizes the reflection of the ultrasonic wave from the interface between the overlay clad and the low alloy steel. According to the ultrasonic wave measurement method of a prior art, since the densities of and the sonic velocities in the stainless steel and the low alloy steel are respectively almost the same as each other, the reflection of the ultrasonic wave from the interface of the stainless steel and the low alloy steel where they are well integrated by means of weld penetration is not sufficient to allow accurate measurement. In fact, the thickness of the overlay clad of the stainless steel applied to the inner surface of the pressure vessel of a nuclear reactor is in the order of 5-10 mm and measurement of the thickness of that overlay clad is preferably executed with a resolution in the order of 0.1 mm for practical usage. It is to be pointed out, however, that the ultrasonic wave measurement method according to a prior art is unable to attain such a desired accuracy. SUMMARY OF THE INVENTION An object of the present invention is to provide a method of and an apparatus for measuring the thickness of the overlay clad of the pressure vessel of a nuclear reactor with a favorable accuracy by solving the drawbacks above pointed out. In order to attain the above-mentioned object, the method according to the present invention of measuring the thickness of the overlay clad of the pressure vessel of a nuclear reactor comprises the steps of: placing a magnetic yoke having an appropriate length and cross-sectional area of the magnetic path closely in contact with or in proximity to the surface of the overlay clad of said pressure vessel of the nuclear reactor to form a magnetic path by said magnetic yoke and said pressure vessel of the nuclear reactor; magnetizing said magnetic yoke; measuring the distribution of the magnetic field on or in proximity of the surface of said overlay clad closely in contact with which or in proximity with said magnetic yoke is placed; and identifying from the distribution of the magnetic field thus measured the thickness of the overlay clad of said pressure vessel of the nuclear reactor. In order to attain the above-mentioned object, the apparatus according to the present invention for measuring the thickness of the overlay clad of the pressure vessel of a nuclear reactor comprises: a magnetic yoke having an appropriate length and cross-sectional area of the magnetic path, said magnetic yoke being placed closely in contact with or in proximity with the surface of said overlay clad of said pressure vessel of the nuclear reactor to form a magnetic path by said magnetic yoke and said pressure vessel of the nuclear reactor and said magnetic yoke further having at least one exciting coil for causing said magnetic yoke to be magnetized; and a means for measuring the distribution of the magnetic field on or in proximity with the surface of said overlay clad closely in contact with which or in proximity with which said magnetic yoke is placed. The present invention applies the fact as described later that a magnetic yoke having an appropriate length and cross-sectional .area of the magnetic path is placed closely in contact with or in proximity with the surface of the overlay clad of the pressure vessel of a nuclear reactor to form a magnetic path by said magnetic yoke and said pressure vessel of the nuclear reactor, and the magnetic yoke is then magnetized, whereby the distribution of the magnetic field thus generated on or in proximity of the surface of the overlay clad closely in contact with which or in proximity with which the magnetic yoke is placed will be subject to variation only by the thickness of the overlay clad if the geometrical dimension and the magnetic characteristics of the material of the magnetic yoke and the pressure vessel of the nuclear reactor are decided. Accordingly, in the method according to the present invention for measuring the thickness of the overlay clad of the pressure vessel of a nuclear reactor, in accordance with the construction as described above, the magnetic yoke is placed closely in contact with or in proximity with the surface of the overlay clad of the pressure vessel of a nuclear reactor to form a magnetic path by means of the magnetic yoke and the pressure vessel of the nuclear reactor, and the magnetic yoke is then magnetized, whereby the distribution of the magnetic field thus generated on or in proximity with the surface of the overlay clad closely in contact with which or in proximity with which the magnetic yoke is placed is measured, and, because the distribution of the magnetic field thus measured varies only depending on the thickness of the overlay clad due to the geometrical dimension and the magnetic characteristics of the material of the magnetic yoke and the pressure vessel of the nuclear reactor being determined in advance, the thickness of the overlay clad of the pressure vessel of the nuclear reactor is identified. Therefore, the method according to the present invention of the measurement makes it possible to measure the thickness of the overlay clad of the pressure vessel of a nuclear rector with a practically desired accuracy, for example, the resolution of 0.1 mm relative to the thickness of the overlay clad of 5-10 mm. Furthermore, in the apparatus according to the present invention for measuring the thickness of the overlay clad of the pressure vessel of a nuclear reactor, the magnetic yoke is placed closely in contact with or in proximity with the surface of the overlay clad of the pressure vessel of a nuclear reactor to form a magnetic path by means of the magnetic yoke and the pressure vessel of the nuclear reactor, and the magnetic yoke is then magnetized by the exciting coil to generate a magnetic field on or in proximity with the surface of the overlay clad closely in contact with which or in proximity with which the magnetic yoke is placed, and the distribution of the magnetic field thus generated is measured by the measuring means of the distribution of the magnetic field. Accordingly, the measuring apparatus is capable of accurately measuring the distribution of the magnetic field generated on or in proximity with the surface of the overlay clad closely in contact with which or in proximity with which the magnetic yoke in placed, the distribution of the magnetic field being variable only depending on the thickness of the overlay clad of the pressure vessel of the nuclear reactor.