Patent Number: 055442072
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of the present invention will firstly be explained. FIG. 1 is a perspective view showing a magnetic yoke having an appropriate length and cross-sectional area of the magnetic path and an exciting coil, the magnetic yoke being placed closely in contact with the interior of the pressure vessel of a nuclear reactor, the pressure vessel being shown in a fragmentary sectional view. In FIG. 1, the reference numeral 101 designates a magnetic yoke, the numeral 102 an exciting coil, the numerals 103a and 103b the portions of the magnetic yoke 101 adapted to be closely placed in contact with the pressure vessel of the nuclear reactor, the numeral 104 the inner wall surface of the pressure vessel of the nuclear reactor, the numeral 105 an overlay clad of the inner wall of the pressure vessel of the nuclear reactor and the numeral 106 a vessel section of the pressure vessel of the nuclear reactor. FIG. 2 is a sectional view taken along the line C--C' which passes through the center of the magnetic yoke 101 shown in FIG. 1. In FIGS. 1 and 2, the overlay clad 105 is made of non-magnetic stainless steel while the vessel section 106 is made of a low carbon, ferromagnetic low alloy steel. In the condition shown in FIGS. 1 and 2, if an exciting current is caused to flow through the exciting coil 102 and the magnetic yoke 101 is magnetized thereby, the magnetic fluxes flow as shown in FIG. 3 from the magnetic yoke 101 to the overlay clad 105 of the non-magnetic stainless steel and the vessel section 106 of the low carbon steel. In FIG. 3, the reference numerals 301a through 301i designate the flow of the magnetic fluxes, the symbol g the thickness of the overlay clad 105, the symbol l the length in the direction of the line C--C' of the closely contract portion 103a of the magnetic yoke 101 placed closely in contact with the pressure vessel of the nuclear reactor and the symbol x the distance in the direction of the line C--C' from the half point of l as the reference. In this case, the spatial distribution Hv(x) of the magnetic field component orthogonally crossing with the inner wall surface 104 of the pressure vessel of a nuclear reactor is represented by the curve 401 in FIG. 4. And it is to be noted that the medium value h of the spatial distribution Hv(x) at the point x=0, and the half value width W.sub.1/2 indicating the distance between two points in the direction of x where the magnitude of the spatial distribution Hv(x) are h/2 varies only along with the thickness g of the overlay clad 105 if the geometrical dimension and the magnetic characteristics of the material of the magnetic yoke 101 and of the pressure vessel of the nuclear reactor are determined, and such a relation can be as shown in FIG. 5. This relation can be readily predetermined by using a static magnetic field analysis method such as the definite element method, the boundary element method or the like. Accordingly, if the measured value .sup.m h or .sup.m W.sub.1/2 respectively of the medium value h or the half value width W.sub.1/2 of the spatial distribution Hv(x) of the magnetic field are obtained by measuring the distribution of the magnetic field, the value .sup.m g of the thickness of the overlay clad 105 can be known. The first embodiment of the present invention based on the above-mentioned principle is illustrated in FIG. 6. In FIG. 6, reference numerals identical to those shown in FIG. 1 designate; the same components as those designated by the same reference numerals shown in FIG. 1. The present embodiment is characterized in that a plurality of magnetic field sensors 601 are disposed along the straight line defined on the inner wall surface 104 of the pressure vessel of the nuclear reactor by the central plane C--C' of the magnetic yoke in the system shown in FIG. 1. FIG. 7 is a sectional view taken along the line C--C' passing through the center of the magnetic yoke 101 shown in FIG. 6. The spatial distribution of the magnetic field component orthogonally crossing with the inner wall surface 104 of the pressure vessel of the nuclear reactor is measured by a group of the magnetic field sensors 601. Then, the thickness of the overlay clad 105 can be obtained from the medium value h or the half value width W.sub.1/2 of the measured spatial distribution of the magnetic flux component, by using the relationship between the medium value h or the half value width W.sub.1/2 and the thickness g of the overlay clad 105 which has been determined in advance by the static field analysis. For the magnetic field sensors comprising the group of the magnetic field sensors 601, such comparatively cheap elements as Hall elements, magnetic resistance elements or the like can be used. The second embodiment of the present invention based on the above-mentioned principle is illustrated in FIG. 8. The present embodiment is characterized in that one magnetic field sensor 801 is used in place of a plurality of the magnetic field sensors 601 as a group employed in the first embodiment shown in FIG. 6 in such a manner as the magnetic field sensor 801 is travelled on the inner wall surface 104 of the pressure vessel of the nuclear reactor so as to measure the spatial distribution of the magnetic field. In FIG. 8, reference numerals identical to those shown in FIG. 6 designate identical components to those designated by the same reference numerals as those shown in FIG. 6. According to the present embodiment, a clearance g.sub.o is required to allow the magnetic field sensor 801 to travel therethrough. However, since the clearance g.sub.o can be incorporated in the computation process to the static magnetic field analysis and then the relation between the medium value h or the half value width W.sub.1/2 of the distribution of the magnetic field and the thickness g of the overlay clad 105 can be predetermined, there will be no problems. The present invention having been described in a detailed by referring to certain preferred embodiments, it will be understood that changes and the modifications can be made within the spirit and the scope of the claims of the present invention.