Patent Number: 055966128
Section: summary

The present invention relates to a testing arrangement for materials testing at lead-throughs, especially for control rods, in pressurized-water reactors, to a probe sword suitable for the mentioned testing arrangement and to a method for materials testing at lead-throughs. BACKGROUND OF THE INVENTION At a pressurized-water reactor in a nuclear power plant there is a dome-shaped cap, with cap lead-throughs for a control rod mechanism, which is detachable attached to the cap. In the pressurized-water reactor there are also, in the cap or in the bottom, other lead-throughs for different instruments and vent pipes, which are connected to a kind of expansion vessel in the shape of an external pressure vessel with a water column. A lead-through for a control rod consists of a tube passing through the cap, which tube is welded to the cap at its lower side. The lead-through ends at the inside of the cap with a downward inner conical opening having a cone angle of a few degrees, normally about 3 degrees. In the lead-through there is an essentially coaxial tubular sleeve, which ends with a downward essentially funnel-shaped end-piece. The lead-through sleeve is at a seat at the top sealingly joined to the projecting end of the tube. Between the tube and the sleeve, there is a gap with a nominal width of a few millimeters, normally about 3 mm. The purpose of the interior sleeve in a control rod lead-through is to make the longitudinal axis of the lead-through opening adjustable in relation to axial openings in means located inside the reactor. Therefore, the annular gap does not normally have equal width around the sleeve, due to the fact that it is used as a trimming allowance. In a cap, there is normally a total of about 60-75 control rod lead-throughs. It has been found that leakage can arise at the control rod lead-throughs and that the leakage is due to cracks in the tubes of the lead-throughs, more particularly in the area close to the weld that connects a tube with the cap. If there is a crack in the tube above the weld, water can come up on the outside of the cap when the reactor is pressurized, while a crack below the weld is less dangerous since it does not give any external leakage. For the sake of security, it is necessary to test the material at the lead-throughs in order to detect cracks and determine their characteristics, such as size, position and orientation. According to prior art, inspection of a reactor cap is carried out by dismounting the cap with the control rod mechanism from the reactor and place it on a biological concrete shield. The occurrence of defects at each lead-through is checked with the aid of a manipulator and by means of eddy current technology, whereby an eddy current probe is inserted into said gap between the sleeve and the tube. The eddy current probe is inserted into the gap by means of a thin metal sword, the probe being mounted at the top of the sword. The sword is bent past the end-piece of the sleeve and into the gap by means of reels. The sword can only be led forwards in the vertical direction because of the flexibility of the sword, and has to be pulled out of the gap before a displacement around the sleeve can be carried out. If defects are detected at one or more lead-throughs with said eddy current testing, the whole control rod mechanism and the lead-through sleeve with its end-piece are dismounted for the lead-through at issue. For the dismounting of the sleeve, the control rod mechanism with the associated sealing means at the top of the lead-through is disassemble, the end-piece is dismounted and the sleeve is pulled out of the tube. Thereafter, the characteristics, such as depth, length, position and orientation of each defect is determined by means of ultrasonic testing and with the aid of a manipulator placed above the cap, whereby the interest today is mostly focused on longitudinal cracks. Such a reactor inspection is expensive, time-consuming, entails the dismounting of the sleeves only for the ultrasonic testing of the insides of the tubes and entails manual handling which results in radiation exposure of the persons carrying out the inspection. Today, there is in a number of countries an acute need for testing pressurized water reactors with respect to cracks in the lead-throughs and a need for an appropriate testing equipment that eliminates unnecessary irradiation of people. Thus, it is desirable to be able to determine the characteristics of the defects without the disadvantageous dismounting of the sleeves. It is a purpose of the present invention to provide a simplified and improved arrangement and method for testing a pressurized-water reactor. In particular, it should be possible to carry out both the detection of the cracks and the determination of their characteristics without the expensive and hazardous disassembling of the control rod mechanism and the lead-through sleeve, and thereby reduce the dose of radiation for people. In accordance with the preferred embodiment, it should be possible to carry out the entire testing by means of the manipulator which, according to prior art, is placed inside the dismounted cap. The above mentioned and other purposes and advantages are obtained with a testing arrangement, a probe sword and a method as set out in the independent claims 1, 4 and 10, respectively. The invention will now be described further by the explanation of an embodiment and in conjunction with the drawings. It is shown in: FIG. 1--an explanatory overview of a pressurized-water reactor cap with a testing arrangement according to the invention mounted at a manipulator placed under the cap, FIG. 2--an explanatory sketch of a lead-through, FIG. 3--a sketch of a weld joint unfolded in a plane, FIG. 4--a sketch of a preferred embodiment of a testing arrangement according to the invention, FIG. 5--an elevating motor arrangement in a lifting arrangement of a testing arrangement according to the invention, FIG. 5A--is a top plan view of FIG. 5, FIG. 6--a brake arrangement in a lifting arrangement of a testing arrangement according to the invention, FIG. 6A--is a top plan view of FIG. 6, FIG. 7--an explanatory sketch of the testing arrangement according to the invention with associated peripheral equipment, whereby the testing arrangement is docked to a cap lead-through, FIG. 8--the upper part of a testing arrangement according to the invention docked to a cap lead-through, FIG. 9a--an explanatory cross-section of a lead-through after a deflection of the sleeve, FIG. 9b--an explanatory longitudinal cut through a lead-through, with the essential shape of a probe sword in an inserted position drawn in the gap, FIG. 10--an explanatory sketch of a probe sword according to the invention, FIG. 11--an explanatory sketch of the upper part of a probe sword according to the invention, with five cross-sections of the sword, FIGS. 12a and 12b--a probe sword head with a cleaning probe according to the invention, FIGS. 13a and 13b--a probe sword head with an eddy current probe according to the invention, FIGS. 14a and 14b--a probe sword head with an ultrasonic probe according to the invention.