Abstract
In order to validate capabilities of ultrasonic testing (hereinafter, UT) from outside surfaces of reactor pressure vessel (RPV) (ex-vessel UT) to detect cracks and determine the cracks size, Hamaoka nuclear power station unit 1 (Hamaoka unit-1) owned by Chubu Electric Power Co., Inc., which is under decommissioning work was selected to be used as a validation plant for ex-vessel UT since there were a lot of cracks found on the nickel based alloy material welds of shroud support plate to RPV and of shroud support plate leg to RPV, and in the future the capabilities of the ex-vessel UT will be evaluated by comparing with direct measurement results of the crack samples taken out from the plant. Examination procedures of ex-vessel UT for those welds in Hamaoka unit-1 were established separately by two Japanese RPV suppliers and each company performed ex-vessel UT to the weld of shroud support plate and shroud support leg jointing to RPV by their own procedures. In the validation test, the results of crack sizing showed significant differences between two companies. However, the differences could become smaller by conforming relative UT parameters of two companies to each other. It was assumed that the reasons to cause the differences were due to differences of detectability on actual field work, beam orientation and angle, beam frequencies, automatic or encoded manual ultrasonic examinations（EMUT）of scanning and decision criteria of boundary between low alloy steel (RPV) and buildup welds. The reason for the discrepancy in the crack sizing results would be clarified in the evaluation by comparing with direct measurement results of the crack samples.
