고온화학세정환경에서 20 % EDTA 용액이 결함 전열관 (Alloy600)에 미치는 영향

Abstract

The transport and deposition of corrosion products in pressurized water nuclear reactor (PWR) steam generatorshave led to corrosion (SCC, denting etc.) problems. Lancing, mechanical cleaning and chemical cleaninghave been used to reduce these problems. The methods of lancing and mechanical cleaning have limitationsin removing corrosion products due to the structure of steam generator tubes. But high temperature chemicalcleaning (HTCC) with EDTA is the most effective method to remove corrosion products regardless of thestructure. However, EDTA in chemical cleaning aqueous solution and chemical cleaning environments affectsthe integrity of materials used in steam generators. The nuclear power plants have to perform the pre-test(also called as qualification test (QT)) that confirms the effect on the integrity of materials after HTCC. This is one of the series studies that assess the effect, and this study determines the effects of 20 % EDTAaqueous solution on defective tubes in high temperature chemical cleaning environments. The depth andmagnitude of defects in steam generator (SG) tubes were measured by eddy current test (ECT) signals. Surface analysis and magnitude of defects were performed by using SEM/EDS. Corrosion rate was assessedby weight loss of specimens. The ECT signals (potential and depth %) of defective tubes increased marginally. But the lengths of defects, oxides on the surface and weights of specimens did not change. The averagecorrosion rate of standard corrosion specimens was negligible. But the surfaces on specimens showed tracesof etching. The depth of etching showed a range on the nanometer. After comprehensive evaluation ofall the results, it is concluded that 20 % EDTA aqueous solution in high temperature chemical cleaningenvironments does not have a negative effect on defective tubes.