Electrochemical aspects on corrosion in Swedish reactor containments

Post-stressed concrete is used in all Swedish nuclear reactor containments. Steel in concrete is normally protected from corrosion by the highly alkaline pore solution in concrete. A passive film develops on the surface of steel in contact with the pore solution. However, corrosion may still occur under special circumstances. It is therefore desirable to monitor the corrosion status of the containment. A review of the corrosion experience with steel in concrete strongly suggests that the potential problem of most concern for the Swedish reactor containments is cavity formation during grouting of tendons and of penetrations in the containment wall. Cavities break the contact between alkaline grout and steel. Corrosion is then possible, provided the relative humidity is high enough. Normal methods for inspection of the corrosion status of steel reinforcement in concrete are not applicable to very heavy structures like reactor containments. Since inspections are difficult to carry out, it is important that they be focused on the most susceptible portions of the containment. This report is an attempt to assemble potentially useful background information. The original intention was to focus on electrochemical methods of investigation. When it was realized that the potential use of electrochemical methods was limited, the scope of the review was broadened. The present as well as previous investigations indicate that nondestructive testing of grouted tendons is the outstanding problem in the condition assessment of Swedish nuclear reactor containments. Grouted tendons are also used in a very large number of bridges built since the early 1950s. The experience gained in connection with bridges has therefore been investigated. The need for a testing method for grouted tendons in bridges has long been strongly felt and development work has been in progress since the early 1970-ies, for example within the Strategic Highway Research Project in the Unite States. Potential methods for nondestructive testing have been evaluated a number of times. No practical testing method for grouted tendons in bridges has thus far been developed or is in its final state of development. To investigate the corrosion status of a grouted tendon, typically a nondestructive method is first used to detect cavities in the duct. An inspection hole is then drilled and the corrosion status of the tendons investigated using an endoscope. Finally, the cavity is grouted using vacuum injection. Selected nondestructive methods have been tested on the Barsebaeck 1 containment within the CONMOD project. High energy radiography was found to be a useful method for detecting cavities in grout. To inspect the corrosion status within cavities, an inspection hole was first drilled. An endoscope was then used. A factor strongly affecting the corrosion rate of a non-passivated steel surface in concrete is the relative humidity. The corrosion rate is high at a relative humidity of 95-98 %, but negligible at a relative humidity of 70 %. In the CONMOD project, a relative humidity of approx. 80 % was measured at the position of the tendons in the Barsebaeck 1 containment. Relative humidity is initially high in concrete, but decreases with time. In heavy designs, like nuclear reactor containments, the concrete dries extremely slowly. In the opinion of the author, a nondestructive testing method for grouted tendons cannot be expected to be available in the foreseeable future. The available alternative is to use intrusive testing in a limited number of places. It is then particularly important to select the places where testing is performed in the best possible way. The selection should be made in a stepwise process. The following procedure is suggested. 1. Calculate the temperature distribution in the containment wall. 2. Measure the relative humidity at selected points within the wall. 3. Map out where cavities in the grout are most probable (vents, etc). 4. Select a number of points where at the same time the relative humidity is high and cavity formation probable. 5. Perform radiography. 6. Drill an inspection hole and investigate the corrosion status using fiber optics if a cavity is indicated. 7. Grout the cavity using vacuum injection. Condition assessment of grouted tendons is a major challenge. However, a well performed grouting provides superior protection against corrosion. Non-grouted tendons are considerably more at risk. The condition assessment of non-grouted tendons in the Swedish nuclear reactor containments must be adapted accordingly