Inspection of the Steam Generator Heat Transfer Tubes for FBR Monju Restart

Abstract

Japanese prototype FBR Monju will restart its operation for the first time since the occurrence of the sodium leak accident in 1995. The integrity of its components after the long suspension needs to be confirmed before restarting. One of the most important components of Monju is steam generator (SG) unit that was first tested in the 2007.11–2008.3 period during plant-confirmation test. The safety authority required that Monju components should have as same integrity as at the beginning of the first operation in 1994. Now the SG tubes have been installed in the vessel and it is impossible to test them directly. In order to satisfy the safety authority requirement, presumable deteriorations of SG tubes in the long-term suspension were investigated at first and these were limited to only the lack of thickness by corrosion, which are detectable by three tests: eddy current testing (ECT), visual testing (VT), and leak check. These tests were performed with customized procedures and evaluated for appropriate criteria. The ECT was applied to check for the lack of thickness of SG tubes caused by regional corrosions. The ECT probe passes through the full length of a tube with the speed of 200mm/s. Integrity of the tube was checked by amplitude and phase chart. Threshold of the approval thickness was decided based on the tube data at the time of manufacturing. The thickness of the tube is 3.5mm and it cannot be reduced less than 7% lack of the thickness. The characteristic signal at special points like bends or SG support plates were identified by their phase chart. Based on these evaluations, no significant deterioration was detected in SG tubes excepting the special undetectable region. The VT checked for corrosions on the inner surface of tubes. It covers the undetectable regions by ECT. The testing region is between the tube-plate and the bottom U tube along the down comer. The condition of the other regions, subjected to the same environment conditions (temperature, humidity, time not being in service, others) is expected to be similar to the tested region. A customized CCD camera was operated with about 10 mm/s along the tube. The test did not reveal any significant corrosion. The leak test checked for penetrating holes in SG tubes. After keeping a high pressure argon gas outside of SG tube, the nitrogen gas inside of SG was pumped to a mass spectrometer. In this test, no argon gas was detected by the mass spectrometer. After evaluation of all results of three tests, it was confirmed that there is no significant lack of thickness and penetrating holes in SG tubes. Therefore, it was proved that SG heat transfer tubes have the required integrity for reoperation.