Effects of process-generated hydrogen on RPV walls

2.2. Flaw indications in Doel 3 and Tihange 2

2.2 Flaw indications in Doel 3 and Tihange 2 The UT inspections on the Doel 3 RPV in June 2012 were performed over a height of 4080 mm, starting from 3320 mm under the transition flange RPV up to 7400 mm, and cover the complete circumference of the RPV. The complete height of the core was covered by this examination. This inspection showed no presence of underclad defects over the complete investigated area. However, 158 indications of a different type of defect were found. These indications were found to be quasi-parallel to the RPV wall surface, were “almost circular” in shape, had a mean diameter of 15 mm with a maximum up to 30 mm and a flaw density up to 40 indications per dm 3 . However, as this inspection technique is only qualified for the detection of underclad defects, it is not reliable for the measurement of indications, which are different in shape, orientation and depth compared to underclad defects. [7] To overcome the limitations of the inspection method used before, a new UT inspection over the complete height of the RPV is performed in July 2012. This UT inspection is the same as used for the inspection of the welds and HAZ of the RPV performed on a 10-yearly basis and investigates the full thickness of the RPV wall. The so called “MIS-B”, Machine d’inspection en Service Belge , used is shown in Figure 2.4. [1] The inspection confirms the presence of a large amount of quasi-parallel indications, especially in the core shells of the RPV. The highest number of these indications were found in the lower core shell, where 7205 indications were detected. In the upper core shell 857 indications were detected. All indications are found in the base material, outside the weld regions, of which the major part is situated at a depth between 10 and 100 mm from the inner surface. Their distribution as a function of depth can be seen in Figure 2.5. [1] Similarly, the distribution as a function of depth and height is shown in Figure 2.6 [1]. The figure shows the detected indications over a cumulative angle of 20 ◦ , where the orange indications are the larger ones compared to the pink colored indications. One can see that the indications are found deeper in the RPV wall with increasing height, along a diagonal. The indications are interpreted to be parallel to the wall surface and slightly smaller than estimated from the first UT inspection, with a mean diameter between 8 and 10 mm, while some indications are measured to be up to 70 mm. [13, 14] A quantitative control of the UT inspection on independent samples in 2014 revealed that the detection parameters of the machine and the interpretation method of the signals might result in an underestimation of the real number of indications and their size. Therefore, the detection parameters were adapted in order to have a conservative measurement of the indications, i.e. to guaranty that the measured size of the indication is equal to or larger than its real size and that every indication is detected. A result of this adaptation is that closely neighboring indications might be detected as one big indication. [15] During the outage of Doel 3 in 2014, this improved UT inspection was performed. An approximate increase of 60% more indications were detected compared to the inspection in 2012, 1440 indications in the upper core shell and 11607 indications in the lower core shell. Together with this increase in number of indications, also the dimension of the indications was found to be larger. The average size of the indications was found to be 12 to 16 mm, while 13