Effects of process-generated hydrogen on RPV walls

2.3. Origin of the indications

the largest indications was measured being 179 mm large. In the mean time, these results are converted to the detection parameters used during the inspection in 2012, in order to be able to compare them and see any possible evolution of the indications over this period. It was concluded by Electrabel that the indications were stable and did not grow between both inspections. [14] For the examination of the Tihange 2 RPV, a similar timeline can be constructed. First UT inspection of the RPV was performed in September 2012 during a planned outage of the reactor. The inspection resulted in a detection of 1931 indications in the upper core shell and 80 indications in the lower core shell. Similar to the procedure in Doel 3, a new improved UT inspection was performed on the Tihange 2 RPV in 2014, resulting in 1901 indications in the upper core shell and 76 indications in the lower core shell. The decrease of the number of indications between both inspections is attributed to the merged detection of some closely neighbouring indications for the 2014 UT inspection. As for Doel 3, an increase in the mean indication size is observed going from 8 – 10 mm in 2012 to 14 – 16 mm in 2014 with maximum dimensions up to 155 mm. [16] Table 2.1 gives an overview of the number and dimension of the indications measured in the core shells of the Doel 3 and Tihange 2 RPV. Table 2.1: Number and size of indications measured in the core shells of the Doel 3 and Tihange 2 RPV by the UT inspection methods used in 2012 and 2014. The size of the indications is given in mm and expressed as dimension in X and Y direction. [7, 13, 14, 16]

Reactor Unit Core shell

number/size

2012 inspection 2014 inspection

number

857

1440

upper

average size [mm] (X–Y) maximum size [mm] (X–Y)

8.8 – 7.6

13.7 – 12.3 56.4 – 45.3

31.0 – 26.4

Doel 3 [7, 14]

number

7205

11607

lower

average size [mm] (X–Y) maximum size [mm] (X–Y)

9.6 – 7.6

16.0 – 12.7 179.0 – 72.3

67.9 – 38.4

number

1931

1901

upper

average size [mm] (X–Y) maximum size [mm] (X–Y)

9.8 – 7.9

14.8 – 13.8 154.5 – 70.9

38.0 – 25.4

Tihange 2 [13, 16]

number

80

76

lower

average size [mm] (X–Y) maximum size [mm] (X–Y)

10.2 – 9.3 27.4 – 19.1

15.5 – 15.4 33.1 – 27.6

2.3 Origin of the indications Lots of investigations have been carried out to find the origin of these indications. [1, 14, 16–22] These investigations pointed so called “hydrogen flakes” to be the root 15