Critical Reflections about Doel3 & Tihange2

Integrity reactor vessels Doel 3 and Tihange 2

Page: 13

sampled and the hydrogen content was measured [ 14 ] p43. After cooling down the ingot in a controlled manner, samples of the metal are taken and the hydrogen content was measured again. The amount of hydrogen present in the steel of the Doel3 lower core shell was for all measurements 1.5 ppm [ 14 ] p44. During cooling of the ingot, impurities and alloying elements segregates into so called ghost lines which are sensitive to hydrogen cracking [ 9 ] . As solidification progresses from the outside wall inwards, the internal part of the ingot will be enriched with impurities and alloying elements, while the outside part will be poor in alloying elements, due to the higher solubility of the alloying ele- ments in the liquid phase. During forging, the parts of the ingot with the high segregation areas have been removed by cutting off the bottom and the top from the ingot and in a next step the centre part of the ingot which also contain high segregation areas has been pierced out. From the resulting hollow cylinder, the reactor vessel shell has been forged. After forging and cooling until room temperature, the inner and the outer wall will be machined by cutting away about 40 mm at each side on a lathe. Hypothesis of Electrabel. The hypothesis of Electrabel, as presented in the paper [ 9 ] and also in their communication on page 28, is that when the segregation areas finally transform from the γ -phase to the α -phase during cooling of the ingot, they become supersaturated in hydrogen which will recombine to hydrogen gas H 2 at trapping sites in the metal, building up an internal pressure. The combination of this internal pressure and local stresses can lead to cracking during or shortly after fabrication of the shells. So, Electrabel concludes that the cracks were already present at the start-up of the reactor. Available amount of hydrogen in case of a uniform distribution over the shell. Figure 10 presents the solubility of hydrogen in iron, where the curve of 1 atm should be followed. The necessary hydrogen concentration on the curve in figure 10 is 5 ppm at 900 o C before the γ - α - transformation starts to be supersaturated in the α -phase. However, as the hydrogen concentration has been measured in the melt to be 1.5ppm and afterwards in the cooled metal also to be 1.5ppm, concentration of hydrogen did not occur as the metal has never become saturated, the level of hydrogen present in the metal remains at least 3 times below the saturation level. The saturation of hydrogen occurs on the curve in figure 10 at 400 o C at a hydrogen level of 1.5ppm. It is unrealistic to

Total dissolved available H 2 volume

concentration H 2 volume

for cracking (Nml / dm 3 )

(Nml / dm 3 )

(ppm)

threshold concentration uniform concentration

0.8 1.5

70

–

131

61

Table 1: Volume of normal cm 3 H

3 steel

2 in 1 dm

assume that each individual atom hydrogen diffuses towards the crack zones. Some of the hydrogen will escape to the atmosphere, another part will be trapped on the crystal boundaries and inclusions in the metal without initiating crack formation, and a part remains in solution. In order to have an estimate of the amount of hydrogen which does not take part to crack formation, the amount of

R.Boonen & J.Peirs

May 18, 2017